Subj : Battery charger.
To   : JEAN PARROT
From : TOM WALKER
Date : Thu Jun 16 2005 07:56 pm

Jean;
Here is some good general Information on Charging NiCAD and NiMH Batteries
The differences are small but Important.

Tom

**NiCAD Battery Charging**
*Basics
The coulometric charging efficiency of nickel cadmium is about 83% for a
fast (C/1 to C/.24) charge, and 63% for a C/5 charge. This means that at
C/1 you must put in 120 amp hours in for every 100 amp hours you get out.
The slower you charge the worse this gets. At C/10 it is 55%, at C/20 it
can get less than 50%. (These numbers are just to give you an idea, battery
manufacturers differ).
When the charge is complete oxygen starts being generated at the nickel
electrode. This oxygen diffuses through the separator and reacts with the
cadmium electrode to form cadmium hydroxide. This causes a lowering of the
cell voltage which can be used to detect the end of charge. This so-called
minus delta V/ delta t bump that is indicative of end-of-charge is much
less pronounced in NiMH than NiCad, and it is very temperature dependent. 
As the battery reaches end-of-charge oxygen starts to form at the
electrodes, and be recombined at the catalyst. This new chemical reaction
creates heat, which can be easily measured with a thermistor.. This is the
safest way to detect end-of-charge during a fast charge.
Nickel cadmium battery chargers should cut the charge off when the
termperture exceeds the maximum charging temperature, typically 45 degrees
C for a controlled fast charge, and 50 degrees C for an overnight or fast
charge.

*Overnight Charging
The cheapest way to charge a nickel cadmium battery is to charge at C/10
(10% of the rated capacity per hour) for 16 hours.. So a 100 mAH battery
would be charged at 10 mA for 16 hours. This method does not require an
end-of-charge sensor and ensures a full charge. Cells can be charged at
this rate no matter what the initial state of charge is. The minimum
voltage you need to get a full charge varies with temperature--at least
1.41 volts per cell at 20 degrees C. The best charging practice is to use a
timer to prevent overcharging to continue past 16 hours. 

*Trickle Charging
In a standby mode you might want to keep a nickel cadmium battery topped up
without damaging the battery. This can be done safely at a current of
between  C/5  and  C/6. The voltage required for this is dependent on
temperature, so be sure to regulate the current in the charger.

**NiMH Battery Charging**  

*Basics
The coulometric charging efficiency of nickel metal hydride batteries is
typically 66%, meaning that you must put 150 amp hours into the battery for
every 100 amp hours you get out. The faster you charge the worse this gets. 
The minus delta V bump that is indicative of end-of-charge is much less
pronounced in NiMH than NiCad, and it is very temperature dependent. To
make matters worse, new NiMH batteries can exhibit bumps in the curve early
in the cycle, particularly when cold. Also, NiMH are sensitive to damage on
overcharge when the charge rate is over C/10. Since the delta V bump is not
always easy to see, slight overcharge is probable. For this reason
PowerStream does not recommend using minus delta V as a termination
criterion for nickel metal hydride batteries.

*Overnight Charging
The cheapest way to charge a nickel metal hydride battery is to charge at
C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery
would be charged at 10 mA for 15 hours. This method does not require an
end-of-charge sensor and ensures a full charge. Modern cells have an oxygen
recycling catalyst which prevents damage to the battery on overcharge, but
this recycling cannot keep up if the charge rate is over C/10. The minimum
voltage you need to get a full charge varies with temperature--at least
1.41 volts per cell at 20 degrees C. Even though continued charging at C/10
does not cause venting, it does warm the battery slightly. To preserve
battery life the best practice is to use a timer to prevent overcharging to
continue past 13 to 15 hours.

*Trickle Charging
In a standby mode you might want to keep a nickel metal hydride battery
topped up without damaging the battery. This can be done safely at a
current of between C/3 and C/5. The voltage required for this is dependent
on temperature, so be sure to regulate the current in the charger.

**Automatic Chargers**  A Quick Summary
dT/dt versus -dV/dt

These two termination methods work well for NiCads, and are both applied to
NiMH as well. dT/dt measures the temperature rise at the end of charge.
After the battery is fully charged it starts new chemical reactions in
order to absorb the unneeded current. In nickel hydroxide style batteries
this consists in generating and recombining oxygen. This process heats the
battery. The sudden increase in temperature rise can be used to terminate
the charge.
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