Choosing Rechargeable Batteries
Weight-to-Power Ratios
One way of measuring the cost and effectiveness of a rechargeable battery is to measure its weight-to-power ratio. This measurement is exactly what it sounds like; that is, how much power a specific battery can pack into a certain amount of weight. One such way to measure weight-to-power is with watt-hours per kilogram (W-h/kg), or how many watts of energy a one kilogram battery can produce in an hour. Here are some examples using the most common types of rechargeable batteries:
- Lithium Ion (Li-ion) - 150 watt hours per kilogram
- Nickel Metal Hydride (NiMH) - 60-70 watt hours per kilogram
- Nickel Cadmium (NiCd or Nicad) - 42 watt hours per kilogram
- Lead Acid - 25 watt hours per kilogram
As you can see, lithium ion batteries have a significantly higher weight-to-power ratio than the other types of batteries out there.
Discharge
Another way to measure batteries is by their discharge. Here, when we talk about discharge, we are referring to how fast a rechargeable battery will lose its stored charge while not in use. As you might expect, different types of rechargeable batteries lose their charge at different rates, all based on their chemical composition and structure. Once again, here are some examples of how different types of batteries lose charge at different rates:
- Li-ion - 5% loss per month
- NiMH - 20% loss per month
- NiCd - 10% in the first 24 hours after recharge, followed by 10% per month after that
Again, lithium-ion batteries win out over nickel metal hydride and nickel cadmium.
So why aren't lithium ion batteries the one and only rechargeable batteries being used? Well, although nickel cadmium and nickel metal hydride batteries have a lower power-to-weight ratio and a higher discharge rate, they also are a bit tougher and therefore are more suited for things like industrial settings or use in machinery or power tools.
But what does this mean in plain English? How long is a battery going to last?
Unfortunately this is a very difficult thing to gauge. The best way to try to figure out how long a battery will last is to look at how much power you need from the battery, then look at its Amp-hours. For instance, let's say that you need to run a laptop which needs 1 Amp per hour to operate. A laptop battery that has 4 Amp-hours will power your laptop for... 4 hours! Similarly, a camera that needs 300 milliAmps per hour to work will get approximately 6 hours of use from a 1800 milliAmp-hour battery. All these numbers, however, are subject to change and don't take into account things like powering on and off or periodic use.
Now you know!
