Which battery?

[billvolz]

I have a small remote telemetry system that runs off 12 volts which are recharged from a solar panel through a Victron charge controller. The telemetry uses about 1 amp. The solar panel can produce up to 330 watts. When I first installed the system, I used two 6 volt Vmaxtanks golf cart deep cycle 225 AH AGM batteries (V6-225) in series. The design was for the batteries to provide power for 7 days to 50% discharge with no charging.

I thought that the batteries should have lasted about 7 years, but they actually only lasted 3 years. On average, the system would be on battery 12 hours and consume 12 AH of power or about 5% of the total available power. The system would be on battery longer in December with reduced sun and increased cloudiness (essentially they should have been on float). I assumed the batteries needed replacing when the overnight minimum voltage dropped under 11.8 volts. When I removed them a battery tester said two of them were marginal, the others two were OK. I replaced all the batteries with Duracell 6 volt deep cycle 230 AH flooded lead acid. The thought process was if have to replace them every 3 years I might as well go with something cheaper.

The questions are:

1) Am I using the correct battery for this application?
2) Would Li-Iron battery work better, though far more expensive?
3) Are there settings on the charge controller to look into (Statistics from the charge controller said it was in bulk charging longer than I would have expected (3+ hours) and I wonder if that was over charging the batteries).
4) Would a single 12 volt 200 AH battery work better (though cost is higher than two 6 volt batteries).

[solardreamer]

AGM and lead-acid batteries in general must be charged soon after discharge to prevent sulfation or they won't last. You might try desulfation but that's hit and miss. Frankly, given the recent price drop for LiFePo4 batteries, it's cheaper to use them over the long run. As long as you keep them in the 20-90% state of charge range they will last much longer than typical AGM batteries for use cases like yours.

[billvolz]

AGM and lead-acid batteries in general must be charged soon after discharge to prevent sulfation or they won't last. You might try desulfation but that's hit and miss. Frankly, given the recent price drop for LiFePo4 batteries, it's cheaper to use them over the long run. As long as you keep them in the 20-90% state of charge range they will last much longer than typical AGM batteries for use cases like yours.

Ideally, the battery charges the next morning. In reality, there may be 2, 3 and more rarely 4 days when it's cloudy and they don't charge. The cheapest LiFePo4 200 AH 12V battery i could find with a few minutes of search was $569. Two Duracell 215 AH 6V batteries were $300 for the pair. So the LI batteries are more expensive unless they last twice as long in which case the Li's are only slightly cheaper. Another benefit for the Li's is that I only have to haul one battery up a 25 foot water tank half as often.

[SunEagle]

Ideally, the battery charges the next morning. In reality, there may be 2, 3 and more rarely 4 days when it's cloudy and they don't charge. The cheapest LiFePo4 200 AH 12V battery i could find with a few minutes of search was $569. Two Duracell 215 AH 6V batteries were $300 for the pair. So the LI batteries are more expensive unless they last twice as long in which case the Li's are only slightly cheaper. Another benefit for the Li's is that I only have to haul one battery up a 25 foot water tank half as often.

Based on the manufacturers data it should indicate how many cycles the 6V batteries will last based on how deep they are discharged. A Duracell should last you a long time but if they get cold and are too deeply discharged then you shorten their lifespan. Most Lead chemistry batteries do not like to be discharged below 80% but again the life should be based on the manufacturers data.

If the system is critical then going with an Li chemistry is usually worth the extra cost if it means more cycles and less worry that they will still work for you when needed.

[organic farmer]

One of my neighbors experienced his lead-acid batteries freezing. When the electrolyte freezes it expands, and in his case, they shattered the plastic housing.

If lead-acid batteries are used in a location where they might be exposed to cold temps, they need to be kept fully charged and warm.

[solardreamer]

Ideally, the battery charges the next morning. In reality, there may be 2, 3 and more rarely 4 days when it's cloudy and they don't charge. The cheapest LiFePo4 200 AH 12V battery i could find with a few minutes of search was $569. Two Duracell 215 AH 6V batteries were $300 for the pair. So the LI batteries are more expensive unless they last twice as long in which case the Li's are only slightly cheaper. Another benefit for the Li's is that I only have to haul one battery up a 25 foot water tank half as often.

For AGM, even charging the next morning is too long to avoid sulfation. You don't need as big of a LFP battery since you can have 90% DOD vs 50% DOD for AGM. For long life, it's more like 80% DOD vs 25% DOD.

[billvolz]

One of my neighbors experienced his lead-acid batteries freezing. When the electrolyte freezes it expands, and in his case, they shattered the plastic housing.

If lead-acid batteries are used in a location where they might be exposed to cold temps, they need to be kept fully charged and warm.

The batteries are outside in a battery box and not heated. I don't think the DOD gets much below 80% and for that the freezing point is around -40F. The coldest we got here was just above 0F which would allow the battery to get to 50% DOD before freezing. Thanks

[billvolz]

For AGM, even charging the next morning is too long to avoid sulfation. You don't need as big of a LFP battery since you can have 90% DOD vs 50% DOD for AGM. For long life, it's more like 80% DOD vs 25% DOD.

Ok you convinced me. Next time I replace the batteries I'll us LFP at about 150 AH.

i found a setting on the charge controller that deals with the absorption time. I don't recall if I turned that off but if I did it might be over charging the batteries. I'll check next time I'm up there.