health of 48v battery

mahendra
mahendra Solar Expert Posts: 188 ✭✭✭
in Off Grid Solar & Battery Systems #1
Hi i know i have been over and over again on battery system voltage so i have now decided that i have no other choice than to use 48 v system.i know that it is unhealthy. for batteries to function above 100 amps dc.But my concern is that if i am to set up a 48 v system with 8000 watts pure sine wave inverter 120/240 volts ac and would need to run appliances that would use a total of around 6000 watts ac(125 amps DC) for a maximum of 4 hours on a daily basis is this possible with out causing any major concerns.for the remaining 20 hours of the day the system would run below a 1000 watts .thanks in advance.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: health of 48v battery

    It is not unhealthy for batteries to have a draw of more than 100 Amp on them specifically, it is more a matter of what percentage capacity that represents. For instance on a 200 Amp hour battery that sort of load will "flatten" it almost immediately. For a 1000 Am hour battery it's a mere 10%. The issue here is that the higher the current draw at any given moment the lower the functional capacity of the battery under that load.

    So with your load of 6000 Watts on a 48 Volt system that is roughly 125 Amps. That needs to be compared to the total Amp hour capacity of the proposed battery bank. If you follow the recommendation of a minimum 100 Amp hours per 1kW of inverter capacity (a somewhat arbitrary rule-of-thumb) you would have an 800 Amp hours 48 Volt battery. That 125 Amp draw would represent 15% which is not out of the question for short usage. That much battery would supply up to about 19 kW hours of power, so compare that with your daily usage.
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  • mahendra
    mahendra Solar Expert Posts: 188 ✭✭✭
    #3
    Re: health of 48v battery

    well i dont require that much kwh i am just trying to make sure i am not maxing out the batteries or overheating them.my battery bank would be 490ah at 48v
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: health of 48v battery
    mahendra wrote: »
    well i dont require that much kwh i am just trying to make sure i am not maxing out the batteries or overheating them.my battery bank would be 490ah at 48v

    125 Amps on 490 Amp hours would not be good: 25% That is really about the upper limit of current draw you'd want to have. Any significant amount of time at that rate will deplete the batteries quickly, and if you try to pull that much power at lower SOC it will happen even faster. It would not be surprising to see that cause low Voltage shutdown of the inverter when the load is applied.
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  • mahendra
    mahendra Solar Expert Posts: 188 ✭✭✭
    #5
    Re: health of 48v battery

    also these high usage will be when the sun is out on a 3600 array,and the 6000w for 4hours would be a very unlikely i was just anticipating a worst case scenario,a realistic scenario for me would definitely be half of what i mentioned.appliance that would be operated are well pump,washer dryer,refrigerator and ac ,just wanted to know if it is safe to operate when these appliances are starting up or operating at peak at the same time.Of course when ever it is stat up or peaking it would only be for short periods which would be in minutes
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: health of 48v battery

    The refrigerator start-up will be a blip that will go unnoticed. You can control when the washing machine is run (again, the motor start is the worst but doesn't last long). If possible use a very large pressure tank on the well pump and use the charge controller or a Voltage controlled relay to prevent it turning on if batteries aren't up/solar not producing. That leave only the A/C which in theory is most likely to come on when the sun is shining down on those panels. They ought to be good for 50 Amps under full sun and that will help.

    Try it. If it doesn't work you'll soon figure out how to keep loads separated so that it does. That will be easier and cheaper than doubling the battery bank. :D
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