Battery Charging
I will discuss charging Nicasds and Metal Hydrides
Both batteries have nominal voltages of 1.2V but when fully charged(peaked), the V goes to 1.4V. Thus a flight pack of 4 cells should have a voltage of 5.6V when freshly charged. the voltage drops suddenly when the V reaches 1.1V. Thus 4.4V is a critical level.
How to charge them
In technospeak, C is charging rate. Thus a charger is known to have percentC charging rate. A .10C means the charging rate is 10% of capacity(less the hr). The safest charging rate is .10 C. for example, if the pack is a 1000mah pack, the charging rate at 10% is 100ma. For a fully discharge pack, the time to restore the battery to full charge is 15 hours. It means 50% more is inserted into the battery to charge it.(15 hoursx100ma is 1500 mah). .10 is also called C10.
The enemy of the battery is high temperature. As the C rate goes up, heat build up goes up. And it is really bad to leave a battery under charge beyond the time needed at high C rates. The safest is the .10C charger.
Most of the radio sold in the market with batteries come with a 500 mah packl. And the call charger is usually a .10C charger. (You might say charger for dummies who will surely forget to unplus the charger after 15 hours.) . One other good reason why a .10C charger is used is because the pack is not really depleted before it is recharged. So, the 10C charger is safe.
Modern chargers with peak detection allows us to plug in the charger and batt anytime without worry. It dos not matter how much charge was there at the start.
We can make a very simple rugged charger like the ones used on RC cars charging a fully discharged pack. Usually, there was a 15 mins timer in those charger. Tose chargers were as high a 3C chargers.
15 mins RC chargers are no longer in voque but once in while, there is a need to make a simple rugged charger to charge metal hydrides used in model planes. Normally 7-8 cells are used. The peaked voltage os 8 cells is 8 x 1.4 or 11.2V. We can charge them in series from the car battery.
A simple rule of thumb is to charge it at 50% more opf capacity. If we use a charger that is 1C, we need a timer for 1.5 hours. or if we charge it at 1.5C, we charge it for 1 hour. The formula is rate x time = 1.5 x capacity.
For example, for a 1000 mah pack, we can charge at 1.5 amps (1500 ma) for one hour. If we want it for 30 mins, then we double the rate to 3 amps.
A good good guideline is to decide on how many packs one can afford in the field and use the longest time to charge acceptable. for example, using 4 packs, each pack lasting 10 minutes, with a rest in between. When the first pack is used, the nest 3 packs is good for 30 minutes plus rest time of 30 minutes. Then a 1 hour charger is good.
Can we charge more than 8 cells from the 12V car battery. Well, yes. Say for a 10 cell pack, we break them up into 5 cell packs. We charge them as parallel packs. After they are charged, they can be plugged together in series. Only one timer has to be used and there is no need for two chargers really.
Current is the critical parameter. The charger voltage has to be higher than the peak V of the pack. Current is calibrated using resistors in series with the right wattage.
BK
