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Not wanting to write an article on this (I have far too much to do and too little time
already), I decided to respond to Peter directly and include the text in the ‘Letters
to the Editor’ feature.
I appreciate your concerns, but they are unfounded when using a balancing
charger or external balancer. There is no need to perform any soldering or
change pack connections to achieve balanced charging. Hopefully the following
will explain it properly.
Firstly, the limiting voltage for a 3S pack should be 12.6V (4.2V/cell) and
therefore 13V should never be seen. If a LiPo charger is at 13V when set for 3S
there is a serious safety fault with it and it should not be used.
I have modified your spreadsheet to show the action of the balancer circuits.
16
Charging at 5A with balancers connected
Each balancer is a variable resistance provided by a FET, which dissipates the
unwanted current as heat. Typically they start to operate at 4.21V and limit
individual cell groups to 4.25V. This is so that they don't interfere with the
function of the charger. As the pack nears fully charged, the pack voltage
rises to 12.6V and the charger will limit it to that.
If one group on a 3S pack is extremely low it could be at 4.1V, with the others
at 4.25V. As the voltage on this group increases above 4.1V, the voltages
across the other packs will drop, maintaining the 12.6V overall.
You will notice on your diagram that cells 4 and 5 are exceeding the 1C charge
current of 2.5A. I produced another chart at 4.5A (0.9C), but cell 5 still exceeds
the 1C cell rate at a pack rate of 0.8C. Reducing the current further to 4A
(0.8C) drops the current to an acceptable level. For this last diagram I had to
E.F.-U.K.