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Small Cabin Forum / Off-Grid Living / Sizing generators for solar backup
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# Posted: 6 Mar 2016 20:08

I am planning on a 1500 watt solar system. Eight 12v 100ah batteries. What size generator am I going to need to recharge the batteries if there is not much sun for some time?

# Posted: 6 Mar 2016 21:24

How are these eight 12 volt batteries to be wired? Into a 12 volt system with all eight batteries in parallel?

Pardon my bluntness, but if that is the plan it is a poor one. Eight parallel batteries will suffer with unequal discharge and unequal charging which will result in a shorter than necessary lifespan for the batteries. This is caused by the variable internal resistance with lead acid batteries. With batteries in parallel current flow follows the path of least resistance. This occurs under discharge and under charge. One or more batteries will recharge before the others. The artificially high voltage will be see by a 'smart" charger (solar or other) and cause the other not-yet full batteries to remain partially charged. These differences get magnified over time and batteries begin failing early.

If you need, or simply want, that much capacity you will be better off with larger batteries.

Things change a bit if some if those batteries are to be series connected to make a 24 volt system. Even then though 4 parallel strings will lead to the same problems, just on a slightly reduced scale.

Eight 12 volt 100 AH batteries, in parallel = 800 AH capacity.
800 AH * 10% = 80 amps which is the generally recommended amperage of the charger for most efficient charging of the batteries from a fossil fuel drinking generator. But that is not a recommendation for the use of 8 in parallel.


800 AH @ 12 volts = 9600 watt-hours capacity

9600 watt hours, or close to it, would be better achieved with something like six 6 volt golf cart batteries, three pairs to provide 12 volts and 675 AH total (2 x 6 volt 225 AH in series, three times in parallel) or maybe 4 parallel strings, but more than three parallel strings is really pushing it.

OR change to a 24 volt system and run eight six volt 225 AH in a series / parallel for 24 volts at 450 AH (24 volts x 450 AH = 10800 watt-hours.

OR if a 12 volt system is needed and you need the 9600 watt -hour capacity go to a larger battery like an L-16; 6 volts, 370 to 420 AH each. Four of them; two pairs in series / two parallel strings = 12 volts and 840 AH.

# Posted: 6 Mar 2016 22:37 - Edited by: Rickkrus

This is what I was thinking of. &qid=1457321744&sr=8-3&keywords=agm+battery

With this:

I want agm batteries so I can keep them inside and warm in winter.

# Posted: 6 Mar 2016 23:53 - Edited by: MtnDon

OK, at least the solar kit can be used for a 24 volt battery system, cutting the parallel strings to 4. I stand on that being too many in parallel for good battery life though. There are better, larger capacity AGM batteries.

If you Google the battery number, UB121000 you will get many returns. They show the same battery picture. Most refrain from calling it a deep cycle battery; a few do. That does not make a real deep cycle. No matter what the amazon page calls it I seriously doubt that is a true deep cycle battery. I would be very careful; hate to see money wasted.

NAWS sells that battery; here is their page. No mention of deep cycle. They call it a universal battery; like the one that starts our tractor.

Seriously 4 parallel strings of batteries is not a recipe for a long life. Being AGM's also means that you can not take remedial action like you could with FLA batteries. With FLA you can at least use an equalization charge to bring the low cells or batteries up. That can not be done with AGM, unless the manufacturer has special instructions for special conditions.

For an AGM you would be much better served with a Sunextender, a honest deep cycle. NAWS has some of those as well. Four of the PV2240's in series would eliminate all the parallel strings.

amazon price for the battery you linked to... = 1280 w/shipping
NAWS price on the sunextender = 1380 w/shipping to where I am. That will vary depending on where you are.

The spec sheet cycle / discharge charts for both indicate the sunextender should last twice as long.

Better batteries, fewer cables, no parallels, hard to go wrong.

The sunextender batteries are rated at 246 AH @ 24 hr rate (usually 20 hours is used, sunextender is different but that is okay. 246 AH at 24 volts. Sunextender recommends a charge rate of .2C when cycling to 50% DOD. For the battery I pointed to that means 246 AH x .2 = 50 amps.

Samlex makes a nice charger in 24 volts, the Model: SEC-2440UL It is 40 amps but would do. It is a true multi stage charger. The spec sheet states it has an internal 20 amp, slow blow fuse. That means it may draw more than 20 amps when the battery charge is first started. A surge current. They don't say how much . maybe call or write them ans ask. However, 20 amps x 120 VAC = 2400 watts. A guess would be that a generator that puts out an honest 3000 watts might start the charger.

The solar kit presents a quandry to me. The description names the Tristar charge controller. One of the pictures shows the Tristar MPPT; not the same thing. Just raises a flag to me. The non MPPT model is inferior in performance.

# Posted: 8 Mar 2016 18:26

Is he da man or what????

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