Overcharging rechargable batteries.

[Pulsar]

Back in the day when I used to use rechargeable nicad batteries for RC Cars, I noticed that if I charged them fully, I can then hit them with full charge voltage for another 5 minutes or and get a much higher output voltage out of them when I used them (for a short period of time).

What are the physics behind it, and why does it damage the battery when you "overcharge" it? From my limited understanding of batteries and the chemical reactions, why don't they simply stop accepting a charge (stop the reversal of the initial chemical reaction) when they have no more chemical to break apart into it's original constituents?

One final question - I also know they get quite hot when overcharging them. If I were to use an ice bath or some other way of keeping them at the proper temperature (dry ice perhaps) would that mitigate the damage I'm doing by overcharging them?

 

[Ninjahedge]

Its the difference between NiCd and LiIon.....

NiCd does not have too many problems with being heated up a bit. You can charge them up to "full" and possibly a bit more than rated full by trickle charge. I BELIEVE this is only due to the battery being stocked to store a bit more than what it is rated at AND the fact that NiCd is notorious for developing "memory" (when you do not fully charge or discharge, there is almost like a electrochemical speed bump left behind. Get enough of them and it lowers what you can store... I am not sure if it is charge or voltage that is effected, but still).

Trickle charge may help to clean these "bumps".

Noe LiIon batteries, I believe, have a substrate that evaporates when you leave them hooked up too long. The battery heats up and it loses some of the stuff it needs to operate. They are also prone to something similar when fully discharged (cross contamination? Leakage?). So, unlike a NiCd (or NiMH) battery, it is not recommended to fully discharge or trickle charge a LiIon.....

 

[jamesmcuk]

1. I noticed that if I charged them fully, I can then hit them with full charge voltage for another 5 minutes or and get a much higher output voltage out of them

2. why does it damage the battery when you "overcharge" it?

3. would that mitigate the damage I'm doing by overcharging them?

1. This is indeed a phenomenom and is not easily explained but must be to do with a short term effect in the chemicals which pushes out a voltage equal to the charge voltage.

2. It Doesn't (with NiCd and an appropriate SLOW charger) a battery can only do a finite amount of charge and discharge cycles in its life and so does degrade over time as a fact however (in the case of nicd at least the "Memory" effect is kind of a myth) it comes down to the charging technology. NiCd chargers tend to charge based on either time or temperature. When they are full the temp will rise and most fast charger will detect this and turn off. Time based usually SLOW (10hour type) ones though will continue to charge based on the time it take to put a full charge in to a flat battery at a given current. Therefore if you charge a partially discharged battery it will be overcharged and due to the chemistry of a NiCd this will cause it to provide a lower average voltage but the capacity is unchanged. Fortunately this can be restored with a few full discharge and recharge cycles. The real damage occurs with fast chargers where the battery heats up and damages the chemical makeup of the battery.

3. Its the heat that damages them so I would guess "yes" but since it won't produce a beneficial effect other than allow you to use a fast charger to overcharge them then it may not be worthwhile.

 

[Ninjahedge]

James, I think the slow chargers just work on resistance.

Once the battery is close to being charged, the resistance goes up causing the battery to heat up.

I am not sure if fast charge sets have a temperature gage to tell when it is charged, or just as a safety, but I know it is pretty easy to design a circuit to click off once a given voltage is reached....

Slow charge would not really need that as they put so little oomph (amperage) through it to begin with, they will not cook your batteries even if 100% of the power is dissapated through heat from the batteries resistance.


BTW, I don't believe it is physically possible to charge a battery above its components electrochemical potential difference between the component parts. I think when you do that last trickle, you are only getting it closer and closer to the max it can hold (if you have the exact same voltage as the difference, wouln't the charge curve be asymtotic?).

I think you would only get a smidge more charge on the battery.

Look at how your charger is rated, then start looking around for the absolute maximum voltage your battery can be (not what it is officially rated for) to get a better idea of what might be going on....

 

[jamesmcuk]

James, I think the slow chargers just work on resistance.

Once the battery is close to being charged, the resistance goes up causing the battery to heat up...

Most slow chargers are simply time based some more complex ones are time based with trickle charge after a set charge time to keep the battery topped off. Only fast chargers or very very good slow chargers (as the chance of a slow charger doing permanent damage is small) have any detection. If they do have detection it is voltage based and it is detected on the slowing of the charge voltage curve. Or on the temperature of the battery which due to over charging heats up the battery and causes a reaction, I can't remember what it is though just now but is some sort of gassing. Maybe it could be a resistance effect between the battery and charger causing the heat but the charger simply detects the heat.

 

[Modelworks]

You definitely do not want to overcharge lithium based batteries, even worse would be to have a failure while one of the packs is in contact with water.

 

[jamesmcuk]

You definitely do not want to overcharge lithium based batteries, even worse would be to have a failure while one of the packs is in contact with water.

Yeah that would be a total FAIL lol

 

[Ninjahedge]

James, I am still not sure of that. Almost all of the ones I have had and seen have been 3 level.

1: No timer or shutoff whatsoever. It will keep charging for as long as you leave it in.
2: Timer. Old school RC cars had this in abundence.
3.: Voltage. They have a little LED/LCD gage on the side (or even a simple "done" light) that changes once it gets to "full". This is incredibly cheap and easy to do, so I have no real doubt that most of them have it (if not the gage).

Unless the charger had a literal wind up timer on it, it has never charged in the same amount of time for me (on about a half dozen chargers over the years).