Battery Life Calculation

[vne147]

Ok, so this is probably a stupid question but I just want someone to verify that I'm not out to lunch on this one. I have a circuit that I made that will draw about 500 mA. The power souce is 8 AA batteries wired in series so the supply voltage will be approximatley 12V. I'm trying to figure out how much runtime I will get out of the batteries. I am assuming that each battery will supply 500mA/8 = 62.5mA of current or roughly 1/8 of the total current draw, correct? If the capacity of each AA is 2100 mAh then the 8AAs should be able to power the circuit for 2100mAh/62.5mA = 33.6 hrs, correct? I understand that the assumptions and thought process that I used are for ideal circumstances and that the real world is not ideal but is my thinking correct? Thanks.

 

[ecerfoglio]

I have a circuit that I made that will draw about 500 mA. The power souce is 8 AA batteries wired in series so the supply voltage will be approximatley 12V. I'm trying to figure out how much runtime I will get out of the batteries. I am assuming that each battery will supply 500mA/8 = 62.5mA of current or roughly 1/8 of the total current draw, correct?

NO, it isn't. If the batteries (cells) are in series each one has to provide the full 500 mA that the load demmands.

Each cell provides 1.5 V and yes, the voltages add up to aproximatelly 12 V (8 times 1.5 = 12)

If the capacity of each AA is 2100 mAh then the 8AAs should be able to power the circuit for 2100mAh/62.5mA = 33.6 hrs, correct?

2100 mAh / 500 mA = 4.2 hours

 

[audioguru]

No.
Batteries or anything in series all carry the same current. In your case the battery is discharging quickly with 500mA. It might last 4.2 hours.

Eight 1.25V rechargeable battery cells in series is only 10V, not 12V.
Eight AA alkaline battery cells in series drop to 9.6V in 2.25 hours with a load of 500mA.

 

[Willbe]

And the actual mAh you get depends on the mA you are drawing. The mAh spec is probably for a 20 hour discharge time.

 

[vne147]

Ok everyone, thanks for the correction. I'm glad I asked.