Duramax Diesels Forum Truck of the Week
  #1  
Old 10-26-2009, 04:21 PM
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Default different size batteries

ok so for the first time in close to 2 years I might put my second battery back in my truck. Yes, it started in -10* on one battery without being plugged in but is cranked very slowly...

The only thing is the battery thats in my truck now is an advance auto platinum whatever, 875cca. The other battery I have (same age and same advance auto platinum brand as the 875cca one) is a 1000cca.

Yes I know you are "supposed to" have batteries with matching cca ratings, but is there an actual electrical engineer (or someone who knows a real answer other than "ive read..." and "everyone says..." or "just because...) out there who can tell me/give me a valid reason why I cant do this?

thanks
ben
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Old 10-26-2009, 06:47 PM
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FWIW On my gasser I run two different part number Optima Red tops (each have a different CCA rating) with no isolator (both positive side terminals are tied together and both use the frame/chassis for ground) and I have had no issue to date. They were even purchased over a year apart. On another GM truck (non diesel) site I belong to, I asked what the issue would be with running them this way and was told that if one battery dies it could draw the other one down without an isolator but if both batteries are quality/new it should not be an issue.

Disclaimer This is only 1 man's experience, yours may differ....
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Last edited by Leadfoot; 10-26-2009 at 06:48 PM. Reason: clarification
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Old 10-26-2009, 08:02 PM
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Good!! I like that answer... thanks Chris. If my truck blows up you're responsible

ben
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Old 10-27-2009, 01:49 AM
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we'll come help you again
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Old 10-27-2009, 07:25 AM
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While you may never see a problem it really isn't a good idea. The batteries will drain and charge at a different rate and their lifes could be shortened. I doubt that anything will blow up. It's kinda like running with a battery that is really old and near the end of its life and one that is brand new.
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Old 10-27-2009, 09:35 AM
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lol i owe ya some truck work too ben

BTW made a little headway on gettin the trans parts issue worked out too will keep you in the loop
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Old 10-27-2009, 11:52 AM
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So they run all the electronics on one battery? It seems like something would overload in the long run, maybe you get some time out of it but its like doing holeshots with the tuner maxed out its only a matter of time before something goes bad. I am not an engineer but I did stay at a holiday inn express last night.
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Old 10-27-2009, 02:30 PM
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Quote:
Originally Posted by mmangels22 View Post
So they run all the electronics on one battery? It seems like something would overload in the long run, maybe you get some time out of it but its like doing holeshots with the tuner maxed out its only a matter of time before something goes bad. I am not an engineer but I did stay at a holiday inn express last night.
no...the batteries are wired in parallel... You can run the entire truck on [either] battery, it just cranks more slowly and is a little bit sketchy below 0* outside air temp.
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Old 10-27-2009, 02:39 PM
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I have a 690, and a 770 in my truck
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Old 10-27-2009, 06:55 PM
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Quote:
Originally Posted by clayt171 View Post
While you may never see a problem it really isn't a good idea. The batteries will drain and charge at a different rate and their lifes could be shortened. I doubt that anything will blow up. It's kinda like running with a battery that is really old and near the end of its life and one that is brand new.

They will drain and charge at the same rate. It should not matter one bit unless one of the two batteries is not big enough to handle its part of the load or if the battery cables were way mismatched in size. The only difference between the 2 is one has more amps available than the other. It will not use more amps it just has more available.
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Old 10-27-2009, 07:28 PM
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ive had 2 different batteries in mine for well over a yr . ones an autozone gold and other is a optima yellow top. i have never had any issues
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Old 10-27-2009, 07:42 PM
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I have one OEM AC Delco and one optima red top with no problems to date. Just to cheap to go buy another optima
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Old 10-28-2009, 12:39 PM
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Quote:
Originally Posted by duratothemax View Post
no...the batteries are wired in parallel... You can run the entire truck on [either] battery, it just cranks more slowly and is a little bit sketchy below 0* outside air temp.
You probably could run the whole entire truck on just one battery but the question is how long will it run on just one battery? If we can run 5-7 years with the two battery set up you would figure that the time would at least be shortened a year or so maybe more because the truck was designed to run on two batteries not one.

Do it and see what happens. I am subscribing to this.
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Old 10-28-2009, 01:53 PM
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Quote:
Originally Posted by mmangels22 View Post
You probably could run the whole entire truck on just one battery
probably???? Re-read the first post and you'll see thats what ive been doing for the past (almost) two years.

Ive ran it on the same single battery (from advance auto, not some special battery) and ive never had a problem with it. The only thing is it just cranks a little slower at all temperatures and cranks really slowly below 0*.

Quote:
Originally Posted by mmangels22 View Post
but the question is how long will it run on just one battery?
My single battery is just as good (load test, crank speed, etc) as it was when I put it in a long time ago. Am I shortening the life of it by putting a BIG load on it when I start the truck? Im not really convinced. Remember the BIG load is on the battery for all of what, 5 seconds? Its not like im sitting there cranking it until the battery is almost dead, then the engine starts and blasts the battery with full charge current from the alternator. THATS what kills batteries. Draining them down to nothing with a big load, then charging them up again really fast.

Quote:
Originally Posted by mmangels22 View Post
because the truck was designed to run on two batteries not one.
the two batteries is because when you start a diesel, you are running the starter and the glow plugs (or, sorry, excuse me cummins guys, the grid heater). Especially when in very cold temperatures. So far ive proven that with only 1 battery, yes, it sure cranks a lot faster below 0*, but it still starts just fine and so far there are no long term effects on the battery's health.

Im just wondering because I have two batteries sitting here. One 875cca thats in my truck, and another 1000cca sitting under a bench in the shop. (yes, its still fine, the old myth about batteries dieing when you leave them sitting on concrete is buffalo chips) Its a big PITA to put my second battery

Here is how it starts at 7 degrees below zero on only one battery. Yeah, it cranks slowly, but its still acceptable in my own opinion.

http://www.youtube.com/watch?v=6iA72Mzv9ek

Ben
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Last edited by duratothemax; 10-28-2009 at 01:54 PM.
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Old 03-31-2011, 03:57 AM
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I believe there was a SCARE many years ago that batteries wired in parallel and not PERFECTLY matched would somehow discharge into each other.

OK, that is probably moot;
The wiring schematics for '03 and on show an aux battery relay and the way that I have been reading them that battery is only connected (via the aux battery relay) to the regular battery (as in single battery gas trucks) when the key is ON.

So for the few seconds of pre-heat and starting a weakened battery is a LOAD to a good battery. After that each draws from the 14 volt or so line from the alternator, e.g. each draws according to it's need, neither gets "boiled" by the high charge rate of the other.
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