THE MEANING BEHIND DEFICIT CYCLING 

The simple definition of deficit cycling refers to the batteries, when the system – be it solar, Wind or 4x4, consumes more power than it creates. When this occurs, your batteries - which store the power, will be on a daily discharge cycle and ultimately go ‘flat and thus damage your battery.

The life expectancy of a battery listed in Cycles, is directly related to its operating costs. Not ‘all batteries are created equal’ and you would be well advised to check whether you are, in fact, being sold a proper leisure battery with a good deep cycle curve, as opposed to a stand-by battery which has a much shorter life expectancy (but is cheaper).

The depth of discharge will thus determine how many days (i.e. cycles) you will get from your batteries. The greater level to which you discharge the battery, the less life you will get. For solar, in S.A. anyway, most system designers appear to work on around the 40% discharge level for their battery banks (it’s 20% in the USA!). For the more casual 4x4 application (unless it’s for daily use!) one can work on a discharge rate of 60-70% because of the infrequent use and thus balancing age of battery against amount of use.

So, having decided that we wish to limit the discharge level to 65%, we now look at our daily consumption to see how many batteries we will need for such a benchmark.

Let us say that we have a 100AH battery and we are consuming 50AH per day from our system(fridge average). This then means that we can operate the system for just over 1 day before we hit the 65% discharge level. Fridges can be set to shut off at different levels of discharge, and the worst level is around 75%, by which time you will have damaged the average battery’s life expectancy.

If you have solar support from an 80W panel, then you may be inputting some 450WH (36AH) per day (in Gauteng area) but consuming 630WH (50AH) – thus a deficit of 180WH (14A) per day. With this combination you can now last 4-5 days before you reach the 65% discharge benchmark. The bigger the panel and the farther north you are, will increase the amount of time you will get from your fridge before it shuts off.

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