Batteries in Parallel behavior

[strnjss]

I've got a question about connecting batteries in Parallel, such as when jumping a car.

I know that when connecting two batteries in parallel that you are increasing the amp hours of the batteries.

But how do they behave when one battery is bad and the other battery is good?

Does the battery that's bad lower the output of the battery that's good?

For example, I have a booster pack that reads around 13v when not attached to the another battery (and load tests an output of around 800A).

However, when attaching it to a dead battery (reading 11.9v), the output of both together seems to be around 12.5v

The two combined batteries do not have the power to start the vehicle they are in.

If the dead battery was working right, it was rated for 615CCA, so I would assume just my jumper alone (which puts out around 800A (not cca since I haven't tested it at 0 degrees)) should be able to start the car, yet when I attach it to the other battery, it's as if there's not enough power still.

How do batteries in parallel behave when one is bad? shouldn't the other one be able to compensate, or is the bad battery causing too much resistance or something?

We're going to replace the bad battery, but I'm more curious about what's happening here with the batteries than the actual car itself.

 

[Gary S]

Anytime you put two batteries in parallel, it is best to use two batteries of the same size and age. As you found out, if one battery is weak, it will attempt to charge itself from the strong battery, and you lose power from the strong battery.
If one battery developes a shorted cell, it will drag the good one down with it.

 

[strnjss]

Anytime you put two batteries in parallel, it is best to use two batteries of the same size and age. As you found out, if one battery is weak, it will attempt to charge itself from the strong battery, and you lose power from the strong battery.
If one battery developes a shorted cell, it will drag the good one down with it.

Thanks!

 

[socapots]

like he said..
good answer

 

[RPH]

He nailed it!

 

[ddawg16]

That was a good answer....the only thing I can add is that if the weaker of the 2 batteries still has somewhat of a charge....the two batteries together will have a lower 'apparent' internal resistance......translation, they will be able to deliver a bit more current than just one battery......withing the limitation of the connecting cables....

Bottom line....if there is any significant differenc in charge between the two batteries, most of the current will flow to the weaker battery vs the 'other' load. Reason....path of least resistance....

Caution....if the weaker battery is bad...it could cause a fire or explosion....especially if your using heavy cables....nothing to limit the current....

 

[Lotek]

Same thing will happen with a dual battery setup. Had a customer's 07 diesel truck towed in, 1 original battery, one brand new sears battery. No crank, no start, couldn't even jump it. Hooked up our fancy new capacitance tester/charger to the new battery, tested 0cca 10v, hooked it to the original battery, showed battery discharged, it wanted to charge it an hour and a half and retest, about 10 minutes in, the battery started to smell like sulfer and get hot. Fail. Customer wasn't happy about needing 2 new batteries, but he should be able to get his money back from Sears, since they recommended only replacing the one, the other one tested good for them. Moral of the story? Change both batteries at the same time, with batteries that are the same size, even if only one tests bad, not a place to mix and match.

 

[strnjss]

This is great information everybody, thanks!

 

[larry_g]

Same thing will happen with a dual battery setup. Had a customer's 07 diesel truck towed in, 1 original battery, one brand new sears battery. No crank, no start, couldn't even jump it. Hooked up our fancy new capacitance tester/charger to the new battery, tested 0cca 10v, hooked it to the original battery, showed battery discharged, it wanted to charge it an hour and a half and retest, about 10 minutes in, the battery started to smell like sulfer and get hot. Fail. Customer wasn't happy about needing 2 new batteries, but he should be able to get his money back from Sears, since they recommended only replacing the one, the other one tested good for them. Moral of the story? Change both batteries at the same time, with batteries that are the same size, even if only one tests bad, not a place to mix and match.

I agree with you on the matched batteries however I had a Chevrolet diesel van that had dual batteries that were not the same from the factory. Looked in the book and sure enough they were correct. So don't assume that mismatched batteries are incorrect.

lg
no neat sig line

 

[Torque1st]

I agree with you on the matched batteries however I had a Chevrolet diesel van that had dual batteries that were not the same from the factory. Looked in the book and sure enough they were correct. So don't assume that mismatched batteries are incorrect.

lg
no neat sig line

No wonder Chevrolet became Government Motors.

 

[tcianci]

The reason you can have "mismatched" batteries in a dual battery system is because if the system is properly designed and connected, the batteries are isolated from each other with respect to being able to draw current from each other. They will charge from the same source but will not be able to draw each other down. Some dual battery systems connect both batteries in parallel while in the cranking mode. Many dual battery systems, in RV's for instance utilize a standard battery for cranking and a deep cycle battery for running the electrical devices in the camper. So to your dismay, it has nothing to do with General Motors taking government loans.

 

[Lotek]

The reason you can have "mismatched" batteries in a dual battery system is because if the system is properly designed and connected, the batteries are isolated from each other with respect to being able to draw current from each other. They will charge from the same source but will not be able to draw each other down. Some dual battery systems connect both batteries in parallel while in the cranking mode. Many dual battery systems, in RV's for instance utilize a standard battery for cranking and a deep cycle battery for running the electrical devices in the camper. So to your dismay, it has nothing to do with General Motors taking government loans.

All true, but the setup for the late diesel trucks has no isolator, and both batteries function as one bank. I have an isolated dual battery setup on my 75 with one battery functioning as a house battery for radios and trailer etc, but my 07 is one bank, hence the recommendation to replace both batteries at the same time.

What year van? Conversion?

 

[Torque1st]

I know ALL about battery isolators. This thread was about connecting batteries in parallel -directly-. Now if the GM system was using an isolator and not connecting them -directly- then the GM system with mismatched batteries was off topic.

 

[larry_g]

Jeeze guys your making me dig up very old memories. The van was ~1982 with a 6.2 and I believe that there was no isolator. If memory serves me correctly one battery was something like 1100cca and the other was 900cca. I also remember replacing them with a couple of 900cca as the 1100cca was an oddball to find. This is near 25 year old memory so take that into consideration.

lg
no neat sig line

 

[Lotek]

Well if it didn't have an isolator it isn't off topic at least.

Probably had a relay though, most older systems did.

 

[tcianci]

Isolator, relay no big deal. A relay can beused to isolate one battery from another by only putting them in parallel when they are being discharged. I have not seen any schematic diagrams for the type of install you are referring to but I highly doubt that ANY dual battery setup leaves the batteries in parallel when the truck is not operating. It is near to impossible to match batteries so perfectly that there would be no current flowing between them when they were at rest. The recommendation to replace them both at the same time is most likely because if the batteries are constantly used in parrallel, one battery had done everything that the other battery has done and if one has failed the other is not far behind and when either of the batteries have a change in internal resistance or an internal short, there would be a reduction in total power as one battery becomes part of the load. I hope someone can come up with a diagram. From my training, I can't see how you could leave the batteries connected constantly.