NiMH AAs - when to replace them?
 

When do you folks replace your NiMH batts? I've been using mine for a year now, and I think they're getting a bit wimpy in terms of holding charge.

Depending on the quality of the batteries, NIMH should last between 400 and 2000 charge cycles. Even if you charge them every day, they should last longer than a year. Then again, operating them at extreme temps may mitigate the number of charge cycles.

I use NiMH AA's in my video game controllers and such, but those only get recharged about once a month, so I'm nowhere near the charge cycle count. I have them in my Dinotte light, but only for 6 months now; so I am unsure of the "right" answer. I expect mine to last about 2 years, but we'll see.

Wouldn't it matter which charging system you use? I mean those 15 minute chargers versus the MAHA C-9000.

The thing is that those quick charging systems may be doing a disservice to the rechargeable batteries.

That's a definite; they heat up the batteries much more than they ought to be. I've heard that they can halve the servicable life of the battery. The original design of NIMH batteries was for them to be charged at a rate of 8 or 10 hours. While even 4-hour chargers heat up the batteries somewhat, I think that's a good compromise between life and convenience; 1-hour and 15 minute chargers are definitely a bit extreme.

How good is your charger, BTW? I've had much better behaved batteries ever since I got my LaCrosse BC-900 charger that keeps each individual cell topped off and lets me see how much juice it actually charged them with so I can see how much life is left...

You replace them when they seem a bit too wimpy at holding a charge, of course!

More seriously, if you have something like a BC-900 or one of the many really good chargers/cyclers made for R/C use, you can measure how much capacity they have left. Fully charge it, then discharge at a moderate rate down to 1.0 volts/cell and see how long it lasts compared to how long it lasted when new. If it's lost more than x%, replace it, where x% depends on how critical the application is.

But most people don't have a cycler, so you estimate. Carry a spare pack of batteries with you when you ride, and when you start needing to use them when you didn't need them before, replace the batteries. Replace the batteries in your rear light more quickly than the ones in your headlight -- if your headlight goes out or goes dim, you notice that right away. If your rear light goes dim or off, you may not notice until you get home.

And don't forget that you might be able to re-use the old batteries in something less less important -- your kid's toys, for example.

When the LaCrosse BC-900 refresh mode can no longer bring them back to a useful capacity. Let them fully discharge once a quarter and they will retain capacity longer.

Usually the rule of thumb s replace when they are down to holding 80% of capacity. If your using a poor charger such as a 15 minute or on made by rayovac or radio shack or any other company that wants to rip people off then 80 % could occur at less than 100 cycles. A good battery with a good charger should last a lot longer. I have batteries that are several years old now that have just moved into kid toys.

Well, for R/C use, the `rule of thumb' is 80% or 90%. But R/C is a bit more critical than most uses -- if the battery goes dead mid-flight, your plane crashes, and you may not have much warning that this is happening. If your headlight batteries die, you usually have it gradually getting dimmer and dimmer before it goes out and that's very obvious. And if you're properly equipped, you've got a spare set of batteries with you -- or a spare light. So there's no need to be that careful about your batteries -- even 50% life left is OK as long as that gives you enough juice for a typical ride without any drama. And yes, batteries that have lost a significant amount of capacity are more likely to go suddenly dead with no warning, but that's rare and immediately obvious, and usually not a crisis. (Assuming you have more batteries or lights, of course.)

(Outside of R/C use, I'm not sure there really is a rule of thumb. People replace batteries when they stop working. Often they ditch the entire device!)

Ultimately, replace them when they don't last long enough for you anymore, with `long enough' being something that you decide based on your needs.

Do be a little more careful about rear-lights, however -- especially if you only have one. It's easy to miss them getting dim and going out.

But yes, don't use those 15 minute chargers -- a full charge (from empty to full) should take at least an hour, and even that's on the fast side for AA sized NiMH cells. (It's usually fine for sub-C sized cells, however.) It's OK if the battery gets warm, but not if it gets hot.

Also, get a charger that turns itself off. The good ones detect the `peak' when the battery is fully charged and turn off automatically. The cheap ones just have a timer and turn off after X hours -- not as good, but still better than the ones that have no timer at all. Leaving NiCd/NiMH cells on a slow charger that never turns off wears them out over time. (Putting them on a fast charger that never turns off ruins them quickly.)

LiPo and Li-ion chargers all turn off when the battery is full, so it's not a concern there. (If they didn't turn off, the battery would be ruined the first time it happened and perhaps catch fire.)

Considering that the MAHA chargers take upwards of 40 hours to break-in a new uncharged battery, I'd say the simple "cooker" units probably do cut down the life of your batteries.

most smart chargers will tell you during the recondition cycle if a battery is kaput.

the Maha will 'condition' and drain all batts, then fully charge them. if one is out of whack it will flag that battery as junk

yes the dumb blister pack chargers you get with batteries at the quik-e-mart are the ones which
bake the battery and cut life in half or worse.


anyhow...if my dinotte on high, craps out in 2 hours, them I know one or more, of the 4 AA's, is end-of-lifed

There is no probably in quick chargers will shorten the life, it is a verified fact that they shorten the life and can greatly shorten the life.
80% is a rule of thumb, when you think your batteries are not lasting as long as the used to test them and see. If one or more is below the 80% mark then it can probably be considere bad. I tested all my NiMH's from back to 1999 in early 2008 when I got my c9000 and most of sanyos were just gettinig down to 80% with the rayovacs from 2005 at 80% and the crap brands like radio shack, energizer, etc all down much less than that.