Cast 117 (Nov 15)
Rechargeable Batteries vs Standard Batteries
In Show 60, one of our regular listeners asked us whether buying rechargeable batteries really offered a saving, when consideration's given to the cost of running that charger. Interesting question, so we bought a charger and did the maths.
Here's our findings...
Does it make sense to get rechargeable batteries?
For this calculation, we're going to assume our electronic device needs 4 x AA batteries.
We'll use Duracell AA 1.5V Alkaline batteries rated at 2200mAh. Cost per 4 pack is £3.50 at Tesco
Total cost: 25 packs at £3.50 = £87.50
Rather that the older and less reliable NiCad batteries, we found a handy charger from Duracell that uses NiMH (the type of battery used in a mobile phone). The charger is the Duracell Rechargeable Value Charger. This comes with a pack of 4 AA 1.2V Alkaline batteries rated at 1800mAh.
The charger, plus the 4 batteries costs £15 at Tesco - Duracell Value Charger Pack
As the mAh on the rechargeable batteries is lower, we need to find out how many charging cycles will be needed to get the same consumption as 25 packs of Alkaline batteries.
We then need to work out the cost per charging cycle. On our power meter, we see that the charger draws 3 watts. Recommended charge time is listed on the charger as 6 hours. 18 watts per charging cycle.
If we multiply this by the number of cycles, we get the total Watt Hour. Divide by 1000 to get kWh, then look on an electricity bill to get the cost of a kilowatt hour (12p per kWh with EDF at time of writing). Multiply the total kWh of the charges by the unit cost. Then add the cost of the charger and batteries. Here are Pete's scribblings:
Winner: Rechargeables: You'll be better off after just 5 charges
Our Pete's done the maths. For charging 4 x AA batteries 30 times, you'll pay less than 10p. With the cost of the rechargeable batteries and the charger, the total is £15.07, compared to £87.50 for the Alkaline battery equivalent
|Rechargeable Batteries||Alkaline Batteries|
|Cost of 4 x AA batteries||£15 with charger||£3.50|
|Cost per recharge||£0.002||N/a|
|COST OF 5 CHANGES||£15.01 (4 batteries, charger and 5 charges)||£17.50 (5 packs of new batteries)|
|COST OF 10 CHANGES||£15.02 (4 batteries, charger and 10 charges)||£35.50 (10 packs of new batteries)|
Your results so far...
Thanks to Richard for the following:
The formulas are correct.... BUT they exclude various other issues. You need to take into account the length of time that you propose to use these 25 packs of batteries. If they are going to used over short period of time, say 2 years, then the formulas are OK. If you are going to use them over 5 years then the rechargeables suffer from greater and greater internal discharge so the charge does last as long, they don't hold as much charge, and unless you use the recharged batteries straight away then they will need to be charged again.
The other factor to take into account is that it is not likely that people will get to the recharger just when the batteries are fully charged, so you need to add the power used in the period between when the batteries are charged and when the person gets the batteries and switches off the charger. But also take into account that any heat produced from any electrical equipment goes into heating the house, which is a complete waste in the summer but is of benefit in winter as the central heating is used less, only a bit but yes less central heating power will be used.
Thanks to another Richard, Richard Paice, for this:
Your maths seems pretty sound to me. I would guess the cost of the power drawn from the mains would be pennies. But on environmental grounds alone, it's gotta make sense. I remember doing similar calcs for petrol vs diesel cars many years ago. Diesel cars were dearer to buy upfront, but then the fuel was cheaper and you got more mpg. Also they were less likely to break down, but when they did they were dearer to fix. It all got horribly complicated!
Also thanks to Rob Dickson for his observations:
Your sums look OK to me, but there are a few more things worth considering.
- People will probably buy more than one pack of rechargeable batteries.
- Rechargeable batteries can be charged more than 25 times.
- The environmental impact of throwing away standard batteries.
- My electricity at home is from renewable resources, so no CO2 is emitted to charge the batteries.
Got a comment? Get in touch!
Thanks to Michael Lees for the following question on rechargeable batteries vs conventional batteries:
"Using standard Duracell Batteries in my Camera, 2 off, last a lot longer than using rechargeable batteries in the same device, I have also noted the same for Battery Clocks. Do you have any thoughts on this?
Perhaps this has to do with the internal resistance of the units or something. I understand from your findings the Rechargeable Battery is perhaps more economical but is this offset by the Standard Battery lasting longer."
Here's our (rather limited) understanding: There are two types of rechargeable batteries - the older NiCad (now being phased out) and the NiMH types. These have different chemical properties, and they differ to standard Alkaline batteries. Rechargeable batteries have a voltage of 1.2V (compared to 1.5V in non-rechargeable batteries). Rechargeable batteries generally hold their voltage at 1.2V, whilst non-rechargeable batteries show a voltage drop as they're discharged. Factors such as efficiency, discharge rate and internal resistance all play a part in performance of rechargeables. Some equipment doesn't like some types of rechargeable batteries, where the sudden fall-off in voltage or behaviour under heavy load, can't be so easily monitored / measured. Some other notes.
- NiCd / NiCad - Nickel-cadmium: The older NiCad batteries weren't great for high-drain applications (like a camera flash), and because of their discharge rate, weren't great in clocks
- NiMH - Nickel-metal hydride: These normally have a higher capacity than NiCd and are better able to cope with high-discharge applications such as in cameras
- How Many Watts? Check the cost of running your gadgets