Lipo Balancing issues

[rocky79]

Hello,
I am experimenting with a cell balance. Conceptually, when you are charging lipo's from a charger and attaching the lithium balancer to the cells. if one cell exceeds the 4.2 v then current will be dumped in a power resistor. But since the internal resistance of a Tp pack is in the order of 75 mohms. And current will flow through the least resistive path. in that case current will only flow to the battery. Correct ?

Also how to size the resistor. and how would you determine the discharge time for a cell so it comes back to 4.2 v.?

when you apply a load to the cell you discharge it but you have to unload the cell wait some time measure the voltage and then see if it needs some more balance.how long does it take for the battery to recover from a load. Am i making sense or missing something. Help me understand.


Thanks

[AndyOne]

Rocky,

The current doesn't just flow through the lowest resistance. Any two resistances in parallel will share current in inverse proportion to the resistances. Also current only flows in the internal resistance of a cell when there is an external load (discharge condition) , the internal resistance is a resistance in series with the electochemistry not in parallel with it.

The bleed resistor in a balance circuit needs to be sized to allow enough of the current to be bypassed around the cell. If you are using hard balancing i.e. capable of more current than your charger can put out then all you need to do is to divide 4.2V by the charger maximum current to get the resistor in ohms. R=V/I simple Ohms law. Soft balancing will usually be enough to correct most packs during normal useage, a bleed current of c/10 is usually all that is required.

Most balance circuits you will find (read this thread...http://www.rcgroups.com/forums/showthread.php?t=157641)
have a voltage sensitive control which will turn off the current below 4.2V. They will in effect apply a duty cycle from zero to 100% over a very small range of voltage around 4.2V. the device controls itself so there is no need to measure the voltage once it is correctly adjusted.

I hope this helps.

Andy.

[rocky79]

Thanks Andy. HAve you built one yourself ?.

Rocky,

The current doesn't just flow through the lowest resistance. Any two resistances in parallel will share current in inverse proportion to the resistances. Also current only flows in the internal resistance of a cell when there is an external load (discharge condition) , the internal resistance is a resistance in series with the electochemistry not in parallel with it.

The bleed resistor in a balance circuit needs to be sized to allow enough of the current to be bypassed around the cell. If you are using hard balancing i.e. capable of more current than your charger can put out then all you need to do is to divide 4.2V by the charger maximum current to get the resistor in ohms. R=V/I simple Ohms law. Soft balancing will usually be enough to correct most packs during normal useage, a bleed current of c/10 is usually all that is required.

Most balance circuits you will find (read this thread...http://www.rcgroups.com/forums/showthread.php?t=157641)
have a voltage sensitive control which will turn off the current below 4.2V. They will in effect apply a duty cycle from zero to 100% over a very small range of voltage around 4.2V. the device controls itself so there is no need to measure the voltage once it is correctly adjusted.

I hope this helps.

Andy.

[AndyOne]

Rocky,

This may sound really strange but I do this kind of thing all the time at work but I've never actually bothered with my hobby stuff. I guess it's a bit of a "busman's holiday". None of my li-po packs have cell output connectors but I do probe them with a meter from time to time to check on their balance. My cells tend to stay in balance most of the time without help.

Andy.

[rocky79]

What kind of lipos do you use? I bought the TP Prolite 2100mah. Maybe i should do the same thing just check them once in a while.

[AndyOne]

Rocky,

I use Kokams exclusively up to now but I'm just about to branch out into some of the cheaper types.

Andy.