Saturday, 9 January 2016

Charging cells using Flyback PSU type circuits

experiments during the last year have focussed on pulse motor operation adapting brushless fan motors, and also battery charging using flyback switchmode power supply type circuits (Boost Converters)

an interesting development has occurred with one particular variant of these test circits:

the circuit was being used to charge a battery of 2 NiMH AAA cells using a similar battery as input; the initial offload voltages of the i/p & o/p pairs were 2.62V & 2.63V (ie. both batteries approximately 50% charged)

after the first test, the end voltages were 2.3V (i/p) & 2.9V (o/p), as might be expected, input nearly fully discharged, output fully charged; the batteries were then swapped and the test repeated

observing the in-circuit terminal voltages as the test progressed the run was interrupted at an appropriate point to determine the 'crossover' voltage where the two batteries become equally charged (after a rest offline)

this voltage was found to be approximately 2.65V (within a few millivolts) - higher than both the original offline battery voltages

it appears that this circuit/battery combination is able to charge its output batteries slightly faster than it discharges its input batteries

more tests are underway to investigate this behaviour - the next test certainly appears to confirm that first observation:

this test circuit is similar to a small flashlight - it has a single white LED which is bright enough to cast a clear shadow on a white surface approx 7' away (>2 metres) in a darkened room; the circuit is powered by a single AAA NiMH (750mAh) and it is also charging a second similar cell on its output







the in-circuit voltage of the input cell (B1) dropped by 3mV (holding around approx 1.3V), while the output cell (B2) has increased by 30mV (from approx 1.29V)

i'll upload some of the graphical output from the datalogging PC, showing the voltage traces, as soon as i can prepare images & co-ordinate file transfers between my Windows & Linux systems



Update: the graph here shows the in-circuit terminal voltages logged for the input & output batteries, B1 & B2.  It can be seen that B2 charges whilst B1 discharges, as the LED is illuminated. After the flashlight has been operated for a while, switch S1 can be used to swap the 2 batteries between i/p & o/p for the next time that the flashlight is used.  In this way, the operation of the flashlight can be extended from the original charge of the NiMHs.  My application here has been for a small flashlight, using 1 run battery, 1 charging battery and a single LED lamp;  the same principle could be extended to use more cells per battery and a larger number of LEDs. VR1 can be used to alter the intensity of the light output (which will alter the discharge/charge rates accordingly) Switch S1 has a central 'Off' position, in addition to the the 2 'On' positions which select the current input battery


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