Depth of discharge: what going deeper actually costs

Lead acid batteries can safely be discharged down to 50%

WRONG

This is by far the most common misconception, incorrect advice, and lie I see about anything to do with a battery.

It most likely started from some advice given to a person and them trying to make a gross simplification and then give justification for that simplification. The misconception and incorrect advice is that lead-acid batteries experience no wear when their depth of discharge is less than 50%. And then some intense matter happens immediately at the 50% mark and beyond the depth. Hence, under this advice, if you go to a maximum of 50%, you will have a battery that lasts a very long amount of time.

Reality is something completely different. In fact, any time you charge or discharge a battery at all to any amount, you cause an amount of wear to the battery. Whether it be 1% or 99%, you're causing wear. What is a factor is that greater depths of discharge cause greater amounts of wear on the battery. And that wear is a nonlinear effect. For example, discharging the battery twice to 25% would induce less wear than going to 50% once. Similarly, going to 50% twice would cause less wear than going to a 100% depth of discharge once.

Let's delve into some of the technical principles here.

Depth of discharge (DoD) is how much of a battery's capacity you draw out before you recharge it, and it's the biggest single lever you have on how long the battery will last you. There is no one correct depth. Every deep-cycle battery we sell publishes its cycle life at more than one depth, so the price in wear of going deeper is printed on the page: a NOMAD E2 is rated ~1500 cycles at 30 percent DoD and ~620 at 50 percent, a C2 CARBON 4500 and 3000 cycles for those same depths, and the PROFLEX lead-carbon 6000 at 30 percent, 4000 at 50 and 1500 at 80. Deeper isn't wrong, it's a trade, and you pick the trade that suits you.

The chemistry behind it: each discharge converts active material on the lead plates to lead sulphate and recharge converts it back, but the deeper the discharge the more mechanical stress each cycle puts on the plates, and the more sulphate is left behind when the recharge is anything less than fully complete. Going from 30 percent depth to 50 percent costs a standard AGM most of its cycle count, ~1500 down to ~620 on a NOMAD E2 and ~2600 down to ~1250 on an E2 ULTRA G2. The same step barely troubles lead-carbon: 4500 down to ~3000 on a C2 CARBON, 6000 down to 4000 on the PROFLEX. That difference, and not a fixed percentage, is why we put lead-carbon into banks that get worked hard.

Practical guidance: decide the depth as part of the design rather than discovering it. Fit a shunt based monitor so depth of discharge is a number you can read, because voltage under load is a poor indicator of the state of charge of the battery. Treat the inverter's low-voltage cutoff as the backstop for a bad day and not as the thing that ends every evening; every decent inverter has one, and our low-voltage cutoff guide explains why it fires earlier than people expect. Recharge promptly and completely after a deep discharge day, because a lead battery left sitting part discharged sulphates while it waits, and that's the damage. Lead-carbon is far more forgiving of the weeks when a full charge never arrives, and its own guide explains why.