Are Rechargeable Batteries a Good Deal?
With two boys - ages 9 and 8 - and all their toys, we go through a ton of batteries. So, I finally broke down and bought an Energizer Universal Recharger ($19.99) and a 4-pack of Energizer AA NiMH batteries ($9.99), making my upfront investment $29.98.
Now, the question is: how long before I recoup my investment? I am going to assume that the rechargeable batteries last as long between charges as the regular batteries do (I have an email in to Energizer about this but have not yet heard back from them). I can get a 24-pack of regular Energizer AAs for $12.99 ($.54 per battery). The NiMH batteries can be charged 500 times, so that's like 2,000 batteries. Divide $29.98 by 2,000 and you get $.015 cost per battery over the long-term.
So, although it seems like a pretty hefty upfront expense, if my math is correct, over the long-run I will save about $.525 per battery by going with the rechargeables. Oh, and to top it off, I am helping the environment!
JLP
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You're forgetting to factor in the extra cost of electricity to charge them. Granted, this is difficult to calculate, but this plus the hassle reduces the benefit.
The cost of electricity would be tiny. That's why I didn't include it. When you have two boys with lots and lots of toys, rechargeable is the only way to go.
energytech and JLP,
Calculating the cost per charge, to an order of magnitude is easy, which will show you that the cost really is negligible:
Quick and Dirty
===============
One penny per charge for a AA battery. Now read the rest if you are interested.
Note first that:
* A single AA NiMH cell is 1.5 volts (due to NiMH chemistry, does not vary by brand)
* Duracell brand AA NiMH cells store 2050 mAh (milli Ampere-hours) [1]
* Your electricy is billed in kilowatt-hours, which is 1000 watt-hours
* A watt-hour is equal to volt * ampere-hours
* An ampere-hour is the number of culombs of electrons that pass a given spot in a wire during an hour
* An ampere-hour rating for a battery means "if you drain all of the energy from this battery, you could make an X amp current flow for one hour"
Calculation
===========
Watt-hours of battery: 1.5 V * 2.050 Ah = 3.075 Wh
Watt-hours required for charging: 10 * 3.075 Wh = 30.75 Wh
kWh required for charging: 30.75 Wh / (1000 Wh / 1 kWh) = 0.03075 kWh
Cost of electricity for one charge: ($0.25 / 1 kWh) * 0.03075 kWh = $0.008
Result
======
Charging one AA cell costs no more than $0.01, or one penny per charge.
Assumptions
===========
* Electricity cost: $0.25 / 1 kWh (Kilowatt-Hours) (basis: even in California I'm paying around $0.13/kWh)
* Charger efficiency: 10% (basis: there is a fair amount of energy lost to heat, but the basic operation of a charger is to convert AC to DC and to step down the voltage; such products typically range between 25% and 70% efficiency, so we will choose the low end of the spectrum and adjust for heat losses)
Sources
=======
[1] http://duracell.com/products/rechargeable.asp?id=42&
Oops, I lied about one thing: the voltage of NiMH cells is 1.2 volts, not 1.5 volts as I stated above. However, that only makes it less expensive to charge NiMH cells by 20% so it isn't a big deal.
Harold
