When you see information on batteries you often see their performance talked about as their Depth of Discharge or DOD. This is the percentage of battery capacity that can be used. For some batteries you can use 80 or 90% of their capacity but others only allow you to use 30%. It is very important to understand this when you are looking at the output of any battery.
You can have a range of batteries, all having 10 KW capacity but if one battery has a DOD of 80% where you would actually have 8KW of available storage another one might have a DOD of only 30% which means you would in fact only be able to use 3KW of storage. This means the lower the depth of discharge the more batteries you would need to achieve the same storage capacity.
You can’t fully discharge batteries for various reasons but largely because it affects the chemistry of different batteries in different ways. For example most lead acid batteries have quite a low depth of discharge, often around 30%. If you take more out of the battery than 30% the chemistry of the battery starts to deteriorate. You will often hear people complaining that they have to replace their lead acid batteries every couple of years and this can be one of the reasons why. At the same time at the opposite end of the scale Sodium Nickel and zinc bromide batteries can actually be discharged to 80 – 90% of their capacity without deterioration to the battery chemistry.
Depth of Discharge is interrelated to the number of cycles you can get out of your battery (see article on “The Cycle Life of Your Battery”). If you increase the DOD above what is recommended for the particular battery type you will reduce the number of cycles that you can get out of the battery. This means your battery will have a shorter lifespan.
Battery specifications usually state a specific depth of discharge per number of cycles expected over a batteries lifetime. This might be 4,000 cycles at a DoD of 80% or 4,500 cycles at a DoD of 30%. If you were to only look at the Depth of Discharge the first option of 80% would look like a better option. You would, however, need to take into account all the other characteristics of the battery and make sure they fit your particular requirements.
However, there are many other factors that also affect the lifetime of a battery and it is because there are so many interrelated aspects to the longevity of your battery that it is often difficult to give a definitive length of lifetime for any battery. This is also why most battery manufacturers have a much smaller guarantee period compared to the number of years that they expect a battery to last under ideal operating conditions.
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