# calculate Amp Hours when voltage is given

I'm trying to wrap my head around how exactly Amp Hours work. I have been trying to figure out this question for a while now, but can't seem to find the information that I need. This is the circuit, and the information given is that the battery has 20AH capacity, and has a constant voltage 11.5 V. The question is to calculate how long it would take the battery to fully charge from being completely discharged.

What exactly is Amp Hours, and what is the formula to calculate it? I have noticed on many web pages that this is being calculated when the current of the circuit is given, but nothing about what to do when the voltage is given.

Edit: I know I can calculate the current in the circuit using ohms law, but I don't know what to do to calculate the time it takes to calculate how long it will take to charge to full.

• Hint: given the schematic and the information you've posted, you can figure out the current. – The Photon Jun 16 '15 at 4:22
• Hint: What is a volt-ampere? – Ignacio Vazquez-Abrams Jun 16 '15 at 4:24
• @ThePhoton Ah okay so I have the current of the circuit, but I still don't know what to do to find the time it takes to charge the battery. – Vaderico Jun 16 '15 at 4:31
• @IgnacioVazquez-Abrams No I don't :( I am new to this – Vaderico Jun 16 '15 at 4:31
• What do websites say about how to calculate the charging time, once you know the current? – The Photon Jun 16 '15 at 4:32

12v - 11.5v = 0.5v

i=v/r i=0.5/1 = 0.5Amps charge current.

battery = 20Ah (note: not A/hrs!)

20/0.5= 40 hours to charge.

amp hours= 20 amps for 1 hour or 10 amps for 2 hours or .5 amps for 40 hours.

it is a measurement for how much capacity the battery has and is useful for calculating how long it will last.

• Ah! right yes I completely understand now, I got to the part of calculating the 0.5 amperes but the rest was not clicking instantly. Thank you very much Steve! – Vaderico Jun 16 '15 at 4:33
• I would read "A/hrs" as "amps per hours" rather than amp-hours. – The Photon Jun 16 '15 at 5:14
• @ThePhoton - agree. A/hrs is incorrect, it is Ah, which means 'amps time hours' or, 'amps for so many hours' as in '0.5 amps going into the battery for 40 hours'. I've edited the answer. – RJR Jun 16 '15 at 5:40