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Dual Batteries in a 2003 Ford Excursion Diesel

Morning starts on my Excursion were beginning to show signs of a weak battery. The existing batteries were 2 Die Hard Platinums, one less than 2 yrs old and the other less than 3 yrs old. I took it to Sears today and the 3yr old battery tested bad, so they replaced it. Unfortunately, they don’t sell the Platinums any more so they gave me a Gold AGM replacement. The Platinums are around 930 CCA while the Golds are around 775 CCA. Since the 2yr old battery tested good, they were only willing to replace one battery under the 4yr replacement warranty. The good news is I got a $50 refund. Is there any bad news running these 2 batteries together?


Because the remaining battery is only 2 years old and tested good, there shouldn’t be a problem right now, but as the 2 year old begins to weaken the new battery might end up supporting the engine AND the 2 year old. Your effective power will be somewhere between the new battery and the old battery. IMHO you’d be better to just get another 775CCA to match the current new battery and in the coming years the weaker one won’t require support from the newer one.

However, I’m assuming this is a simple parallel circuit, using both batteries in parallel to light the glowplugs and turn the big diesel engine with the high compression over. If the second battery is there to operate auxiliary systems only and is only recharged by the alternator but not drawn on for starting, you’ll be fine.

I myself would like this setup explained for future reference.

The truck is pretty much stock. The two batteries run in parallel, mainly to support the starting requirements of the diesel engine. The Sears mechanic did test the charging system and it tested fine.

I wouldn’t have guessed the batteries would be connected directly in parallel, positive to positive, negative to negative. You’d think the batteries would never be exactly match, so one battery would tend to drain the other. But maybe that effect is so small that it still works ok.

Just took a look under the hood to make sure. There is a cable directly connecting the 2 positive terminals. The negative terminals are both grounded to the vehicle body. Don’t think you can get any more parallel than that.

Parallel is as parallel does … you are right, there’s nothing more parallel than that. In fact you may have proved the fifth postulate of arithmetic there … lol …

Paralleled lead-acid batteries should be replaced in pairs, period, IMHO.
Otherwise you’ll get less than you paid for.

That’s the way my Olds diesel was too-neg to neg with a long cable going to the second battery. I always replaced mine in pairs though. If one was bad it seemed to screw up the other one. I never had much warning either and used the Die Hards then. So I guess I’d just replace the other one at the same time, but that’s me and my experience. What’s another $100?

I kinda figured it was a simple parallel circuit enabling higher current draw than a single battery would.

For the record, two batteries in series would provide 24 volts…
What I wasn’t sure of was whether the circuits were in some manner controlled such that the second battery was only drawn on for auxiliary equipment. I could be wrong, but I believe cop cars are set up that way so that if the blue, red, and/or yellow lights draw down the battery (like say in an accident situation or for a construction “detail”), the primary battery is still charged to start the car’s engine again… wherein the engine can recharge the “auxiliary” battery. I believe the way this is done is to disable the primary battery circuit if the lights are operating without the engine operating, and the secondary battery circuit is disabled when the starting circuit is enabled, such that the primary battery doesn’t have the added load of a dead secondary battery while starting the engine.

The best way I can describe it, your batteries can put out x amount of amps. No matter what the capacity is the system will only draw the amps needed, I myself would prefer 2 batteries with the same cca, but as long as both batteries are up to specs, 750 cca will not drain the 930 cca as they are both providing needed power. The cca is an as needed situation, and like a big brother if the 775 cca is not enough the 930 will kick in, You have a depricated total CCA, but not a problem otherwise. One battery is fine for most vehicles, so no worries.

Barkydog: The problem with parallel batteries is the charging situation. If the chemistry of the two is a bit different and the full charge voltage therefore a bit different, then the charger will never fully charge the one with the higher voltage.

It appears no one here has this concern, but I wouldn’t recommend using one AGM and one lead-acid battery in a system that charges batteries in parallel like this one does.

All I can relate to is my diesel was fine in the morning but when I went to start it in the inside parking at work at noon, all I got was a spark and then nothing from either battery. I borrowed a car and went and picked up two new batteries from Sears and all was fine. Another time it barely started so let it run while I picked up two new batteries. I always replaced them in pairs with the identical battery for both.

So who knows but as said in two years, you’ll have a 4 year old battery and a 2 year old battery and will always be out of sync. Just like replacing one bad tire, you’ll forever have a mismatch until just putting a new set on. For the extra $100, I would just replace the other one.

asemaster - Both batteries are AGM.
Bing - I guess if the DieHard Platinum was out of warranty, it wouldn’t be a big deal to replace it. I sure hate to replace a battery that still has 2 yrs left on a 4 yr warranty!

This is slightly off topic but I can’t help but wonder why anyone still uses Sears for automotive things.

Ahh ok. I’m not entirely familiar with their battery line. Didn’t know they were both AGM.

This is slightly off topic but I can’t help but wonder why anyone still uses Sears for automotive things.

Because they’re conveniently located, open when some other places aren’t, the prices can be competitive, and the quality of merchandise is often comparable to others.

I could ask why anyone still buys food at McDonalds.


while I probably wouldn’t let Sears work on my car(s), I see no reason why a Diehard battery should be any worse than one bought from Costco, for example

Well, just to clarify but the last Sears batteries I bought were about 1987.