Michael Ross wrote:
I will put my JLD 404 in to provide a lower limit (I think it can also do
an upper limit too). The 60VDC, 6A charger I have stops at some point, but
it is one of those mysterious, cheap Chinese, jackleg things with some LEDs
added and stickers peeled off. It has an IEC plug to go into the BMS sent
with it (for another pack). Very not kosher. The JLD also provides me
with an estimate of Ah used.
The first thing I do when I get a new chargers is to put my own meters
on it, and watch what it *really* does. Far too often, the advertising
is wrong; sometimes blatantly wrong!
I do get some extra protection if I use 17 cells, which is probable.
Agreed. It's always a prudent strategy to get a couple extra batteries
or cells. If you don't, Murphy will almost certainly make sure you have
bad ones!
It also helps extend the life of just about everything if you don't push
it right to the maximum. Don't run a "72-120v" controller at 120v. Don't
put 60 amps through a "60 amp" transistor. You get the idea. :-)
With batteries, it means don't take 100ah out of a 100ah battery. And
don't charge it to the maximum rated charging voltage; it may maximize
amphour capacity in the short run, but will shorten life in the long run.
Why is 2.75V right for BB'ing a LiFePO4 pack? What is that based on?
Experimentation.
- Suppose you discharge three 100ah cells to (say) 3.0v. Are they at the
same state of charge? Let's see... *Keep* discharging them while
counting amphours to 2.5v. You'll find they deliver noticeably different
numbers of amphours (like 10ah, 15ah, and 20ah). So you haven't really
bottom balanced them.
- Suppose you discharge them to 2.75v. Now if continue discharging them,
you get something like 2ah, 4ah, and 6ah more. They still aren't
identical; but they're close to dead and much closer together.
- You could try it at 2.5v too. That would get them even closer. But at
2.5v, the evidence suggests that you have also discharged them so deeply
that you're shortening their life.
So, 2.75v is a "good enough" number; not too high, and not too low. It's
a compromise.
--
ICEs have the same problem as lightbulbs. Why innovate and make
better ones when the current ones burn out often enough to keep
you in business? -- Hunter Cressall
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/projects.htm
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