Excellent points Eric.
I think what I take out of this is that while I can get by with my tired set of T105’s that probably only have 80-100ah capacity, given that I sometimes find myself drawing 80a for as much as ½-hr at a time or more, if I went with the cheap Chinese Lithium batteries I’d want to go with 160ah rated ones as you have done. Still, it’s interesting hearing from the EV community early adopters of these batteries that their expectations as regards performance not compromising cycle life were not met.
-Myles
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Eric
Sent: Friday, December 17, 2010 11:29 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: FW: [EVDL] Cycle life of batteries
Myles,
I think that you've raised an important factor in cycle life that is never really discussed. That is "How does discharge rate affect cycle life?" Virtually all data and discussion about cycle life for any battery (FLA, AGM, Li, etc.) refers to depth of discharge, not discharge rate.
But if you think about it, what do you think that a 1.5C load would do to an FLA battery? Would you run a consistent 75A load on a 50Ah FLA battery bank and what do you think that usage like that would do to the cycle life, even if you kept every cycle around the 50% DoD level (about 8.4 minutes from 100%). Perhaps more relevent would be consistently running a 300A load against a 200Ah AGM bank. Still, I would expect that the battery cycle life would suffer even while maintaining a reasonable DoD (21 minutes to 75% DoD) each cycle.
All of the life cycle spec sheets that I have seen for LiFePO4 batteries have been done for 0.5C loads. I guess that the problem is that the Li batteries themselves will survive 3C loads or even 5C that are way beyond the abilities of lead acid batteries. But just because that they won't die immediately, it makes sense that their cycle life would be affected through this type of use (abuse).
Of course, these high loads are much more likely in electric cars than in our boats. My relatively small 8kWh Li battery bank (160Ah) will rarely see 80A loads and I certainly wouldn't run at 80A (4kW) for the 90 minutes that it would take to drain the batteries from full to 80% DoD.
So your note is well taken, one can't assume that any new technology is impervious to some of our earlier constraints, even if some of the constraints have changed for the better.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Myles Twete" <matwete@...> wrote:
>
> Cross-posting this post on "abused lithium batteries" from the EVDL this
> week.
> It's from a thread on lithium battery life experienced by several users.
> One user noted his 2007 Thundersky lithium pack that was only ever
> discharged between 30-80% and didn't last but 500 cycles. Another noted
> that the cycle life for these batteries is inversely proportional to
> discharge (and perhaps charge) current levels, having noted that 0.5C
> discharge rates were fine, but that the cycle life drops exponentially with
> current such that at 1.4C discharge rates, the user measured less than a
> 500-cycle life and 16,000ah total energy delivered---presumably these were
> 40ah-80ah rated cells.
>
> The post below points out that the cycle life risks of battery neglect do
> not disappear when you switch from PbA batteries to lithium...
>
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