Floating the Engine Battery Class C? UPDATE 3

for the Safari
i used a small Drock buck converter from Amazon
i fused the input at 10amps,
connected input to house DC, adjusted output to 13.xx with DMM
connected to output to the chassis fuse BUSS (batteries are in the rear of coach)

after blowing the fuse during cold engine starts
i replaced with 10amp fuse, with a 10 amp auto reset breaker

the chassis batteries stayed up, and start the engine
but the high draw would sometimes cause enough drop that fully topped off house batteries with Solar charging would give and in rush surge thru my little Drock maintainer and blow the fuse, then it could not do its job

since it was a buck converter stepping down from the house voltage
if the house volts dropped below the output setup, it did not drain the house to charge the chassis

if the house got low from heavy inverter use, the chassis batteries were effectively removed from circuit and not affected, they just stopped being on float

but if the converter was charging or the solar topping off, then chassis batteries were on float

pnichols wrote:
.....

However, your idea of going 12V to 12V directly using the RV's built-in charger/inverter is very interesting. You might have to hand-make a special cable with a male 12V plug on each end to be able to easily go from RV 12V receptacle to cab dash 12V receptacle........

.

Yes, in our Class C the house 12v socket in the "entertainment" area for TV etc (instead of a bed up there) is just above the dash socket, so not that long a run for a male-male jumper.

The TV 12v up there is confirmed not to be on the engine battery, but is on the house battery bank.

Very low amps for this job, so skinny wiring ok.

I wouldn't use something through the cig lighter... those wires are a bit spindly.

What about a Trik-L-Start or Amp-L-Start system? These will charge the engine batteries from the coach batteries and maintain them.

DrewE wrote:
If you have a solenoid based battery combiner that's controlled by the engine run circuit, you can just put the key in the run position (without starting the engine) for a spell to connect the batteries and give the chassis battery a little boost. I often do this when exercising my generator during storage periods. Yes, it does leave the engine computer and vent fan and so forth energized, but if you're on shore power that's no big deal.

I thought leaving the key turned burnt out the points. :) Oh wait---.

If you have a solenoid based battery combiner that's controlled by the engine run circuit, you can just put the key in the run position (without starting the engine) for a spell to connect the batteries and give the chassis battery a little boost. I often do this when exercising my generator during storage periods. Yes, it does leave the engine computer and vent fan and so forth energized, but if you're on shore power that's no big deal.

Mutha Fletcha's Do It Herself Cheater Cord

Use a fistful of dirt cheap Schottky axial diodes.

Look here comes some now...

https://www.ebay.com/itm/10x-15A-45V-Blocking-Schottky-Barrier-Diode-Rectifier-15AMP-For-Solar-Cell-Panel/201851510453?epid=697039277&hash=item2eff499eb5:g:EewAAOSwxEpYxpcd

The band on the diode case goes toward the chassis battery. Use a handful in parallel and crimp into a large butt connector -- one on each end.

Make up the cigarette lighter plug. Good a good one so it's easy to yank before starting the engine.

Heat-shrink both ends up to but not including the barrels of the axial diodes. Leave the barrels open to dissipate minor amounts of heat.

ANTI GYRO GEARLOOSE ANSWERS TO ARGUMENTS...

Yes this works fine and is dirt cheap

Forget voltage drop arguments. A glob of Schottky 15-amp diodes will drop .2 volts on a bad day with a hangover. Sue me if dropping from 13.3 volts to 13.1 will negatively affect the storage life of a low antimony engine starting battery, it won't because some 40-years of doing exactly that proves you wrong. Keeping a battery mind you a low antimony battery that is not cycled at 13.1 volts is wonderful for the battery. The Delta T between 12.6 and 13.1 is a universe apart as far as effects on longevity.

If you forget to unplug and blow the cigarette light fuse, the hassle of replacing it puts you on a vertical learning curve experience.

There is nothing wrong with expensive bells, whistles, and flashing lights plug in battery maintainers except they needlessly separate me from my cash. Spend 3 dollars or spend 30 dollars. It makes no difference -- for this specific use your battery will not live ten seconds longer except if you live at McMurdo Sound, or Timbuctu Mali where temperatures are as crazy as the people who live there.

Yeah I'm a bit testy. I went to the doctor in the USA and forgot the cardinal rule of never eating there -- I am getting over a randy case of E-Coli and sometime today I have to scrub the sandal skid-marks off the tile between the bedroom and bathroom.

Our motorhome is always powered up while being stored, so I just plug a BatteryMinder's input side into one of the RV's 120V receptacles and plug it's 12V output receptacle into an always-hot 12V receptacle in the motorhome's dash. The BatteryMinder then automatically takes care of the engine starting battery 24/7.

However, your idea of going 12V to 12V directly using the RV's built-in charger/inverter is very interesting. You might have to hand-make a special cable with a male 12V plug on each end to be able to easily go from RV 12V receptacle to cab dash 12V receptacle.

A BatteryMinder does not supply a steady voltage for battery maintenance, like an RV's stock fixed voltage inverter does. The BatteryMinder uses a varying voltage for maintaining batteries - which is supposed to be superior to a steady float voltage.

time2roll wrote:
Depends if the cigar plug is switched with the ignition or always on.
Seems a bit makeshift compared to Trik-L-Start battery combiner.

So it would work if the cig plug is always on? Makeshift yes.

I am not certain I want to float the engine battery in parallel with my bank of AGMs for any long period, even if the 13.6ish volts would keep them all happy, but want to know if this is an option.

Depends if the cigar plug is switched with the ignition or always on.
Seems a bit makeshift compared to Trik-L-Start battery combiner.