SilverFoxCPF
Posts: 58
Joined: 1/5/2008
From: Bellingham, WA, USA
Status: offline
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Hello Chris,
Battery basics...
At the time a battery is manufactured, there are a series of tests done to determine the capacity of the battery. This involve following standards set for the battery industry.
A standard charge involves taking the battery capacity, dividing it by 10, and charging at that rate for 16 hours. This is referred to as a 0.1C charge.
A standard discharge involves discharging the battery over 5 hours. This is referred to as a 0.2C discharge.
This is how battery capacity is determined.
This testing is done by sample testing. Not every individual battery is tested. Sometimes the manufacturer will sell batteries to someone that re-labeles the battery with their own brand name. The re-labelers look at the manufacturers data sheets, then it seems the "make up" some capacity numbers that tend to inflate the capacity rating. This may be why some batteries are unable to live up to their capacity ratings.
If a battery is loaded with a 660 milliamp load, and continues to provide that load for 5 hours, we then can do some math and come up with the capacity of that battery.
We have 660 mA X 5 hours = 3300 milliAmphours.
In an ideal world, this same cell would be able to power a load of 3300 milliAmps for 1 hour, but there are some losses, so it actually comes in a little less than 1 hour.
If we refer to the capacity of the cell as "C," then we can play with multiples of C when talking about charging and discharging.
If you have a 3300 mAh cell, and your load is 10 amps, you are loading the cell at 10 divided by 3.3 = 3.03C.
If you charge the same cell at 6 amps, you are charging at 6 amps divided by 3.3 mAh = 1.82C.
Now things get a little fuzzy...
A higher capacity cell may offer more capacity, but it may not be able to deliver that capacity under high loads. This is where testing comes in.
When you see graphs showing a discharge curve, it has a lot of information in it. A 3300 mAh cell may hold a voltage of 1.1 volts under a 10C load. In comparison, a 4400 mAh cell may only be capable of holding 1.0 volts under a 33 amp load.
In this case the 4400 mAh cell will give you longer runtime, at the expense of punch and top speed.
The challenge is to hold voltage under load and still get decent runtime.
Charging your cells and packs has a lot to do with your charger. NiMh cells, when charged with a charger that uses peak detection to terminate the charge, need to be charge in the 0.5 - 1.0C range. For a 3300 mAh cell, that works out to charging at 1.65 - 3.3 amps. To get better performance, most people charge at 1C. Some cells can be charged at higher rates, but it is hard on the cells. If used "hot off the charger," faster charged cells can give you a little more "punch" right off the line.
NiMh cells do not tolerate overcharging. If you overcharge your cells, you kill them. The same goes for heat. If your cells get too hot, you kill them. NiCd chemistry is more tolerant of heat and overcharging, but eventually those cells will also die from abuse.
There are situations when you can't charge at 1C. If your transmitter uses a battery carrier, it may not be able to handle the higher currents of 1C charging. In this case, you charge at 0.1C for 14 - 16 hours. The cells may receive a little bit of overcharge, but at this low charge rate, the damage from the overcharge is minimal.
Sanyo, and other battery manufacturers, have a pretty good technical handbook on their web site. It may be worth your while taking a look at that while you are trying to figure all of this out.
Tom
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