battery backup system

[ches2443]

Is there a formula or calulation for finding the size battery amperage needed for a backup system.

 

[hillbilly]

Is there a formula or calulation for finding the size battery amperage needed for a backup system.

Yep, lots of them.:smile:

You need to be a little more specific.

steve

 

[ches2443]

I have 12 volt 85amp charger with a avg. amp draw of 62amps at the battery.
Daily, the battery needs to hold this draw for 4hrs. before it can be recharged. What size cold cranking amp battery would I need?

 

[steved]

You need a deep-cycle battery, which will be rated in amp-hours. Starting batteries, which are rated in Cold Cranking Amps, won't last long if they're heavily discharged.

This RV battery sizing calculator may give you some idea of what size batteries you need.

It's generally recommended to discharge deep cycle batteries to 50% of their capacity, for reasons explained here.

 

[dereckbc]

Yes there are several ways to go about it all depends on how accurate you want to be. Best or most accurate way is to use manufacture discharge curves. To get in the ballpark is:

AH = TA

Where AH = Amp-Hours
T = Time in Hours
A = Amps

But be warned the above formula only gets you in the ballpark if the battery is discharged at its published discharge rate of 8 to 20 hours.

For example let's say you have a battery rated at 100 AH @ 20 hours means you can discharge it at 5 amps for 20 hours. However let's say you want to discharge it at say 20-amps, you will not get 5-hours, more like 3 or 4 (60 to 80 AH)

 

[steved]

There is a Peukert's Equation spreadsheet that can estimate the capacity of a battery at different discharge rates.

 

[iwire]

Merged.