Electrical – Can you really overcharge a battery at its rated voltage

Good question. Big question. Partial answer ...

Reputable manufacturers provide specification sheets (yes, even for batteries) and these will provide recommended maximum continuous currents and may provide peak allowable discharge currents.

The maximum value is NOT a hard and fast limit which may not be exceeded, and how much it matters depends on the battery chemistry, the specific implementation and on how much you care about the result. Slight but continuous over current discharge may led to reduced cycle life at a rate disproportionately high compared to the amount of over discharge.

There are many Li (Lithium) chemistry based systems. Some are primary (non rechargeable), and some secondary (rechargeable).

Starting with LiIon (Lithium Ion) which is probably what you meant. These are the most common Li secondary cells available and have related "spinoffs' such as LiPo (Lithium Polymer). They have close cousins in eg Lithium Ferro Phosphate (LiFePO4) whch is a lower capacity but MUCH better behaved variant.

LiIon have a charming "feature" known euphemistically as "vent with flame" (VWF) (to which you can append :-) :-( !!!! ) ie when used in modes outside spec (or sometime just because they can) they will self destroy with heat flame smoke and general hilarity.

LiIon are generally rated at 1C max charge rate and 1C to 2C max discharge rate depending on manufacturer, model etc. Exceeding the max recommended discharge rate modestly is not liable to cause problems. 10% or 20% is probably OK and maybe even 50% or 100% MAY be OK . YMMV and you can have no complaint if it does VWF.

Charging LiIon above their specified rate is a really bad idea [tm]. As above, it may work OK but certainly may result in "vent with flame". Again, I'd hazard that 10% or 20% is liable to be fine and maybe double may be OK. Or not.

If you use LiIon at rates beyond rated values you will generally degrade their cycle life by accelerated amounts. eg I'd guess that a consistent 20% overcharge may halve cycle life. Informed guess only. Similarly, by running LiIon at somewhat below spec the cycle life can be usefully extended.

LiIon also have a very tightly specified upper charge voltage - usually 4.2V with some variation specified across temperature. Exceeding this by 0.1 volt is "unwise" and by 0.2 v is very very unwise. eg 4.2V std, 4.3V hazardous, 4.4V stupid. BUT lowering the max charge voltage slightly to say 4.1V or 4.0V will greatly improve the cycle life and also lower the charge capacity. eg 4.1V max charge voltage may be 80% - 90% of capacity.

At the bottom end, lifetime is also affected by Vmin. There is very little energy left below about 3.0V* and stopping discharge at 3V or even above can be a very good idea for lifetime purposes. (* Discharge curves not to hand - look at manufacturer's graphs. Note that voltage depends heavily on load. Heavy load will drop acceptably lower than light load.

There are numerous "new" versions of liIon being announced regularly. Few have yet got to market. These may have charge or discharge rates of 10C or even 100C. ie at the top end of claims, charging in under 1 minute is claimed.

Lithium Polymer (LiPo - NOT to be confused with LFP / LiFePo) uses "plastic" materials for electrolyte retention and generally have somewhat superior electrical characteristics and somewhat greater resistance to VWF destruction. Somewhat.

A very worthwhile variant of LiIon is LiFePO4 / Lithium FerroPhosphate. Sometimes referred to as LiFe which is OK enough as long as this is not taken to be the chemistry. I'll use LFP. LFP allows charging at 1C to 2C (some manufacturers 0.5C) but discharging at 10C or more (some eg 30C) Energy contant is low wrt LiIon (about 60% or less) but cycle life is vastly superior and performance at high and low temperatures may be superior. Properly managed LFP offers 2000 deep discharge cycles (against 300-500 for LiIon) and vastly greater figures are claimed by some for larger batteries with good management.

As always - see spec sheets. Top manufacturers provide a large amount of information re rates, voltages, cycle life etc.

LiIon is a joy to manage charge wise.

NimH (see below) is an ornery pig. You can get OK results with NimH using simple methods but best results need rocket science or necromancy.

Reputable manufacturers equip LiIon cells with internal protection circuitry. When a single cell MUST have electronics inside to make it half safe you know you have a fun product. Very low voltage LiIon need to be coaxed into normal range with great care. Very very low LiIon are usually declared dead by their controllers. Insisting on charging such (bypass protection( may result in death (usually just the cell) but can work with due care. People make special bags for charging liIon cells in. What does this tell you?

All that said, an excellent technology. Treat with due care.

Lightly:

NimH: Charging up to 1C Ok with monitoing of negative delta V or delta temperature or absolute temperature for termination. Some allow 2C with speial batteries. Smart monitoring may allow 2C+ with care. Radio control model fans charge NimH at 4C or more using capacity x 1xx% overcharge as charge termination. eg they may charge a 4Ah pack at 20 A for 15 minutes. This is 5C and 125% energy input. Lifetimes suffer. They don't care.

NimH may be more or less discharged at whatever rate they will bear. Internal cell resistance drops voltage increasingly at high current making battery less useful unless designed accordingly. Discharge should be stopped at say 1V at lowish loads and no less than say 0.9V at very high loads. I'd err on the high side. You can discharge them to utterly empty (0.8-0.9V ) but you gain little and will very severely impact lifetimes. NimH is good for 300-500 deep discharge cycles but can be taken to 2000 or so by taking 10% off top and bottom (stop discharge early, terminate charge early).

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/