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Wrenches, I am working extensive cleanup on a five year old 24V Outback off-grid system for a neighbor. The original installation work was pretty amateur. The BOS is standard PS2, with 2100 watts of tracked PV and a single VFX3524 and MX60 (which will need to be replaced with an FM80 to handle this much array). The batteries are GNB Absolyte II, the 100/33s with "1600 amp-hours, 8 hour rate" on each. I believe the 8 hour rate multiplies by 1.23 to determine our more familiar 20 hour rate, so each set is rated 1968 amp-hours. There are two sets; apparently a crane was used and the stairs removed (and replaced) in order to get these bad boys settled in the basement. The batteries are used telecomm batteries, and the one string currently in operation appears to be in quite good condition, as it is holding up well under extended discharge due to cloudy weather and weak input during rewiring. My questions: I can safely assume that when the rework is done PV input will be about 70A PV input in good sun. That's a C/28 charge rate on one string; C/56 if both are hooked up. 1. What is the minimum charge rate needed to properly charge these batteries? Flooded batteries like at least a C/20 or higher to overcome internal resistance and get the voltage well up, but I don't have much experience with these. 2. Put the question another way: given that the batteries are already in the basement, will the customer get better performance and longer life with one string or both connected in this particular situation? Back story: I tried to get answers first from GNB. I asked for the minimum necessary charge rate and got this back: "I did discuss this with engineering, and they recommend 2.35 volts per cell as the charge rate." So I replied: " I'm sorry to be a pain, but let's try again. A cell voltage is not a charge rate. That's a charge voltage. We already discussed this last week. I am asking for a minimum current - in amps, not volts - necessary for good battery care. Could you ask them again, please?" And got back: "I spoke with Sr. App Engineer, and said 9 amps per 100 amp-hours." OK, so if I use the C/20 rate, I need to charge at 177 amps. Double that if both strings are used. Right. I need better advice, so it's time to request it here. Thank you. --
Allan Sindelar |
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