Interesting that a greater increase of resistance in the center will give a greater adjustable range

That's because you tap off a greater fraction of the battery voltage.

I was also thinking how the ignition shutdown works ?

Setting IGN high raises the Q1 base voltage, turns off Q1 and reduces the charging to a trickle. However, when the battery voltage drops during cranking, the Q1 emitter voltage increases relative to the base voltage, Q1 stays on and the charging is at full rate.

Are those two diodes doing a reverse breakdown like a zener?

No. They conduct in the forward direction to raise the Q1 base voltage. The zener still controls the charge termination.

 

I haven't been following this all that closely, so may be wrong on what your trying to do. But to keep three seperate batteries charged, as needed, while at dockside, wouldn't it make more sense to have three window comparators? One for each battery? The charger would be on at all times(at dockside) but the comparators would only charge a battery when it goes below it's 'needs charged voltage'. Each comparator would turn on a mosfet that feeds that battery. All three mosfets would be connected to the charger, but only turn on as needed.

Sorry if what I suggested is not what your trying to do.

Interesting idea. Such a thing could be worthwhile. Have you got a circuit idea to look at? What your saying is a fully charged battery does not really need a trickle charge. As long as the comparator constantly monitors the state of charge volts off the battery and comes on again immediately. I have a lot of IRF 1010E power mosfets which check out good.

I think determining SOC on a big charging battery like these might be difficult to do. Consider that an actively charging battery has a cool down time after you take it off the charger while the voltage output steadily drops to it's fully charged resting state of around 12.6 to 12.7

Couple years ago, I talked to Vic at Raritan since retired who actually made these chargers. He did say they would sometimes run them off simple on and off timers to give the batteries a charging rest. This circuit design was based off a 1958 magazine article in Popular Electronics Magazine. When he heard I was still using it and it was working, He was pleased about how robust it has been. And when I gave him the serial number, he said he may have been the one to build it.