The choice of batteries is possibly the biggest decision to be made if you are planning a solar power system of any size. 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 amp hours. A 100 amp hour 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 amp hour 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 amp hours (c10) that infers that] it will supply 100 amp hours when discharged over a 10 period.
100 amp hours (c100) would indicate that the battery will supply 100 amp hours 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 amp hour rating will be same as each individual battery (which should be the same size).
If two batteries are connected in parallel, the total amp hour 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.
Deep Cycle Batteries