Matching batterys

[TheBizMan]

Do the motor battery and the coach batterys have to be the same make and model?

[bigdipper]

Do the motor battery and the coach batterys have to be the same make and model?

No,but coach batteries ought to be identical, including in age.
Ralph

[whemme]

Also, there is a difference in the type of batteries used in the cab and coach applications. The cab battery is a normal starting type battery and the coach batteries are of the deep cycle type.

[TheBizMan]

Thank you, Thank you, Thank you. That is exactly the information I needed. Now I can get a new cab battery.

[Trix]

Newbie RV owner here, learning about batteries as I go along.

What I have is two 12v brand new (well replaced just before purchase in April 2012) Duralast Marine series 27 batteries. However one is a 27DC (deep cycle) with CCA 675 and AmpHrs 180. The other one is a 27DP-DL (dual purpose DL??) with CCA 800 and AH 115.

Imagine my horror to find out my coach batteries should be identical! That one is either being overcharged, or the other not getting enough charge! Good, reliable batteries are important to me, as I intend to boondock whenever possible.

Batteries were purchased and installed (many states away) by a national chain of auto repair shops. Is it reasonable for me to ask them to test the batteries, and request a replacement of the 27DP-DL to match the 27DC if it hasn't been damaged? Or do I need two brand new matching Deep Cycle batteries?

Thanks,

[bigdipper]

This is at odds with a more conservative position, but I would buy a new Duralast Marine.
Ralph

[whemme]

Trix,

Since the two coach batteries are charged in parallel, I would recommend that they be identical batteries (brand and model) and also of similar age. That would be the only way to insure that they share the charging current equally.

[Roger]

As a general rule, I wouldn't recommend connecting batteries in parallel. If you need more capacity, two 6 volt batteries in series is a better way.

[Trix]

Thanks for the replies.

Would testing the specific gravity of the electrolytes in these two batteries be a definitive test for damage? Or could they 'test ok' and still be damaged?

They are less than 60 days old, and under warranty.

[whemme]

Trix,

I would guess that neither of your two different types of batteries are damaged yet because they are still quite new. It is just that since they are different model batteries that since they are charged in a parallel connection, one battery may receive a major portion of the charging current and the other battery a minor portion. Having this resulting unbalance in state of charge may shorten the life of both batteries.

The shop that installed your two different model batteries should have known better since they are connected in parallel.

[Roger]

I've been pondering my answer for a while. I don't want to overload you with useless information.

First, a true deep cycle battery is designed to produce a reasonable amount of power for a long time such as in golf carts, floor machines, etc. A starting battery is designed to produce large amounts of power for a brief time. if a battery has a CCA spec, it's not a true deep cycle battery!

When paralleling batteries, the important criteria is the battery's internal resistance. This resistance produces a voltage drop when the battery is supplying power to a load. When two batteries are connected in parallel, the battery with the lowest resistance will supply more power to the load and will take more current when charging. Is this important? I think a lot depends on the application. For normal RV use where you are powering a TV, some lights and occasionally, the water pump, I don't think it is very important. If you are powering a 2000 watt inverter that draws 150 amps, then I think it could be a problem.

What I would do is keep a record of the cells specific gravity and water useage. A battery that is undercharged will have a lower SG while one that is being overcharged will use more water. Hopefully, there will not be a big disparity between your batteries.

A big factor in battery life is the type of charger you have in your coach. Mine came with a single stage charger that has been proven to fry batteries when left on for long periods of time. I changed to a modern three stage charger which tailors it's output to the battery's condition. These types of chargers can be left on continuously to keep the batts in top share.

[Mike Jean Bandfield]

IMO there is no likelihood of a dangerous situation or catastrophic failure. To the coach and its equipment, there is also no problem. However, two batteries of differing internal resistance will charge and discharge at different rates and will have parasitic currents flowing back and forth as they seek equilibrium when not charging. Even once they find equilibrium daily changes in temperature will change the balance and cause them to see each other as a resistive load or charging voltage depending on whether the temperature is rising or falling. Two matching batteries in parallel tend to remain in sync as they charge or discharge. However, even matching batteries arranged in parallel have small parasitic currents due to minor differences in mfg or materials. Hooking two 6 volt batteries avoids this problem. Hooking two dissimilar 12 volt batteries in parallel exacerbates the problem.

This will definitely shorten their life and they should never have installed dissimilar batteries in parallel.

Mike