Looking beyond batteries, I've also seen people that have been similarly disapponinted by their motors. I've heard compaints about overheated motors only to find out that the user was running a 125A rated motor at 200A continuous. The response has been "it didn't fail right away, so I figured that it would be OK..."
I find it interesting when people are surprised that pushing components or systems beyond what the manufacturer recommends results in less than optimal results. Looking at the battery adopters, they are surprised that they got less than stated cycle life when pushing the batteries at 3 to 6 times the measured rating. I would figure the same with my Honda, while I expect that it will last well over 100,000 miles with normal operation, if I operated it regularly at 110 mph, I would expect that it won't last as long.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Myles Twete" <matwete@...> wrote:
>
> 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
>
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