A proper explanation of Peukert's Equation (Peukert's Law)

Proper monitoring of an off-grid battery pack requires good instruments. A necessary item is a "gas gauge", a way to track amps into, and out of the pack, showing current "fuel level". The issue is capacity varies with the rate of discharge. Just like a car uses more fuel at higher speeds. Good meters compute a user entered Peukert number into the equation. The Peukert number describes the internal resistance of the battery, and it's ability to discharge at various rates. The following (from Smart Gauge) explains the issue:

Mr Peukert first devised a formula that showed numerically how discharging at higher rates actually removes more power from the battery than a simple calculation would show it to do. For instance discharging at 10 amps does not remove twice as much power as discharging at 5 amps. It removes slightly more. Therefore a 100 amp hour battery (at the 20hr rating) could provide 5 amps for 20 hours, but it could not provide 10 amps for 10 hours. The available time would actually be slightly less.

They have a very cool spreadsheet that allows you to play "what if" with your ah rating, and the Peukert number for your batteries (or for batteries you'd like to have). For example, my Trojan T-105's are rated at 1.24 Peukert, where Concorde AGM's are rated at 1.18. The Concordes would give me an additional 40ah at a 135a draw.

For this "gas gauge" function, we use a CruzPro VAH30. Other good ones are the Trimetric, Smart Gauge, and the E-Meter.