I have a 12V "non-spillable sealed lead acid battery" (the UPS type) that is labeled to be 9Ah (it is almost new). When I fully charge it and then discharge using a discharger that counts ampere-hours (at 0.5A), it measures only about 6Ah. Is this expected behavior, or should it be closer to the rated capacity for a properly functioning battery?

Also, how much does the discharge energy depend on the current at which it is being discharged? Label says "9Ah/20HR", does this mean that optimal capacity can be reached when discharging for 20 hours (which would be 0.45 A)?

  • \$\begingroup\$ Have you got the data sheet for your battery? \$\endgroup\$
    – HandyHowie
    Commented May 6, 2021 at 16:25
  • \$\begingroup\$ Here it is: gme.cz/data/attachments/dsh.540-521.2.pdf \$\endgroup\$ Commented May 6, 2021 at 16:35
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    \$\begingroup\$ It also depends on what is the voltage that the battery is considered as discharged. But yes, batteries are typically rated at 20 hour discharge or 0.45A, and discharging faster results into less Ah available, but 6Ah at 0.5A sounds quite low. \$\endgroup\$
    – Justme
    Commented May 6, 2021 at 16:44
  • \$\begingroup\$ Discharging ends at 10.5V, charging at 14.7V. \$\endgroup\$ Commented May 6, 2021 at 16:46
  • \$\begingroup\$ The battery SHOULD meet the datasheet specs. I would cycle it a few more times to confirm. In practice, though, it is typical for people to plan on only using half of the capacity for a lead acid battery in order to preserve cycle life. So when sizing lead acid batteries, usually you will size it at 2x the amp hours you actually need. Most other popular rechargeable battery chemistries do not have this problem. Only lead acid. \$\endgroup\$
    – user57037
    Commented May 6, 2021 at 17:14

1 Answer 1


According to your datasheet, even at 8h discharge rate (at about 1A) down to 10.5V the capacity should be about 8Ah.

The 6 Ah measurement you made at 0.5A is lower than expected.

  • 3
    \$\begingroup\$ Agree. It is worth it to cycle the battery several times just to confirm, though. See if the capacity comes up a bit after a few cycles. Like 5 or something. \$\endgroup\$
    – user57037
    Commented May 6, 2021 at 17:12
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    \$\begingroup\$ ^^ This... If a lead-acid cell sits without being periodically top-charged for months, it develops sulfation on the plates, which interferes with ion flow. You remove this by going through at least 5 full charge/discharge cycles. Do your test a few more times and see if the capacity improves. \$\endgroup\$
    – Kyle B
    Commented May 6, 2021 at 23:36

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