Dual battery diode isolator

[freda]

i'm just asking if anyone out there have real experience with any of these?
and maybe measured the Vf across them, at load?
because i'm yet to see any specs of such devices mention those parameters.
 
so if they say use Schottky diodes, still if it needs to pass 30amp, the  Vf drop could still be 0.5V or so, the battery, either of them are never going to get fully charged, right?
(just looking at some random high current schottky diodes (>100amp) on Mouser)
I do recall as a young lad, took an alternator apart and delved into the transistor that controlled the field winding, I guess, if a voltage sense could be wired up past the diode isolator,........
the loss could be compensated for?
as an aside, i wonder if doing so would remove the problem so called "smart" alternators of newer vehicles introduce, not my problem though.

[iMo]

The forward voltage drop on any diode goes up with the current, thus with 30A it could be more than 0.5V.
The Vf drop there depends on the diode types, the If current, temperature, and number of diodes wired in parallel or in series (if that is the case).
Btw., most probably you do not know the exact voltage at the alternator's output, it may vary significantly, thus the Vf drop plays minor role when talking "charging", imho. In that simple scheme above you simply push a current (pulsing) into the battery and it charges the battery to some 14.5V when alternator is running.

[freda]

no, the alternator assumes it is connected directly to the battery, they never output more than 14.1/14.2 volts, so the intervening diode will always hamper the full voltage to top up the battery.

That's why i speculated about doing a hack to the alternator, so the voltage sensing bypasses the diode....

[iMo]

When the alternator includes the voltage regulator you may simply increase the voltage of the regulator by aprox 0.5V, for example (better to measure the Vf in-situ). 
Another option is to feed the battery voltage back into the alternator's voltage regulator (like the "sense" voltage) and use the sense voltage such it compensates for "any" Vf drop (the diode isolator will be "inside the regulator's control loop"). You would need to have the schematics of the regulator and a good understanding how the regulator works handy, however.

[freda]

I'm not sure you actually read the previous reply. The hack I mentioned is exactly that

(the diode isolator will be "inside the regulator's control loop")

.

I don't think you can

may simply increase the voltage of the regulator by aprox 0.5V,

, if i recall about the alternator i dismantled oh so long ago, there is nothing
adjustable, the PCB completely sealed with some sort of plastic paint. A couple of transistors,one canned, smaller than TO3, probably not so hard to reverse engineer. I imagine just need the +V supply of such PCB, with another wire to externally bypassing the diode to V+
.
But anyway, as I hinted in the first post, I don't actually have one of these diode isolators atm.
i'm keen to know how well they work though, if someone happens to read the post and supply actual experience.
I know the in vogue way now seems to be an in vehicle DC/DC converter for the 2nd battery (must be a V booster I guess), but gee a very costly solution.

BTW, what I have actually done is diy'ed my own MOSFET nchannel switch, and in my purpose i'm only needing < 10amp, Voltage drop i experience with it is about 20-35mV.