What cutoff voltage should be adopted for a given current using a lead acid battery?

Question

From what I understood about lead acid batteries their capacity is related to the current drawn: the more current I draw from a battery, the less "effective" capacity I obtain, which means that the battery will last less than expected. Usually manifacturers state the nominal capacity of a battery for some C ratings: if for instance I have a 10 Ah C20 battery, this means that it should last for 20 hours assuming I draw 0.5 A (10 Ah / 20). If I try to draw 1 A, that same battery will not last for 10 hours, but it will actually last less time.

The battery is considered completely discharged when it reaches the cutoff voltage: at that point the battery is essentially dead, and I can no longer draw current from it. It could have some residual capacity, so if I allow it to rest and then try to draw less current I could "squeeze" that capacity, but in general is not advisable to further discharge the battery in order to avoid permanent damage or at least severely limit its life.

I took a look at some datasheets for different batteries from different companies, and some give more informations than others: most of them offer discharge curve graphs, ampere and watt tables, pretty much all of them give at least the ampere table.

Here is where I get confused: if both discharge curves and amp tables are present, by eyeballing the graph I can confirm what I see in the amp table, but in said table there are several cutoff voltages for a given use time. As an example, these are the discharge curves and the ampere table for Panasonic LC-XC1222,12V 22Ah C20 (here is the original datasheet for the battery):

Looking at the curve for 1.1 A, the cutoff voltage seems to be ~10.8V, reached in 20 hours, and by looking in the table the intersection between 10.8V and 20h I see that the current draw listed is 1.09 A: so it seems almost spot on, given the fact that I'm guessing the cutoff voltage from the graph. However, there are other cutoff voltages listed for the same use time, spanning from 9.6V at 1.11 A to 10.8V at 1.09.

If the battery is considered completely discharged when it reaches the cutoff voltage, then why there are several values listed? Is the battery "more discharged" if I let the voltage drop to a lower cutoff point, or maybe I should be more conservative and choose a higher cutoff point in order to minimize damage to the battery?

Also, as a side note, I know that it's not advisable to discharge the battery more than 50% of its capacity, but is this figure related to the nominal capacity or is it related to the capacity for a given discharge rate?

Answer 1

Ordinarily one only needs to ask “Why are there several lines in the Ampere Table of a lead-acid battery’s datasheet? What do those several lines represent?” and get a useful answer, rather than a seven paragraph wall of text telling us what you know abut PbA batteries ;), however this is a topic where people’s knowledge varies dramatically, and misinformation and confusion with other battery chemistries abounds.

All of what you say is broadly correct for PbA batteries, although the max discharge % is very specific to the design of the battery, as articulated in its datasheet. Some PbA batteries (for ‘starter’ applications) will allow only 5% within their warranty offering, others 25%, some 50%.

What the manufacturer(s) is telling you with those 5 lines of the Ampere Table is a range of End-of-Discharge-Voltage (EoDV) that will result in a range of battery lifespans (discharge/recharge cycles), with basically two realistic extremes, and a few points inbetween.

As for how that will translate into 5 actual cycle counts depends entirely on the design and intended application of the battery, for example whether it’s a battery designed for a (ICEngine) ‘starter’ application where deep discharge is NOT expected (and not covered by the warranty, and doing so will kill the battery in as few as dozens of such deep discharges), through to genuine ‘deep cycle’ designs, where discharges to 50% or even less is expected and supported and still yield cycle life in the hundreds or thousands.

In some applications with ‘intelligent’ &/or human control (e.g. micro-grids, RAPS, life support, etc), a conservatively high EoDV will be normally selected, to preserve battery life (cycles), but in certain circumstances the owner might wish to discharge to a lower voltage and accept the hit on long-term cycle life, if it ‘keeps things going’ through some unusual event.

To your last question, ‘nominal capacity’ = “capacity when you operate the battery in the specific conditions advised”, so in your example, the C20 discharge rate. If you operate the battery at a faster discharge rate, then you get less capacity (because of internal resistance), and so that lower capacity is your range for targeting End-of-Discharge-Voltage.