Boat Electrics

Deep Cycle Battery 

by Kurt Küpper*

Determining Deep Cycle Battery Capacity

Last month we discussed how to determine the total daily electrical loads of a boat. This information will allow us to calculate what deep cycle battery capacity is required, i.e. how big the batteries need to be.
The loads could in part be carried by sources of electrical power that are available at the time and may thus not necessarily have to be provided by the battery.

As in a vehicle, the engine could be running and power could therefore be supplied directly from the alternator. Similarly, a generator, wind or solar power could be available. But one does not necessarily want to be dependant on having an engine running, and there may be insufficient wind and sunlight at times. It is therefore best to size the batteries so that they are capable of supplying the total requirements without assistance from other sources.

The next important factor to consider is the depth to which the batteries are to be allowed to be discharged before recharging. Discharging a battery until it is flat, i.e. until the voltage falls to about 10.5V, will severely reduce the number of charge/discharge cycles that the battery is capable of. Generally, the shallower the discharges, the more cycles the battery will be capable of.

A typical 120Ah deep cycle battery may be specified to be capable of 5,000 charge/discharge cycles if drawn down by 10% of its capacity, and 2,500 cycles if drawn down by 50% of its capacity.

The first conclusion may be to say that one should try to never let the battery be discharged by more than 10%. But if we look at the useful power supplied by this battery over its life cycle, we see that with 10% discharges we could draw 5,000 x 12Ah = 60,000Ah. With 50% discharges, we would draw 2,500 x 60Ah = 150,000Ah.

So, while the battery will last for fewer cycles, we will get more usable Ah out of a battery over its life if we cycle it down to 50% before recharging. Without going into full details, the trend does not continue beyond 50% discharges, i.e. once we cycle even deeper than 50%, the total life cycle Ah start diminishing. 50% is therefore what one should try to limit discharges to and what our calculations should be based on.

The next aspect to consider is recharging. Charging batteries is like inflating a bicycle tyre. At first, when the tyre pressure is low, it is easy to pump in a lot of air and to quickly raise the pressure. The longer we pump, the harder it gets to force air into the tyre and to raise the pressure the last bit.

With an appropriately sized alternator, a battery that has been discharged to 50% of capacity will accept charge up to 80% of capacity within an hour or two. But to raise the state of charge to 100% of capacity could take another six to ten hours. This becomes an expensive exercise if the engine is being run solely or primarily to recharge the batteries.

The conclusion to be drawn is that one should provide enough capacity to be able to make do with 30% of battery capacity between recharges, i.e. routinely charge the battery to 80% of capacity and then draw 30% of capacity, bringing the battery down to 50% of capacity, before recharging. (N.B. charge batteries to 100% before leaving the boat for a while to prevent sulphation.)

Lastly ageing must be considered. Batteries deteriorate with time. An ageing allowance must therefore be factored in so that the batteries will still be able to meet requirements towards the end of their required life.

In conclusion, if we are going to use only 30% of battery capacity, the overall size of the battery bank would have to be 333% of load requirements of one discharge cycle. Adding an ageing allowance of around 20%, we end up with rule of thumb that total battery capacity should be four times anticipated demand between recharges.

This allows for occasional deeper discharges without unduly compromising battery life. Note that if potentially very heavy loads like an inverter are going to be applied, battery capacity must be checked to be at least four times what the greatest anticipated single discharge for such a load would be.


*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455