I have a 40 volt, 5 amp-hour battery for my battery powered lawn mower. Is that 5 amp-hour measurement at 40 volts? I have a USB charging adapter for the battery – that is, you put the battery in the adapter and it acts as a USB charger for your phone, speaker, whatever. Is it still a 5,000 mAh charger at 5V (or whatever USB is), or is it 8 times that?
–Embarrassed former electrical engineer
Depends on how the voltage is stepped down to 5V. With an inefficient linear regulator 1A to the load is 1A drawn from the source, so still a 5,000mAh charger. With a switching regulator the efficiency is much higher and you would get closer to the 8x number (probably more around 7x).
This is right. It might be helpful to think of the battery capacity in terms of Watt-hours. As said, there are losses with the conversion but they can be made fairly minimal with careful design.
OK, assuming away inefficiencies for the moment, 5 Ah at 40 V is equivalent to 40 Ah (40,000 mAh) at 5 volts, right? What unit is VAh? I guess it’s the same as W-h, which is, uh, some multiple of joules or something?
Yea, it would be (40 volts * 5 amp-hours) 200 Watt-hours. I guess you could call that Volt-Amp-hours though VA is usually for reactive loads. Most loads aren’t too reactive so watts and volt-amps should be similar.
The AH (amp-hour) rate of most batteries is measured over 20 hours. As you can imagine, depending on the type of battery (agm or lithium) the voltage and the current falls over the 20 hours. So, if you integrate the voltage x amps over the 20 hour period it will give you 5 amp hour. It does not mean it will give you 5 amps for an hour and also the 40V will fall (especially for a non-lithium battery) over time.
The USB charger charges typically at 2.4 amps or 2.4 * 5 = 12 W. The USB charger doesn’t deliver more to your phone just because you have a bigger battery on the supply side. Whatever is the max rated capacity of the USB charger, that’s the rate it will deliver.
Right. I’m just trying to figure out how many phones it could charge.
Thanks, everyone! I guess I haven’t forgotten everything.
Don’t be too hard on yourself.
Knowing how to ask the question (or even to ask the question) is a hurdle a lot of people never get over.
Heh, thanks! Still, you’d think four years at CMU would have stuck a little better.