# Figuring out battery life from amp draw?

I decided to take apart one of those cheap handheld fans. It takes 2AA Alkaline batteries. I wanted to hook it up to an arduino as a way to get into electronics, and I wanted to know what kind of battery would be best for the setup.

Hopefully, I measured the amps correctly, and it read out ~.70 when the multimeter was set to 10 and @ DC current. So, the device has a 7A draw. From wikipedia, Alkaline batteries have about ~1800mAh, so 2AA would give 3600mAh.

So, total battery hours from 2AA would be 3600mAh/7000A = .514... which ends up being only half an hour.

If I wanted the setup that lasted roughly 2 hours, then that means I would need at least a 14000mhA battery without considering the arduino amp draw. That means I need at least 1 D battery or 8 AA batteries.

Does that seem right?

0.7 means 0.7 amps! (i.e. 700mA). I would be very worried if it was drawing 7 amps :)

You are nearly there with your calculation though: with two AA batteries in series, the total capacity is still 1800mAh. If you had it in parallel, then it would be 3600mAh (but batteries in parallel is not a good idea in general, the exception being lead acid batteris and similar, under certain conditions). So 0.7 amps with 1800mAh capacity means it would last 1.8/0.7 = 2.57 hours (2 hours and 34 minutes)

• 2.57 hours = 2 hours and 34 minutes. Otherwise, take my +1. Apr 21 '15 at 16:55
• @Jodes, I'm curious why you said putting batteries in parallel is not a good idea. Can you elaborate? Is it based on the (probably fair) assumption that the average consumer can't be trusted to put two equally charged batteries together? Apr 21 '15 at 17:26
• Your assumption is correct; but also batteries may not be identical, even if they are the same brand, same chemistry, same batch. So their discharge curves, voltages, etc will differ. Not only would it be bad to essentially charge a non-rechargeable battery, but similarly for rechargeable batteries, dangerous to over-charge, over-discharge, etc etc, which putting batteries in parallel can easily do. Just do a youtube search for exploding batteries!
– CL22
Apr 21 '15 at 18:08
• @Dan, take a look at electronics.stackexchange.com/a/16354/49251 Apr 21 '15 at 18:13

If you configure your Multimeter to 10A, this is the maximum that can be measured. If it shows 0.7 this means 0.7A (700mA).

Now to the Batteries: If you connect them in Series, you add up the voltage, but the capacity (mAh) stays the same. If you connect them in parallel, the capacity is added but the voltage stays the same. This is normaly only done with rechargeable Battery.

I don't know, but I think your motor will need 3V (2 Cell in serie).