Check out http://www.windsun.com/Batteries/Battery_FAQ.htm#Cycles%20vs%20Life
Ok, You got me, I was exaggerating or at least speaking of worst case. If you
ran the the bike till it wouldn't move anymore and reverse cells, you might not
get much more that 50 cycles. If you stop everything at the 1.75V/cell point
specs sheets say 250 cycles in your case. As you note, that is for a 1C
discharge rate or 35 amps in that case. You will be doing more in the 80-300
amp range so how that is going to affect the cycle life. Not in a good way
certainly! That is also with perfect charging. Charging a string is more
difficult. I just wanted to point out that with neglect, extreme discharging,
and sub-optimal charging 50 cycle life will be the sad reality.
Now stopping the discharge when you are at 1.75V/ cell sounds good, but what
would you do when that happens and you are just a mile from home? You probably
ride it home, cringing all the way if you're like me. Ideally we would have a
BMS that could communicate with the controller to cut back max current draw
when the batteries are very far down to reduce the damage.
If I may quote sage Lee Hart from the BMS list about Optimas
"For a 200 cycle life, you don't need any balancing. Just charge until
the worst ones are full, and let the best ones get "pounded down" to
match by overcharging.
For a 400 cycle life, you have to do some kind of balancing. Early in
their life, keeping them balanced without overcharging will extend their
useful life. Later in life, the balancing lets you keep using batteries
that would have otherwise differed too much to have been used.
For an 800 cycle life, aggressive balancing will be needed. I don't
think this is possible with AGMs and just clampers/shunts/ dissipative
regulators -- you have to be able to move charge between batteries.
That's because the differences between batteries will get quite large,
so large daily corrections will be needed. Also, with packs this old,
you're likely to have a "stinker" or two that fail early. You want a
balancer that is strong enough to be able to replace one bad one, and
keep going."
Thanks for calling me on it Andrew and making my clarify what I meant.
-Garret
I wrote:
Now you don't want to take them to 100% DoD if you want more than 50 cycles out
of them so lets say 80% which is still hard on them.
I don't mean to discredit what you are saying, but do you have any data to
support this? And how do you define 100% DOD?
Maybe there is a common misconception about what 100% DOD actually means. It
does not mean the battery is completely "dead" which is very bad. 100%
discharge is usually defined as ~10v. This means you stop discharging when the
voltage reaches this threshold. As you can see from the above spec sheet I
posted, my batteries should survive >250 100% discharge cycles. Probably a
little less, as cycle life tends to diminish with higher discharge rates. Note
the following data from Hawker on Genesis batteries to demonstrate this:
Cycle life at 25 degrees C
@ 15 min rate: 250 for 100% DOD and 2.45 vpc charge for 16-12 hrs w/initial
inrush current as high as possible
350 for 80% DOD (charged under same conditions)
@ 3 to 5 hr rate: 400 for 100% DOD
500 for 80% DOD
Regards,
Andrew
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