Lead Acid vs. Lithium: Depth of Discharge
Depth of Discharge, or DoD, is how much of your battery bank`s stored energy can actually be used without dramatically reducing its life. For example, a 100Ah (amp hour) lead acid battery rated for 25% DoD means you need to plan to use only ¼ of its rated capacity (so 25Ah), leaving the other ¾ in the battery, unused.
DoD for lead acid batteries – both flooded (which you have to add water to periodically) and sealed ([maintenance-free") – is typically in the 25% – 50% range. Your battery will last at least twice as long if you regularly discharge it 25% than if you regularly discharge it 50%. Keep in mind that if you don`t have a sunny day to recharge your batteries after a day of use, the DoD will go down again the next day – so planning to use 25% per day will allow you to use less than the 50% maximum after two days of use.
On the other hand, DoD for lithium ion batteries is 80% or more, allowing you to use most or even all of the battery`s stored energy. That means a 100Ah lithium battery rated for 80% DoD can safely provide you with 80Ah without being harmed.
As a result, a lithium battery bank can be much smaller than a lead acid battery bank to provide the same amount of usable energy. For example, if you need 100Ah of energy a day, you would need a 400Ah lead acid battery bank to stay at 25% DoD, but would only need 125Ah of lithium at 80% DoD. That is a significantly smaller battery bank with lithium batteries.