[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?
>
>http://914ev.blogspot.com/2007/02/battery-equalizer-schematics.html
>http://www.smartsparkenergy.com/pdf/batteq1.pdf
>(http://www.smartsparkenergy.com/pdf/batteq1.pdf)
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:
http://914ev.blogspot.com/2007/02/battery-equalizer-schematics.html
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
http://914ev.blogspot.com
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