Battery FAQ discussion

Hedges

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The "Battery FAQ" has graphs for lead-acid cycle live vs. DoD, compared to lithium capacity vs. number of cycles.
That's an apples vs. oranges comparison.
I've heard that the cycles quoted for lead-acid are not same DoD of original capacity, but DoD of remaining capacity.
What I'd like to see are cycles vs. DoD for both chemistries, and remaining capacity vs. cycles for both.



As for that being a curve for lead-acid, I note that it shows 1300 cycles to 70% DoD vs. 3000 cycles to 35% DoD.
Compare 70% DoD is twice the Wh or 35%, so compare 3000/2 = 1500 with 1300, you get 15% more total lifetime Wh from the battery at the more shallow 35% DoD.
The graph would be a curve even if there was no penalty for deeper cycling.
 

svetz

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You're very possibly right, the remaining capacity vs. cycle life isn't listed in the SPRE 12 225 datasheet. I'm by no means an expert and the approach/formula (cost / kWh / DoD / number of cycles @ DoD) is parroted from Will's video on it.

There's fairly good data for Lithium (left graph), but only vague data on capacity Vs. Cycles that I could find for "Smart Carbon"; although from that data it doesn't look like capacity is decreasing with cycles (the image is also a link to where I found the data).

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Hedges

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I'm suspicious of graphs that don't put units on axis, but I don't think Trojan would try to pull a fast one.
The linked paper explains PSOC refers to not fully recharging the battery each cycle.
"Smart Carbon" reduced degradation rate of battery when 50% cycled without fully recharging, as compared to previous chemistry.
But it still seems odd capacity is shown as increasing and leveling off, then decreases only to initial capacity.
I think we'd be fine with some reduction in capacity at end of life, e.g. 80% of initial capacity.

I could imagine having a chart that reports number of cycles each 50% of initial capacity, with end of life when you can no longer pull that much current because capacity has dropped below 50%. But no indication that's what's being reported. "Capacity" wouldn't be the Y axis in that case, maybe voltage or specific gravity.

I think for solar we would generally recharge fully, but for a period of less sunlight you could have a few days of partial SOC cycles, followed by fully recharging. We want lead-acid bank to tolerate that. Lithium of course would have no problem being at reduced SOC, but cell imbalance will eventually reduce usable capacity and require rebalancing at top or bottom.
 

svetz

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it still seems odd capacity is shown as increasing and leveling off, then decreases only to initial capacity
I vaguely recall somone on the forums saying that a lot of lead-acid battery testing you saw on YouTube was poorly done because there was a "break-in" period for LA. I also remember Will saying he didn't like carbon batteries, but not why (possibly the fiddly maintenance?).

We want lead-acid bank to tolerate that. Lithium of course would have no problem being at reduced SOC, but cell imbalance will eventually reduce usable capacity and require rebalancing at top or bottom.
The datasheet does say "Operating at a Partial State of Charge (PSOC) on a regular basis can quickly diminish the overall life of a battery". But, I haven't seen data for it (other than the chart above) like there is for lithium (LiFePO4 is the opposite, it degrades faster (although still fairly minor) if kept at 100% charge).
 
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