Floating the Engine Battery Class C? UPDATE 3

[BFL13]

Updated 19 May. and 20 May twice
------

(I know about the other ways like Trickle etc.)

Question here is, can I put a 12v plug into the dash cig lighter socket and connect that plug to the house 12v system, which is on its converter on shore power (or off-grid on solar), and thus put the engine battery in with the house battery bank for floating?

[BFL13]

It took about three hours to get the engine battery up to the same voltage as the house battery bank, so there they are in parallel floating.

I prefer to float the Wet engine battery separately from the AGMs, even if there is no good reason, so my actual method will be to use the little charger inside to the cig plug. That is better for me than using it outside clamped on, powered from the RV outside receptacle and extension cord.

The jumper could also be used when on solar.

So time to declare victory and quit while ahead!

[BFL13]

This time, wired right, the jumper did not blow any fuses. And engine battery voltage rose slowly, but it is rising for sure. Started with 13.8 vs 12.7.

I tried the same plug the jumper used in the cig socket, with the little charger (at 14.2v), and engine battery rose slowly that way too .

[MrWizard]

Reverse polarity, dead short ?

[BFL13]

No mystery at all now. I woke up in the night with a clear picture of what I had done--funny how the mind works. Anyway here is the sad story:

My little charger came with an output wire that has an MC4 on the end. You get two sets of cables to plug into that. One has clamps, one has a 12v plug. I have two of those little chargers and only ever use the clamp sets with them.

So I have these two 12v plugs each with an MC4 on the end of its wire. To make my jumper, I just plugged them together. 😞

I'll try it all again later.

[MEXICOWANDERER]

• The cigarette lighter works normally
• When a jumper plug is fitted it blows a 10-amp fuse
• The chassis battery is slow to run down when parked
• The engine starts

Ys gots ya a mysterious one there BFL13

[pnichols]

Here's all I've used for years to maintain our motorhome's V10 engine starting battery ... about 8 years maintaining the Ford E450's original starting battery and about another 4 years maintaining Ford's largest starting battery that's actually intended for their diesel trucks, but I wanted starting battery overkill in our E450 motorhome.

This little maintainer has done the job just fine and it's not expensive. I have another one in the shop for maintaining various equipment batteries: www.northerntool.com/shop/tools/product_200332201_200332201

[BFL13]

Yes it might have a chance to work if the engine battery were already up to the house battery bank in voltage. Then it would just need to keep it there.

There is not so much of a draw on the engine battery in the 1991 as there is in newer vehicles when parked.

[MrWizard]

last time checked the safari, many months ago
the (2) chassis/start batteries drew 1.3 to 3 amps when being maintained
there is tranny cpu and radio etc.. plus the normal self discharge

you might start the engine, run for a few minutes then plug in your jumper
before shutting of the engine

i'm guessing, the voltage difference was enough for a spike, you were not expecting

just a test ?

you need a clamp on ampmeter or inline meter to see how much the normal draw is
with fully charge engine battery

[BFL13]

MEXICOWANDERER wrote:
Cabin lights and loads off?

Yes, but it might be a wiring issue of some sort behind the dash, no idea. Previous owner(s) swapped out the radio for newer one(s), and there was also a CB (10-4 Good Buddy!) I yanked.

Doesn't matter, I don't need to maintain the engine battery that way; I am doing it the traditional way with its own little charger. I just thought this way would work, but no big deal. Might work in somebody else's rig.

[MEXICOWANDERER]

Cabin lights and loads off?

[BFL13]

Could be something else going on, no idea. 1991 rig.

The cig lighter in the dash socket works ok and pops out red hot. (Engine off, no key in ignition) The TV panel 12v socket (on house batts) works ok. Joining them up, nope. Worth a try.

[MEXICOWANDERER]

I thought the subject of quiescent power needed to maintain a group 31 battery at 13.6 volts was somewhere in the neighborhood of 600 to 700 milliamps. This was discussed at length on this forum including instrument verification a couple of years ago.

The idea that a less than one volt Delta T would fail a ten ampere fuse for a single battery has my eyebrows raised. Ten amperes is what an old Harbor Fright charger was rated for. Could it be the chassis electrical demand was inadvertently raised by a significant factor?

[landyacht318]

Long ago I tried recharging a 12Ah AGM battery through a double ended ciggy plug on 16awg. It would work if the battery was not too depleted. but blown glass fuses would get expensive, and I just started wrapping them in aluminum foil.

But the battery never seemed to get much charge. I did not have ammeters or voltemters then, just a digital multimeter.

Now I only trust ciggy connectors to pass ~20 watts or less.

My OEM dash ciggy receptacle got its ground through nearby metal dash and had a 18 or 20 gauge wire from fuse panel.

I've tried running 10AWg to receptacles and connectors. glad I left enough slack to cut them off and install anderson powerpoles.

I still employ ciggy plugs and receptacleson low wattage devices, as they can be connected or disconnected with one hand in the dark.

One 22 watt LED light has a PWM LED dimmer on it. The ciggy receptacle and plug are the Blue Seas version, pretty much as good as a ciggy plug connector can get. The LED light when dimmed, will have the ciggy plug/receptacle emit a whining sound, and pushing the plug deeper into the receptacle can change the volume of the whining, and pushing it very firmly together will stop it together.

So even the best 12v ciggy connection is revealed to be a horrible electrical connection worthy only of contempt.

[MEXICOWANDERER]

............and............

that's why I recommended inserting Schottky rectifiers in the cord. Amazing what a .2 - .3 volt decrease in Delta T will do...