electric flight uk - British Electric Flight Association - Jan Bassett's

uy a PC board with assembled FETs, say for a 2S or a 3S or a 4S (or
whatever the limiting series charging is today) and plug/unplug this to/
from the factory-installed battery flying leads. Or, how do I specify and
where do I buy such FETs, to make my own balancer?
Finally, how can any model shop claim to sell a “balancing charger” to plug
into a battery? I cannot visualise any such thing. In spite of you saying there
is no need for a soldering iron I remain certain that connection to each cell is
essential and if not factory supplied we must do it ourselves. Yes, if my Method
2 works you would be able just to connect up a FET-fitted battery to a suitable
charger - but such a charger would simply have a limiting capability not any
balancing capability and should not be so described.
But again, if I’ve got it wrong I would be delighted to be told.
OK, a lot to cover, so here goes.
In my last response I did simplify things in an attempt to make it easier to
understand the principle. I also erred slightly, what I should have said is that
each balancer circuit contains a FET. Peter is correct that FETs have 3 leads
as identified. However, FETs are available in a number of different varieties
with very different characteristics. The majority of FETs available do not
conduct when the Gate voltage = Drain voltage, which means they are “off”
when power is initially applied.
Over the past few years the “on” resistance has reduced markedly, with surface
mount FETs available as low as 6mÙ for a single FET. This ultra low resistance
means very low loses and that these small packages can handle higher currents
than their predecessors. This is one of the main reasons for the reduction in
cost and size of speed controllers as less FETs are required. For brushless
controllers this is a major advantage when 6 banks of FETs are required.
Peter’s Method 1 could be used, but if that was to chosen method it would be
better incorporated in the charger. Provided the voltage across each cell can
be maintained it would work. I consider that this method is unnecessarily
complicated and would not be cost effective.
Method 2 is along the lines used by commercial balancers, although you can
do it as simply as this. I’ll describe how a typical LiPo balancer circuit works
in the following paragraphs and hopefully everything will become clear.
A LiPo balancer typically contains a precision voltage reference, a voltage
comparator, a number of discharge resistors, a pulse-width modulator (PWM),
and a FET. The voltage reference is used by the comparator to produce an
error signal. This error signal controls the PWM, which turns the FET on
and off. As the voltage rises above 4.21v, the PWM starts with short pulses,
which increase in width until, the FET is always on at 4.25v.
Typically they also have a LED on the board, which indicates what the balancer
E.F.-U.K. 21