# What is a "charge cycle" of a lead-acid battery?

I'm trying to develop an algorithm that would roughly estimate the health of a lead-acid battery tied to an inverter backup system.

It seems that my 200 Ah battery pack stays at 100% state of charge (SoC) most of the time. And when there is a short (a couple of minutes) power cut, it goes down from 100% to 99% (or worse, 98%). When the power is back on, it goes back to 100%.

Does this count as 0.01 of a charge cycle? And when this happens 100 times, can I consider that it went through a 1 charge cycle?

• check this site, may give you some ideas: batteryuniversity.com/article/… Dec 21, 2022 at 13:13
• "Does this count as 0.01 of a charge cycle? And when this happens 100 times, can I consider that it went through a 1 charge cycle" That would be my first order approximation. For lithium-ion, where you start and end in the SoC makes a lot of difference as to wear and number of charge cycles. Feb 15 at 8:15

I see there is no answer here, so, hopefully this helps you out if you don't already know.

To me, a charge cycle doesn't have such a rigid definition, whether you charge from 20% to 100% or 50% to 100%, each one I would call a charge cycle. But, it depends, because there are some deep cycle lead acids that may not define a charge cycle the way others do, and vice versa. So, I would consult who ever the manufacturer is and see what they say.

Basic/typical lead acid charge cycle looks like the image below Image Source: Research Gate - Paper Name: "Solar Charger Sizing"

Disregard the numbers, just see the terms "Bulk", "Absorption", and "Float".

Bulk is constant current, meant to be your high (Constant) current (relative obviously) portion of the charge cycle, to get you most of the way there (usually around 80% or so) to your target voltage depending on the type of lead acid.

The absorption phase finishes the rest of the 20%'ish (can overcharge relative to your final resting voltage), and, essentially allows the battery to cool down and vent gasses as a result from the Bulk stage. I just found this website and it gives some indication as how to calculate how long the absorption portion of the charge cycle should be (https://www.ecosoch.com/charging-lead-acids-properly/). Hopefully you find that interesting.

Because lead acids relative to li-ion self discharge more, float charging part of the cycle is meant to compensate this intrinsic issue. The current at this point naturally decays, and, once it hits a certain value the charger turns off.

Hopefully this helps!

Cheers

• This is all valid. Additionally, the bulk charge current can be limited below the maximum spec to prolong the battery life. So, battery life is a function of some combination of instantaneous current and the charge moved into/out of the battery b Feb 15 at 19:32

A "charge cycle" is ambiguous. We usually talk about a "full cycle" or a "charge/discharge cycle". That is defined as starting from a full battery, discharging it fully over the rated time (typically 20 hours for lead acid), and charging it fully over the same time.

But this definition doesn't apply at a different charging time or if not charged and discharged fully. In that case, you need a different definition.

For example, instead of "charge/discharge cycle", you can use an "equivalent full cycle" based on the total discharged charge. For your 200 Ah battery, you can define that, after the battery has delivered to the load a total of 200 Ah over many partial cycles, that's the same as if it had done so over a single cycle from full to empty.

Yes, I know, it's not the same because the battery degrades differently over 10 short 10 % cycles than it does over a single 100% to 0 % cycle. But it is your best option.

If you worry that this definition is not accurate, let me point out that number of cycles is a poor indicator of State of Health (SoH). If you only use number of cycles, you would rate two batteries the same after 100 cycles, even though they have very different SoH because they were operated at very different temperatures or they sat fully charged for very different durations.

Instead of considering cycles, if you wanted to be accurate, you would base your SoH estimation on the measurement of the actual performance of the battery over time.