I have an AGM 12V battery which has capacity 100Ah and max peak discharging current 700A.

How can I check the maximum continuus discharging current?

I want to use four of this batteries serial connected (48V) at 5kW UPS and need at least 100A of continuus discharging current.

  • If you want to get anything close to 50Ah out of these batteries (and discharging lead-acid battery past 50% DoD is a good way to destroy it fast) then you probably can't take much more than 5A out of them. 5A is the C/20 discharge rate, the typical rate where lead-acid chemistry comes close to providing the nameplate capacity. If you discharge 100A then you are discharging at the 1C rate. You should probably expect something like a 40% loss of capacity at this rate - leaving you with 30Ah to use. Commented Nov 5, 2022 at 1:28

1 Answer 1


If the datasheet doesn't say this, you can only guess.

My guess however would be that you can safely use it at 100 amperes, provided you take at least a few measures and the batteries have few properties:

  • The batteries must have large terminals, small spade connectors can't handle 100 amperes but large screw terminals or round terminals can. However I would be very surprised to find if a 100 Ah battery has too small terminals.
  • Use at least 25 mm2 wiring, 10-16 mm2 wiring isn't up to the task of those currents
  • Remember to use a fuse, slightly larger than 100 amperes but not much larger, fuses for these high currents are made with bolt holes on both ends to allow using crimp connectors with bolt holes at both ends of the fuse.
  • If the battery has screw terminals or flat terminals with bolt hole, you must use good quality crimp connectors intended for 25 mm2 wire. Use good quality crimp pliers. An alternative could be soldering but then you need a very big soldering iron, so big you probably won't have it already and have to purchase it separately. Buying crimp pliers for 6-50 mm2 cables would be cheaper than the very big soldering iron you would need only for one task.
  • Remember that if you have a 5kW UPS with small internal battery, and you are going to use it with a large external battery bank, the UPS itself could overheat. Maybe it was rated for only few minutes at max load and with your current batteries, you are capable of using it for maybe 40 minutes now at max load. So be careful at max load. If you use only 3 kW though, I'd say something that withstands 5 kW for few minutes is very likely to withstand 3 kW indefinitely.
  • If the UPS has Anderson SB50 connectors, I would actually forget using at 100 amps continuous, and fit a slightly smaller fuse to prevent you from using the full 5 kW. The largest SB50 connectors you can find are for 16 mm2 wire which isn't up to the task of 100 amperes continuous.

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