The term depth of discharge (DOD), and this refers to how much of the total capacity can be used before the battery needs to be recharged again.
To help explain this one, it’s time for a quick exercise:
Suppose you have a 10-litre bucket, and you fill the bucket with water. Now, how much water can you take out of the bucket?
10 litres, right? Of course, you are correct.
Now, suppose you have a 10 kWh (kilowatt-hour) battery, and you fill the battery with electricity. How much electricity can you take out of the battery?
10kWh, right? Well, in this case, not quite.
Batteries don’t work in quite the same way buckets do. It’s important to understand that the capacity of a battery is not the same amount of energy you can draw from it. You’ll often hear the term depth of discharge (DOD), and this refers to how much of the total capacity can be used before the battery needs to be recharged again.
Lead acid batteries have a somewhat shallow DOD, which is generally recommended around 20-30%. This means if your battery bank can hold 10 kWh of energy, you can only access 2-3 kWh of usable energy. You can draw more than this, but you risk damaging the batteries and shortening their lifespan. To this end, most systems have control systems to prevent this from happening.
Lithium batteries can be discharged much further – as much as 80% or in some cases even 100%. This means that with a 10 kWh battery, you’ll get at least 8 kWh of usable energy – or maybe even the full 10 kWh. The Tesla Powerwall 2, for example, permits a 100% DOD without any adverse effects on the battery lifespan or warranty. At Off Grid Energy, we recommend around an 80% DOD to extend the battery lifespan as long as possible.
In summary, lithium batteries easily have the advantage when it comes to how much electricity you can draw from them. With some batteries able to discharge all the way down to 0% capacity and recharge back up to 100%, they give you access to more usable energy, more often.
See the below article for the difference between Lithium & Lead Acid Batteries
