Charging a 3rd battery from the alternator

There shouldn't be any problem. Assuming you don't wire it with 00 welding cable, the resistance of the connecting cable will limit the current to the battery. You'll probably use at least 10 ft of wire so say, #4 should be about right.

You might consider a battery interconnect solenoid (looks just like a starter solenoid but with a continuous duty coil) instead of the T-handle. Connect the coil to an ignition-hot circuit so that it energizes and connects the battery to the truck electrical system whenever the ignition is on. That way the battery is always charged and if you leave your lights on and run down the main battery, the aux battery is available to jump with.

John

I like Johns answer to this only thing I would do different is run a minimum of #2 guage cable from the front to the rear battery. The only reason diesel engines are equipped with dual batteries is due to the large amount of power required to start the truck in the cold. There are alot of trucks out there with only one battery with diesel engines. You will be fine with that deep cycle.

Ford Tech

I ran a spare battery in a Dodge van for many years. It was inside. behind the seats, connected with a piece of RG-8 to a starter solenoid and then to the regular battery. No, the starter solenoid worked great, it did not overheat, etc. Yes, the battery charged up and lasted darn near forever, powering a quantity of radios and gadgets. I produced more gas than it did. I never used it to start the engine as the engine battery never failed.

It was a great system that worked perfectly for many years.

Steve

I have a different take on all of this. The poster stated that the purpose of the battery was to run tools not start vehicle. Because of this I would connect it to the altenator directly with a 12 ga wire and a in line 30 to 35 amp breaker (you can install a cutout switch too). The reason for this is that the 12 gas wire would limt charge rate due to wire resistance and therefor the load on charging system so that priority is given to charging vehicle battieres first. Also by limiting charge rate you would extend deep cycle battery life and a cutout switch (or pulling break) would let you isolate battery for truck and run it flat and not pull down truck batteries. On my plow trucks which are always gas powered I run dual batteries and power plow off of second battery and charge it with a 12 ga wire hooked to altentor output for same reason as it keeps battery topped off but keeps plow loads mostly off main system due to wire resistance tending to block big loads transfering back to main system and voltage on truck for accessory stays more stable too.

----------------- TheSnoMan.com

Am I correct in assuming I would only have to run a single hot power cable front to back and simply connect the ground terminal on the deep-cycle battery directly to the truck chassis?

Also, I got an email response from somebody who suggests a 2-battery isolator as an automatic way of making sure using the deep-cycle battery did not discharge the starter battery. Apparently the isolator does something to protect the generator. In a slightly more expensive way, that makes sense to me.

I assume the 12 ga wire trick would also work in that arrangement for limiting charge rate to the deep-cycle?

Regards, John Kinney

IMHO, if you take advice from SnoMan, you at risk. Get some GOOD advice.

Lon

On Sun, 07 Jan 2007 09:13:23 -0700, Lon VanOstran wrote:

You are such a child Lon,

A isolator can have a blocking diode setup that will let it charge while preventing discharge. If you want a no brainer but myslf I have been using a breaker cutout setup for many years. If something shorts out the breaker will pop and a single 12 ga wire can safely handle over 40 amps without heating issues (when it is in a sheath with other wires carry same load it can carry less power safely) I have put together a few charts on voltage drops in circuits and I have some for single wire feeds for SUV's and such that I need to get posted sometimes. If you look at the chart in link below it covers extention cord voltage drop which will show you the voltage drop with distance under load and you will see how as load increases at a given distance so does voltage drop and since charge rate it limited or controlled by applied voltage vs internal battery resistance in it current state the drop in the feed circuit will limit charge rate to battery. Basically all battery now are low maintaince (they have calcium in plates to limit gassing which also raises voltages a bits) so no need to disconnect when sitting overnight or for a few days. There can be a problem with old stlye non low maintainance batteies and new low maintanance ones because the old style would tend to discharge the new style a bit due to differing voltage requirements for charge. YOu can also connect negative on battery to chassis ground so that one wire is all that is needed.

TheSnoMan.com

It's not that hard to do it right. Go to your local RV parts house and get a battery isolater. It wires in to the charge circuit, has three terminals, one terminal is the main fed from the alternator, continuing on to the normal circuits. The second terminal feeds the spare battery. when the engine is running, and the third terminal is a switrched battery feed. when the ignition switch is off, the spare battery is "isolated" from the others so you dont have to worry about accidently draining your starting battery. Thy come with very clear wiring instructions.

Whitelightning

On Sun, 07 Jan 2007 23:59:05 +0000, Whitelightning rearranged some electrons to form:

Agreed. A battery isolator is what you want. Don't listen to the Snowblower, his bad advice (on many topics) will get someone killed.

You know the wise man avoids the advise of name callers because it shows inmaturity of responce as does the anominity of your posts as well also show lack of accountablity. I do not hide at all. Never take advise for someone that is mere smoke in the wind.

----------------- TheSnoMan.com

Anonimity? You don't see his sig or something? I notice you snipped it out....

Ask him what he does for a living and then you'll know he's more than qualified to make determinations about battery isolators or anything else that has to do with the flow, management and utilization of electrons...

Remember, no >

John... I have a E 250 van made into a camper. At first used the isolator type. The diodes have a voltage drop of 7 tenths of a volt. So the battery would never get full charge ( this was observed with a installed voltmeter. I changed this to the solenoid type (one made for this purpose) and now get a full charge. Also a 35 amp circuit at the solenoid to prevent fire in case of short to ground through to wiring to 2nd battery located in rear of van. Made negative connection to chassis ground. W W

John, I agree with the posts that say you shouldn't harm your alternator with your proposed set up, however, I do have some concern about the battery. A deep cycle battery is constructed differently from an automotive battery. They are designed for a low discharge (relatively speaking) over a long period of time as opposed to a high discharge rate over a short period of time such as an starting battery in an auto. The deep cycle battery (ideally) should be charge at a low rate (2A or so) over a long period of time. Most makers recommend several shorter periods with rest periods in between charges. Admittedly, this is an ideal situation and would not cause the deep cycle battery to fail immediately. Just my observations. DaveD

>

None of that is true as a generality and particularly not in this case since John said that he'd bought an AGM battery. Even wet cell batteries can be charged at 1C or better during the bulk stage. Companies like Trojan will tell you that if you convince the application engineer that you know what you're doing. I do it routinely.

AGMs as a class are capable of much higher charge and discharge rates than ordinary wet cells by virtue of the construction. 5C charging and >10C discharging is common. Hawker Genesis, the Orbitals and other similar batteries are rated for thousands of amps of discharge and have proven to hold up in electric drag racing applications for both charge and discharge rates like that. The 30AH Hawkers in my hotrod electric scooter, for example, will sustain a discharge rate of greater than 500 amps and have been doing so for years.

In this particular application, no charge rate the alternator is capable of will harm the AGM.

John

A wise man knows that he must present himself well if he intends to be taken seriously. Perhaps you could consider using a spell checker, and in longer posts, the key.

What you describe is exactly what happens when you connect a travel trailer. The trailer batteries are charged by the alternator. My

2000 250 7.3L PSD has the trailer towing package, and there is a relay that connects the charging power to the trailer connector when the engine is running to prevent the trailer from discharging the starting batteries.

I discussed this and some other suggestions with my friend and he agrees that a solenoid arrangement is a good idea if I can get the parts at a reasonable cost. He also says that battery isolators will work, but apparently all of them seriously limit the charge that the alternator will pass on to the deep-cycle battery. I checked this with several sources on the Internet and it appears to be true. He agrees that if I'm only using the connecting wiring to charge the battery, that #4 wire should work. I looked up the F-550 alternator, which appears to be rated at 12V

105A. I haven't measured exactly how far I have to run, but 10-12 feet seems about right.

Now I gotta go figure out solenoids!

Regards, John Kinney

That's the usual setup.

The solenoid you are looking for, looks like a starter solenoid, BUT must be desigated as a "continuous duty" solenoid. Readily available at any auto or RV parts outlet for less than $20.

True.

Sounds good.

Not to worry about overloading the alternator: They are thermally regulated. If they get overheated, the regulator reduces the output voltage (and consequently, current output) to compensate. the alternator circuit will typically start up at around 14.2 to 14.5 volts when the engine is cold. As it gets warmed up, the output voltage will be reduced to somewhere between 13.2 and 13.8 volts.

Everything you need to know.

You can get the solenoid at NAPA for under $20 even if you get the sucker-just-walked-in-off-the-street price column. Camping world sells it for about $14 last time I looked. Our local RV dealer gets $11.

John