The modern LiPo (lithium polymer) rechargeable storage battery
offers advantages of low weight, high capacity and large available output current,
when compared with its predecessors (Lead Acid, SLA, Ni-Cad etc)
However, particular care must be exercised when re-charging a depleted "battery" of LiPo cells.
Charging a series string of LiPo cells by "conventional" constant voltage (or constant current)
may mean that one or more individual cells reaches the critical
4.2V 'knee-point' before others, which could then result in it being overcharged.
Overcharged cells can be permanently damaged and may even rupture and catch fire!
According to manufacturers specs' Lithium Polymer cells require a constant current charge until
terminal voltage reaches 4.2 volt, and then switch over to a constant voltage charge.
Charging should be shut-off when charge current drops to 10%
of initial constant current value, and also, final float voltage being acheived (4.2V)
See graph:
Additionally; if the cell voltage is below 3.0 volt a "pre-charge" of 10% max is applied until
the voltage rises above the threshold whence full constant current charge is applied (shown on graph).
LiPo cells can be safely charged at rates like 0.5C (0.5 AHC)i.e. a 2AH battery can accept 1A.
This means a rapid recharge compared to say NiCad's which were 1/10 AHC for 14 hrs!
Later improved "sintered" NiCad and NiMH cells could safely accept up to 1/3 AHC charge.
Fortunately there is a cheap TC4056A chip which will handle all charging voltages/currents and changover points
The battery charger design as shown below, uses 4 off TC4056 boards each powered by an independent DC supply.
The DC inputs to each TC4056 board must be galvanically isolated, for the design to work.
To acheive this; I had to re-purpose a mains transformer.
I did this by removing the secondary of a 240/110v stepdown transformer,
(salvaged from an analogue Color TV) and rewinding four new 6 volt secondaries.
The four, low voltage AC secondaries are bridge rectified using 1N5817 schottky diodes,
which are rated at 1Amp PIV 20V and low fwd voltage drop 0.42V approx
Single electrolytic filter cap' gives about 8 volt DC to each TP4056 board.
This allows each cell within the battery to be charged correctly in compliance with the makers spec'
You might think that a simple parallel connection of the DC input supply
of each of the four TP4056 boards would work?
but it doesn't because of shunt paths across the cells which would then become short-circuited!
Most (but not all?) LiPo batteries have facility for "balanced charging" which means
an accessible connection is provided for each cell interconnection, see picture
I am intending to use a 4 cell (4S) 14.8 volt LiPo battery to power a WW2 "command receiver"
which needs 2-3 amp at 12~14 volt DC to function. "click to enlarge"
References:
New TP4056 Lithium Cell Charger Module with Battery Protection
Charging a Lithium 18650 Cell using the TP4056
10 worst mistakes for a LiPo battery
TP4056 data sheet
© 4Q 2019