Different sized batteries in series - Is this a problem?

[houdel]

My new Deuce came with two near new 6TL batteries. I tested them with a battery hydrometer, one was great, all cells 1.275 - 1.300 S.G. (it just doesn't get much better than that). Unfortunately, the second one had one bad cell, > 1.100 S.G., while the remaining 5 cells were all 1.275 - 1.300 S.G. As a temporary fix until I can come up with another 6TL, I replaced the bad battery with a fairly fresh car battery I had. The truck cranks & starts fine and the voltmeter reads normally.

The 6TL is rated 875 amps at 32 degrees, 750 amps at 0 degrees. The car battery is a pretty good sized one (I believe is a 34 series) and is rated 720 amps at 32 degrees and 600 amps at 0 degrees.

Is running two different batteries in series as I am going to cause any problems? Is either battery (particularly the smaller one) going to be damaged running like this for an extended period of time? Will both batteries charge properly?

For that matter, would running two 6TLs, one fresh and one older one, cause a problem? Or do batteries need be replaced in pairs at the same time, both batteries of equal age and ampacity?

Any thoughts would be appreciated, especially knowledgeable and/or experienced thoughts!

Thanks, Lee

 

[cranetruck]

The total internal resistance of the batteries determins the charging current.
If this resistance is different for the two series connected batteries, then the the charging voltage across each of the batteries will be different. Not good.
Ideally, series connected batteries should be identical in charge, capacity, brand and age.
Confirm the situation by measuring the voltage across each of the batteries when the engine is running (=charging). One battery may not get full charging voltage.
This is also a potential (no pun intended) problem, when one battery is used for 12 volt accessories.

The best battery maintainer that I have found is the "Chargetek 500", which maintains both batteries within millivolts of each other. It runs on 120VAC, so can obviously only be plugged in when the vehicle is parked.

 

[houdel]

Hey Bjorn, thanks for the info. I checked with my local Interstate dealer, he claims he can order a brand new 6TL for "around" $100, which is a lot better than I've seen in other posts. I'll try that route as soon as I can come up with a little extra pocket change and put the other battery back into my '91 Plymouth Voyager! -- Lee

 

[G744]

A friend used a group 24 and a group 27 in an M37. They didn't last out the year, had a 7 volt and a 17 volt at the end. It can get you by, but don't expect them to go for long.

dg

 

[SixBuy]

I put two new 6TLs in last May and the whoresale price then was $86/ea. And there is a core on the old battery.
Ditto with CraneTruck on the internal resistance of the cells - think of it as one large 12-cell battery!

 

[Recovry4x4]

I'd love to find a supplier of 6TLs or 6TNs without losing an arm. They're about $140 ea here. With that in mind my trucks get the $29 Wallyworld specials. After about 3 yrs just toss em and get more.

 

[cranetruck]

The difference is really only reserve capacity and vents that seal the batteries when fording.

Would be nice to be able to rebuild batteries, not by using chemicals, which doesn't seem to work that well, but by replacing the plates....

 

[rmgill]

Save the extra 6tl and use it for a 12 volt accessory setup being charged by a 24-12 volt charger.

 

[red devils dude]

if you really want to buy one set of batt's for your deuce and never buy new ones again
buy these http://www.milbatteries.com/hawker/index.html there like $240EA but they last a LONG time.

 

[devilman96]

Them be gel cells.... and mighty expensive ones at that!!!

 

[pistolnut]

My experience as a former truck driver is that when one battery goes, replace the other one as well and find some other use for it. When you put a new battery with an older battery the old one will fail soon. I have seen this again and again.

 

[SixBuy]

Yep! That's the fine for trying to break Ohm's law...
The internal resistance of the old battery is much higher than the new so the charge/discharge voltage is not the same across each battery and they both suffer.
Pay me now or pay me later... (Mother Nature)

 

[jayjetseck]

whats the proper way to wire two batteries instead of 4 in my 813??

 

[greenjeepster]

Hey Bjorn, thanks for the info. I checked with my local Interstate dealer, he claims he can order a brand new 6TL for "around" $100, which is a lot better than I've seen in other posts.

yeah.... I was quoted 186.00 yesterday each... I went to autozone and bought a pair of marine batteries instead.

 

[Keith_J]

Them be gel cells.... and mighty expensive ones at that!!!

Actually, they are valve regulated absorbed glass mat sealed lead acid batteries.

Gel is far from ideal in PbSO4 in high current density applications due to off-gassing and internal resistance issues.

 

[cattlerepairman]

I put two new 6TLs in last May and the whoresale price then was $86/ea.

I never bought from ladies of the night, but at that price it could be worth the while!

 

[conductorx]