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Small Cabin Forum / Off-Grid Living / How many batteries?
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Lost arrow
# Posted: 9 Dec 2019 01:33 - Edited by: Lost arrow

Hello everybody
Im somewhat new to the cabin livin world and had a question.
I found a 500 watt invertor at a thrift store and i already own a battery charger. I think my electric needs are small and was wondering how many 12 volt batteries would I need to power a laptop a 20 inch tv a cell phone charger a blue tooth speaker and a lamp with a led bulb for roughly 48 hours a week?

I plan to charge the batteries at home and bring them up to my property for 3 to 4 days at a time. Does this sound like a good idea? Ive not alot of money right now to put into solar panels.

Im currently using an old onan generator that was taken out of a RV. I seem to be using around 12 gallons of gas a day and was hoping to switch to a battery box to save money.

Im a working single father and attend school full time, so im putting off the web calculators and what not until winter break starts. My brain is fried extra crispy most days.
Im thinking 4 12 volt batteries but this is merely a guess. Any help would be amazing.

# Posted: 9 Dec 2019 06:55

Use a solar calculator or battery bank calculator

# Posted: 9 Dec 2019 07:19

Just saying "12 volt battery" isn't enough; they come in many capacities even though they're all 12V. You need to add up all the loads and times to determine what capacity (in amp-hours) you need.

But, for example, a typical group 27 deep cycle battery is 85 amp-hour, which works out to 1020 watt-hours. That means it would run your 500W inverter at full capacity for two hours. But a bulb, a laptop, and a TV probably won't add up to 500W, so do the math. Don't discharge a battery more than 80% if you want it to last.

# Posted: 9 Dec 2019 07:36

Here is a load calculator... fill in the blanks and you will have a better idea of what your needs truly are. From that it will be easier to determine battery bank size. Likely 2 x 6v batteries, AGM if you can afford them, will be better than 1 x 12v battery. BTW, amp-hour x voltage gets you watt hours. But as Fanman points out, you don't get all those watt hours. With flooded lead acid (FLA) you really don't want to discharge more than half/50% of the capacity. With AGM theoretically you can go 80% but you wouldn't want to do that day after day.

Figure what your usage will be then post that.

# Posted: 9 Dec 2019 10:32

Hello all. Maybe you can just get your hands on the best you can. They will have a cold cranking amp number on them. Bigger is better. There are ways to recharge old batteries on th web. You can always add on more batteries. Start with what you can and go from there.
At our cabin I charge up a battery from generator. Then I have an adapter with alligator clips on one end to battery. And a cigarette lighter socket on the other. Then use a 12v charger from car to charge the phone, computer etc. When the battery is dead recharge it. With a battery charger powered by generator. When you leave your cabin for a while always top off the charge in your battery.
If you have several batteries, you can charge them all at once. Use connecting wires in series. Start where you put negative battery clamp from charger. Go pos neg pos neg . As many batteries as you have. Hook up charger to respective pos neg on the ends. Like a train. I always, never plug in charger until everything is hooked up and looked over. Plug in charger and use off on switch to turn on. Don’t plug in with the switch on. Jumper wires are available that have battery end sockets all made up. But I made mine from sturdy alligator clips n ten gauge braided wire. That way I have them to use while diagnosing electrical problems on our vehicles. Usually bad grounds. Multi purpose.
There are usually lots of places to get old batteries free. You can get a tester that tests the specific gravity of the battery acid. Most folks use them to test radiator fluid for freeze level. Little bulb with some beads or little indicator. I have a scope because I used to do it a lot. This will give you a good idea of the integrity of the battery to hold amps. You could get by with a small charger as just a slow two amp charger is best. I’ve bought a few under ten bucks. A role of wire which you will probably find many uses for n the jumper wires made great diagnostic tools for your car.
Hopefully this will help some.
Merry Christmas

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