Frank,
In answer to your relentless search for truth on Ferro's and for you to better understand the beast, here is an excerpt of what one actually is
While being somewhat difficult to describe without going deep into electromagnetic theory, the ferroresonant transformer is a power transformer engineered to operate in a condition of persistent core saturation. That is, its iron core is "stuffed full" of magnetic lines of flux for a large portion of the AC cycle so that variations in supply voltage (primary winding current) have little effect on the core's magnetic flux density, which means the secondary winding outputs a nearly constant voltage despite significant variations in supply (primary winding) voltage. Normally, core saturation in a transformer results in distortion of the sinewave shape, and the ferroresonant transformer is no exception. To combat this side effect, ferroresonant transformers have an auxiliary secondary winding paralleled with one or more capacitors, forming a resonant circuit tuned to the power supply frequency. This "tank circuit" serves as a filter to reject harmonics created by the core saturation, and provides the added benefit of storing energy in the form of AC oscillations, which is available for sustaining output winding voltage for brief periods of input voltage loss (milliseconds' worth of time, but certainly better than nothing)
In addition to blocking harmonics created by the saturated core, this resonant circuit also "filters out" harmonic frequencies generated by nonlinear (switching) loads in the secondary winding circuit and any harmonics present in the source voltage, providing "clean" power to the load.
Ferroresonant transformers offer several features useful in AC power conditioning: constant output voltage given substantial variations in input voltage, harmonic filtering between the power source and the load, and the ability to "ride through" brief losses in power by keeping a reserve of energy in its resonant tank circuit. These transformers are also highly tolerant of excessive loading and transient (momentary) voltage surges. They are so tolerant, in fact, that some may be briefly paralleled with unsynchronized AC power sources, allowing a load to be switched from one source of power to another in a "make-before-break" fashion with no interruption of power on the secondary side!
Unfortunately, these devices have equally noteworthy disadvantages: they waste a lot of energy (due to hysteresis losses in the saturated core), generating significant heat in the process, and are intolerant of frequency variations, which means they don't work very well when powered by small engine-driven generators having poor speed regulation. Voltages produced in the resonant winding/capacitor circuit tend to be very high, necessitating expensive capacitors and presenting the service technician with very dangerous working voltages. Some applications, though, may prioritize the ferroresonant transformer's advantages over its disadvantages. Semiconductor circuits exist to "condition" AC power as an alternative to ferroresonant devices, but none can compete with this transformer in terms of sheer simplicity.
Well that about sums up the tranny for you. The switching freq of the diodes is very very low, so normal ones will be fine. Like I say, It seems that your ones are ok. I'd push the batt voltage to at least 14v per 12v batt with the lester and then look at the lower current small one for finishing charge... I think your batts are barely charged at less than 13v with the lester as the current is pretty high with these things, and pushes the apparent voltage quite high, while really without the charger on, the batts are no where near charged. I 'm guessing if you measure the plate charge 1 hour after disconnect at the present turn off time, your voltage will be quite low.... meaning probably nothing is draining the batts.... disconnect and see.
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If you wish to decrease the output of the lester, change the cap to a smaller value (maybe 4uf) and your current will drop to a more reasonable 15A or so for your bulk charge. In this form you may well charge the batts to 14v/12v batt and then let it run at (guessing here 5A) for another hour to gass them mildly.
Anyway, I think at this stage the lester is fine, your problems may be your charging technique. Isolate the batts after charging them as you normally do and see if the low volt problem goes away, or remains the same. If the problem goes away, then look for a bleed from the truck somewhere else. If all else fails, disconnect the batts after charging till you find the problem, and get your milliamp meter back on line (is it internal fuse problem?)
best of luck
..........oztules
