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# Sizing Battery Bank for no less than X volts after Y hours with a load of Z amps

## Sizing Battery Bank for no less than X volts after Y hours with a load of Z amps

(OP)
I've been doing some reading on Peukert's exponent but I'm still confused. I'm familiar with sizing a battery bank based on the system load, but in my current application, I want to make sure that at the end of 24 hours, the batteries are lower than "X" volts. My reason for this is that components in the field have a minimum operating voltage (18 volts) and when I do the voltage drop calculations, I'm ending up with 8 AWG wire with is too large for the device screw terminals, and expensive to buy and install.

The voltage drop is based on a starting volt of 20.4, with is 85% of the power supplies rating. The assumption is that if the system looses AC power, I'll be running on batteries and should assume a voltage loss after 24 hours on batteries, down to 85% of the system rating (i.e. 20.4 volts).

If I can demonstrate the math to show a final voltage of say 23 volts (or something close), I can use a higher starting voltage for my voltage drop calculations thereby decreasing my wire size.

I've been reading SmartGauge Discussion on Peukert's Equation, but still can't figure out what the equation would be, or if that's even where I should be looking. They have several spreadsheets:
I'm hoping someone can help me out on the equation for this...

Regards,
DM

"Real world Knowledge isn't dropped from a parachute in the sky but rather acquired in tiny increments from a variety of sources including panic and curiosity."

Replies continue below

### RE: Sizing Battery Bank for no less than X volts after Y hours with a load of Z amps

You have two separate problems:

> You need to have a voltage vs. SoC (capacity) curve to figure out how much you can afford to discharge. Your math is not right, because 20.4V does not mean that you're at 85% of capacity

> You need to have discharge curves, and/or Peukert exponent information to determine how much of the raw capacity you're actually going to consume.

TTFN (ta ta for now)
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### RE: Sizing Battery Bank for no less than X volts after Y hours with a load of Z amps

For emergency backup, voltage supply wiring, low volt DC applications, I'm surprised you're assuming AWG 8 wire "too big"

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