C-ratings and real usable capacity
A lead-acid battery's amp-hour rating is only correct at the discharge rate it was measured at, the faster you drain the current from the battery, the less total energy you're going to get from the battery. Our own spec tables show it plainly: the 12V 300AH NOMAD E2 ULTRA G2 is 300Ah at the 100-hour rate and 234.4Ah at the 20-hour rate, and it keeps falling as the current rises. That's Peukert's effect, it applies to every lead battery on the market.
The C-rating means this, C100 means the capacity measured over a 100-hour discharge, C20 over 20 hours, C10 over 10. The faster the discharge the less total current you will get out of the battery, because at higher currents the chemistry can't convert active material fast enough. You can read the ratings straight off our product pages, as we publish all figures on every deep-cycle battery. At C20 the 300Ah E2 ULTRA G2 is 234.4Ah, the 270Ah E2 is 220.4Ah, the 220Ah C2 CARBON is 184Ah, the 160Ah E2 ULTRA G2 is 127.4Ah. Across every battery big enough to build a bank from, the 20-hour figure sits 14 to 23 percent below the 100-hour figure. Drain faster than the 20-hour rate and the capacity lowers further.
Use C-ratings when comparing and when sizing. Comparing: check both batteries at the same rate, because a 120Ah rated at C100 can be smaller in service than a 110Ah rated at C20, and the bigger number on the label is usually the gentler rate. We rate ours at C100 and publish the C20 figure beside it on every battery page, which is the only way to make the comparison accurate, so if a competitor gives you one number, ask which rate it is. Sizing: if the design has sustained big loads, size the lead bank from the rate nearest your real draw rather than from the label.
How deep you cycle the bank is the other half of the same question, and the depth of discharge guide covers it.
Common questions
What rate are deep-cycle batteries advertised at?
It varies, which is the problem. Some are advertised at the 20-hour rate, plenty at the gentler 100-hour rate, which is on the label. Ours are rated at the 100-hour rate with the 20-hour figure published in the spec table on the same page, so you can see both before you buy. Always compare two batteries at the same rate, and if you run big inverter loads look at the shorter rates the manufacturer publishes.
Does the C-rate effect apply to lithium?
Only weakly. LiFePO4 delivers close to its rated capacity across normal discharge rates and holds its voltage much flatter while doing it. Its limits are the BMS current rating limitation rather than Peukert shrinkage.
My inverter cuts out but the battery is not empty, why?
High current makes lead battery voltage sag, so the inverter's low-voltage cutoff triggers early. The capacity is there but it isn't deliverable at that rate. A larger bank, a higher system voltage, or lithium each solve it structurally.