16 Jul Rechargeable Batteries 101
We use rechargeable batteries in all sorts of things every day — power tools, solar lights, solar fans, flashlights, my camera, our tablets, some kitchen appliances, and more. Basically, anything that’s “cordless” and powered. Heck — even my toothbrush has a rechargeable battery!
NOTE: In this post, I’m discussing batteries that are in various consumer devices, not the main house or starting batteries for the boat.
Rechargeable battery technology has changed tremendously in the past 10 or so years and we’ve learned to look carefully at what’s “inside” various items as we decide what to buy, as well when we buy replacement batteries. Four primary factors play a role in how long a battery will last — both for use today and how soon you’ll have to replace it. Below is a summary of these factors at what I’ll call a “101” level — it’s not an exhaustive look at every aspect of rechargeable batteries, but hits the important things to know.
I’m not an “expert” on rechargeable batteries; it’s just that we’ve learned a lot about them over the years . . . and a certain amount of that learning has been by doing the wrong thing and then figuring out why it was wrong.
Whether you’re talking about a AA, AAA, C, or D cell battery, or a specialized battery pack for a particular item, or even a non-replaceable built-in battery, the same considerations apply.
Type of Battery
Rechargeable batteries are one of three technologies: nickel-cadmium (generally shorted to NiCad); nickel metal hydride (usually referred to as “metal hydride” and abbreviated as NiMH) and lithium-ion (usually just called “lithium” and abbreviated as Li or Li-ion).
The battery technology determines three important characteristics of the battery:
- Memory. Without going into a long technical discussion, “memory” refers to the fact that some battery technologies won’t fully charge a battery if the battery is only partially depleted before being recharged. If a battery is subject to memory, you should fully discharge the battery before recharging it. Most people prefer to recharge a battery when it’s convenient (say between uses) than getting halfway through a project and needing to recharge.
NiCads have the biggest problem with memory (and they can be damaged by discharging them too far, making it very difficult to know exactly when they should be charged for the longest life). Metal hydrides have less of a problem with memory and it’s generally considered okay to recharge them at any time. Lithium batteries have no memory issues.
- Self-Discharge. Self-discharge refers to the rate at which batteries lose their charge when stored with no load on them. Metal hydrides have the fastest rate of self-discharge, at about 1% a day — meaning that in a month without use, they’ll be down 30%. Consequently, never use NiMH batteries for things such as smoke detectors as there may not be any charge left by the time it’s needed.
NiCads have about half the self-discharge of NiMH, or about 15% a month. Lithium have the lowest rate of self-discharge, at about 10% a month. (NOTE: lithium batteries that are not rechargeable employ various technologies to reduce the self-discharge further, making them suitable for EPIRBs, emergency flashers and more).
Being stored at higher temperatures will make the self-discharge problem worse; lithium batteries in particular do not like heat (keeping them in a hot location is about the quickest way to destroy them). Both NiCads and Li have bigger problems with self discharge when fully charged; thus they should be discharged about halfway before being stored.
- Rate of Charge/Discharge. NiCads can take a fast charge and fast discharge, while Li and NiMH do better with slower charging. Both NiCad and Li are good in applications such as power tools that don’t tolerate a voltage drop well (that is, you want your cordless drill or vacuum to have full power when you use it, even if the battery is partially discharged). Metal hydrides are better in high-drain devices such as most electronics.
While designations such as “AA” or “D” refer to a battery size and voltage, different brands/models will vary by battery capacity, which is expressed in amp-hours or, for small batteries, milli-amp-hours (mAH or mAh). Bigger numbers mean that you go longer before recharges, and bigger will likely have a longer useful life as it will still provide sufficient power even as it loses some capacity due to age.
- Lithium batteries typically weight 20 to 35% less than comparable NiCad or NiMH batteries. For power tools, this can be a deciding factor!
- Li and NiMH batteries are much more environmentally friendly when they reach the end of their useful life: NiCads contain cadmium, a very toxic heavy metal.
- Lithium rechargeables cost about 3 times as much as NiCads and NiMHs.
- Always check to make sure that the batteries you are buying are compatible with the charger you’ll be using — the wrong one can harm the battery, not provide a charge or, in rare instances, cause a battery to explode.
Over and over, I read that heat is the biggest enemy of batteries. While you can’t change the weather, at least try to store your batteries in the coolest possible location. If you store your boat in a hot climate for the summer, consider taking anything with batteries home with you. I know, it’s not always possible. But if you leave them in a hot boat, realize that you cannot expect “normal” life from them.
Running a Charger on an Inverter
One last thing about rechargeable batteries, and it’s important. If you run the chargers off an inverter (as opposed to a 12-volt charger running off a cigarette lighter plug), it’s much, much better to have a pure sine wave inverter. Using a modified sine wave inverter can both damage the charger and poorly charge the batteries.
We totally destroyed the charger for a nice set of power tools the first time we tried to use it on a modified sine wave inverter, a very expensive lesson made even worse by the fact that we couldn’t get a replacement for nearly three months. Read more about pure sine wave inverters vs. modified sine wave inverters.
Pure sine wave inverters used to be frightfully expensive but have come down in price considerably in the past 10 years. If you’re in the market for an inverter, I’d spend the extra for a pure sine wave as not just battery chargers, but all electronics and variable speed motors will run much better on it.