NiMH charging advice

[Muse]

The internal rechargable NiCD battery of my old Philips Norelco 6843XL shaver died. I use the shaver very infrequently, so I figured it would be smart to not replace the battery with another NiCD but with an Eneloop AA (~2000mah), and I did that maybe 1 year ago. Meantime the shaver worked like a champ when I used it but the initial charge is finally gone.

I normally shave with a blade shaver + shaving cream, but once in a while I use the electric razor if my beard is more than 2 day or if I want to shave without first showering (it happens).

OK, so I've never charged the Eneloop while in the shaver. The instructions in the manual for this shaver say to charge the (NiCD) battery by plugging in the supplied cord for 12 hours initially, 8 hours subsequently when needed (for maximum battery life they recommend letting the battery run down, not leave it charging daily). They say if you haven't used the shaver for a month, charge it the full 12 hours.

Of course, the Eneloop retains charge very very well.

My concern is over-charging the Eneloop NiMH. It would be a hassle to remove the battery and charge in an NiMH charger and then reassemble (I think I may have soldered it in, anyway). So, using the cord (which has no polarity associated with it, it can go into the shaver either direction), how long should I leave the shaver plugged into the wall on the rare occasions when it needs a charge?

 

[Viper GTS]

You should not plug it into the wall at all. Pull the battery out and put it into a charger that knows what to do with NIMH.

Viper GTS

 

[Muse]

You should not plug it into the wall at all. Pull the battery out and put it into a charger that knows what to do with NIMH.

Viper GTS

I don't remember if I soldered it in, I'll have to open the shaver to find out. Maybe I can work out having it installed without soldering. I'll open it. Meantime, it's been charging using the wall charger for the last ~2 hours, I'm sure it's not overcharged at this point. I think the important thing is that the cell hasn't reached full capacity. It's at that point that overcharging could occur if the charger doesn't cut current, I figure.

 

[Muse]

I got the shaver open. The AA is soldered in there. I don't want to unsolder it. I don't see a reasonable way to get it in there without soldering. The charging cord had been on it around 2:45, considerably less than the 8 hours specified in the manual. I measured the voltage across the AA's terminals with a multimeter and it was 1.36 volts. That's not far from full charge. I think if I charge it in the future for under 3 hours it will be alright. If it isn't fully charged, that's no big deal, it shouldn't affect the battery life and the shaver should function fine.

 

[manly]

You're overthinking this.

Doesn't the shaver stop charging when it thinks the battery is full? (This point varies as the battery ages.) It's not like people who leave their Norelco shavers plugged in over 12 hours return to a molten pile of plastic?

So I don't think you'll be able to overcharge the non-original battery. FWIW charging for a few hours at a time is quite reasonable. Supposedly NiMH doesn't exhibit the memory effect like NiCd but if I'm not mistaken, no rechargeable batteries benefit from full charge/discharge cycles.

 

[Muse]

You're overthinking this.

Doesn't the shaver stop charging when it thinks the battery is full? (This point varies as the battery ages.) It's not like people who leave their Norelco shavers plugged in over 12 hours return to a molten pile of plastic?

So I don't think you'll be able to overcharge the non-original battery. FWIW charging for a few hours at a time is quite reasonable. Supposedly NiMH doesn't exhibit the memory effect like NiCd but if I'm not mistaken, no rechargeable batteries benefit from full charge/discharge cycles.

Yeah, I don't know. I think a NiCD will be OK continuous charging if it's a true trickle charge, i.e. slow enough charge. NiMH, I'm not so sure. I know that NiMH designed chargers look for a sign from the battery and then reduce the current to a maintenance current. So, NiMH can be overcharged if the signal is missed, which happens sometimes with some chargers. What's going on with the electronics in this shaver I don't know.

 

[Imp]

Sounds to me like the shaver's built-in charger is a "dumb charger" with no charge termination circuity -- because it's telling YOU to stop the charging.

It'd be nice if you could figure out the original capacity of the NiCd. Sounds like it's too late now. An 8 hour long recommended charge sounds pretty slow though. But to get to 1.36 volts on the NiMH under 3 hours sounds quick for a charger that recommended 8 hours.

FYI: My Panasonic charger that came with 4 AA Eneloops takes about 8 hours to charge an almost empty ~2000 mAh AA battery -- it self-terminates. The math works out with the mA specs too.

This might be useful:
http://batteryuniversity.com/learn/article/nickel_based_batteries

 

[Muse]

Sounds to me like the shaver's built-in charger is a "dumb charger" with no charge termination circuity -- because it's telling YOU to stop the charging.

It'd be nice if you could figure out the original capacity of the NiCd. Sounds like it's too late now. An 8 hour long recommended charge sounds pretty slow though. But to get to 1.36 volts on the NiMH under 3 hours sounds quick for a charger that recommended 8 hours.

FYI: My Panasonic charger that came with 4 AA Eneloops takes about 8 hours to charge an almost empty ~2000 mAh AA battery -- it self-terminates. The math works out with the mA specs too.

This might be useful:
http://batteryuniversity.com/learn/article/nickel_based_batteries

I was thinking the exact same thing. Now, I measured that voltage pretty soon after the charge process. In my experience, rechargeable batteries settle down some if you let them sit a few hours after being charged, so the 1.36v reading might be higher than what it in practice is, which might be more like 1.32v. However, that's a decent charge. I want to be conservative in this. I have visited and read Battery University's info several times in the past. My impression from what I've read (I've been a rechargeables mavin for many years) is that NiMH batteries have no memory effect, are not harmed by recharging anywhere in their cycle, it doesn't matter if you don't fully charge them. All that might not be entirely true.

Actually, I could maybe find out the capacity of the original NiCd because I have another of this same shaver. I bought it used off Ebay a few months ago. I can open it up and inspect the battery. However, it's soldered in there and the orientation might not be correct for reading the capacity.

Anyway, what I do know has me thinking that this shaver with the Eneloop NiMH soldered in there and charged ~3 hours at a time, as infrequently as I'll need to do that with my usage pattern, is apt to serve me well for many years!