# How does the terminal voltage vs discharge time curve of an SLA battery change with time as the battery ages?

As a battery ages, it loses its ability to recharge back to its original capacity. How does this change the terminal voltage vs discharge time curve of an SLA battery? Since this curve represents the capacity of a battery when it is in a relatively "new" condition.

For example of this SLA battery.

Here is what I think. Since the capacity of a battery decreases with time, the discharge rate for any of the given periods (for example C20) will also decrease; this is because less capacity means less total charge and therefore a smaller rate of discharge for a given period. And the cutoff voltage will increase due to the decrease in discharge rate (for any given period) hence the curve will shift upwards with time.

As a battery ages, it loses its ability to recharge back to its original capacity. How does this change the terminal voltage vs discharge time curve of an SLA battery?

At any given discharge current the voltage will drop below cutoff earlier. The shape of the curve will in general be similar, but may be modified by how the battery has aged.

Since the capacity of a battery decreases with time, the discharge rate for any of the given periods (for example C20) will also decrease; this is because less capacity means less total charge and therefore a smaller rate of discharge for a given period.

This is true, but not useful. A battery will always last the same time when discharged at a proportion of its actual capacity, by definition.

But the actual capacity of the battery is unknown until fully discharged, so it will be tested at a discharge current which is a proportion of the rated capacity. Discharge current is not reduced just because the battery may have aged.

And the cutoff voltage will increase due to the decrease in discharge rate (for any given period) hence the curve will shift upwards with time.

No, the cutoff voltage is not raised just because the battery may have reduced capacity. The battery is rated to maintain voltage above a cutoff voltage that varies depending on discharge rate according to the values specified in the datasheet. As the battery ages its capacity, as measured against those cutoff voltages, reduces. When measured capacity drops below 80% the battery is considered to be 'worn out' - even if it could deliver higher capacity at lower current.

A common example of this is when the battery suffers from sulfation, which increases resistance causing voltage to drop more at high current, reducing the realizable capacity at rated discharge current. However at lower current, where the the high internal resistance doesn't drop so much voltage, the realizable capacity may be higher. Of course if it needs to supply high current for a certain application this higher capacity at low current is not useful.

• "At any given discharge current the voltage will drop below cutoff earlier. The shape of the curve will in general be similar, but may be modified by how the battery has aged." This means, for example, if the battery originally was able to discharge 2A over a 1hr period before reaching cutoff, it would now only be able to discharge 2A over, for example, a 50min period before reaching cutoff as the batteries actual capacity has reduced due to age, right? Commented Mar 5, 2023 at 13:12
• "This is true, but not useful. A battery will always last the same time when discharged at a proportion of its actual capacity, by definition." Oh yea, your right about that. Commented Mar 5, 2023 at 13:13
• "The battery is rated to maintain voltage above a cutoff voltage". Does this statement mean that the stated battery capacity (based on some conditions, for example 35Ah @ 10hr-rate and so on) is stated such that it can provide that capacity before the cutoff voltage is reached. Commented Mar 5, 2023 at 13:13
• "cutoff voltage that varies depending on discharge rate according to the values specified in the datasheet." So this mean that if you discharge a battery at 0.05C of its capacity (even if changes due to age) it will always have a cutoff voltage of 10.5V. For example, if C=35Ah, at 0.05C, its cutoff voltage will be about 10.5V. And, if C=30Ah, at 0.05C. its cutoff voltage will also be about 10.5V. I'm saying this because you said that the cutoff voltage depends on the discharge rate according to the values (of the discharge rate and its corresponding cutoff voltage) specified in the datasheet. Commented Mar 5, 2023 at 13:15
• 'C' is equal to the rated capacity of the battery. If the rating is 35Ah then the specified cutoff voltage at 1.75A is 10.5V. If it's rated at 35Ah but the actual capacity is 30Ah (or 1Ah), the cutoff voltage at 1.75A is still 10.5V. Commented Mar 5, 2023 at 18:54