The FT817/818 charge circuit would not really cause issues if "overcharging". The radio is built for NiCD / NiMH cells and the charging regime is 14 hours times 0.1C, so 14 hours times 10% of the capacity of the cells. The "fourteen times 10%" is to make up for losses in the charge / discharge process; "ten times 10%" would charge the cells but the process has losses. Hence 14 hours.
However, NiCD cells no longer exist (environment!), and NiMH cells have grown in technology. I think the original packs had a capacity of roughly 700 mAH, so a charge current of 70 mA. Today's cells are often closer to 3000 mAH, and hence a 70mA charge current won't charge the cell in 14 hours, you need much more than that.
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With a low charge current like this, "overcharging" won't affect the cells unless you do it continuously. The cells will dissipate the energy they can't store. They get warm, but not hot. The amount of energy is not very high.
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The charge circuit is just a current source. If you look at the schematic, it is Q1103 with R1435 defining the charge current and Q1101 enabling the circuit.
There is a second charge circuit, Q1105 and R1436 with Q1102 enabling. This is a trickle-charge as NiCD/NiMH are known to self-discharge.
The "green wire" disables both charging circuits (charge and trickle) which is just as well because trying to charge alkalines is likely to cause them to leak.
I have changed R1435 to match my cells; you will need to revise yourself if you want to do this, and please mind the dissipation and max Ic for Q1101 and Q1103; I have not blown my finals but I *have* blown the charging circuitry at some time.
To add, the FT817/818 doesn't charge 14 hours, but selectable 6-8-10 hours. The result is that even a 10-hour cycle won't get close to charging them modern high-capacity NiMH cells and two cycles won't hurt the cells. We're not talking "ultra-speed 30 minute fill charge", the battery chemistry is 30 years older.
