Size of amp load as a percentage of battery amps?

bbbuddy · September 2010

I thought I read this somewhere on the forum, but now cannot find it...what is the recommended maximum amp load as a percentage of battery amp?

For example only, if your battery is 400 amps, maximum sized load should be 10%, or 40 amps at any one time...
ie. a 40 amp motor load would work, but a 60 amp would not...

Is there a rule of thumb?
Does it vary according to voltage of battery, rather than just looking at amps stored/amps pulled?

Kamala · September 2010

Re: Size of amp load as a percentage of battery amps?

Batteries are not usually rated in amps. They are rated in amp-hours. This rating is usually given at a 20 hour rate. So, a 400 amp-hour battery would become completely discharged after being connected to a 20 amp load for 20 hours. For a 12 volt battery this would be a 240 watt load.

My answer to your question is, no. There is not a direct relation between capacity (rating) and the amps that can be delivered by any given battery (perhaps there are thermal runaway issues.) It is a matter of how long the battery can supply a certain current (amps.) That is why the capacity is given in amp-hours.

The battery capacity (rating) decreases as the time to total discharge decreases. If the load is heavy enough to discharge the battery in, say, 5 hours, the battery capacity is significantly less than 400 amp-hours. This characteristic is an exponential function, and for the sake of demonstration, will be disregarded here. If you were to put a 100 amp load on a 400 AH battery (1.2 kWH @ 12V) it would be totally discharged in less than 4 hours.

Also, know that no battery should be discharged to less than 50% of capacity, so cut the hours given above in half.

HTH
K

BB. · September 2010

Re: Size of amp load as a percentage of battery amps?

For a typical flooded cell battery bank (we use the 20 Hour Rate for battery capacity--It seems to best represent the "average" Off-Grid system's average current draw for successful design/operations):

C/20 (or 5%) of the battery 20 Hour Rate is a good efficient average. Also the minimum charge rate rule of thumb we use for charging a battery bank too.
C/10 (or 10%) is probably about the most you want to consume on average. Batteries are less efficient at the higher rates and don't deliver as much power (and battery can get warm).
C/8 (12.5%) Call it the maximum "safe rate" for continuous discharge current. Over that rate, batteries may need forced air cooling (for discharging or charging currents). 13% is also the typical maximum recommended charging current for a typical flooded cell battery bank.
C/4 (25%) About the maximum current that you can efficiently design for with a standard battery bank.
C/2.5 (40%) Maximum Surge Current Operation--At this point, you can start to damage/warp the plates, heat up the battery, draw down below operating voltage (even if fully charged)... Drawing this much current or more and you can have flaky inverter operation and such.

Now, the above is just from what I have read trolling around here and the web... And is a good starting point for a a rule of thumb for design.

Obviously, there are batteries designed for all kinds of applications--Car batteries are designed for high surge currents (but not deep discharge).

AGM's have very low internal resistance and at least one vendor (Concorde AGM) claims C*4 (400% current rating) for charging/discharging. Obviously, that amount of current requires a lot of copper bus bars and is very difficult to design for.

-Bill "my two cents" B.

bbbuddy · September 2010

Re: Size of amp load as a percentage of battery amps?

BB. wrote: »

For a typical flooded cell battery bank (we use the 20 Hour Rate for battery capacity--It seems to best represent the "average" Off-Grid system's average current draw for successful design/operations):
C/20 (or 5%) of the battery 20 Hour Rate is a good efficient average. Also the minimum charge rate rule of thumb we use for charging a battery bank too.

C/10 (or 10%) is probably about the most you want to consume on average. Batteries are less efficient at the higher rates and don't deliver as much power (and battery can get warm).

C/8 (12.5%) Call it the maximum "safe rate" for continuous discharge current. Over that rate, batteries may need forced air cooling (for discharging or charging currents). 13% is also the typical maximum recommended charging current for a typical flooded cell battery bank.

C/4 About the maximum current that you can efficiently design for with a standard battery bank.

C/2.5 (40%) Maximum Surge Current Operation--At this point, you can start to damage/warp the plates, heat up the battery, draw down below operating voltage (even if fully charged)... Drawing this much current or more and you can have flaky inverter operation and such.

Now, the above is just from what I have read trolling around here and the web... And is a good starting point for a a rule of thumb for design.

Obviously, there are batteries designed for all kinds of applications--Car batteries are designed for high surge currents (but not deep discharge).

AGM's have very low internal resistance and at least one vendor (Concorde AGM) claims C*4 (400% current rating) for charging/discharging. Obviously, that amount of current requires a lot of copper bus bars and is very difficult to design for.

-Bill "my two cents" B.

Thanks Bill, I knew I had read some "rule of thumb" around here, just wasn't finding it in search.

The way you have stated it make it easier to remember, taking watts out is the same as putting watts in for the battery.

This makes needing a higher voltage battery bank for larger loads logical, as the amp hours used goes down with them.

BB. · September 2010

Re: Size of amp load as a percentage of battery amps?

You are very welcome bbbuddy,

By the way, roughly, a 6 volts * 100 AH battery = 600 Watt*Hours ; and the same size (weight) 12 volt @ 50 AH battery also = 600 Watt*hours...

We believe in the conservation of energy and mass around here.

Basically, the higher voltage battery allows lower current for the same power--so the wiring is cheaper and easier to do.

-Bill

Size of amp load as a percentage of battery amps?

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