Battery power capacity storage

[Tim Miller]

This last week I bought some rechargeable batteries. The baby monitors run on 9V batteries and we were going through the alkaline batteries fast. Now the rechargeable batteries go even faster. Then re-charge. So here is my question: how much energy (kw) does it take to fully re-charge a 9V battery with a 175 mAh rated capacity?

My utilimate goal is have a little wind/solar power unit helping supply the house with energy. But I'm doing lots of reading on that first. Like to do it myself. Any help with the above question is appreciated!

[Rob Beckers]

Hi Tim,

Electrical energy is measured in kWh (kilo-Watt-hours), or Wh (Watt-hours). Where 1000 Wh is 1 kWh. Electrical power is measured in Watt, after the guy that made a practical implementation of the steam engine. kW (kilo-Watt) is simply 1000 Watt. As you can see the difference between the two is time. Power is Energy-per-second, or the rate of energy. Multiply power by time and you have energy. In practical terms, run a 1000 Watt heater for 1 hour, and you've used up 1 kWh of electrical energy.

Batteries are a bit funny, in that they use Ah (Ampere-hours) instead of Wh or kWh to express energy. The difference between the two is voltage. For your battery the stored energy, E = 175 mAh x 9 V = 1575 mWh (milli-Watt-hours) = 1.575 Wh = 0.001575 kWh. How much energy it takes to recharge a battery depends a great deal on how it's recharged, but say you loose around 25% in the process for small batteries on a quick-charger, that makes it just about 0.002 kWh, or 2 Wh.

That's very little...

-RoB-

[Tim Miller]

Thanks Rob! I was getting that too, but I was starting to think I moved the decimals one too many spots. Seemed awful small. The local utility company is charging $.13/Kwh. So it isn't costing much to recharge these batteries.

What really gets me on these batteries, is the rated capacities and estimated charging time. AA, C, D are rated at 2500 mAh. Charge times are 3-6 hrs. The more batteries you put in, the longer it takes. But that 9V battery with a 175 mAh takes 7 hours to charge.

[Rob Beckers]

175 mAh is a very small battery, even if we both got the decimal point in the wrong place it will only store a very small amount of energy.

There are quick-chargers for NiMH batteries, in fact I didn't know they still make the traditional 'normal' charge type. As a point of reference, the original specs of NiMH and NiCd called for 12 hours of charging at 1/10 the capacity (in current). By carefully monitoring temperature, current, and voltage charging can be done a whole lot faster. The little charger I have takes about half an hour from empty to fully recharged. The miracles of technology... :)

-RoB-