Is the 4.25V figure in the OP the voltage of a cell after it has been charged or is it the voltage of the charger taken while the charger is empty while plugged and measured the the + and - ends?
The measured values around 4.25V is the end voltages during charging. I have a piece of alu foil in each end of the battery and a Fluke 179 voltmeter with MINMAX function enabled connected to the foils. It stores the max voltage on the display.
The main IC is a 14 pin unknown circuit (there are no markings on it). There is a separate 6 pin booster to make 5V marked AL579 but I have not yet found it by Googling. There are 3 MosFets marked 2300, a 3 pin Voltage reference TL431, marked 431, and a lot of other components. Interresting is that the charging current comes from an extra switcher, not directly from the 5V input.
I don't have that information, that is up to the type and age of battery, especially the inner resistance. I use different batteries, old and newer. It is not the battery, but the charger I am evaluating.
4.25V for charging is quite normal, so other than standardizing end voltage and increase battery endurance per OP wish, other members with regular charging use might not need to do this procedure.
Just my viewpoint, this mod slows down charging a bit and leave you with less charge capacity (and of course in doing so increase battery lifespan).
You can also use this fix for the 4.35V position if you only have 4.30V cells. Remember that all voltages are changed by the same percentage.
It seems to be a good charger. I just miss lower charge currents. 0.5A is not really good for many batteries. I can only think of 18500, but for 14500 and smaller, I want 0,25A or lower.
Have you checked what those changes do to the NiMN charging behavior?
With an unmodified one, AAA NiMH cells get warm on the 0.5A setting.
I have no clue what method this charger is using charge NiMH cells though.
I think it is quite normal that AAA NiMH get warm at 0.5A.
I just made myself a mod with 50/100 mA charge current for charging some incoming 10180. I have no intention of using that unit for charging NiMH. Let's see what HKJ finds out about the NiMH charging method and we can act accordingly.
Yes, that will also do it, but it takes two solder irons and a lot of skill to unsolder R6 and get it fast away from the tip of either iron in healthy state (and find it afterwards ). But at least the board has a better support by this procedure.
I tried to charge a half full Eneloop XX, size AAA with 200mA in a unit modded for 200/400 mA. Happy to see that the charging stopped. Max voltage was 1.559V and cell voltage right after was 1.45V. The cell did not get warm.