[EVDL] BMS and my first battery pack

Grant Clinch wrote:
> Now, is there an off the shelf battery monitoring system that is
> configurable to a 48V pack, consisting of 8 12V batteries, and
> having two strips of four wired in parallel? Do they monitor each
> individual battery ?

My Battery Balancer can do this. It monitors and charges up to 80
individual batteries or cells, regardless of whether they are in series
or parallel. It is not an "off the shelf" system, but rather a
do-it-yourself project. The old version is documented at:

http://www.geocities.com/sorefeets/balancerland/

We're making a new version right now, which is almost the same. About a
dozen people have ordered them. The relay boards (that select the
battery) are done, and being shipped to those with prepaid orders. Some
bought bare boards (will get their own parts), some bought kits (with
all the parts but they assemble them themselves), and some bought
assembled boards (which are due in by Sept. 1).

I'm working to finalize the design on the Control board now. It will
probably be done in September, and shipped later this year.

> How does one charge such a pack? That is to say, how do you monitor
> the amount of charge going into each battery if you're charging the
> whole pack?

Lead-acid batteries are fairly good at self-limiting their charge.
So, most systems just connect all the batteries in series, and charge
them as one big string. If "full" is considered 2.5v/cell, then a 48v
pack (24 cells) is "full" when it reaches 2.5v x 24 = 60 volts.

The trouble with this is that all cells don't reach "full" at exactly
the same time. Some will reach 2.5v while others are still at 2.4v. With
flooded batteries, the 2.5v cells will just start bubbling and gassing.
The charger just keeps going until the last cells finally reach 2.5v,
then it shuts off. The most-full cells will have lost water from all
that gassing, but you can add more.

With sealed batteries (AGM or gels), the charger needs to strictly limit
the time and current during this time, or the most-full cells will vent.
In this case, the water lost can't be replaced, so it shortens the life
of the battery.

The whole point of a BMS is to watch each battery individually, and only
charge it as much as necessary, to minimize the overcharging and thus
extend life. The main drawback is cost and complexity.

There are other ways to accomplish the same thing. You can use
individual chargers; one per battery. Or, you can deliberately
undercharge, by setting a low "fully charged" voltage. These have their
own strengths and weaknesses.
--
"Never doubt that the work of a small group of thoughtful, committed
citizens can change the world. Indeed, it's the only thing that ever
has!" -- Margaret Mead
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net


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On 19 Aug 2007 at 11:08, Lee Hart wrote:

> The trouble with this is that all cells don't reach "full" at exactly
> the same time. ...
>
> The whole point of a BMS is to watch each battery individually, and only
> charge it as much as necessary, to minimize the overcharging and thus
> extend life.

Read the above statement again. What's wrong with this picture?

The first paragraph discusses imbalance of CELLS, and the second one
discusses balancing BATTERIES.

The sad fact is that to really do the job right, every cell should be
separately monitored. However, almost all lead batteries give us no access
to individual cells. The best we can do is try to equalize the voltage on
each battery (module), usually comprising 3, 4, or 6 cells. This helps a
lot when different batteries operate at different conditions, such as when
some batteries are under the hood and others in the cabin.

However, it still allows significant imbalance among the individual cells in
a module. You still have to commit some level of gross overcharge to carry
out cell equalization. In fact many of the equalizers available have some
provision for temporary defeat so you can perform equalization.

What's more, most balancers or equalizers aim for the same voltage on each
battery. I'm not entirely convinced that "all batteries are at the same
voltage" is close enough to "all batteries are full," especially as the
batteries age.

I'm not saying balancers or equalizers aren't helpful for lead batteries,
just that they leave something to be desired. One of the intriguing
benefits of lithium batteries is that (as I understand it) they almost have
to have charge control at the individual cell level.

David Roden - Akron, Ohio, USA
EVDL Administrator

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David Roden wrote:
> The sad fact is that to really do the job right, every cell should be
> separately monitored. However, almost all lead batteries give us no
> access to individual cells. The best we can do is try to equalize
> the voltage on each battery (module), usually comprising 3, 4, or 6
> cells.

Yes; so we do the best we can. Balance however few cells the design of
the modules allows. With lithiums, you need to balance right down to the
individual cell level, as they have *no* tolerance for overcharging.
With other chemistries, we have to tolerate some degree of overcharging
to balance all the cells within a module.

> What's more, most balancers or equalizers aim for the same voltage
> on each battery. I'm not entirely convinced that "all batteries are
> at the same voltage" is close enough to "all batteries are full,"
> especially as the batteries age.

Agreed. Voltage is easy, so that's all most systems bother with.

My Balancer has a microcomputer, and I *can* measure more than just
voltage. My algorithm tries to keep all batteries at the same energy
storage, not the same voltage or state of charge. For instance, while
you are driving, it is moving charge from the stronger batteries to the
weaker ones, so they all reach "dead" at the same time.

> I'm not saying balancers or equalizers aren't helpful for lead
> batteries, just that they leave something to be desired.

Well, my Balancer is home-made and user-programmed. If you can figure
out what is desired, you can program it to *do* it!

--
"Never doubt that the work of a small group of thoughtful, committed
citizens can change the world. Indeed, it's the only thing that ever
has!" -- Margaret Mead
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net


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

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