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Choosing Your Batteries

The choice of batteries possibly the biggest decision to be made if planning a solar power system of any size.
Unless you are looking at a very small system, possibly using a truck battery, upgrading your battery capacity is likely to be difficult and expensive. If you are using a 24 volt battery and decide you need more storage,you will either need to replace the battery for a larger one or connect a second battery of the same size in parallel with the first.
When connecting batteries in parallel however, it is important that the batteries are similar. For this reason it may not be advisable to add to the battery after you have been using the system for a while, by which time there would have been some reduction in battery performance.

Measurement of Battery Capacity

The capacity of a battery or cell is measured in amphours. A 100 amphour battery in theory can supply a current of 1 amp for 100hrs before becoming fully discharged (although a battery should never be discharged below 20% of full capacity, and on a regular daily basis should not be allowed to go below 60% of full capacity)
As the efficiency of a battery, and therefore it's amphour rating varies according to how quickly it is discharged, a standard discharge time is used when quoting a batteries capacity.
If a battery is quoted as having a capacity of 100 amphours (c10), that infers that the it will supply 100 amphours when discharged over a 10 period.
100 amphours (c100) would indicate that the battery will supply 100 amphours when discharged over a 100 hour period. For a particular battery, the c100 rating would be expected to be significantly higher than the c10 rating.

If two batteries are connected in series, the total amphour rating will be same as each individual battery (which should be the same size).
If two batteries are connected in parallel, the total amphour rating will be the sum of that of the two batteries.

Factors Influencing Size of Battery Required

Don't underestimate the size of battery required. It can be frustrating to have a system where the battery size does not give you enough stored electricity to get you through a dull day and also finding that your battery rapidly becomes fully charged on a sunny day and further output from your panels cannot be used.

Factors to Consider:

You will also need to consider when you might use any appliances that have a high power requirement. The highest power requirement in a solar powered home may be the washing machine, using something approaching 3 Kw when it is heating water. A battery that is not fully charged may be able to run low energy lighting for many hours but may not be capable of providing 3Kw without the battery voltage dropping to the point that the inverter cuts out for protection.
Drawing a high current causes the battery voltage to drop, possibly returning back to it's original level as soon as the appliance is switched off.
With a larger battery, together with having more electrical power stored, the voltage drop resulting from drawing a particular current level will be less, possibly enabling more of the battery's capacity to be used.

Another fact to consider is that, although we are talking about batteries that are designed for deep cycle use (they are expected to cope being regularly drained and then recharged), the greater depth to which they are discharged, the shorter their life is likely to be. Deep cycle batteries can generally cope with regular discharging down to approximately 60% of their capacity and occasional discharging down to 20%.
Therefore, all else being equal, a larger capacity battery should have a longer life.