[Oeva-list] Battery Balancer

Theoldcars at aol.com Theoldcars at aol.com
Sat Mar 3 22:58:21 PST 2007

There has been a few posts on this and I wanted to follow up with some  
emails from Lee about the equalizer your working on Tim. 
I am interested in doing a large run of Lee's battery balancer. To get the  
prices down and the quality up. It is going to require a larger quantity then  
has ever been built. I would be interested in some feed back. One if anyone  
would either buy one and at what cost. Also is anyone interested in helping out 
 doing this even though the goal is not to make money. 
Below is Lee's response to my email.
Don Blazer
From: Theoldcars at aol.com
> I was wondering if you could take a look  at this? I value your opinion ? 
What would the plus or minus be of going  with  either?

Be glad to, Don!

This is the old "flying capacitor" circuit. It's  been around for a very long 
time. It works, but not very well. There are a  number of problems:

- The peak current each time the capacitors switch is  very high. Thus,
the losses in the circuit resistances (MOSFET  Rds, capacitor ESR,
battery internal resistance, etc.) are  high.

My Balancer uses essentially pure DC from its DC/DC  converter to charge
the batteries, so switching losses are  negligible.

- If you hold batteries at the same voltage, the current that  flow between
them falls almost immediately to a very low level.  It takes many days
for them to reach the same state of charge.  Thus, it balances very slowly.

My Balancer applies a higher  voltage to the battery being charged, so it
charges much faster.  For example, it can load the pack as a whole to 12.5v
to charge  the low battery at 13.5v.

- It takes an unreasonably large capacitance to  transfer any significant
amount of charge.

I  used a DC/DC so you can charge at any reasonable current (the  parts
are sized for up to 30 amps).

- Efficiency is low;  on the order of 50-75%. It *appears* high if you make
casual  measurments with average-reading meters, because they ignore
the  high peaks.

DC/DCs can easily be 80-90% efficient. The  Batmod I'm using is around
88% efficient.

- It only  transfers charge between adjacent batteries. When the "high" and
"low" batteries are far apart in the string, the efficiency losses of  each
stage multiply together. For example, if you have a 10%  loss per stage,
and you need to move charge across 10 batteries,  you have essentially
nothing left at the  end.

There is only a single transfer in my Balancer; no  cascade effect.

From: Tim Kutscha   <tim_kutscha at yahoo.com>

>Hi Don,
> Thanks for  your  response.  Lee Hart's balancer uses a digital multimeter
>  to read the voltage on all the batteries in your system, picks the  lowest
> one and then uses an isolated DC-DC converter to charge just the  low
> battery for a period of  time.  It then rescans the pack  and starts again
> with the next lowest  battery.
> The  equalizer described in the whitepaper is far simpler.  It doesn't 
>  require a controller or multimeter. It just requires a waveform  generator
> (in this case a 555 timer) and a bunch of isolated FETs and  capacitors.
> In short, it shares charge between adjacent  batteries,  thereby taking
> charge off of higher voltage batteries and dumping  it  into lower voltage
> batteries.  When all batteries are  equal, no charge sharing happens and
> no power (except the 555) is  consumed.

His description of my Balancer is correct. However, he's  comparing a 
full-featured circuit to a partially developed breadboard. The  Balancer includes the 
PC boards, fuses, filters, connectors, and all the other  incidental parts to 
make it actually work His circuit leaves all of these  out.

When he actually builds it, he will find that he needs a lot more  parts to 
make it work and be safe. It also consumes power all the time, due to  
switching losses. From his circuit, I would guess that it draws over 10ma from  every 
battery even when it isn't doing anything.

It's possible to scale  the Balancer down. Paul Compton built one of my 
Balancers, but downsized  everything to make it cheaper. He eliminated the BASIC 
Stamp, Batmod, and 30a  relays; replacing them with a $3 PIC, a $10 5w DC/DC, 
and 2a relays. It costs  under $10/battery, but only transfers about 0.3 amps 
(which is still several  times more than this capacitive balancer). [In my view, 
a 4:1 reduction in price  wasn't worth the 10:1 reduction in performance.]

> I find it   easily expandable to large strings too.

It is straightforward to expand,  but the charge transfer between widely 
separated batteries gets so low as to be  negligible.

I suspect that once he adds up the cost of all these part, he  will find that 
his balancer costs more than the method I'm using.

Hope  this helps!
Lee Hart
In a message dated 2/19/2007 1:16:50 PM Pacific Standard Time,  
oeva-list-request at oeva.org writes:

Message:  2
Date: Sun, 18 Feb 2007 22:13:36 -0800 (PST)
From: Tim Kutscha  <tim_kutscha at yahoo.com>
Subject: [Oeva-list] Request for feedback on  battery equalizer
To: OEVA <oeva-list at oeva.org>
Message-ID:  <174824.31402.qm at web36908.mail.mud.yahoo.com>
Content-Type:  text/plain; charset="iso-8859-1"

Hello EV folks,

I'm working with some other engineers on an economical home-brew battery  
equalizer at the following  link:


I've  prototyped this on my desk and it seems to work fine with 12V 
batteries.   I would appreciate any feedback you might have.  My hope is to get this  
working and provide the design for free to help EV folks get their battery  
packs to last longer.

Thanks for your time,
Tim  Kutscha

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