[EVDL List]

RE: parallel batteries

>As I understand it, lead acid batteries work okay being in parallel
>and NiMH don't work well in parallel.

Question for those who have more knowledge on this point... NiMH don't
parallel well, but what about having several strings that are
either higher or lower in voltage than the motor+controller
need, and a buck/boost bidirectional converter for each string,
dumping power into (or out of, in the case of regen) the main
'bus'. Say, 6 or 10 192 volt strings, with a 156 volt bus. Each
converter would need to be capable of maybe between 40 and 100 amps,
depending on your power level. Obviously, synchronizing them might
be a bit of effort, but could this work? I would think buck for
driving, boost for regen would make the most sense to me from an
efficiency and minimizing switching loss point of view. Obviously
the output capacitors on the bus would need to handle full ripple
current of the motor and controller.

 

[EVDL List]

Re: parallel batteries

NiMH parallel no problem to discharge. They can't be charged in
parallel because their voltage drops once fully charged, but they still
accept current, so the other strings will continue to overcharged the
weakest one to death. The solution is to use a switch or diode so a
string can't be charged by another one, and charge each string
independently. You need a BMS to handle it, its not difficult, just
adds expense, but adds a lot of other value as well, like balancing and
monitoring.
Jack

Dale Ulan wrote:
>>As I understand it, lead acid batteries work okay being in parallel
>>and NiMH don't work well in parallel.
>
>
> Question for those who have more knowledge on this point... NiMH don't
> parallel well, but what about having several strings that are
> either higher or lower in voltage than the motor+controller
> need, and a buck/boost bidirectional converter for each string,
> dumping power into (or out of, in the case of regen) the main
> 'bus'. Say, 6 or 10 192 volt strings, with a 156 volt bus. Each
> converter would need to be capable of maybe between 40 and 100 amps,
> depending on your power level. Obviously, synchronizing them might
> be a bit of effort, but could this work? I would think buck for
> driving, boost for regen would make the most sense to me from an
> efficiency and minimizing switching loss point of view. Obviously
> the output capacitors on the bus would need to handle full ripple
> current of the motor and controller.
>
>

 

[EVDL List]

Re: parallel batteries

Dale Ulan wrote:
> what about having several strings that are
> either higher or lower in voltage than the motor+controller
> needs, and a buck/boost bidirectional converter for each string,
> dumping power into (or out of, in the case of regen) the main
> 'bus'... could this work?

Yes, I think it could. We've discussed it before. It's a hybrid battery
pack, with two different types of batteries. Because in general the two
packs are different, you can't just connect them in parallel, and need
some means for controlling the rate that each charges and discharges.
This could be a bidirectional converter as you suggest, or two motor
controllers and two chargers.

You probably don't want the converters to be boost-buck, as this is one
of the less efficient types. But you could arrange things so pure buck
(pack voltage always higher than motor voltage) or pure boost (for
charging, where motor or AC line voltage is always less than pack
voltage) was sufficient.
--
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

 

[EVDL List]

RE: parallel batteries

>of the less efficient types. But you could arrange things so pure buck
>(pack voltage always higher than motor voltage) or pure boost (for
>charging, where motor or AC line voltage is always less than pack
>voltage) was sufficient.


That's what I had in mind - in driving, say, pack voltage is 192
volts and bus voltage is 156 volts, so buck. When in regen, bus voltage
rises to maybe 170 volts, so boost into the battery pack at whatever
it takes to charge the batteries. That way you could discharge each
string fairly evenly, and charge them in the same way. So I guess
by buck/boost, the supply is in buck mode when driving, and in boost
mode during either battery charging or regeneration.....


BATT+ -C E --+--- inductor ------ BUS +
Q1 |
C
Q2
E
BATT- ------------+-------------------- BUS -


Where each IGBT (or MOSFET) contains its freewheeling diode too.
In motoring, buck is by Q1 = switch, Q2 = freewheel diode,
inductor = energy storage.
In charging/regen, boost is by Q2 = switch, Q1 = freewheel
diode. I didn't show the bus capacitors but those are essential.
Note that Q1 and Q2 are a standard 'half-bridge' so you could
just use pre-existing parts that already exist.

Then you parallel packs as you want provided the voltages are
somewhere close (for efficiency), and you have enough current
capacity in whatever you use to hold up the bus. If you have
enough battery strings and converters to support the full
motor current, then you only need caps on the bus, otherwise,
it's a hybrid pack with maybe some AGM's or that sort of
thing on the bus, as you mentioned. Possibly with smart
phasing of each converter on the bus, you can limit the
ripple currents on the caps?

-Dale