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Battery capacity...should i add more

R

Rod64

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Joined
Aug 20, 2020
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19
Hi all,
I have an off-grid cabin that i use mostly on weekends from spring to fall. I have 1KW of panels charging a 24V 470Ah battery bank (8x6V 235Ah). Works fine so far and when i arrive the batteries are always fully charged. When at the camp using power the morning SOC is around 60% before the sun starts charging again. I am concerned that if i want to extend my stays to 3-4 days and if the weather is overcast that i will run out of battery power. Should I add more Ah to my battery bank? and if so do I have an issue with my solar capacity to provide the necessary amps to keep the batteries happy. thank you for responses.
 
Link #1 in my sig. Do an energy audit. You may find that you're using excessive power. Are you using electric heat? If so, that's murder on batteries.

That's a lot of capacity for a small cabin. We have two RVs on off-grid "shore" power with a similarly sized battery. With 5 of us using power in the evenings, we don't drop below about 75% overnight.

If you add more capacity, you'll need to add more panels. Your optimal charge current is 47A, and you're sitting at 42A peak if the panels are actually producing their 1000W. You're really on the low side.
 
I will check your link but essentially i am providing wattage to 6 LED lights (not used at same time), a 1/6 hp piston pump and satellite wifi with a laptop and LED TV that we use for music. Refrigerator, oven and water heater are all propane. Heat is a wood stove. No high amperage usage like toaster or coffee maker. The batteries are new to this year. I am wondering if my charge controller is underestimating the SOC? The peak charge i get on bright days is 960W max.
 
1/6hp piston pump? What's that?

Oh... this is the SCC SoC rating? I wouldn't put much faith in that. What SCC? What is your low voltage in the morning?
 
And the pump only runs when needed to refill the pressure tank?

Hmmm... Depending on the load at the time of the low voltage reading, it's probably not too far off.

Are you certain that you have the charge controller properly programmed for your batteries and that they are getting fully charged per the manuf recommendations?
 
Yes. Duty cycle for pump is about 2 minutes to fill the tank and it triggers about 4-5 times per day and not at night. I have all the settings for FLA batteries according to manual. One thing that irks me is that the reading on the SCC is at least 10% lower than the reading on the inverter. I guess my next step is to get a hydrometer reading early morning. As for fully charging the batteries i believe that the do get filled up in the 4-5 day interval when i am away.
 
There you go. SG is the best way to determine actual SoC. I'd do that before anything else.

4-5 days to charge up? Yikes. That just doesn't seem right.
 
No the batteries are back up to 100% after an hour or two of sunlight. What i was referring to is that when i return to cabin after 4-5 days the SoC is always 100%. My feeling is that my 60% in the morning is more like 70% or so...which i can live with.
 
Gotcha. If you can get the manufacturer's battery data, program the absorption duration and tail current to custom values, that can make certain you're getting fully charged. The default programs aren't necessarily good for all batteries or sizes.
 
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