Looking for the right battery for a backup system.

I'm looking for a deep cycle battery to run 2 small fans totalling 97 watts or
0.81 amps draw.

They will be run from a power inverter.

I want them to last about 10 hours.

What would the AH Amp-Hour rating on the battery that I would need to do this.

I would like to stay with a 12v battery because my generator manual says it will not charge 6v batteries on this outlet.

My plan is to use them at night in the case of a hurricane power outage after I turn my generator of.

Then in the morning disconnect then and hook the battery up to the 12v outlet on my generator and recharge the battery.

I guess I could get a 115v charger for the battery and plug it into a regular outlet on the generator.

Also I would like this to be relatively maintenance free.

Thinking of a trickle charger or solar charger for when the battery is in storage, which will most likely be the majority of the time.




  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Looking for the right battery for a backup system.

    First realize the less you discharge the batteries per cycle the longer they will last.

    You should try to avoid going beyond 40% or 50% depth of discharge (60%, 50% rated capacity left).

    If you want to run 100 watts for 10 hrs, that is 1 kWh.

    Assume 90% efficiency for your inverter then you will be drawing 100/.9 = 111 watts from inverter. If it is a 12 v inverter then battery amperage will be about 111 watts/ 12.3 vdc = 9.0 amps.

    For ten hours that is 90 A-H's. For 40% depth of discharge battery capacity needs to be 225 A-H.

    Two 6 volt T-105 golf cart batteries will be about right or about a 8D sized 12 volt battery.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Re: Looking for the right battery for a backup system.

    Another suggestion... If you can afford it, AGM batteries are about the best (low self discharge, no maintenance, high current output and input).

    And, if you are using the 12 volt charger on a generator--take a look at the ratings. The Honda eu2000i I have only puts out about 10 amps (120 watts). The generator can put out 1,600 watts which, in theory, a good charger can recharge at almost 100 amps... Even if you only use 800 watts (50 amps), that is still 5x faster than the 12 volt generator charger.

    That 1kWHr of discharge will take upwards of 10 hours of generator runtime (and, pretty much wasted gasoline) to recharge. 1-2 hours of recharging time with a "real" battery charger is much better and will use much less fuel:

    Honda eu2000i can run about 15 hours on a gallon of fuel and 400 watts output.

    Or, run 1 hour at 1,600 watts on a bit less than 1/4 gallon of fuel (1.1 gallons, 4 hours, 1,600 watts).

    In the end, after a storm (or for me earthquake), fuel will be gold and you will not want to waste any of it.

    For example (never seen one, never used one), Xantrex has a 4kVA inverter (24 or 48 VDC battery bank models) that can charge the batteries up to 110 amps (@24 vdc, supposed to be adjustable). And you can just connect it to Generator AC, and your emergency circuits to the output (and to your battery bank) and it will power your appliances and charge the batteries (using a 3 stage charger) with the generator running, or automatically turn on the inverter and power your AC loads with the generator shut down...


    Probably pretty expensive--but others here can probably suggest some smaller units that would better match your requirements.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Looking for the right battery for a backup system.


    The fans may draw 97 W from a 120 VAC sine wave power source, but they’ll likely draw up to 20% more if powered from an MSW inverter. Assuming 90% inverter efficiency, they’ll draw ~113 Ah over 10 hours.

    Limiting discharge to 50% and looking for an essentially maintenance free solution, you’ll need something like a size 8D AGM battery. Manufacturers include Deka (MK) and Concorde.

    Although I haven’t researched this point, I doubt the generator’s 12 V battery charger is particularly good. An appropriate 3-stage charger for your location and planned use would be something like the Xantrex Truecharge 10 (not the 10B).

    Neither the AGM battery nor the charger is inexpensive. A 60 W PV module and a small controller should work as a trickle charger for a battery this size.

    An alternative solution might be a pair of Fan-tastic Endless Breeze 12 V DC fans. These DC fans would eliminate the inefficiencies associated with the MSW inverter. A pair of these fans running on “HI” would draw 5.16 A directly from the battery, or ~52 Ah over 10 hours.

    A size 31 AGM battery and the same charger would work nicely, and a 40 W PV module would be big enough for a trickle charger. Although you'd have to buy new fans, this approach would avoid the cost of an inverter, and the smaller battery and PV module are much less expensive than the larger ones I mentioned. I just don’t know how these fans would perform (as in moving air) compared to the ones that you already have.

    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: Looking for the right battery for a backup system.

    Thanks to everyone for the suggestions and advise.
    I'm not looking to spend a fortune on this, so I still have many things to consider.
    I'll look into crewser's suggestion.

    Feel free to continue to post suggestion since I havn'e decided on anything yet.

    Thanks again;