The 50% rule for deep cycle batteries

The 50% rule - A Layman's explanation

This is a rule that states quite simply, the most economical use of deep cycle batteries comes about when they are, on average, discharged to 50% capacity then recharged.

It seems like one of those rules that someone "just thought up". In actual fact it has a sound scientific basis and really does work.

An explanation is in order.

Let's go to 2 extremes of battery bank use in the case of a 200Ahr wet cell lead acid battery bank made up of 2 X 100Ahr batteries.

First consider the case of extremely heavy use.

The battery is discharged at 100 amps for an hour. The terminal voltage has fallen to 8 volts. It is then recharged at 100 amps until the terminal voltage is 14.4 volts, then kept at 14.4 volts until the charge current falls to 4 amps (this is a typical 3 stage charge) at which time the charger drops to float charge at 13.3 volts. This cycle is repeated 50 times.

At the other extreme, the same battery bank is discharged at 2 amps for 20 hours, the terminal voltage falls to 12.2 volts. The battery is then recharged at 10 amps until the terminal voltage reaches 14.4 volts, this is then maintained until the charge current falls to 4 amps at which time the charger switches to float at 13.3 volts (again, typical 3 stage charging). The cycle is repeated 50 times.

Most battery usage falls somewhere between these 2 extremes.

Which battery do you think will last the longest?

It's obvious to us. It also should be obvious to anyone that the second battery will last much longer. It will have a longer life.

But why?

Well, every battery has a finite life. Each discharge and recharge cycle uses up some of the battery's life. The deeper the discharge, the heavier the discharge current, the heavier the charge current, the more life it uses up.

In the first example the battery is being very heavily discharged (high discharge current) to a very deep depth of discharge. This severely shortens it's life. On the face of it, it would seem that doubling the size of the battery bank would double the life of the bank as a whole. However this is not the case. Doubling the size of the battery bank could increase the life of the battery bank as a whole by say 3 times. So double the initial outlay (twice as many batteries to buy) results in a saving of 50% (they last 3 times longer).

In the second example, this batterybank is going to last a long time. It is being well treated, lightly discharged to a reasonable depth of discharge and charged at a sensible rate. However, reducing this battery bank by 50% (to a single 100Ahr battery) would perhaps only reduce the life of the bank as a whole by say 30% because the single battery would still be being relatively well treated. So in this case a saving could have been made by buying half as many batteries (so half the initial outlay) and getting a battery life of 70%. A saving of 30% in monetary terms.

This explains it in plain English. It can be shown mathematically, by graphing the cost of each used amp hour against the initial monetary outlay. The result is a graph with a peak in the middle at, you guessed it, 50% depth of discharge.

Discharging deep cycle batteries to 50% results in the most economical use of the batteries in terms of battery life and monetary outlay.