When an electrical current is applied to the reacted (discharged) battery, the lead sulphate which has coated the plates is broken down. This combines with oxygen produced from the ionised (acid rich) water and the resulting lead oxide is deposited on the postive electrode and lead is deposited on the negative electrode.

Thus the application of a charge current reverses the reaction that took place inside the battery to originally produce electricity.

Now, a common rule of thumb is not to discharge a lead acid battery below 75% of it’s original capacity. We would go so far as to say that for the common 12v marine/leisure batteries that are generally used in narrow boats it is not advisable to regularly discharge below 50% of the original capacity. This nominal figure of 50% discharge provides the best balance of lifespan vs economy for these standard batteries.

A discharge/recharge process is known as a cycle. The heavier duty the construction of the battery internals, the more cycles they can take. This is why genuine traction batteries, those designed for electric vehicles such as forklifts or golf carts, tend to be individual 2 volt units or 6 volt units. Further information on the charging cycle can be found in our Charging Section.

They simply use thicker plate combinations to provide higher amperage outputs and as a result of the heavier construction they can be discharged deeper and can be put through considerably more cycles. 2 volt or 6 volt cells can be combined in series to reach the desired voltage and in parallel to reach the desired ampere output. Many narrowboats now use traction batteries due to these qualities. Naturally, they are more expensive.

Charge cycles are used to indicate a batteries expected life. Due to the chemistry of a lead-acid battery, during the charge/discharge cycle the battery will experience some degradation over time.

It is impossible to predict how long a battery will last for you, as in practise your discharge percentages will vary from the theoretical laboratory predictions used to rate batteries.

A characteristic of lead-acid batteries is that as the rate of discharge (application of load) increases, so the battery capacity decreases. Peukert’s law describes this exponential relationship so those of you who are interested in such things may wish to undertake further research. For the purposes of our FitOutPontoon article, we will try and simply give a relative overview of what to expect from the different types of lead-acid battery we commonly use on a narrowboat.

Due to the less rugged construction of dedicated starter batteries, they can only stand 10-20 deep discharge cycles.

Midrange leisure/dual purpose batteries can cope with up to 500 cycles.

Heavier duty deep cycle traction batteries can cope with over 1000 cycles.

There will always be exceptions to these rough figures and it has to be said that battery life is very much dependent upon how you use the batteries. For wet cells deep states of discharge, regularly below 50%, will shorten battery life no question. AGM batteries should not be taken below 70% discharge and gel batteries can stand up to 80% discharge.

Whatever the theoretical values are, deep discharge will eventually ruin all lead-acid battery types. AGM batteries will withstand more abuse than gel batteries but are more expensive than wet cells. Gel batteries require precise charge control rates but can be discharged to a lower rate. Wet cell batteries require regular maintenance and produce the  most economical £ to amp ratio.

Generally speaking, you get what you are prepared to pay for. The key is to be able to balance your budget against what you need from a narrowboat battery bank.

Some words of caution. It is not advisable to mix battery cases of different ages or types. We have seen that the battery bank should be viewed as a complete entity, regardless of how many cases are used. With this in mind, a case or cell failure will upset the chemistry of the whole bank and therefore the bank should be replaced in its entirety in the instance of a failure.

Likewise we have seen how different battery types use slightly different chemistries to produce power. As such they charge and discharge differently and mixing types and ages can lead to uneven performance and premature failure.