Hey guys

What should be a straight forward question, is giving me a bit of a tickle. This is really only out of interest really, but is applicable I reckon.

What exactly constitutes a "battery cycle"? I know that one should only use different cell type technologies to differing states of charge, which may influence this question's answer. If a LiFePo4 is usable to 10% SOC, that then constitutes 1 cycle obviously. What about 90%, or 40% etc? Should one see anything more than 25% used as a full cycle in this case?

Then what about Lead Acid? only recommended to use down to 70% SOC (if I recall correctly), so again, using the above example, if one gets down to 90%, or 80, or the "depleted" 70%, where does one count as cycles=n+1 ?

Secondly, on lead acid, from what I recall reading at some stage is that one wants the SOC to be brought back to 100% ASAP, as a LA battery does not do well to be in a discharged state for extended periods of time. But should one worry about 100% SOC, or depleting 1 cycle if you ultimately only used it down to 95% the previous cycle?

I am running a "toy" setup at home currently, just as a proof of concept, and it is actually working quite nicely, even if it is only 3x 7.2AH batteries, with a 20A PWM charge controller, and a 30w china mall special panel running 4 lights throughout the house on a daily basis. I do still want to know how to treat the batteries to ensure best results.

18 hours ago, Minerva10210 said:

What exactly constitutes a "battery cycle"?

What constitutes a cycle depends on the depth of the cycle.

If I say a battery has 2000 cycles at 20% DoD, then for this particular specification, a "cycle" means 20%. If I say a battery has 1000 cycles to 50%, than for this specification it means 50%. The two are always interlinked.

Since batteries are subjected to various cycles of different depth, some of them will reduce the life more than others.

The cycle life they specify in the spec sheet is the outcome of a controlled test where the batteries are repeatedly discharged to the stated DoD (usually at C/20 rates, sometimes C/10 rates), and then recharged to 100% again. The original Ah capacity is used to calculate how much energy is taken out of the battery, and the test ends when the battery can no longer supply that amount of energy without the voltage dropping too low. The number of cycles it has done is then quoted as its cycle life at that DoD.

Of course batteries are never cycled in such a controlled manner in real life, so this is only a (fairly good!) indication of what you can expect of it.

Thanks guys, appreciate the input 👍🏼