Dumb***** question of the day - what is balancing cells and why is important to the Lithium Polymer type cells? Do the A123 cells need it as well? I'm a long time glow flier that knows very little of electric flight, please be gentle...

Mark

The following is the general idea. Of course there are tolerances on all the numbers.

1) Lithium cells cannot be charged above 4.2 volts or they will be destroyed. That means no trickle charge after they reach 4.2 volts per cell.
2) If you charge two or more cells in series, you charge the pack to 8.4, 12.6, etc.
3) If one cell has more charge than another it will get to 4.2 before the other and continue to charge beyond 4.2 until the pack gets to 8.4,12.6, etc., and then it might be destroyed if the cells were unbalanced enough.
4) A balancer assures that all cells are at nearly the same voltage and state of charge so they all get to 4.2 at the same time when being charged.

The Thunder Power charger/balancer system will shut down if any cell gets too high. An important safety feature.

Jim O

Jim,

Thanks for the help, I figured that was the case (matching voltages, that is) but I wanted to be sure. Another related question - how does a balancer manage to charge each cell individually in a series connected pack?

Thanks,
Mark

It doesn't.....What it does is monitor the voltages of each cell and when one gets a certain percent higher than the others it shorts that cell....Effectively draining off the excess. different balancers have different ratings. I believe the Astro Blinky shunts off up to 250ma where the Thunder Balancer can shunt twice that much. so if you have a 4 cell pack and one cell reaches the optimum charge, the balancer shunts off 250ma of the charge from that cell. when a pack gets close to full charge the charge rate drops off, substantially. So at the end the pack is charging at, say 500ma but the balancer will be shunting off the excess to the higher charged cells. The 500ma is not all going into the pack....That's why it take so long at the end of the charge cycle....Pat

the other option would be a balancing charger. In effect a balancing charger is many small chargers in one case. In the case of a balance charger you would charge through the balancing tap. The balancing tap has a positive lead from each cell and a common negative lead. Using this method, each section of the balance charger charges a single cell. since Lipo chargers automatically trickle down as the cells charge, all cells will charge to the same value, before the charger signals the pack is fully charged.....Pat

Mark, there are three informative articles here:
http://www.rctoys.com/pr/category/rc...balancer-info/
Lithium Polymer Battery Balancer Information and Benefits
A Guide to Charging Lithium Polymer Batteries
Thunder Power RC 205v & 210v LiPoly Balancer Tips & Tricks

For me balancing is a bit more. I measure voltages of every cell after flight to verify if any has depleted lower. Voltages rise fast but sometimes I can see difference and I charge that cell more mAh running extra 1/20 C and usual 1/10C for the rest.

I have never used a balancer but I think it is wrong to charge thru balancer leads because first and last cell balancer circuit starts to limit their charging when constant current phase reaches constant voltage 4,2V because of voltage drop caused by usually rather thin wire and lousy connectors.