The batteries in my electric razor are wearing out and I would like to replace them. There are two AA size cells with solder tags solders to a PCB incorporating a charger. But, nowhere does it say what type of batteries they are. Is there any easy way to find out what type of replacements I need. I would rather not order the relevant part from Braun because they are certain to cost more than generics from Maplins.
Thanks,
Andrew
I'd just use NiMH. If it originally had NiCd, then it will just take longer to charge in proportion to how much higher capacity the new cells had to the old, unless it has a sophisticated charger, which will probably work out what to do itself.
Christian.
Just came across a link to Braun shaver service manuals on sci.electronics if that's any help working out the battery type.
-- Adrian
If in doubt use NiCd, a charger designed for NiMH will always function correctly on NiMH, one designed for NiCd may well overcharge NiMH. Moreover capacity isn't usually an issue with shavers and NiCd will withstand a third or more recharge cycles than an equivalent NiMH.
NimH batteries tend to weigh less than NiCd. use a digital scale to weigh your existing pack. Go to a shop and take the scale with you. Weigh a couple of AA Nimh. You should be able to tell the difference.
NimH are not neccessarily better or worse than NiCd. They are just different. Different battery chemistries suit different applictions.
have a look at this link for some answers
NiMh don't like overcharging, even a trickle charge, so measure voltage/current at end of charge to see what the charging circuit does.
You might need to time the charge carefully but high capacity NiMh will give a lot more use for each charge.
cheers, Pete.
As a side note you should buy batteries that already have tags on them and be very careful when soldering... If you heat the batteries to much they tend to leak which will damage the battery and also spit some nasty chemicals at you.
Have you checked to see if the batteries are rebranded generics, I've seem normal batteries wrapped in plastic coating which can be removed with possibly more information underneth.
Thanks to all. I have download the relevent service manual and it confirms that the batteries are indeed NiMH. I now have some new ones waiting to be fitted. So here's hoping that everything works OK.
Andrew
I was going to say use NiMh it won't matter. But someone said they'r no better or worse. If you are looking at it from an environmenta viewpoint, NiMh are less polluting than NiCd.
Someone else said NiMh don't like overcharging, I was under th impression NiMh could be charged at any time irrespective of thei charge status, whereas NiCd should only be charged when empty. And NiM could be charged for as long as you like. whilst NiCd don't lik overcharging. I await confirmation from a higher source.
Can I also hijack this thread, now that it has been answered, and sa that Braun used to sell just the foils as replacements, now you have t buy the foil and top plastic curvy thing (connected) along with a ne cutting blade, costing about 13 - 16 pounds (which happens to be 1/3r the price of the new machine!) My shaver is stuck in a draw after th foil lasted 3 - 4 weeks, no way am I going to pay all that money whe all I need is a pack of 5 foils like we used to be able to buy!
Has anyone else noticed this and/or know how to obtain just the foils?
Thank
-- wig
There are basically two ways of charging NiCd and NiMH cells. The first is to use a charge rate of no more than 1 tenth of the cells capacity. So for a 1500mA hr cell you could use a charge current of no more than 150mA. The cell will take about 12 hours (150 x 10 = 10 hours plus some extra for energy losses) to charge from empty to full. The cell can be left on this charge for well over 12 hours without harm although for standby use (where they are always on charge for months on end) a rate of one twentieth of capacity (so
75mA in the example given) would usually be used.This method of charging is reliable, cheap and safe however it takes a long time.
The second method is to charge the battery at a much higher rate and detect when it is fully charged by sensing one or both of the small voltage change which takes place when it becomes fully charged or cell temperature.
Voltage change sensing is usually called Delta V (change in voltage) or DV sensing. It can work either by sensing the small voltage fall which happens as overcharging starts (-dV) or the rate of change of charge which happens just before full charge (+dV). With NiCd cells this voltage change is of greater amplitude than with NiMH so a charger designed for NiCd will usually fail to sense if a NiMH battery is fully charged and overcharge it. Most chargers relying upon dV sensing also use temperature sensing as a backup. The problem wit h this is that the temperature rises only when the cell is overcharged so thermal sensing is a bit late to avoid some cell damage. NiMH batteries also heat up throughout the charging process so again older NiCd chargers with thermal sensing may fail to detect the rate of change of temperature which occurs on overcharge.
Putting a NiCd cell in an NiMH charger is always safe for the battery, putting an NiMH cell in a NiCd charger often isn't. Neither type of cell will tolerate overcharging at high rates.
THe mythical "memory effect" you will see mentioned everywhere in connection with NiCd is actually nothing of the sort but something properly called voltage depression and is simply a reaction of the battery to _overcharging_. It has nothing to do with under-discharging (which was the cause of the original "memory effect" only ever seen on one uncommon type of cell construction in outer space!).
As an aside the "chargers" supplied with cheap electric drills are the worst of both worlds. They are unregulated and usually charge at about one quarter of the batteries capacity. This gives a charge time of about 5 hours, too long to be useful but with a charge current far too high to be maintained for more than 5 hours without causing damage. Leaving one of these batteries on charge overnight once is usually enough to damage it irreparably.
In general NiCd are better if the application means they are going to be treated roughly and charged often. They also usually have a capability to survive more recharge cycles than NiMH. NiMH are better if the best power/weight ratio is required. So things like cameras, where a lot of energy is needed in a small package, suit NiMH whereas electric power tools are usually better suited to NiCd.
If you have a electronic thermometer, with an alarm, this can be a good use for it. Simply stick on top of the battery to be charged, with a foam scrubbing pad (insulation) and a small weight to keep it there, and turn it off when it beeps (at room temp +5C)
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