Re: (ET) Time for a NiMH Elec-trak - Amp hours

[Christopher Zach <czach computer org>]

> Never heard of a "saddle method."  can you explain?
Saddle method is to watch for the relative minimum voltage between when 
the battery is fully charged (and the voltage drops a bit) before it 
comes out of the saddle and starts climbing into thermal runaway. That 
can probably be done for small strings of batteries, but I wonder if the 
effect would be masked by some batteries in a string coming full before 
others. I'll have to see if the Prius checks voltage on each prismatic 
module; perhaps one way to do it would be to ladder-step the pack 10 
times a second looking for batteries that have stopped climbing, then 
backing off the charge.

Monitoring each battery module may sound like a nightmare, but I'm 
finding that the Basic/Stamp modules are great at doing the same thing 
over and over and multiplexing the sense wires. Opens possibilities.

> While the Prius's battery processor does monitor battery temperature, I 
> strongly doubt that it is the only or even the predominant method of 
> determining SOC.  The controlling processor, like everything else in the 
> car, is very sophisticated.  Besides, the battery in a Prius is not normally 
> charged beyond 80% SOC, so I don't think that method would be usable.  Most 
> of the heat it does generate comes from its own internal resistance as it's 
> cycled anyway.

*nod* What's beginning to interest me though is a report I read about 
the RAV4 EV. Toyota sent a reminder letter to lease-holders explaining 
that when support ends parts are going to be hard to get. One thing that 
caught my eye though was the prices:

$2,000 for a battery "module"
$50,000 for a complete new "pack"

This implies that they are using parallel strings. I know the voltage is 
around 300 volts, and they are using NimH and get around 140 miles of 
range. Thus by doing the math (300wh/mile) that's about an Ah per mile. 
Or a 120 Ah pack.

Which would be 25 (25 2k modules per pack)*6ah=150ah.

I think they're using 25 NiMH battery packs in parallel. They also 
mention a complex "contactor board" for several thousand; my guess is 
they have a contactor per pack and drop packs off the charger as they 
come full. Thus they can charge them all in parallel, and drop packs out 
 to keep them from overcharging. Or something.

> The Prius pack comprises very high quality, closely matched modules and 
> cells.  They are definitely not cheap Chinese sweatshop cells!  I think if 
> you charge those packs you have until they get warm, you will ruin them in 
> short order, and that would be a shame.

Agreed. Thus the attempt to try a few modules at a time in the 
Elec-trak. Learn from there and scale up a possible solution.

> A little advice on charging NiMH batteries has been posted right here.  
> There's also quite a bit more of it to be found on the web and in textbooks. 
>  Over the last few years, engineers have put a lot of time and effort into 
> learning how to charge NiMH batteries for long life and good performance.  
> IMO, you will be wasting the money you spent on the pack if you don't listen 
> to them. 

I'm more than willing to learn. If you know of any good sites, pls let 
me know.

Chris