I have 4 tubular C20 batteries, 225 Ah each (2 series-parallel-2 series) for a residential solar system.

I suppose the below is true:

C20 gives us the recommended current to obtain the advertised cycle life (900 cycles at 80% DoD) using the below equation:

My battery bank is 450 Ah divided by 20 hours; 450/20 = 22.5 A.

If I need to discharge for 15 to 20 minutes (less than 20 hours), can I use more than 22.5 A and still maintain the same cycle life?

What is the maximum current in this case?

  • 3
    \$\begingroup\$ The battery data sheet will give you this information. \$\endgroup\$
    – Andy aka
    Commented Jan 4, 2023 at 13:44
  • 1
    \$\begingroup\$ More than rated discharge current reduces lifecycle. Less than rated DoD increases lifecycle. Graphs in the datasheet should be extrapolatable to hint at which effect wins. No datasheet? You're on your own. \$\endgroup\$
    – Neil_UK
    Commented Jan 4, 2023 at 13:50

1 Answer 1


There is no such thing as a "maximum current" for a cell (the way there is for a fuse, for example). If you read that a cell has a "maximum current of 1 A" it doesn't mean that it's perfectly fine at 0.99 A and that at 1.01 A it's destroyed. Instead, it's a continuum. The higher the current, the more the degradation. 0.1 A is better than 1 A which is better than 10 A, all the way to the short-circuit current of a cell. When they say "maximum current of 1 A", that's just a compromise value that they picked. They could have picked 0.5 A and increased the expected life, or 2 A and decreased the expected life.

So, when you ask "What is a safe max. discharge current for a tubular lead acid battery?", there is no specific answer. You want long life? Then either reduce the current or add more cells in parallel. You want high current? Then be prepared to have to replace the cell sooner.

If I need to discharge for 15 to 20 minutes (less than 20 hours) can I use more than 22.5 A and still maintain the same cycle life?

Do you mean "fully discharge from 100 % to 0 % in 15 minutes"? And you're asking if you would get the same life as if you fully discharged over 20 hours? Then, no, you would not.

  • 1
    \$\begingroup\$ I remember there's such thing as "Calendar life". Even if you minimize the current below a certain value, the battery will eventually die because of the number of years you used it. So, it makes sense to size the battery in a way to obtain the optimal cost-to-utilization ratio. And no, I meant to use ~5% of the battery and still be able to recharge it before sunset. At this rate, the battery will be empty within ~5 hours. \$\endgroup\$
    – Jean
    Commented Jan 5, 2023 at 9:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.