John,
What you're suggesting is kind of like driving your car way past empty on the gauge, until there is less than a cup of gas in the tank before you think about looking for a gas station.
You can interpret the graphs any way you like. However, the spec sheet clearly states that the batteries should deliver greater than 3000 cycles at 0.5C if kept above 80% DoD, and 5000 cycles if kept above 70% DoD.
I take most manufacturer claims with a grain of salt, the specs also show that the cells can take an impulse load of 20C (1800A for the 90Ah cells) without damage. That seems a bit optimistic to me, but might be true. Personally, I try to look at these specs conservatively. I'm setting my expectations to getting only 1500 cycles for my pack if kept above 80%, though they will probably die of old age long before then.
You do need to be careful with lithium though. A single event of discharging a cell to less than 2.5V can ruin it completely. The difference between 2.8V and 2.5V is less than 2% of the rating or less than 1.8Ah for these cells. That can happen pretty fast in real life.
BTW, The batteries that I suggested significantly exceed the performance of your original battery at the weight that you wanted. I'm surprised that you're willing to risk this kind of investement for such a small percentage more range. If you want more range, buy bigger cells like 160Ah that would still be lighter than your original set up. But like I said before, if you buy them, you can treat them any way you like.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "john_casperson" <john_casperson@...> wrote:
>
> Eric--
>
> When I looked at the discharge curve in the link provided below, the LFP battery doesn't fall to the recommended cutoff voltage of 2.8V at a discharge rate of less than 1C until more than 100% capacity is reached.
>
> Since I would be operating at 30 amps (0.33C for the 90AH battery), shouldn't I be able to discharge beyond 80% without damaging the battery, provided I stop prior to reaching 2.8V?
>
> John Casperson
>
> --- In electricboats@yahoogroups.com, "Eric" <ewdysar@> wrote:
> >
> > Here's a quick comparison that I recently wrote: "As a practical comparison, the 160Ah 48V lithium battery bank in my recently converted 1964 Cheoy Lee Bermuda 30 ketch is 60% lighter, 50% smaller and cost 50% more than an high quality 200Ah 48V AGM battery bank that would deliver a similar range under normal 4-5kt operation."
> >
> > So here's what I would do if I was aiming for the performance equivalent of your battery. I have no idea what the Ah capacity of your battery is, so I'll have to focus on the weight.
> >
> > Thundersky 90Ah LiFeYPO4 3.2V cells weigh 7.05 pounds, so 4 cells with strapping would be right around 30 pounds. The cells cost $112.50 a piece for a total cost of $450 dollars. Her's a vendor that sells them. http://currentevtech.com/Lithium-Batteries/Thundersky/Thundersky-90ah-cell-p23.html
> >
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