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340Ah Lithium LifeP04 battery: How many AC watts?

  1. Dec 3, 2015 #1
    Sorry for a newbie Q.

    If I have a 340ah Lithium LifeP04 12V battery in my camping trailer...
    its BMS set to 5% (for a drainage of 95% = 323 usable amp hours)...
    it is hooked to an inverter claiming to have 88% efficiency...
    how many AC watts (@ 120V) can I expect in theory, or... heaven-forbid, in reality... before the battery drains to that 5% level?

    I thought this was a simple question but I am getting a wide range of answers from different sources and everyone is 110% confident that they are correct and everyone else is wrong. WhoDaThunk?!

    Is there an online calculator to do this for lithiums (meaning, not hardcoded to a max drainage of 50%)?

    Thanks very much for any help you can give me.

    BTW, I know that I am severely reducing my charging cycles by draining to 5% but even with that reduction, I don't boondock enough that I would expect to use up all those charging cycles for 7-10 years. Thus, I would rather get the maximum amp hours I possibly can on each charge can rather than extend the life of the battery far beyond what I will ever be able to use.
  2. jcsd
  3. Dec 3, 2015 #2


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    Gold Member

    Hello Karl,

    The number of watts you can expect will be listed on your inverter.
    If you meant to ask; "how many watt hours can I expect...", then the answer is roughly:
    (323 amp hours) * (12 volts) * (0.88) = 3410 watt hours
  4. Dec 4, 2015 #3


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    Gold Member

    The capacity of your battery is specified at a specific discharge rate. Assuming you are at that rate you can assume (323Ah times 12.8 volts) watt hours.
    If you exceed the rate, you get fewer Wh, and vice versa. You should look at the specifications for your specific battery. It also specifies a minimum allowable discharge voltage. Hopefully you obey that. You get very little additional capacity from a LifeP04 battery after it drops below 3.2 volts - 2.5V.

    So, depending on your load at 120VAC, you get different times. Those are reduced by your 88% inverter efficiency (but, careful, this is also specified at a specific power load).
    Say you are supplying 500 watts constant at 120V. 500 watts / 0.88 = 568 watts in. Battery Capacity is 4134Wh, so it will run for 4134/568 = 7.2 hours. You are drawing 323Ah/7.2h = 45 amps from the battery (also, 568 watts / 12.8 volts = 44 amps) which may or may not reduce its capacity.

    So, at what load is the battery capacity specified, and at what load is the inverter efficiency specified.
  5. Dec 4, 2015 #4


    Staff: Mentor

    As you drain the battery, it's voltage will slowly go down. At some point, the inverter will shut off because of low voltage. My inverter cuts out at 11.8 volts. That will prevent you from using all the battery's capacity to run the inverter.

    As mrBigGuy pointed out, the inverter efficiency is quoted at a specific load, usually full load. At low loads, Inverter efficiency can drop far below 50%. Therefore, you should always try to match inverter rating to the actual load. Using an oversized inverter robs you of efficiency.

    In boats and RVs, I often see people using a 3000 watt inverter to power a 10 watt cell phone charger. Their efficiency is probably down to single digits. It would be far more energy efficient for them to use a $5 cigarette lighter USB charger and to leave the 3000 watt inverter turned off.
  6. Dec 4, 2015 #5


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    Gold Member

    I did not know that. Thanks!

    I've been looking for a 3000 watt inverter, as my 400 watt inverter is VERY limited in what it can power. Now I know better than to toss it in my "obsolete e-pile".
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