My wife recently purchased for me a fine new electric razor. It’s the kind where you ordinarily run it off the battery, and only plug it in to recharge it. The manual suggests that to preserve the power cell’s capacity, it’s a good idea to let it run out completely before recharging it. Fine, no problem.
Except that getting the last erg of juice out of it is taking forever. I mean that it’s reached the point where you turn it on and it only runs for a second or two before running down. Then if I let it rest for ten or fifteen minutes and turn it on again, then it runs another few seconds before petering out again. Repeat X100.
Do I really have to wait until it’s stone cold dead before recharging it? If I just left the switch on, would the residual charge leak out even though the motor isn’t running, or is giving it that rest period essential?
You’ve already gone too far. Deep discharging a battery pack can reverse the charge of one cell, ruining the pack. As soon as you see a significant drop in perfomrance it’s time to recharge.
Look up “battery memory” and “voltage depression”. The short answer is that you should probably recharge when it drops below the point of usability, but not recharge it in the middle just for the heck of it. That’s what they don’t want people to do. This advice page seems pretty good:
In essence the manufacturer doesn’t want you to leave it on the charger all of the time. Wait until it’s low, then recharge it.
My mom has a cordless phone that she leaves on the charger when it’s not in use (despite my pleas to the contrary) and now it’s battery is about a 1/3 of capacity.
The reality of memory effect is not what the myth says but deep discharging and/or overcharging can damage batteries. Some chargers are a little smarter and know to shut off or switch to trickle when the battery is fully charged. I’ve got a Vtech cordless phone that is left on the cradle all the time and battery performance hasn’t suffered. If I forget to put it back on the cradle it will still last for 2-3 days before the battery runs low.
I have to agree with BioBrat on the cordless phone thing. I leave my cordless phone off the charger until the little red “charge battery” light comes on. It will last at least a month to six weeks this way, before the charge runs down. I just pop it on the charger overnight and it’s good for another month or so. Much better than 2-3 days, like Padeye said. FWIW, mine is a Panasonic. I don’t know about the Vtech brand.
I know I ruined my Dustbuster by leaving it on the charger all the time. Now it will only run for about 3 minutes before the charge noticeably drops to nothing. I need new batteries for it.
My girlfriend and I received a cordless phone two x-masses ago. I always put it back on the charger, and she always said that that was bad. We looked in the owners manual and it specifically said something like, “The phone or the battery will not be damaged by being left in the charging cradle after the battery is charged.” I’m paraphrasing this, but you get the idea.
I also read an article, by an obscure author, on this very subject. You can read it here.
I tend to stay away from rechargable appliances that either don’t allow use while plugged in or have built-in batteries that can’t be changed. I’ve had a couple (electric toothbrush & a handheld mixer) that eventually lost their ability to recharge & became completely worthless.
I have one of those rechargeable razors, too. At first, I would let the razor run continuously once the red “recharge” light went on (to discharge the battery), then I reasoned that at one charge every two weeks or so, I was at greater risk of killing the motor (I usually pulled the blades so I didn’t wear them down) sooner than the batteries. Since NiCds are usually good for at least 500 charges if treated well, that’s at least 18 years of shaving. Probably longer than I’ll want to keep the Norelco, anyway (and the batteries probably won’t last that long no matter how nice you are to them).
Some razors (Braun, I think) use NiMH batteries, which is nice. Unfortunately, the Braun razor I tried didn’t work as well on me as the Norelco I eventually settled on. LiON batteries are too expensive right now to put in a < $100 device; I don’t think current draw is a problem, I’d wager a laptop computer uses more electricity than a razor anyway.
While we’re on the subject of rechargeable batteries, can someone tell me how rechargers determine when they’re done charging? I get the impression that some (the cheaper ones?) are timer-based, but the batteries on my Sony camcorder have a third terminal, marked T (temperature?). And the charger is supposed to be good for many different sizes of battery packs, including both NiCd and NiMH types. I know lead-acid chargers check the amount of current the battery is accepting, but that’s a totally different animal.
Lead-acid batteries are easy - as you charge them, their voltage increases to about 2.4 volts per cell. So a charger can cut out when the battery reaches this charge level, or “float”, that is, provide a constant 2.4 volts per cell across the battery. (You can actually charge to 2.65 volts per cell, which is recommended periodically to clean the plates. This is called “equalisation”.)
NiCads and NiMH are tougher because they have a very flat voltage-vs.-time charging profile. There are three approaches which are used:
Constant-current trickle charging. The batteries are designed to dissipate the energy as heat when fully charged, so if charged at a low enough rate you can theoretically leave them connected to the charger indefinitely.
Chargers which look for a slight characteristic drop in cell voltage after a maximum has been reached. These are good, but require comparatively complicated electronics.
As you guessed, temperature. The same technique as (1) except you can use higher current levels. Thermistors detect the batteries heating up when fully charged and cut the current off, or reduce it to the trickle charge level.