I have a device with integrated NiCd charger. Would it be feasible to modify it to work with NiMH batteries? If necessary, I'm ready to rebuild the whole circuit.
Also what are main differences between NiMH and NiCd battery charging cycles?
Electronic – How related are NiCd and NiMH chargers
battery-charging
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The use of 4 x AA Alkaline would usually be safe BUT does exceed the USB spec and damage may occur in some cases. I have seen IC's in this role with max operating voltages of 5.5V (which is ludicrous) - you'd hope designers had more sense, but it can't be guaranteed.
While some devices may use converters between charge input and battery, many don't (probably most). A LiIon battery has max charging voltage of 4.2V so a 5V nominal USB input will usually meet this need with enough headroom for a linear regulator.
An Alkaline cell can be nearly 1.6V when fully charged - about 1.55V is common or 6.2V for 4, and up to 6.4V may be seen. There is not much energy in this initial high voltage "tail" and voltage falls to 1.5V or below very quickly.
So, you should be safe, but YMMV, alas.
A solution would be to use an LDO (low dropout voltage) regulator OR a clamp regulator which takes the peak energy out of the battery or a series diode to drop 0.4 to 0.8V (Schotky / Silicon).
LDO is best solution but you want as little drop as possible.
Clamp to drain peak battery voltage is unusual but viable. A zener could be used but is too inexact. An eg TL431 clamp regulator in a TO92 or other largish package (to get OK dissipation capability) ould do. A TL431 plus a transistor would be safer.
Series diode is cheap and easy but prevents full battery use. Say minimum usable battery voltage is 4.6V (may be higher). At 1.15V/cell there is still some battery capacity left. Adding a Schottky diode increases minimum battery voltage to 4.6 + 0.4 = 5V or 1.25 V/cell. Some capacity wasted. At the top end a 0.4V drop diode results in Vbattmax of say (1.55V x 4 - 0.4) = 5.8V or 1.45V/cell."Almost certainly safe".
Using NimH works but is more marginal at bottom end and safer at top end. At 4.6V, V per cell is 1.15V where NimH still has modest energy left. At top end Vmax = say 1.35V, maybe 1.4V for short periods at start. 4 x 1.4V = 5.6V. Very probably safe.
I very seldom disagree with Olin technically. In this case there may be special circumstances which make part of his advice correct in general but specifically wrong in this case.
As he notes, first it is necessary to establish the voltage across the battery to ensure it is in fact a single cell and not a number in series. As you say that the razor operates OK on the new battery then it is extremely likely that the old one is also a single cell.
15 VDC at 420 mA sounds just plain wrong. The voltage is high by a factor of about ten times, so maybe it's 1.5V.
For a 2300 mAh cell the 420 mA would be C/(2300/420) ~= C/5.
This is an OK charging rate BUT if the charging is not COMPLETELY terminated when the cell is charged the cell will "cook" in short order.
For capacities up to 1500 mAh, maybe 1800 mAh NimH calls had special arrangements (chemicals and structures) which allowed recomination of Hydrogen when "gassing" occurred when a cell was left on charge when fully charged. This allowed manufcxaturers to specify a trickle-charge rate of say C/10 (230 mA for a 2300 mAh cell). At or below this rate the cell could be left on charge indefinitely with little or no damage. HOWEVER as the typical battery capacity arms-race occurred and capacities were pushed up to 2100 2300 many_lies 2500 2600 all_lies ... mAh the manufacturers looked for more space to fit active material into. Something had to go, and it was the gas recombination mechanism. Modern NimH cells above about 2000 mAh from reputable manufacturers have data sheet advice of the form:
- Do not trickle charge at all! or
Trickle charge at no more than C/20 or whatever for some_very_small_period or
Can be trickle charged at <= C/100 on a good day downhill with the wind behind you.
Any battery manufacturer whose data sheet says ... 2500 mAh ... trickle charge at <= C/10 can be safely shunned as a source of supply for all future time.
SO when Olin says " ... In that case, the highest capacity battery is best since it will be abused less at the same current." - this is good advice in the general case BUT not so when using NimH where the charger is badly behaved. In such cases use of an older style 1500 mAh cell would probably [tm] give a much longer life.
However - IF the charger really is a true 1.5V charger and if this is tightly controlled (rather than edging upwards as load current drops, then it MAY be OK.
At say C/10 the terminal voltage of a NimH cell at room temperature at the end of charge will be ~= 1.45 V. 1.4 is safer and 1.5 is a bit high. Actual value varies slightly with manufacturer. Temperature much above 25C vary this voltage BUT also are best avoided. Higher charge rate lead to higher voltage st end of charge.
SO - measure charger output. If it is 1.5V and no more your battery may last OK. If it rises to > 1.5V at light loads you MAY be able to load it down with a suitable resistor. But using a 1500 mAh cell is probably wise.
Added:
The 1.46 Volts after 4 hours sounds very good. That's 420 x 4 = 1680 mAh BUT the 1.46 volts sounds like a fully charged cell so presumably the cell was partially or filly charged originally.
Try an overnight charge - if it's still at 1.46V they seem likely to have done a reasonable job of charge control.
If you are able to measure the battery current on charge at the end of an overnight charge you will be able to tell if it is trickle charging. This can be accomplished by eg a battery interceptor / continuity break insulator against the +ve battery terminal and add a conductor on either side and take wires out to an ammeter. OR locate the battery externally and bring out two wires to it via an ammeter.
Here's an example of a battery interceptor, From here
= http://www.instructables.com/id/Remote-Power-Control-For-Battery-Powered-Devices/
Best Answer
Its possible but hard to answer without knowing the charger topology.
NiCd can be handle very high charge/discharge currents. Such charge currents could damage a NiMH battery.
Now it MAY be OK, not all NiCd chargers are of the high current/peak detect variety, sometimes called "Rapid Charge".
Some are simply timer based in which case they will very likely undercharge a NiMH battery.
Still other NiCd chargers are "trickle chargers" made to slow charge overnight, such a charger would work with NiMH but would likely take a very long time to charge since NiMH batteries generally have a much higher capacity.
Yet another charger type is the "smart charger" which monitors the battery as it charges and adjusts its charge rate, stopping or going into a trickle mode when the battery is determined to be full. The "full" trigger and charge profiles are different between NiCd and NiMH so I would assume this type would need to be modified (possibly with new firmware) to handle NiMH batteries.
EDIT: Ancillary question: are you sure switching battery chemistry is a good idea for this application? The mAh rating doesn't tell the entire story of battery life. If the device is high current then a NiCd battery may last much longer than a NiMH battery even though the NiMH battery likely has much higher mAh rating.