My neighbor's golf cart is showing signs of chronic undercharging of its flooded lead-acid deep-cycle batteries (uneven voltages between groups of cells, low capacity). It has 18 cells; nominally 36V. Under charge using the Lester 09611 charger, the batteries barely reach 38.0V (2.12V/cell), which is not enough to top them up. The end-of-charge voltage should be at least 45V (2.5V/cell). The charging current tapers to zero prematurely (too soon)

The Lester charger is dirt simple. Here is the schematic. Big transformer, 115Vac primary, center-tapped secondary, two rectifiers with common cathodes which make the +; - taken from the c.t. i.e. full-wave rectifier. The tranny has an extra winding which is paralleled with an 6uF 660Vac AC capacitor, with no other connections. AFIK, it is some sort of crude ferro-resonant regulator.

I have checked continuity, and everything appears good. Diodes good, fuse good, transformer windings all show continuity, no shorts between windings. The only thing that looks suspicious is that the resonating capacitor measures 2.94uF instead of the marked 6uF.

Is this the reason why it is not charging the batteries? Not resonating properly? How does this regulate?

 

Can't explain how it works, but experience tells me that's the problem.

Common problem with chargers and also generators.

I'll be watching for the explanation, myself.

ps.
That reminds me.
Was going to post but thought better of it.
I test most things for shorts with a variac.
A similar charger that I had on the bench seemed to be shorted.
Turns out that it tries so hard to regulate at lower voltages it looks like a short when slowly bringing up to voltage.

 

My neighbor's golf cart is showing signs of chronic undercharging of its flooded lead-acid deep-cycle batteries (uneven voltages between groups of cells, low capacity). It has 18 cells; nominally 36V. Under charge using the Lester 09611 charger, the batteries barely reach 38.0V (2.12V/cell), which is not enough to top them up. The end-of-charge voltage should be at least 45V (2.5V/cell). The charging current tapers to zero prematurely (too soon)

The Lester charger is dirt simple. Here is the schematic. Big transformer, 115Vac primary, center-tapped secondary, two rectifiers with common cathodes which make the +; - taken from the c.t. i.e. full-wave rectifier. The tranny has an extra winding which is paralleled with an 6uF 660Vac AC capacitor, with no other connections. AFIK, it is some sort of crude ferro-resonant regulator.

I have checked continuity, and everything appears good. Diodes good, fuse good, transformer windings all show continuity, no shorts between windings. The only thing that looks suspicious is that the resonating capacitor measures 2.94uF instead of the marked 6uF.

Is this the reason why it is not charging the batteries? Not resonating properly? How does this regulate?

I would say it is likely the cap (or a bad cell in the battery)- that cap and tranny tap (inductor) resonate and allow the voltage to climb as current draw drops. It is kind of odd and it works - really old school with no active parts.

 

I would say it is likely the cap (or a bad cell in the battery)...

Not a bad cell as indicated by the ammeter on the charger, and by measuring battery voltages. The charging current tapers to zero before the batteries are charged. If there were an open cell, the battery terminal voltage would be high (it isn't). If there were a shorted cell, the charger current wouldn't taper. The charger tapers without getting the battery voltage high enough.

The link above didn't resolve: Here are the specs:

 

I knew nothing of ferroresonant chargers so I had a look around.

A rough explanation of how it works is here:

http://www.catalina-capri-25s.org/tech/Batterychg.pdf

I guess if the cap loses it's value then the resonant circuit will be off tune and won't be driving the transformer to saturation, hence the output voltage will drop.

 

Replace the weak capacitor.

On a ferroresonant type charger that capacitor sort of works as your peak voltage adjustment.

Smaller value lower peak voltage. Larger Value higher peak voltage (up to a point.)