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Introduction
1) Beginner issues
2) Safety
3) Batteries
4) Electric motors
5) Speed controllers (ESC's)
6) Propellers
7) Chargers
8) Model info
9) Electrical formulas
Reference resources
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How do I match cells without spending a fortune on expensive equipment?
Q. How do I match cells without spending a fortune on expensive equipment? A. RC Groups Forums Since few of us can afford a turbo matcher and it really isn't that important for sport flying here is what I do to make semi- matched packs from inexpensive cells. The lower tech way requires only a DVM and your regular charging gear plus a notepad. 1) Check the individual cell voltage with a DVM. If the cell is at Zero volts then it will probably never be any good for any use. 2) Discharge the individual cells to <0.5 volts. I use a 0.5 ohm power resistor for this but you can use around 30 ohms overnight. 3) build a pack so that cells can be easily changed out. I like 7 cells as a max for this as too many cells makes it hard to do some of the testing. 4) Charge the pack at c/10 for 24 hours. The cells should be warm at the end of this period. 5) Discharge the pack into a load. A motor with a light load works pretty well. 5-10 amps is a good enough load. Watch the voltage of each cell. You are looking for cells that have lower voltage than the others. Keep an eye on it and when it starts to slow down figure out which cells died first. Mark those some way. Stop discharging when one cell drops below 0.5 volts. I usually put a little number on the cell to tell me which ones died in which order. 6) Charge the pack at c/10 for 16 hours. repeat 5 and 6 a few times. note any change in cell order. Sometimes after exercising the cells a few times the worst ones become good. Change out any weak cells and start over. After a few iterations you will have a pack that with cells that will dump close to each other. 7) Fast charge the pack at a 3c or even 4c rate. Touch the cells often during the charge to see if any cells get hot during charge. If all cells get hot then reduce the charge rate. If only one or two you might want to swap out those cells and start over. The cells you pull out are probably fine for the most part. You can generally make a pack out of the culls that works fine but has slightly reduced capacity. I like to take these cells and charge them up and let them sit a couple of weeks. If they still have nearly a full charge I make 4 and 5 cell receiver packs out of them and sell the packs to the 1/4 scale gas guys. They love them cause I only charge $15 for such a pack and they last for years. If you have a charger like the 110d or the 112d that will tell you the energy put into a cell you can do this a different way. This is a slightly higher tech way but requires a smarter charger. You can substitute the charge time for the amp hours figure if your charger displays that. Another method: 1) Discharge the cell to zero volts. I use a 0.5 ohm power resistor. a 25 to 30 ohm resistor overnight will do this as well. 2) Charge the cell in the peak detecting charger at a 3c rate. Use the same rate for all cells of a given type. Watch the temp of the cell and if it gets hot during the charge cull that one out. When the cell peaks you need to record the peak voltage and the ah that went into the cell. 3) Repeat 1 and 2 a couple of times. The values you get will become fairly consistent unless the temperature changes a lot. 4) Select cells with similar capacities and peak voltages in that order. The cells with the lowest peak voltage are the better cells. They will have the higher voltage under load. You learn a lot about batteries when you play with them like this. I hope this helps! |
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