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I just put in a dual batter setup in my truck. When I put in the new batteries, WOW what starting power! Instant crankage! But I've been waiting on a slip yoke, so the truck sat for about 2-3 weeks with the batteries in, and I started the truck oh... 7 times or so while it sat. Came out today and the batteries were dead. Jumped started them, but it took some time. Drove to Speedway to air up the tires and sure enough... tried to start, and the batteries were dead. Once jump started again... which took forever I drove around town for nearly a half hour. Got home, turned it off and tried to start it again and NOTHING. No lights, no power. ZIP, ZERO! What the heck happened to my batteries!? Any suggestions anyone?

 

First step, check all your cable connections and the alternator belt.

Post details on the vehicle type and equipment.

Do you read a discharge on the dash? Put a voltmeter across the battery with the engine off, and read it again with the engine on. Then get a reading with the engine on and the lights on. Post your readings.

 

Also you've got to have some juice in the batteries in order for the alternator to even charge. If they're completely dead you need to charge them awhile before testing and inaccurately blaming the alternator.

 

Are the batteries in paralell or did you use an isolator?

 

If you jump started the truck and let the alternator charge the batteries, there is a good chance you junked the alternator. On a high mileage Ford alternator (or starter) letting it sit unused for a few weeks can result in a bad alt. or starter. Basically, brushes and stuff rust up and on the used components it is enough to kill the part. I learned this taking care of a large fleet of used Ford trucks.

Start with the basic short test. If you have a short, unplug the alt. If you still have a short, undo both cables +/- on one of the batteries to find the short in the battery.

Did you use two of the EXACT same batteries?

If not (gell vs. ASM or cont vs. main. free e.g.), what will happen is the alt. will keep on charging according to one battery, which over charges and destroys the other. Then the destroyed battery will bring the good battery down to its voltage level while sitting if you did not use a battery/charge isolater.

Now the destroyed (shorted) battery will cause the alt. to overcharge all the time, which will kill the other battery or alt. Kind of a vicious circle without isolators.

 

like billybob said, dont rely on your alternator to charge one (let alone two) batteries...read the instructions of any alternator, they will say to make sure the battery has a full charge after installation and before starting the engine to avoid ruining the alternator....sometimes batteries just lose power from sitting, sometimes they dont..your best bet is to unhook the batteries if the truck is gonna sit for more than a week....

EDIT: well looks like rebocardo beat me to it! his version even sounds better..lol...

 

WOW... lots of info, ok cool... let me fill you in on what I've got. 77 F-250. The dual battery setup came out of used parts on an old 80's something F-350. So did the alternator. So they are supposed to work with each other. I am going to take the batteries in to AutoZone today and getting them really charged, bring them home, double check all of my connections and give it another whirl.

Now that I think about it, I used to kinda get low battery power before. And that all started after I installed my tachometer. Maybe its drawing power. I hooked it in through my headlights. That way its either on, or off. Anybody got a better place to hook it up?

 

If you are going to Autozone anyway, why not take the alternator in too and have it checked? Might save you some effort.

 

The batteries tested A OK, they didnt even need charged!? So I went over every connection and sandpapered it down. Even bought a new lead wire for the selenoid to the + terminal. She started right up! I hope it stays this way!

I busted out the volt meter to check for any shorts. Nope! Both batteries read in the high 12 volt range! Thanks for all your help everyone! Hopefully I wont be stranded again!