Is it OK to mix battery types??

[sprintstream]

I have a new trailer along with its accompanying new 12v deep cycle battery. I also have a nearly new large 12v diesel starting battery just sitting in the garage. I would like to hook them up together to improve my available AmpHrs. Is there any problem to doing that I don't see? It irks me to have the diesel battery sitting there doing nothing. (yes, I know to hook them in parallel)

[valhalla360]

wa8yxm wrote:
Well there are a few significant differences but not all that significant,.

The Big D'a cranker is a STARTING battery should be kept 80% or fuller State of charge the "Deep Cycle" is most likely a MARINE/deep cycle. note the change of case with a recommendation to keep it over 75% full

True DEEP CYCLE go down to 50 (or less with some of 'em) before serious damage happens,.

Beyond that I've seen all sorts of posts saying "DO NO MIX" Ignore them.
The stronger one will be dragged down by the weaker NO IT WILL NOT
The smaller one will charge faster and tell the converter to stop charging NO IT WILL NOT in fact that is impossible.

One exception and this applies if batteries are mixed or identical IF you have batteries in parallel, mixed or identical matters not. and one gets a SHORTED CELL then it will drag the entire bank down.. But Mixing makes no difference.

NOW: batteries in SERIES (Example 2 GC-2's to make 12 volts) or a 5 Cell Streamlight or maglight flashlight) The batteries need to be matched.. Replace one replace all.

That is where the myths about parallel hookups come from But for parallel they are myths.

ON one more comment
Mixing gc-2 pairs with 12 volt cans.. I do that. no problem
One pair GC-2
One each group 27 29 and 31 all working together flawlessly

(Note I'd have all 31's save the store did not)

Lead acid batteries are only happy at 100%...deep cycle simply tolerates deep discharges a bit better. They don't like to sit at 50%.

The rest about myths...simple math...if you parallel the batteries the voltage between the incoming and outgoing cable must be at the same voltage...so the terminals on either side of each battery must have the same voltage...if one battery is running a higher voltage, it will force a current to the lower voltage battery. If it's a small difference, you may not notice it but it will be happening.

If you have batteries of different group but otherwise same design, if they are new, you can probably get away with it because they will be in similar condition and not much difference in input/output voltages.

[wa8yxm]

Well there are a few significant differences but not all that significant,.

The Big D'a cranker is a STARTING battery should be kept 80% or fuller State of charge the "Deep Cycle" is most likely a MARINE/deep cycle. note the change of case with a recommendation to keep it over 75% full

True DEEP CYCLE go down to 50 (or less with some of 'em) before serious damage happens,.

Beyond that I've seen all sorts of posts saying "DO NO MIX" Ignore them.
The stronger one will be dragged down by the weaker NO IT WILL NOT
The smaller one will charge faster and tell the converter to stop charging NO IT WILL NOT in fact that is impossible.

One exception and this applies if batteries are mixed or identical IF you have batteries in parallel, mixed or identical matters not. and one gets a SHORTED CELL then it will drag the entire bank down.. But Mixing makes no difference.

NOW: batteries in SERIES (Example 2 GC-2's to make 12 volts) or a 5 Cell Streamlight or maglight flashlight) The batteries need to be matched.. Replace one replace all.

That is where the myths about parallel hookups come from But for parallel they are myths.

ON one more comment
Mixing gc-2 pairs with 12 volt cans.. I do that. no problem
One pair GC-2
One each group 27 29 and 31 all working together flawlessly

(Note I'd have all 31's save the store did not)

[old_guy]

no any battery person will tell you to use same age and same type batteries on any thing. My brother was a battery store owner and he told me that. I trust what he told me. even if you look up rv education 101 or Dr rv they will tell you the same thing

[pianotuna]

Seeing as the "new" battery is a "marine cycle" and not true deep discharge, the "old" diesel battery, while not a perfect match, will work.

In an ideal world all the "house" batteries would be deep cycle, and identical in all possible ways.

[sprintstream]

I really appreciated the "balanced battery" post. Made a lot of sense. I'm thinking if I can figure out a good battery tray (yes, the diesel battery is big) that just might take care of the issue of old/new mix. Thanks for the comments.

[dougrainer]

You can add mixed and mixed age batteries. But, it is not a good idea. A older battery will cause problems with the newer battery and can cause the newer battery to fail prematurely. Interstate Battery, one of the largest makers of Batteries in the USA, They have a policy, If just ONE of their batteries fail during warranty, they replace ALL the batteries in that battery bank. Now to your situation. The RV battery SHOULD be a deep cycle battery. A Chassis battery is NOT. So you will be mixing a Deep Battery with a standard Battery, which is not good. Doug

[valhalla360]

Generally a bad idea. It's unlikely that they will hold a consistent charge which means the weak battery will constantly be drawing the better battery down.

If you want to take the trouble to wire them up as separate banks that are kept isolated except for charging, that's OK.

[2oldman]

Other than what that diesel battery must weigh, it would work.

[Dusty_R]

After we had had our mh for a couple of years, that only had one battery, I wanted to add a second battery. So I added a double throw switch, which allowed me to use only battery at a time and switched them every day. But paralleled them while driving to charge them. That kept them rather equally charged, and worked fine.

[pianotuna]

Be sure to wire the batteries in a balanced manner.

This is what is balanced and best for twin twelve volt batteries.



As it often doesn't cost a dime more to do this, I think it is worth the trouble.

If you wish to understand the "why" surf here:

correctly interconnecting multiple twelve volt batteries

[midnightsadie]

I don,t agree one battery could be weak ,it will draw the other down. you,ll get some input just wait, I,ve all ways been told mixing battery,s a no,no. sell that diesel battery and get two the same.

[time2roll]

Yes it is OK. May as well put them to use.

[Lwiddis]

Starting batteries don’t do well with long, low amp hour discharges. Not built for that type of service.