Download - British Electric Flight Association

Method 2
Three FET devices are connected in parallel with 3 LiPo cells as below. Note
the 3 FET leads, SOURCE, DRAIN and GATE.
The Charger positive is connected to Lead 1, negative to Lead 2 and set up to
charge 3 LiPo cells at 1C.
The Red part of the FET is totally resistive at all times and at the start so is
the Black “conductor channel” which can conduct under certain conditions -
see later - but which is a total resistor at the start.
So current (the same current) passes right through the cells and charges them
up. However, as a cell charges up its -ve voltage becomes greater of course and
something called a “field effect” puts a -ve charge on the Gate.
Now this Increases the “negativeness” of the Gate which effectively “opens up”
the “conductor channel” and when the –ve reaches its “switching” limit (4.2volts
for LiPo cells) the conductor channel actually becomes effectively “open” and
hence a total conductor of no resistance. So the current by-passes the fully
charged cell and so the cell does not overcharge.
Because there is now less resistance in circuit the charge current drops but
the remaining two cells continue to charge up.
The next highest charged cell will then “shutdown” as it fully charges, followed
by the third one.
However, what this means is that for Method 1 the cell manufacturers would
need to supply flying leads. For a 2S there would be 1 positive and 2 negatives
(one to each cell) or for a 3S 1 plus 3 and so on. I can find no evidence of
Kokam or any manufacturer doing this except PolyCell - ignoring simple 2S
systems with centre pole connection. But I am sure you know better so please
let me know, I am sick of soldering and unsoldering LiPo cells, I have wrecked
2 so far and I am supposed to know what I am doing! As to packs with
balancing FETs fitted, sounds good but for me only if connections are visible
and can be reached with meter or clip contacts. I would prefer to be able to
20
E.F.-U.K.