Q: Charging a batt bank w/ alternator & mixing batteries

Hi,

Most alternator wires are a bit undersized, and I think one of the reasons is the auto manufacture really does not wany the alternator to put out a full 14.5 volts and 130 amps. So they use #8 wire from the alternator and run about 8 or 10 feet of wire before it gets to the batery. IT has the effect of limiting the battery charge rate, and that is probably a good thing. I found that when I connected some #4 wire to my alternator and a 125 amp solinoid that is connected to the coach battery, when I turn on that solinoid, then the alternator belt starts to screem "Help". So normally if my battery bank is a little low, I leave the switch off until I am at least out of the camping area, and at freeway speeds. At 800 RPM, the alternator belt is only going about 600 feet per minute, but at 2,000 RPM, it can be going 2,000 feet per minute, so there is less pull on the belt - because the torque is spread out over more feet of belt.

So #8 wire to the alternator will provide the best voltage to your battery bank, while not overloading the alternator to much. I used a forklift relay, rated at 100 amps with silver allow contacts, to isolate the battery from the engine battery when the engine is off. I control it with a switch, because most of the time I don't need to use the engine to recharge the 400 AH battery bank, my 400 Watt solar system does that.

You should run #8 wire both ways, it conducts power better than steel. What I did with my inverter, that sometimes sees over 150 amps into it, is run #000 wire to a two lug terminal that is bolted to the frame, then to the negative to each battery (I also installed a shunt to measure any amperage being used out of or recharged into the battery bank.) So power can take the lease resistant way to make it's way back to the alternator via the frame of the RV, or to the battery via the copper wire thick as my thumb.

You also need to consider fuses. I would recommend a 50 amp automatic reset fuse for #8 wire. This should also be located near the output of all your coach batteries, and near the other end of the wire, the alternator, or battery connection to the tow vehicle. If a short happens in between a pair of batteries, power can come from both directions, and start a fire, while the circuit breaker will overheat, and stop the power flow for a little while.

Yes you will be mixing a pair of 6 volt batteries, and some 12 volt batteries, this is what is in most motorhomes that use 6 volt batteries, and they get along fine.

Fred.

Hi,

I wired a second charging path from the chassis battery to the house batteries. Use #8 to #4 wire and twin 50 amp auto reset circuit breakers, one at each end of the wire within 2 feet of the battery bank (or less).

Between my two charging paths I get up to 70 amps to the house battery bank.

The worse thing that can happen is your truck alternator will not be rated high enough for additional batteries.

The rule of thumb is 20AMPS of charge amps per battery with a bunch of reserve left over.

The big problem with trying to use the truck charging system is the truck battery being only inches away from the alternator usually is the driving factor on how much charge the alternator is going to provide for the batteries further down the line.

My number one rule is always leave the truck starting battery system alone and not bother it. Its purpose is to get me home when I need it too.

I would install a good smart-mode converter/charger system for the house batteries and re-charge them with a 2KW Honda generator direct connected the 30A shore power cable using a proper RV30A-15A adapter plugged into the generator 120VAC receptacle....

just my thoughts
Roy Ken

I owned a B2200 and that isn't anywhere close to a 130 amp alternator, more like 60 amp. Also, remember that you won't get anywhere the full rating of common vehicle alternators at low RPM. I would save myself the wiring expense and put the money towards a solar panel and controller.

Thanks a bunch guys, extremely helpful. I suppose I'll either need a new alternator, an extension cord to charge my batteries, or a solar panel. I wonder if I could charge up at those EV stations..

I like the idea of the truck charging the battery bank because it seems the setup would be efficient and cheap. I also live in the pacific northwest where we don't get much sunlight half of the year. Would a high output alternator get the job done? I think I'll need one anyway for my subwoofers.

Using the truck charging system is a doable thing but it isnt CHEAP. Go look at what the rescue squad and police emergency vehicles do to keep all of their radios and emergency gear working...

Back in the JEEP TRAIL days we got along just fine with the inexpensive diode based battery isolators for having two or more batteries connected to the alternator system. With todays smart-mode technology where having more exact required voltages needed to charge the deep cycle batteries the loss of voltage over the old diode based isolators is a killer. My deep cycle battery wants to see 14.4VDC at 20AMPs to start a charge sequence and half way thru this charge mode process it drops back to 13.6VDC. If I was using a diode based isolator which has a high current .7 VDC voltage drop then those DC charging voltages become 13.7VDC for the intended 14.4VDC and 12.9VDC for the 13.6VDC required. The deep cycle battery charging science does NOT work well at those voltages if you want to recharge your batteries in a three hour time frame..

This make you have to install what are called high current SMART RELAYS/SOLENOIDs which are not cheap. The Blue Sea model SI-Series 7650/7610 smart relay cost around $150


This is a planned method for me to charge up a couple of 110AH-150AH Trojan batteries in the back of my truck. I doubt if my 2010 Ford OEM Alternator is big enough to support even this low of current system I am looking at.


Bottom line is high dollar high capacity alternators are required along with high dollar switching components to make it all work...

Nothing is CHEAP anymore...

I suggest you run by what you are wanting to do with MEXICOWANDERER... He lives and breathes this stuff big time...

just my thought
Roy Ken