Charging deep cycle 12v from truck alternator.. stumped.

[bg86]

Now with all that said the most common thing is still the battery. With age they get sulfation on the plates. Sometimes if you run the battery completely dead, like leaving the lights on or hooking a light across it then charging it at no more than 2 amps till it gets a full charge it'll knock some of the sulfation off the plates and work for a while, but not very long. A higher charge rate won't do the same thing.
If you get a new battery it will probably be an AGM (absorbed glass mat) and it needs to have a long slow charge to start with, 2 amps or less. Putting it directly in the vehicle and using it will shorten the life. There is a chart available that shows how much charge it needs based on the voltage it has in it. A fully charged one will measure about 12.6-12.8 after it has sit for a while, it'll show higher just after a charge.

I have a schumacher smart charger that charges at 2, 12, and 20 amps. I never charge at 20, though. After my truck sat for three days I hooked it up and before the charger when to work it went through a half-hour desulfation phase. It has never done this before as I haven't let the battery sit this long without the alternator doing its thing. Its rare that I let the truck sit a full day so I only have to charge it back from 85% about once a week. The only time I get problems from the battery is when I let it sit for 48 hours and the charge in it isn't enough to get it fired up. I'll go get it tested soon but I've been hard at work on my from-scratch camper trailer for a month now and its easy to forget about it until it doesn't start, and by then I'm on the way to home depot and can't be bothered with it Sounds like an electrical short but it doubt it as I've done nothing to the wiring since the stereo 6 months ago and those wires were soldered, heat shrinked, and taped. Also, the last time I jumped the truck was 3 months ago.

 

[Karl]

Am I missing something?

I think Len said it quite well - you might. I think I may be missing something. What has this to do with solar or (bicycle generator???) charging? Those connections would be made downstream of the relay or diodes, not before them. Bicycle generator? Man, you'd be pedaling your amps off!

You could use a switch to manually control the relay (again, you have to remember to throw it), but I don't know of too many switches that could handle a high current from the alternator by itself when needed. A marine battery switch would do it, but I don't think you want one of those on your dashboard. :

 

[bg86]

What has this to do with solar or (bicycle generator???) charging? Those connections would be made downstream of the relay or diodes, not before them. Bicycle generator? Man, you'd be pedaling your amps off!

Just saying the switch could have more use than just turning on and off, it can give me the option to leave it off and chose alternate sources of battery charging. I can get 100 watts out of a bike generator without too much effort. I suppose I could hold it for 30 minutes or more. 100 watts for 15 minutes would give me 25 watt hours or somewhere around 2 amp hours. If I did this once a day I could keep 1 of my 2 CFL lights on for about 2 hours a day without C02.

Those marine battery switches don't seem to be too obtrusive. I'll go and check for a 50 amp swtich tomorrow at home depot and see what I can find. Good suggestion.

 

[Lou Schneider]

I think you're making this much more complicated than it needs to be.

A #8 charging wire run from the (+) terminal of your truck's battery to your trailer battery is as complicated as you need to go.  Wire resistance will limit the charging rate to your trailer battery to about 20 amps or less, tapering as the battery gains charge.  20 ft. of #10 wire will limit the initial charging rate to 10 amps or less.

If you run the truck long enough to fully charge the battery, the extra voltage will give it the equivalent of an equalizing charge.  No problem, just check the battery's water level every once in a while, which is a good idea whenever you move large amounts of power in and out of a battery.

When you stop, just unplug the trailer from the tow vehicle.  Much simpler than worrying about diode isolator voltage drop, etc.

Ground return can be via the car chassis.  You should run a seperate ground wire to your trailer through the electrical connector - don't rely on maintaining ground continuity through the hitch.

 

[Karl]

Lou, usually we agree on most points, but not on on this one :

Wire resistance will limit the charging rate to your trailer battery to about 20 amps or less, tapering as the battery gains charge.  20 ft. of #10 wire will limit the initial charging rate to 10 amps or less.

A #10/#8 ga,. wire will carry much more than 10/20A for considerably more than 20' without significant voltage/current limitation.
Agreed that we're making this more complicated than necessary, but unplugging /unhooking isn't always the desired choice when in bad weather.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

 

[bg86]

This wouldn't isolate the trailer batteries from being used in cranking. Some RVs have an emergency start switch that utilizes the coach batteries right?

 

[Lou Schneider]

Hi Karl -

Wire loss is pretty horendous at 12 volts - for any given current the percentage of loss is 10 times as great at 12 volts compared to 120 volts, but since any given power requires 10 times as much current at 12 volts vs. 120 volts, the true wire loss for a 12 volt circuit is 100 times more than at 120 volts.

Looking at this 12 volt wire loss chart, 11 feet of #8 wire will give you a 3% voltage drop at 20 amps.  At 22 ft., you'll lose 6% of the source voltage.

Subtract 6% from 14.6 volts and you'll wind up with 13.7 volts at the trailer battery.  Apply 13.7 volts to a moderately discharged battery and you'll limit the charging current to about 20 amps.

All of the above correlates with what I've seen in actual use.

Add in another 0.6  volt loss from a blocking diode or isolator and you'll have even less voltage available to make up for the wire loss, meaning you'll end up with an even lower charging rate.

That's why I say using a diode isolator is a bad idea in a tow vehicle/trailer setup.  The voltage loss caused by the distance between the source (car alternator) and the trailer battery makes the setup self-limiting without the isolator.

The same wire loss will limit the amount of current the trailer battery can supply during cranking.  Yes, it will contribute some but not nearly enough to make a significant difference during the brief period of starter operation.

A solenoid operated cutout is acceptable, as long as it is rated for continuous operation.  Most starter solenoids are not, and will overheat if they are energized for any length of time.

 

[Karl]

Right on, Lou. My point was that a single 8 or 10ga wire would still be better than the usual 12 or 14ga you get with those multi-conductor toad wiring flat cables. Handy little chart!

 

[bg86]

So having the trailer batteries contribute to cranking isn't a bad idea? I never knew wire loss at 8ga would be that significant.  I don't remember Home Depot selling something that I could use for a quick disconnect at the hitch for 8 gauge wire. maybe I need to look again

 

[Karl]

So having the trailer batteries contribute to cranking isn't a bad idea?

Yes, it is a bad idea unless you have heavy duty cables (battery cables) connecting them to the tow vehicle. If your tow vehicle battery is low on charge, the starter will try to draw a lot of current through a too small wire (#8, for example) and will probably get it very hot and melt the insulation and the wire.  A safety fuse or circuit breaker (about 30 amps) in the line would be necessary to prevent that.

 

[rbell]

Lou
There's some very good info on that site, also interesting. Thanks