Lead Acid Battery Life Expectency

Lead acid batteries hate to be discharged.

They especially hate it when you drain them completely. Your battery should come with a datasheet that details the life expectancy vs. the discharge current. This will let you estimate how long your batteries are likely to last based on how much you're going to use them.

Different types of batteries have different lifespans. High cycle batteries may only last a few hundred cycles if you use them with an inverter system.

This is the graph for the batteries I have at home (Stride 100Ah Solar)

So if I drain them completely each time I use them, I can expect 300 cycles (or load-sheds) from them before they give out, defined as when the battery only holds 60% of the original capacity.

I'm drawing roughly 100W per battery for 2 hours at a time, so that's 200Wh, or 20Ah per 12V battery rounded off (There's no point working accurately here because these are rough estimates). The batteries hold 100Ah in total, so I expect to use 20% capacity per load shed.

I estimate that I'll get about 2000 cycles based on 20% usage. Even if I get load-shed every day then the batteries should be good for 5 years.

Even without load-shedding, don't expect low-end batteries to last much past 5 years though.

Battery Types

In this post I'm only going to talk about lead-acid batteries. These are the big heavy batteries you'd typically find in your car.

There are a few different types of lead acid batteries which I'll discuss, however there are other types of batteries like LiFePo4 which I'm going to ignore because they are super expensive in South Africa at the moment. Hopefully in a couple years they will become viable, or let me know if you beg to differ.

So... Of the lead acids you firstly get high cycle and deep cycle batteries. High cycle would be a car battery where you want to draw a lot of power in short bursts with long intervals in between. Deep cycle batteries are used with inverters, where you want to draw a moderate amount of power for extended periods of time.

The next thing to choose is between flooded and sealed batteries. Lead acid batteries can be charged quite quickly at a high voltage, but this has a side effect of releasing hydrogen gas, which isn't a great thing to have in your garage. If gassing occurs you will need to top up the battery 'water', which is only possible in a flooded battery. Sealed batteries tend to last longer but only if you're careful not to overcharge them.

There are then two common types of sealed lead acids
AGM stands for absorbent glass mat: The battery acid lives in a fibreglass mat between the battery electrode plates. This keeps the electrodes covered in electrolyte nicely, but the whole thing is still pretty similar to a flooded lead acid battery.

Gel: The acid is a thick paste instead of a liquid. These batteries can't deliver as much current in one go, but they perform better under deep cycle, ie when most of the power is drained out. These batteries are more expensive, and last a bit longer before the electrodes wear out.