I have a spare car battery that I would like to hook up to the 2 deep cycle batteries of my RV.I want to use it as an emergency battery, so I would not attach it permanently. I'd connect it for charging and then when I dry camp and the other batteries get empty I would use the car battery.I tried to put the + of the car battery to the + of the rv and the - to the -, but I get strange readings on my multimeter.From what I read the + cable of the rv goes in one battery and the - cable leaves from the other battery. I can't do that with the spare battery because I don't want to disconnect the battery pack when I attach the spare battery.What can I do?

The two batteries both appear to have corrosion on the terminals which should be cleaned. What are the strange readings? The wires on the green battery are much to small to do anything useful - maybe trickle charge over a long period of time maybe days. Have the green battery (or even all three) load checked at a auto store to make sure it's OK.+ to + and - to - is correct. Connect the green battery to the upper left terminal and the lower right. It appears that the batteries and the two are connected in a balanced manner which is good. But the cables are much longer than needed.

Wire looks thin. Several terminals could be cleaned.Best might be a battery selector switch to choose the spare, existing or both.

picture finally showedi agree..clean the terminals ..and try again with larger wirebut your spare on a battery chargercharge it and test itmaybe it is Not so good

If you can change out the wire used on those two battery clamps, I would. Use a min. 8 gauge wire. If not, maybe get THESE CLAMPS & 8 guage wire.

Yes I know the wires a too thin, I will buy #8. Thanks for the advise cleaning the connectors, will do...Both battery are somewhat loaded (12.4 volt)When I connect the green (car) battery to the left of the RV batteries the meter jumps from 3-9 volts. I didn't had it connected long, because i am scared to do some damage.If I connect the green battery the reading on the meter should still be 12 something, right?Again this is only for emergency purposes. Or maybe, since I have the battery I could hook it up permanently. Or does it hurt when you mix battery types?

Additional Battery | Good Sam Community

16 Replies

wolfe10
Explorer
Dec 09, 2014
Best practices= http://www.perko.com/catalog/battery_switches/150/medium_duty_battery_disconnect_switch/
MrWizard
Moderator
Dec 09, 2014
i don't think he said jump UP 3~9

i think he means goes DOWN to 3~9 jumping around

this means his "green" interstate car battery likely has a shorted cell
tenbear
Explorer
Dec 09, 2014
RJsfishin wrote:
How did 6v batteries get into this picture ?

Sorry about that. I fouled up.
tenbear
Explorer
Dec 09, 2014
2oldman wrote:
tenbear wrote:
The 12v battery clips in the photo are resting on the same 6v battery.
Those aren't 6v batteries.

Yeah, I just realized that. Sorry for the mistake.:S
RJsfishin
Explorer
Dec 09, 2014
How did 6v batteries get into this picture ?
2oldman
Explorer II
Dec 09, 2014
tenbear wrote:
The 12v battery clips in the photo are resting on the same 6v battery.
Those aren't 6v batteries.
tenbear
Explorer
Dec 09, 2014
Bad post, sorry.
tenbear
Explorer
Dec 09, 2014
Oops, sorry.
time2roll
Nomad
Dec 09, 2014
ThomasM wrote:
Both battery are somewhat loaded (12.4 volt)
When I connect the green (car) battery to the left of the RV batteries the meter jumps from 3-9 volts. I didn't had it connected long, because i am scared to do some damage.
I assume a false reading from poor connections.
No possible way you can properly connect two 12v batteries sitting at 12.4v in parallel and get a voltage rise of 3 to 9 volts.
CA_Traveler
Explorer III
Dec 09, 2014
Yes, 12 something. Where is the meter connected, green battery? Bad batteries can do strange things. Load testing gives you a known good starting point. Are you connecting with the right polarity?