What is depth of discharge (DOD) in solar batteries?

Solar batteries take electrical energy from the solar panels (panels convert sunlight into electric energy) and store it in the form of chemical energy.

The batteries provide electrical energy and run our appliances in the absence of the sunlight.

So, they act as a backup power source.

Solar batteries, like any other batteries, suffer wear and tear with time. They degrade gradually as they are charged and discharged, reducing their ability to store energy.
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If you want your solar batteries to perform better and last longer, it is important to know their depth of discharge (DOD).

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What is depth of discharge (DOD)

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It is the degree or the extent of depletion of charge stored in the battery.

If the battery is fully drained then the depth of discharge is 100%.
A fully charged battery means zero % DOD.

For example, you have a solar battery that holds 8.6 kWh of electric energy and you discharge 7.2 kWh of energy.

Then the DOD is (7.2/8.6) x 100% = 84% approximately

You shouldn’t discharge your solar battery completely, which is 100%, as it affects the lifespan of the battery.

And if the batteries are discharged beyond their maximum DOD then the performance degrades drastically, affecting their storing capacity.

Therefore, manufacturers specify the maximum recommended depth of discharge of the batteries for their optimum performance.

If you are withdrawing current beyond this maximum limit, you are compromising with the life of the battery.

For example, if the supplier of 10 kWH battery recommends the maximum DOD be 80% then you shouldn’t withdraw more than 8 kWh of energy without recharging.

A higher DOD means you can withdraw more current from the battery without recharging.

Therefore, the depth of discharge is an important factor to understand the performance of the battery.
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The battery life is also expressed in terms of number of cycles like 3000 cycles for a lifetime.

First, charging the battery to its maximum limit and then discharging to its DOD, represents one complete cycle.

If you withdraw less current than maximum recommended DOD, you will have more useful cycles in hand than when you frequently drain your battery to its DOD.

It is because the electrolyte degrades more quickly with the degree of discharge and affects the storing capacity of the battery.

A battery bank may have 8000 cycles with 20% DOD but only 1000 useful cycles with 80% DOD.

Example:

A 200Ah battery with maximum DOD of 80% is discharged for 30 minutes at a current of 100 A. Find the current depth of discharge of the battery.

• Energy taken= 100 x 30/60 = 50 Ah
• The Maximum energy of the battery= 200Ah
• Depth of discharge= 50Ah/200 Ah x 100% = 25%

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Sizing DOD

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The batteries with different DODs are available in the market.

Which DOD suits you depend on the configuration of the solar power system, electricity requirements, and the grid supply.

A standalone system or a place with erratic grid supply may require batteries with high DOD values while a small system with regular supply may run well with low DOD values.

If you are planning to install a solar power system with a battery backup, consult the local installer.

He will understand your requirements and advise the battery configuration accordingly.

Don’t forget to ask for the detailed feasibility report of the advised configuration.

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