I’ve been charging the bank gently in an attempt to achieve extreme longevity.
- relatively low Vabs (like 13.6v - 13.8v)
- enough Absorption duration to get us to ~95% state of charge
- at C/5 (~20A)
Let’s see how that’s working out.
proof in the pudding
The batt is performing as expected and is meeting my needs with significant margin left over. The 100Ah is well-suited to my use; in the mornings I am typically at 50% SoC. I can see as low as 30% SoC if I am running a bunch of stuff at night.
Here’s a screenshot of the bank around 95% SoC, my preferred target. Gently overcharged to 13.8v (3.45vpc) cell voltage imbalance is 6mV, less than half of what it takes to trigger the balancing circuit. I won’t say it’s perfect balance, but it’s dang close.
Takeaway: charging at lower voltages and sane currents can help prevent cell imbalance. And prevention of cell imbalance (IMO) is vastly preferable to fixing cell imbalance.
digression: drop-in replacement
This is a copy/paste/edit of a response to an emailed question about the difficulty/ease of dropping in Lithium to replace an AGM bank. The existing system in question charges from 200w of panel + MPPT charger.
Lithium has some benefits in this use case, all related to solar-only charging.
- Li does better with partial state of charge
- would be like having +10% more panel due to better charge efficiency
- Li does better than AGM with the relatively-low current from solar-only charging1
There are ways that the banks would behave similarly.
1x 100Ah Li would be almost exactly the same amount of usable Ah as the existing bank when new: 2x 78Ah AGM @ 50% DoD = 78Ah usable. 1x 100Ah Li @ 80% DoD = 80Ah usable. At the low end it could also be cheaper to buy 1x 100Ah than to replace the existing parallel bank. (see below)
The max charging rate of the two banks is almost identical. C/3 for the AGM would be 52A and C/2 for Li would be 50A.
The actual replacement would be easy enough. In theory you could just drop in the LiFePO4 bank and it would last ≥$warrantyYears. For folks like me who need stuff to really (really) last, it would probably require reconfiguring the controller through the remote to enable more gentle charging. I am working on a sub-article about gentle use but it is still quite rough. And there is no settled consensus; mostly just people experimenting and we won’t know for 10-20 years.
There are some decent Li in the low $300s; the main drawback at that price point is the lack of low-temp cutoff. Chins is a popular low-cost batt in that range. I suspect the batts need to be warmed regardless (for longevity) so low-temp disconnect isn’t necessarily critical.
twitter comment thread for this post
The AGM bank requires C/~10 minimum, or ~16A. I do not think 200w of panel can provide this on a regular basis. (15.6A x 12.1v = 188w) ↩