Originally Posted by jwalker
No. The normal charge voltage provided by a regulator is 13.7 - 14.2v. In practice, I've found them to usually give close to the maximum. The schottky diode will give a voltage drop of about 0.4v so this is within range.
I've run this system for many years and the main battery has been the one to fail first.
Most alternators will output pretty close to 14.3 or 14.4V when battery is fully charged. In any case, even with best case alternator output of 14.4 and diode drop of 0.4 (ignoring any drop in cabling) the max you will get accross the 2nd battery is 14.0V - and this would only be when main battery had stopped pulling current from bulk charge stage.
Lead acid batteries are really fussy about charge voltage. I suspect if you discharged your 2nd battery (one being charged via diode) on a regular basis (to a good percentage, ie 50%), it's capacity will be very quickly reduced by charging in this way as it would never get to absorbtion stage and would not get fully charged. For a backup it's probably fine but if I was using a service battery I'd want it linked direct to the main battery with a switch or split charge relay.
Batteries WILL suplhanate if not allowed to charge properly. If the maximum charge voltage ever applied is 13.8V and the battery is discharged, then sulphanation will occur.
You should be able to find links about charge graphs online. I pulled a few of these a couple of years back when I was looking at building a smart charger
Stage 1 - bulk charge (up to 80% of charge) - battery takes as much current as alternator will provide. When output voltage gets to 14.2 to 14.3 V move to stage 2
Stage 2 - absorption phase - 80-100% charge. Battery voltage is maintained at at constant voltage (approx 14.3V) until battery is fully charged.
Stage 3 - float / trickle. significantly lower voltage (13.4 to 13.8V) to keep battery fully charged.
So, if you are only ever applting 13.8v, you will not correctly charge your battery and it will loose capacity and die early.
(I trust the KISS was not aimed at me - partic the stupid part :-) ) Simplicity is fine but it also has to work. I use a good old fashioned switch to swap batteries on previous boats. No voltage drop, no electronics, complete control)