I have posted this in a few places, so sorry if you have seen it somewhere else

Im working on a battery powered led strip project,
iv put the project together and have been testing with a mains power supply
Iv run some tests and found at max I'd be pulling 4.5 amps
Averaging around 2 ish

I rounded out to about 3.5 amps, and said for 90 mins run time id be looking at the 7ah battery
7/2 = 3.5 so 2 hours runtime
I got a 7ah SLA 12v battery, running though a 12v to 5v converter to power the led strips
(Arduino is running off a usb battery pack)

However after about 45 mins I tested the battery and it said 10.9 v
Which I understand to be too low for SLA batteries (11 should be the minimum?)
so I shut it off

My question is did I mess up somewhere and get my math wrong?
Many thanks in advance

  • 1
    What kind of 12V to 5V converter are you using?
    – Majenko
    Jun 11, 2021 at 10:18
  • 1
    Its listed as a Step Down buck converter regulator 9-35v input, 5v 5A output Jun 11, 2021 at 11:01
  • 1
    In that case 4.5A at 5V will only be around 2A to 2.5A at 12V.
    – Majenko
    Jun 11, 2021 at 11:36
  • So based on that my battery should have lasted longer? not shorter? Jun 11, 2021 at 11:40
  • Yep. If indeed there was even a problem.
    – Majenko
    Jun 11, 2021 at 15:26

1 Answer 1


There are a few things that may contribute to your lower than expected battery lifetime.

Effective capacity: you should aim to never use more than 50% of the capacity of a lead acid battery or you will shorten its lifespan considerably.

Also, note that lead acid battery capacity is usually given for a discharge time of 20 hours (C20 rating). The “C” rating could be different but its value should be on the label of the battery.

In the case of a stated capacity of 7Ah at C20, this means the capacity has been calculated at a discharge rate of 7Ah/20h = 350mA. For higher discharge rates, such as in your application, the effective capacity will be much lower.

Battery voltage: measuring battery voltage under load does not say much. Measure the battery voltage “at rest”, when completely disconnected from the load for about 1 hour.

Charging: Are you charging the battery with a proper charger?

  • I cant see anything about a C rating? I think my understanding ( or at least what i was told ) is wrong, as i was going off the understanding that 7AH means 7 amps over an hour, so 3.5 amps would be 2 hours etc I had no mention of C ratings when researching this For the load, iv charged the battery back up now, but i did check when i was pulling the battery out (would have been about 30 mins after) and was still showing 10.8V more or less and I have a charger which is marked as being for SLA batteries Jun 11, 2021 at 11:12
  • okay so i have done some reading on C values, and can understand what you mean now. my next question would be how do I fix this? EG what battery type would I need, Jun 11, 2021 at 11:44
  • 1
    Next step up in terms of common SLA battery sizes would probably be 17Ah. If you keep to 50% maximum discharge you should probably be able to reach 90 minutes with some spare capacity left, but YMMV.
    – StarCat
    Jun 11, 2021 at 11:53

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