What's the basic definition of an amp-hour? For example:

A 12V battery is rated at 100Ah.

Does this mean that the battery will be discharged in 1 hour if the load is 100A@12V?


  • \$\begingroup\$ 1 Ah = 1A * 1h = 1 Coulomb/second * 3600 seconds = 3600 Coulombs. But that's probably not what you really wanted to know :) \$\endgroup\$ – JimmyB Apr 25 '16 at 11:05
  • \$\begingroup\$ Why was my post edited? I had it exactly like I wanted it. :/ \$\endgroup\$ – TheComputerGeek010101001 Apr 25 '16 at 11:51
  • \$\begingroup\$ What's wrong with the edit? \$\endgroup\$ – JimmyB Apr 25 '16 at 11:53
  • \$\begingroup\$ 12VDC@100A is now 100 A at 12 V. The additional spacing is not necessary, IMO. \$\endgroup\$ – TheComputerGeek010101001 Apr 25 '16 at 11:57
  • \$\begingroup\$ @user94774 All of your most recent edits are wrong. "amp" is not a unit, "ampere" is. There are standards for how to write SI-units, which includes the space between the number and the unit. "@" is not an English word, in the same way as "u" is not a word. \$\endgroup\$ – pipe Apr 25 '16 at 13:07

1) the battery may not remain at 12 V during the whole discharge (and may start from over 12 V when fully charged). However (depending on the selection of the cut-off voltage (lowest voltage it discharges to), 12 V may be a reasonable value.

2) when discharged very quickly (and 1 hour is quick), most batteries tend to deliver less total charge. A more usual discharge rate (especially for claiming capacity) is C/10 -- a 10 hour discharge. This your battery may be expected to deliver 10 A for 10 h before falling to less than 12 V.


In principle, you are correct. But my gut feeling [in lieu of the datasheet for the battery] is that it's more likely that the battery will last for 100h at 1A discharge than 1h at 100A discharge.

The battery has a discharge current that it's rated at. At that current, the battery has the capacity that it's rated for. At the end of the discharge, the battery voltage will drop below a threshold voltage, and the battery is considered discharged.

  • 2
    \$\begingroup\$ The amp-hour rating traditionally was at a 20 hour rate, so at .05 C. That's from the lead-acid days. I don't know if they still do it this way for modern chemistries. \$\endgroup\$ – gbarry Apr 25 '16 at 5:57

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