New camper battery in series with "older" battery?

[LT Traveler]

Hey guys,

Sorry to start another battery discussion but I need some quick advice. I must be getting old because I left the power on to the camper when we took it off the truck after our last trip and put it in storage. Loaded the camper up this past Thursday for a weekend of camping and the battery was DEAD. Drove home, plugged the camper in over night to house power and still bone dead the next morning. We were planning on leaving Friday for the weekend(my birthday weekend) so I scrambled down to Batteries Plus and after some discussion bought a brand new battery so we could then leave for the weekend.

The old battery is an AGM 80Amp Hr and it had 11% on it when I brought it in to Batteries Plus. I bought another AGM 80Amp Hr in the hopes of them being run in series.

Talked to the guys at the store today, the old battery is charged and has an 87% "rating" or usefulness or whatever it's called after being charged on their charger.

Here's the question.....what's the negative to putting the brand new and the old battery with a 87% rating in series? Any negatives, problems, etc? The guys at the store say to go for it, but want to run it past the experts before giving it a shot. This new battery wasn't cheap and don't want to mess it up somehow by going this route.

Thanks all!

 

[Ted]

They say you should only use batteries identical in age and type (not sure who THEY are). That said, about two or three years ago I added an Interstate AGM to my Optima when the Optima was about two years old. No problems so far. When these batteries get tired I'll buy two identical batteries.
BTW, if they are both 12v, wire them parallel, not in series. Wired in series doubles the voltage. Wired parallel doubles the capacity.

 

[UglyScout]

The two batteries will work together to become an '87%' battery (whatever that means).... When hooked together they will only be as good as the worst battery.

I say run it. If they are the same type/rating/capacity/brand/etc you should be fine (assuming the 'old' one is really at 87% of like new condition).

 

[MarkBC]

BTW, if they are both 12v, wire them parallel, not in series. Wired in series doubles the voltage. Wired parallel doubles the capacity.

+1

 

[LT Traveler]

Ok thanks for the help guys. In order to wire them parallel do I wire positive to positive and negative to negative terminals? Thanks, LT.

 

[MarkBC]

In order to wire them parallel do I wire positive to positive and negative to negative terminals?

Yes, that's it.

 

[LT Traveler]

Yes, that's it.


One final clarificatio...guys at the store are advising to run the camper positive and ground to the positive post on one battery and the camper negative wire to the second battery negative post. Then tie the batteries together as described above. Versus tying the camper positive and negative to one battery then tiring the batteries together as described above. Does that all sound right?

Hopefully that makes sense.

 

[pods8]

One final clarificatio...guys at the store are advising to run the camper positive and ground to the positive post on one battery and the camper negative wire to the second battery negative post. Then tie the batteries together as described above. Versus tying the camper positive and negative to one battery then tiring the batteries together as described above. Does that all sound right?

Hopefully that makes sense.


I don't think you meant to try ground in there. For starters there isn't a "ground" on DC systems only a positive and negative. Sometimes folks improperly say ground when talking about negative but regardless you wouldn't tie it into the positive battery terminal.


As for what you're describing I've heard that is the best way to setup dual parallel batteries to try and make sure current goes through the whole battery bank rather that just through the first battery of the bank with the others hanging off the back. I believe it helps make sure the whole thing is getting discharged/charged equally. However not sure how that factors in with different battery capacities.

 

[MarkBC]

One final clarificatio...guys at the store are advising to run the camper positive and ground to the positive post on one battery and the camper negative wire to the second battery negative post. Then tie the batteries together as described above. Versus tying the camper positive and negative to one battery then tiring the batteries together as described above. Does that all sound right?

Hopefully that makes sense.

I think you wouldn't go wrong following those "split the + and - between batteries" recommendation.
But, seems to me: If the two batteries are connected by heavy-enough-gauge cable so that resistance of that short-heavy cable is negligible, then the respective terminals (the two positives or the two negatives) on each battery are at the same potential. So the current will flow through both batteries equally -- i.e., it doesn't matter which battery you connect the campers wiring to or if you split it.
This is my theoretical analysis, which is what I applied in my truck...and I guess a key question would be "What cable gauge is 'heavy-enough-gauge'?"

 

[LT Traveler]

I don't think you meant to try ground in there. For starters there isn't a "ground" on DC systems only a positive and negative. Sometimes folks improperly say ground when talking about negative but regardless you wouldn't tie it into the positive battery terminal.


As for what you're describing I've heard that is the best way to setup dual parallel batteries to try and make sure current goes through the whole battery bank rather that just through the first battery of the bank with the others hanging off the back. I believe it helps make sure the whole thing is getting discharged/charged equally. However not sure how that factors in with different battery capacities.


Thanks Pods8, appreciate the info. I have a negative + positive cable from the camper to the camper battery and a third wire that I assumed was a ground...Now I don't know?

I've set it up with it running parallel and the charge running through the whole bank as you describe above. So far no smoke of flames!

 

[LT Traveler]

I think you wouldn't go wrong following those "split the + and - between batteries" recommendation.
But, seems to me: If the two batteries are connected by heavy-enough-gauge cable so that resistance of that short-heavy cable is negligible, then the respective terminals (the two positives or the two negatives) on each battery are at the same potential. So the current will flow through both batteries equally -- i.e., it doesn't matter which battery you connect the campers wiring to or if you split it.
This is my theoretical analysis, which is what I applied in my truck...and I guess a key question would be "What cable gauge is 'heavy-enough-gauge'?"

Thanks Marc, I appreciate you checking in and helping out. It's all set back up now and I'll report our finding on the next trip.

 

[pods8]

Thanks Pods8, appreciate the info. I have a negative + positive cable from the camper to the camper battery and a third wire that I assumed was a ground...Now I don't know?


These wires are coming from where exactly when you say "from the camper"? My FWC is older and had very limited wiring but your extra wire could be solar prewiring if you're talking inside the camper. If you're talking the camper pigtail to the truck I think the newer campers have a third wire for clearance lights. Lots of different possibilities depending on where you're talking about.

 

[LT Traveler]

These wires are coming from where exactly when you say "from the camper"? My FWC is older and had very limited wiring but your extra wire could be solar prewiring if you're talking inside the camper. If you're talking the camper pigtail to the truck I think the newer campers have a third wire for clearance lights. Lots of different possibilities depending on where you're talking about.


Not exactly sure where it comes from I hate to say...all wires come through the cabinets leading to places unknown. The camper is pre-wired for solar so maybe that's it or clearance lights. Really almost impossible to figure out where they lead back to because they're all hidden.