DIY Electric Car Forums banner

1 - 14 of 14 Posts

·
Registered
Joined
·
70 Posts
Discussion Starter #1
If I go with a Brusa charger with a nicad profile does anyone know if
you can use two 3.3kW parallel for faster charging? Victor mentioned
this and I forgot if it would function the same and if there were
some issues doing this.

Thanks,

mark


_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #3
On 1 Sep 2007 at 13:02, Mark Dutko wrote:

> If I go with a Brusa charger with a nicad profile does anyone know if
> you can use two 3.3kW parallel for faster charging?

Brusa made (and may still make) chargers that could be "stacked." At one
point they offered a 10kW charger actually comprising three NLG4 chargers.

I think you had to order the extras as boosters, however. I don't know
whether you could modify a "normal" charger to act as a booster. Victor may
know, or be able to find out.

David Roden - Akron, Ohio, USA
EVDL Administrator

= = = = = = = = = = = = = = = = = = = = = = = = = = = = =
EVDL Information: http://www.evdl.org/help/
= = = = = = = = = = = = = = = = = = = = = = = = = = = = =
Note: mail sent to "evpost" or "etpost" addresses will not
reach me. To send a private message, please obtain my
email address from the webpage http://www.evdl.org/help/ .
= = = = = = = = = = = = = = = = = = = = = = = = = = = = =


_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #4
On 1 Sep 2007 at 18:02, Morgan LaMoore wrote:

> There might or might not be problems with it in parallel; series would
> probably be a better bet.

I expect you're going to have a LOT of trouble connecting smart chargers in
series. You'd be better off to connect each charger to half the battery
pack.

David Roden - Akron, Ohio, USA
EVDL Administrator

= = = = = = = = = = = = = = = = = = = = = = = = = = = = =
EVDL Information: http://www.evdl.org/help/
= = = = = = = = = = = = = = = = = = = = = = = = = = = = =
Note: mail sent to "evpost" or "etpost" addresses will not
reach me. To send a private message, please obtain my
email address from the webpage http://www.evdl.org/help/ .
= = = = = = = = = = = = = = = = = = = = = = = = = = = = =


_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #5
> I expect you're going to have a LOT of trouble connecting smart chargers in
> series. You'd be better off to connect each charger to half the battery
> pack.
>

That's what I meant; that's what the second paragraph was supposed to say.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #6
Mark,

Yes, you can connect two (or more) BRUSA NLG5 chargers in parallel
as long as you set up different voltages in U section of IUx charging
profile so the chargers don't fight during regulation stage but merely
just add charging power (current) during bulk charge phase.
In general any section of charging profile with fixed voltage should
have different voltage settings for all the chargers in parallel.

This has nothing to do with the battery chemistry, NiCd profile (as well
as any other) is OK for paralleled chargers.

You can also connect two chargers in master-slave configuration, but
it is more complex and expensive (special cables needed) and the outcome
is identical. This is good option only if master charger is not BRUSA
one, the slave is capable of tracking (and so doubling) the current
of any type master charger.

Of course, you can use two separate chargers connected each to the half
of the string. Danger is one half can finish before the other half, and
in general tracking of total Ah charged in this way (which is preferred
for NiCd) is not doable.

Victor

Mark Dutko wrote:
> If I go with a Brusa charger with a nicad profile does anyone know if
> you can use two 3.3kW parallel for faster charging? Victor mentioned
> this and I forgot if it would function the same and if there were
> some issues doing this.
>
> Thanks,
>
> mark
>
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #7
Thanks,

Do you have battery temp sensors available for the chargers or is
there some generic sensor brand or model that works?


Mark


Victor Tikhonov wrote:

> Mark,
>
> Yes, you can connect two (or more) BRUSA NLG5 chargers in parallel
> as long as you set up different voltages in U section of IUx charging
> profile so the chargers don't fight during regulation stage but merely
> just add charging power (current) during bulk charge phase.
> In general any section of charging profile with fixed voltage should
> have different voltage settings for all the chargers in parallel.
>
> This has nothing to do with the battery chemistry, NiCd profile (as
> well
> as any other) is OK for paralleled chargers.
>
> You can also connect two chargers in master-slave configuration, but
> it is more complex and expensive (special cables needed) and the
> outcome
> is identical. This is good option only if master charger is not BRUSA
> one, the slave is capable of tracking (and so doubling) the current
> of any type master charger.
>
> Of course, you can use two separate chargers connected each to the
> half
> of the string. Danger is one half can finish before the other half,
> and
> in general tracking of total Ah charged in this way (which is
> preferred
> for NiCd) is not doable.
>
> Victor
>
> Mark Dutko wrote:
>> If I go with a Brusa charger with a nicad profile does anyone know if
>> you can use two 3.3kW parallel for faster charging? Victor mentioned
>> this and I forgot if it would function the same and if there were
>> some issues doing this.
>>
>> Thanks,
>>
>> mark
>>
>>
>> _______________________________________________
>> For subscription options, see
>> http://lists.sjsu.edu/mailman/listinfo/ev
>>
>>
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #8
Victor Tikhonov wrote:

> Yes, you can connect two (or more) BRUSA NLG5 chargers in parallel
> as long as you set up different voltages in U section of IUx charging
> profile so the chargers don't fight during regulation stage but merely
> just add charging power (current) during bulk charge phase.
> In general any section of charging profile with fixed voltage should
> have different voltage settings for all the chargers in parallel.

Obviously Victor is the authority on the Brusa, however, this doesn't
sound quite correct to me. There should be no problem with connecting a
pair of chargers in parallel without tweaking the voltage setpoints.
Due to manufacturing tolerances, one charger will regulate to a lightly
lower voltage than the other in constant voltage mode, and so the
charger holding the highest voltage will deliver most of the current to
the battery while the other(s) quickly complete this constant voltage
step due to their current falling to the exit threshold criteria.

Certainly, this is how my chargers behave.

The concern with paralleling chargers like this is if their algorithms
implement an IUI-type profile, then the batteries will get hit with 2x
the desired finish current level since neither charger knows about the
other. In practice, if one charger finishes the 'U' step early due to
its output voltage being a bit lower than the other charger(s), then it
will start it's final 'I' step earlier and the result will be a finish
current that steps down to the desired level as each charger in turn
completes this step.

> You can also connect two chargers in master-slave configuration, but
> it is more complex and expensive (special cables needed) and
> the outcome is identical.

I would suggest that springing for the master-slave cable(s) is the
proper way to parallel these chargers. When connected in this manner
they should properly implement the entire charge profile, including the
final I step of an IUI-type profile if applicable.

The cable(s) may not be cheap, but their cost is surely in the noise
compared to the cost of multiple Brusa chargers! ;^>

Cheers,

Roger.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #9
Roger Stockton wrote:
> Victor Tikhonov wrote:
>
>> Yes, you can connect two (or more) BRUSA NLG5 chargers in parallel

> Obviously Victor is the authority on the Brusa, however, this doesn't
> sound quite correct to me. There should be no problem with connecting a
> pair of chargers in parallel without tweaking the voltage setpoints.

If their set points are actually different (while set the same) due to
the tolerances, it's the same as deliberate different settings,
the charger with lower voltage cutoff contributes only during "I" phase
and drops out during U phase. So essentially you complete charging with
one charger where the power of two (or more) is no longer needed anyway.

Problem is if voltages happen to be the same or about the same.
They change during regulation, so interaction between the two
can (this has been observed) lead to unwanted oscillations.

Dumb chargers may ignore it, (may be for better), but BRUSA's
detect abnormality and will shut down.

> The concern with paralleling chargers like this is if their algorithms
> implement an IUI-type profile, then the batteries will get hit with 2x
> the desired finish current level since neither charger knows about the
> other.

Nope. Finished I is after U and U is already done with one charger.
They work as a gang only during initial I when you need power.
Final I can easily be handled by single changer

> In practice, if one charger finishes the 'U' step early due to
> its output voltage being a bit lower than the other charger(s), then it
> will start it's final 'I' step earlier and the result will be a finish
> current that steps down to the desired level as each charger in turn
> completes this step.

Again, they only work together during initial "I" where most of the
time spend and so most of the benefit to have more power.

>> You can also connect two chargers in master-slave configuration, but
>> it is more complex and expensive (special cables needed) and
>> the outcome is identical.
>
> I would suggest that springing for the master-slave cable(s) is the
> proper way to parallel these chargers. When connected in this manner
> they should properly implement the entire charge profile, including the
> final I step of an IUI-type profile if applicable.
>
> The cable(s) may not be cheap, but their cost is surely in the noise
> compared to the cost of multiple Brusa chargers! ;^>

Sorry, this is incorrect. Paralleling is more desirable way to
increase charging power than master-slave config - this info
is from the mouth of designer of these chargers. Master-slave
concept was developed to allow master to be non-BRUSA one,
for instance, it can be PFC50, no problem ;-) and other odd
circumstances. In this config all booster does is monitors
current on *AC* side and tracks it (doubles), so actual type
of the master is irrelevant.

> Cheers,
>
> Roger.

Victor



> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #10
Each charger comes with set of 3 sensors.

Victor

Mark Dutko wrote:
> Thanks,
>
> Do you have battery temp sensors available for the chargers or is
> there some generic sensor brand or model that works?

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #11
Victor Tikhonov wrote:

> If their set points are actually different (while set the same)
> due to the tolerances, it's the same as deliberate different
> settings, the charger with lower voltage cutoff contributes only
> during "I" phase and drops out during U phase. So essentially you
> complete charging with one charger where the power of two
> (or more) is no longer needed anyway.
>
> Problem is if voltages happen to be the same or about the same.
> They change during regulation, so interaction between the two
> can (this has been observed) lead to unwanted oscillations.
>
> Dumb chargers may ignore it, (may be for better), but BRUSA's
> detect abnormality and will shut down.

Interesting; I would take this to reveal a shortcoming in the Brusa
control implementation. My chargers have the same +/-1% voltage
accuracy as the Brusa and I have run as many as 4 units in parallel,
each running the same profile and setpoints with no oscillation or other
misbehaviours.

> > The concern with paralleling chargers like this is if their
> > algorithms implement an IUI-type profile, then the batteries
> > will get hit with 2x the desired finish current level since
> > neither charger knows about the other.
>
> Nope. Finished I is after U and U is already done with one charger.
> They work as a gang only during initial I when you need power.
> Final I can easily be handled by single changer

No, this is incorrect. The final I is after the U, but each charger is
going to complete the entire IUI profile unless a fault condition is
detected that causes it to halt prematurely.

What will happen is that both chargers will run at full current during
the first I phase. One of the chargers will enter the U step before the
other due to tolerances on the voltage sense. Whichever charger
regulates to a lower voltage setpoint than the other(s) will contribute
less current, and if its current falls sufficiently low, it will advance
to the final I step of the profile while the other charger(s) remain in
the U step. If the final I step is sufficiently long in duration, then
this charger will still be delivering constant current when the other
charger(s) finish the U step and also advance to the final I step. The
result depends entirely on how near or far apart the chargers exit the U
step. It can vary from a best case of the finish phase being performed
entirely by one charger to a worst case of the finish step being
performed almost entirely at a current some multiple of the desired
value. The usual case, in my experience (though not with Brusa
chargers), is that the finish phase current will initially start at a
multiple of the desired finish rate and step down to the desired rate as
each charger finishes its final I step and leaves one less charger
running.

> > I would suggest that springing for the master-slave cable(s)
> > is the proper way to parallel these chargers. When connected
> > in this manner they should properly implement the entire
> > charge profile, including the final I step of an IUI-type
> > profile if applicable.
>
> Sorry, this is incorrect. Paralleling is more desirable way to
> increase charging power than master-slave config - this info
> is from the mouth of designer of these chargers. Master-slave
> concept was developed to allow master to be non-BRUSA one,
> for instance, it can be PFC50, no problem ;-) and other odd
> circumstances. In this config all booster does is monitors
> current on *AC* side and tracks it (doubles), so actual type
> of the master is irrelevant.

I have to assume that either you misunderstood the designer or Brusa has
not documented the behaviour of their chargers properly.

According to the NLG5 manual, if a pair of NLG5s are connected in
master-slave, the AC input current of the master is sensed by the slave
and the slave will attempt to match the master's line current draw *IF*
the master's current is within a configurable range. Once the master's
line current fall below a configurable minium, the booster becomes
inactive. This behaviour ensures that the master-slave pair can be
configured such that the master alone will perform the finish step, or
any other steps that deliberately want to be done at a set power/current
level. This is far better than simply paralleling the chargers.

Notice also that when the master is an NLG5, the booster's output
current is passed through the master and is measured by the master. The
manual doesn't go into detail about how this feature is used, but this
capability does suggest that the master is aware of the contribution by
the booster and may even be able to be configured to take advantage of
this knowledge in its charge profile.

Cheers,

Roger.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #12
Roger Stockton wrote:
>
> Interesting; I would take this to reveal a shortcoming in the Brusa
> control implementation. My chargers have the same +/-1% voltage
> accuracy as the Brusa and I have run as many as 4 units in parallel,
> each running the same profile and setpoints with no oscillation or other
> misbehaviours.

It may be viewed as shortcoming, but it's only because these
particular chargers are set up wrong way - voltages must be
different to easily avoid it, so it is not really a shortcoming.

>>> The concern with paralleling chargers like this is if their
>>> algorithms implement an IUI-type profile, then the batteries
>>> will get hit with 2x the desired finish current level since
>>> neither charger knows about the other.
>> Nope. Finished I is after U and U is already done with one charger.
>> They work as a gang only during initial I when you need power.
>> Final I can easily be handled by single changer
>
> No, this is incorrect. The final I is after the U, but each charger is
> going to complete the entire IUI profile unless a fault condition is
> detected that causes it to halt prematurely.
...

No Roger, booster does not have any profiles. To be exact, they do, but
not IUI charging profile. They are set up to only contribute when single
charger draws >5A and gradually increase contribution to available
maximum (3.3kW) by the time mains current increase to 10A. Booster shuts
down below 5A knowing that master can handle this current level on its
own. It is described on page 13 of the manual, take a look.

Completion of IUI by each charger indeed will happen of you connect them
in parallel, not as master-booster, but that doesn't mean you should
load them with identical IUI profiles. One must have full IUI, and since
it is capable to finish "UI" on its own, another [of the two in
parallel] must only have initial "I" in its profile. You cannot take
advantage of final I from both chargers unless this I is huge, more than
one charger can handle. In this case you set them up so initial I
doubles, U - so there are no problems with oscillations (minimal
difference) and final I - again to double the power.

> What will happen is that both chargers will run at full current during
> the first I phase. One of the chargers will enter the U step before the
> other due to tolerances on the voltage sense. Whichever charger
> regulates to a lower voltage setpoint than the other(s) will contribute
> less current, and if its current falls sufficiently low, it will advance
> to the final I step of the profile while the other charger(s) remain in
> the U step.

If you want to prevent it from premature advancing, you program that U
stage to end when two conditions are met: current fell down far enough
AND time spent in U phase is at least X minutes/hours.

> If the final I step is sufficiently long in duration, then
> this charger will still be delivering constant current when the other
> charger(s) finish the U step and also advance to the final I step.

As I mentioned, paralleling is not as trivial - you can force U phases
of both to be arbitrary length, no less. I didn't dig into this, but I'm
sure there are simple solutions for profiles in this case. Largest
amount of chargers in parallel BRUSA ever set up was 6 standard
chargers. No problems, as long as one understands what is going to
happen and set them up accordingly.

> The
> result depends entirely on how near or far apart the chargers exit the U
> step. It can vary from a best case of the finish phase being performed
> entirely by one charger to a worst case of the finish step being
> performed almost entirely at a current some multiple of the desired
> value. The usual case, in my experience (though not with Brusa
> chargers), is that the finish phase current will initially start at a
> multiple of the desired finish rate and step down to the desired rate as
> each charger finishes its final I step and leaves one less charger
> running.

You're right. In this case I'd set up final I to a half of desired
(in case of 2 chargers), 1/3 in case of 3 chargers etc, and
synchronize advancing to every next stage (can be done by using
programmable inputs/outputs)

>> Sorry, this is incorrect. Paralleling is more desirable way to
>> increase charging power than master-slave config - this info
>> is from the mouth of designer of these chargers. Master-slave
>> concept was developed to allow master to be non-BRUSA one,
>> for instance, it can be PFC50, no problem ;-) and other odd
>> circumstances. In this config all booster does is monitors
>> current on *AC* side and tracks it (doubles), so actual type
>> of the master is irrelevant.
>
> I have to assume that either you misunderstood the designer or Brusa has
> not documented the behaviour of their chargers properly.

Well, I understood him well since exchanged many emails on this very
topic. Paralleled configuration is not mentioned in the manual at all,
and was the reason for one of my customers to request additional
info about it. BRUSA agreed that the manuals don't reflect everything
chargers are capable of (opposite of typical these days ;-) ).
I made sure next rev of their manual will include more info about
paralleling.

> According to the NLG5 manual, if a pair of NLG5s are connected in
> master-slave, the AC input current of the master is sensed by the slave
> and the slave will attempt to match the master's line current draw *IF*
> the master's current is within a configurable range. Once the master's
> line current fall below a configurable minium, the booster becomes
> inactive. This behaviour ensures that the master-slave pair can be
> configured such that the master alone will perform the finish step, or
> any other steps that deliberately want to be done at a set power/current
> level. This is far better than simply paralleling the chargers.

Paralleled chargers loaded with proper (non-identical) profiles will do
exactly the same without complications of master-booster cabling.

> Notice also that when the master is an NLG5, the booster's output
> current is passed through the master and is measured by the master. The
> manual doesn't go into detail about how this feature is used, but this
> capability does suggest that the master is aware of the contribution by
> the booster and may even be able to be configured to take advantage of
> this knowledge in its charge profile.

That may be, but whole idea was to be able to work in pair with
non-BRUSA charger, so while BRUSA-master can sense boosters' output
and can be programmed to so something about it, it is not required
for proper operation - else non-BRUSA charger could never be a master.

> Cheers,
>
> Roger.

Victor
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #13
Victor Tikhonov wrote:

> It may be viewed as shortcoming, but it's only because these
> particular chargers are set up wrong way - voltages must be
> different to easily avoid it, so it is not really a shortcoming.

The shortcoming I refer to is that these chargers oscillate when
operated in this way, and apparently do so to such an extent that it is
treated as a fault condition. Other chargers, even intelligent ones, do
not have this limitation.

> No Roger, booster does not have any profiles.

I was describing the behaviour of multiple chargers connected in
parallel, not the master-slave setup. I suppose I was not clear enough
that it is the shortcomings of parallel operation vs the master-slave
setup that I was describing.

> Completion of IUI by each charger indeed will happen of you
> connect them in parallel, not as master-booster, but that
> doesn't mean you should load them with identical IUI profiles.
> One must have full IUI, and since it is capable to finish
> "UI" on its own, another [of the two in parallel] must only
> have initial "I" in its profile.

This is indeed a possibility, but not a desirable one. The problem with
configuring one charger as a "booster" only is that if the "master"
charger with the full IUI profile happens to fail for some reason, the
battery is left only partially charged.

If both chargers have the IUI profile and are connected in parallel,
then if one charger fails the charge time may increase but the battery
will still be fully charged when the charge cycle completes.

> If you want to prevent it from premature advancing, you
> program that U stage to end when two conditions are met:
> current fell down far enough AND time spent in U phase
> is at least X minutes/hours.

This doesn't work, at least not as simply as you suggest. The most
likely outcome of implementing this logic would be to ensure the
worst-case scenario of every charger advancing to the final I step
simultaneously.

My chargers take a different approach and will instead detect if there
are other chargers paralleled, and if so they will either leave it to
the last charger running to perform the finish charge or each charger
will politely wait until the others each complete the finish phase
individually in turn before performing their finish phase. Which
behaviour they exhibit depends on the type of finish phase the algorithm
performs.

> Largest
> amount of chargers in parallel BRUSA ever set up was 6 standard
> chargers. No problems, as long as one understands what is going to
> happen and set them up accordingly.

I haven't had reason to try more than 4 chargers in parallel yet, but no
special setup was required when using 4 in parallel; each charger ran
exactly the same algorithm.

I'm not saying that it isn't possible to parallel Brusas, indeed I am
agreeing that most of what you suggest is possible, however, when you
actually start playing with paralleled chargers you find that some
approaches have distinct advantages over others.

> You're right. In this case I'd set up final I to a half of desired
> (in case of 2 chargers), 1/3 in case of 3 chargers etc, and
> synchronize advancing to every next stage (can be done by using
> programmable inputs/outputs)

While possible, I still consider this less desirable than having each
charger executing the desired profile to ensure the desired profile is
applied to the battery no matter how many of the chargers happen to be
working at the time.

> Well, I understood him well since exchanged many emails on this very
> topic. Paralleled configuration is not mentioned in the manual at all,
> and was the reason for one of my customers to request additional
> info about it. BRUSA agreed that the manuals don't reflect everything
> chargers are capable of (opposite of typical these days ;-) ).
> I made sure next rev of their manual will include more info about
> paralleling.

The NLG5 manual on Brusa's site certainly does include a page or two
(and at least one diagram) describing master-slave ("Mode B") operation;
are you referring to non-master-slave parallel operation?

> > Notice also that when the master is an NLG5, the booster's output
> > current is passed through the master and is measured by the
> > master. The manual doesn't go into detail about how this feature
> > is used, but this capability does suggest that the master is
> > aware of the contribution by the booster and may even be able to
> > be configured to take advantage of this knowledge in its charge
> > profile.
>
> That may be, but whole idea was to be able to work in pair with
> non-BRUSA charger, so while BRUSA-master can sense boosters' output
> and can be programmed to so something about it, it is not required
> for proper operation - else non-BRUSA charger could never be a master.

This thread has specifically been about the paralleling of two Brusa
chargers, not a Brusa and something else. Clearly Brusa's whole idea of
master-slave operation extends beyond allowing a Brusa to be paired with
a non-Brusa master since the NLG5 specifically includes a DC
pass-through so that a Brusa NLG5 master can sense the output current
being contributed by one or more Brusa slaves.

Thank you for the clarification regarding the Brusa's operation; I
certainly have a better appreciation of its capabilties now, and I think
the original poster understands that he has several options available
for operating multiple Brusa's into a single battery.

Cheers,

Roger.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #14
Victor Tikhonov wrote:
>> Problem is if voltages happen to be the same or about the same.
>> They change during regulation, so interaction between the two
>> can (this has been observed) lead to unwanted oscillations.

Roger Stockton wrote:
> Interesting; I would take this to reveal a shortcoming in the Brusa
> control implementation. My chargers have the same +/-1% voltage
> accuracy as the Brusa and I have run as many as 4 units in parallel,
> each running the same profile and setpoints with no oscillation or other
> misbehaviours.

This is a consequence of the output impedance of the charger. In
constant-voltage mode, the ESR (equivalent series resistance) is very
low (in fact, zero for an ideal charger; i.e. its voltage doesn't change
at all from 0 to full current).

But the charger and battery also have inductance and capacitance. When
you have inductance and capacitance and very low resistance, you get a
"high Q" circuit that is prone to oscillate.

Adding a second charger in parallel further reduces the ESR, and adds
more inductance and capacitance. This makes it even more likely to
oscillate, hunt around, or jump between modes. If the charger's designer
knows that they will be run in parallel, he will deliberately add ESR or
take other measures, and test to be sure that it is stable.

Simpler cruder chargers don't have this problem, because they have a
fairly large ESR -- i.e. their output voltage changes a lot as they go
from 0 to full current.

> What will happen is that both chargers will run at full current during
> the first I phase.

I agree. Also, the I phase (constant current) is intrinsically stable,
because a current source has a very high ESR. Its current is not
affected by changes in battery voltage.

> One charger enters the U step before the others... contributes less
> current... when it falls low enough... it advances to the final I step
> while the others remain in the U step... it will still be delivering
> constant current when the others finish their U step and also advance
> to their final I step... worst case finish step [is] performed almost
> entirely at a current some multiple of the desired value.

Yes, this is what I've seen happen with "smart" chargers that weren't
designed smart enough to deal with several in parallel.

There are lots of ways that designers have dealt with this, ranging from
ignoring the problem to elaborate master-slave setups. They are usually
specific to one particular design; i.e. in general you can't parallel
"smart" chargers unless the designers tell you that you can. Even then,
it only works with their particular setup.

I've taken a somewhat different approach. There is no sacred requirement
for any precise charging voltage or current. It is better (and easier)
to let the current and voltage vary with load. For example, don't try to
maintain exactly 14.400v during the U phase from 0-full current;
instead, let it vary from 14.2v at full current to 14.6v at 0 current.
In effect, you are adding ESR, so chargers in parallel naturally share
the load.

Likewise, in the I phase, don't try to maintain a tightly regulated
current. Let the current vary with voltage. For one thing, you can
maintain constant charging *power* instead of constant current, which
makes best use of the AC receptacle's capability. If you try to
literally hold the charging current constant, then the AC line current
rises as the battery voltage rises -- your AC fuse or circuit breaker
may hold when you first start charging, but then blow several hours
later as the pack voltage rises (when you're not around to reset it).

Finally, don't use rigid thresholds at which the charger changes modes.
Rigid thresholds cause the charger to mode-hop due to noise or temporary
transients. For example, if the charger is in the U phase and waiting
for the current to fall below 2.000 amps, a momentary dip in the AC line
voltage, or turning off the car's DC/DC converter, or some other event
unrelated to the battery's state of charge can falsely trigger the
switch to the final I phase. Once it switches, it can't switch back.

Instead, it's better to have it smoothly transition from U to I, so it
can go back again if the cause for the transient goes away. For example,
suppose the charger is in the final I phase, limiting current to 2 amps
and letting the voltage rise as high as 15v. Now you open a car door, so
the dome light comes on, so the DC/DC converter starts drawing current.
This pulls the battery down, so the charger sees the battery current try
to go back over 2 amps. Rather than stay in the I phase, the charger
should transition back into the U phase, maintaining constant voltage at
more than 2 amp. Later, when you close the door, the DC/DC current
drops, and the battery drops back below 2 amps, the charger re-enters
the final I phase.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 
1 - 14 of 14 Posts
Top