Rechargeable AA batteries-Can a NIMH can replace NiCd safely?

[PS85]

Had a Braun shaver passed down to me which I used for awhile, always plugged into the wall, until it stopped. I put it aside for some months, but decided to try to get it going by replacing the rechargeable batteries. Batteries are rated 1.2V.

Don't know too much about rechargeable AA. The official replacement for the shaver model is: https://braun.encompass.com/item/10624718/Braun/67030921/. The shaver model, fwiw, is Type 5742.

Figured I'd rather see what I could pick up locally instead of sending away. Could not find NiCd, so brought home some of these: http://www.rayovac.com/products/rechargeables/recharge/aa-4-pack.aspx . The package said "For all chargers". Since the only charger I use is the shaver itself, not sure if this applies to that or to some separate standalone charging unit. Since Braun does not advise the user to change the batteries themselves at all, but to bring the shaver in for service, I figured no use calling them.

So the question, as it seems to me, is: Can I replace two rechargeable NiCd 1.2V 700 MAh batteries with two rechargeable NiMH 1.2V 1350mAH batteries without foreseeable problems? The new batteries are pre-charged, if that means anything.

 

[WilliamM2]

I replaced the batteries in my Norelco with Eneloops. Which are Nimh. No issues, been about 4 years now, but I don't use that shaver a whole lot any more.

 

[PS85]

I replaced the batteries in my Norelco with Eneloops. Which are Nimh. No issues, been about 4 years now, but I don't use that shaver a whole lot any more.

Thank you for your response. I went ahead ahead and tried. The terminals were slightly different for the stock NiCad battery, which are shaped slightly different at the end, even though they are size AA. I figured I could just bend the terminals slightly to make a connection. Dunno what happened, but I felt the unit started vibrating in my hand with the cover off, but then it stopped, (or I stopped it by doing something I might not have known). Now it doesn't work at all.

As the pushbutton start switch was starting to disintegrate even before the stopped working months ago, the unit is over 8 years old, and the PC board had some corrosion around the edges that looked quite old when I took the cover off, it looks like this shaver's time has come and gone.

Oh well, a little fun and not much money spent on batteries. Maybe Braun wasn't being cheap after all when they said replacing the batteries is not a job for the user, lol.

Thanks again.

 

[Sukhoi]

Nope, a NiCd charger will overcharge NiMH batteries. https://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

 

[WilliamM2]

Nope, a NiCd charger will overcharge NiMH batteries. https://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

Hard to say if the charger in a shaver would overcharge them or not. That article seems to be talking about standard battery chargers. I checked, and it's actually been 7 years since I put the Eneloops in my Norelco.

The shaver was purchased in 2003, the original Nicad batteries stopped holding a good charge after about 5 years, and were only good for about 2 shaves. I had to use it with the cord for a few years before finally getting the Eneloops NiMH. Those Eneloops still hold a good charge after 7 years, so the battery life does not appear to be shortened at all. Although I'm sure they are better quality than the originals.

 

[Sukhoi]

Yeah I'm sure it depends on the charging circuitry used, which is going to vary razor by razor. Yours doesn't appear to hurt NiMH, but that's random chance.

 

[mindless1]

NiCd AA are not hard to find, did you try a google search? They are probably on ebay, amazon, battery websites, and many retailers sold for garden/path solar lights, the cheapest of which often come with a NiCd AA. Ironically the cheapest source might be buying an entire cheap solar light and pulling the NiCd AA out of it, though the quality of the cell might be poor, as might any made in China.

Unless it's a high end shaver, the charging circuit for one NiCd powered is going to just be a crude trickle charge circuit where if you don't unplug it in a timely manner (based on best guess if there isn't some LED that just roughly indicates a level of current flow to indicate it's "near" fully charged) it cooks the cells.

In that case NiCd is more resistant to overcharge damage and offers more recharge cycles but you can still get a reasonable service life out of NiMH if you are mindful to terminate charging when finished rather than allowing overcharge. The worst are probably those designed to sit on a cradle and constantly charge any time you aren't using it. Those too should be unplugged/pulled from the base if you aren't sure it has good charge termination instead of a trickle charger.

Typically to have good charge termination it will have to charge at a rate where it can fully recharge from dead within 4 hours or less, except if you replace NiCd with modern high-cap NiMH, that period could be doubled or more which is too low for reliable charge termination of a smart charge circuit, except I really doubt it had one in a shaver. You can reverse engineer and/or measure the circuit charging to determine this.

If it instead uses a longer charge period, you would have to extend that period by whatever amount the NiMH have higher capacity. You can test this yourself, at full charge and disconnected from the charger for a few minutes they should settle down in a range from high 1.3(n) volts to low 1.4(n)V. You'd be better off erring on the side of conservatism and charging a little less than 100% rather than trickling past 100%. It is not a big deal to not fully recharge them, the memory effect is not applicable even for NiCd on something like this where the discharge level is never going to be exactly the same.