Is this how a TRON 6-station round cell NiMH charger works?

[martys]

Do you really dislike putting up to four NiMH cells that seem to be fully charged into a flashlight or other device and then within a few minutes of use, one of the cells gives out and you're out of business!

What is needed, and not provided by consumer chargers is the charge mAH accepted by the cell.

This is really needed feedback about the cell being charged so you can spot weak cells and and match strong cells  to put in use. This requires a battery/cell analyzer and these are not cheap. So I want to make my own. I now have the case and cell holders and even the power supplies needed from a salvaged TRON 6-cell individual NiMH charger.
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Now I need Ideas and help to make  my custom NiMH Hack-A-TRON Charger

I found a 7-station* AA/AAA/C/D independent cell TRON charger in the electronic dumpster. It seems to be functioning normal, but it is also a cell cooker and I guess this is the reason it was discarded. It charges cells fast, but heats them to 40 to 50 deg C in the process!

(*It also has two 9-V battery connectors for charging 9V batteries, but that is not of any interest here.)

I am trying to figure out how this thing works. I look at the individual charging circuit for each cell  and notice that there is no inductor that would indicate a buck regulator for each cell was PWM'd for controlling the charge current/voltage.

 There are no inductors and no large electrolytic caps used in any of the 6-charging circuits.

I think this TRON smart NiMH charger charges 6 independent cells at once perhaps by this method:

For each charging slot:

Three .1-ohm SMD(1206) resistor are connected in parallel through a power FET  that connects to the common 2.5V high current P/S.

The larger 2nd power FET connects directly to + terminal of the cell being charged trough resistors only.

All the negative ends of all cells inserted are connected directly to a common circuit ground.

The MCU works like this?

The A/D connection is able to measure the voltage drop across the .33-ohm sensing resistors(three 1-ohm in parallel) in order to limit the current to .7-amp Avg(measured) and the peak voltage during charging is always limited to 1.8-V max (measured).

I can only guess End of Charge is determined by somehow measuring DeltaV by comparing cell V when charge current is applied and when charge current is 0.

Does my idea below of how this charger works make sense?


The story so far...


I can spot three 1-ohm resistors connected together in parallel each are 1206 SMD.
This leads me to believe this TRON charger uses only a .33-ohml series resistor to sense and regulate charging current and voltage applied to each cell.

Here's what I know about the charger. It has a singular flyback mains P/S that has several voltage outputs. I see marked on the PCB +23V(LCD), +5V(for precise mcu power, TL431 with pass transistor), high current outputs +2.5V, +2.3V and +10V(for MCU 5V reg).

I can see that about over 19-pins of the mcu are used to drive a primitive LCD display that indicates only the % of charge for each cell.

the mcu is 44-pin chip LFP Holtek BD46R0622 which only runs at 12MHz and has only  eight 12bit A/D pins and only 1 PWM and only 224 bytes of SRAM and 2K of OTP pgm space).

6- I/O pins are used  to light each round cell,  bi-color two-pin Red/Green LEDs, one each to show the status for each of the 6 charging slots and two more I/O for showing status of the two 9V square batteries.

9 I/O pins, one for each charging sub-circuit. I can see  there are for each slot two fairly large active FETs, and at least one of them probably connects to an I/O output of the mcu.

6- A/D pins are used to monitor cell voltage or current.

All charging slot circuits connect directly to the common 2.5V DC output of the single mains switching supply.
 
My plan for this project of mine: I can remove the existing MCU and patch in an infineon PSOC4 mcu dev board. I can replace the LCD display with a LCD or 2 to 4 line LCD ASCII display or a multi-7-seg LED display.

I can take control of this charging circuit to make it charge cells, analyze cells, display their charging mAh on a 7-Seg Display and do all this without cooking them!

Its not as time efficient as just going out and buying an expensive good NiMH charger/battery analyzer, but it makes for me a nice project! (And I am using NiMH round cells in all kinds of things.)