cunakya
Heliman
Location: Sunny Southern California
| When i got my piccolo, I thought that the 5 minute flights it gave me were rather pathetic, so i bought a 2 pack LiPo pack. I love em. It took longer to discharge the battery on the heli than the transmitor.
My dad is a material scientist and one of his partners is studying lithium battteries. When my dad heard i was using LiPo batteries and charging them is series, he got rather worried.
You see, when a LiPo battery is at a voltage above 4.2 volts, it will destroy the battery and, if not caught early, catch on fire. The problem with charging them in series is that the charger can only measure the sum of the voltages. Therefore if one battery pack is at 4.0 volts and the other is at 4.2, it can only measure the sum of the batteries, 8.2 in this example. When the charger measures 8.2, it will continue charging because 8.4 (the voltage of a fully charged 2 cell LiPo) has not yet been reached. Then if one of the cells reached 4.3 volts, it will catch on fire. Neither my dad or his partner could predict exactly what would happen so we emailed an engineer of a battery company.
What he basically said is this:
While charging LiPo batteries in series is a very popular way charge milti cell bateries, it is very dangerous. It is safe for the first couple charges, but because of the manufacturing process, each battery has a slightly different discharge rate, therefore one battery will discharge slightly faster then the other one, and so the diference of the voltages of the two batteries will increase whenever they are used. To solve this problem, you must frequently test the voltages of the individual cells, making sure they are not any more than 30mV apart from each other (.03volts.) If they are, discharge the batery of the higher voltage with a 10k ohm (10000 ohm) resistor until they are the EXACT same voltage.
i have only charged my pack once, so i'm not too woried, but some of you out there might not know this stuff, so do what the chief engineer of a battery manufacturing company tells you to.
(ps. this was a long message (for me anyways) so i didn't bother to check it. Don't complain about spelling or grammar.) |
