Mixing Car and Marine Battery

[canadianllama]

I have a 2year old car battery and was wondering of it's OK to hook it up to my marine battery so I'll have the 2 instead. Make a Little battery bank out of them. Both are 12 volt.

Thanks

[Cheechako]

I've done it with no ill effects. Doesnt mean it's a perfect situation. Do you have a batt switch to use one or both? Your marine battery is likely deep cycle, so with the two combined you wont be cycling it very deep in the discharge. No big deal really.

[canadianllama]

I believe I have that switch, did you use them together or separately.

[Cheechako]

I've done it both ways. When I sailed to mexico in my 30 footer, one of my brand new marine batteries went bad. This was the 80's and I had no fancy charging or monitoring stuff really. I ended up buying a car battery in Mexico and used those two together the whole 1.5 years I was down there.
On another boat I had a car type starting battery that was used with a gel battery house bank. I had no isolator so would charge both at once and then (hopefully) remember to use the switch. In a perfect world it would be nice to have matched batteries in a bank together. But hey,boating aint perfect!

[rwidman]

Batteries to be connected in parallel to increase current capacity should be the same brand, same type, and same chemistry, and the same age. You can connect dissimilar batteries in parallel temporarily (jump starting, for example), but otherwise, they won't discharge evenly and they won't charge correctly.

You can do it and believe you've accomplished something but it's not going to work well in the long run.

[GordMay]

Originally Posted by rwidman

Batteries to be connected in parallel to increase current capacity should be the same brand, same type, and same chemistry, and the same age. You can connect dissimilar batteries in parallel temporarily (jump starting, for example), but otherwise, they won't discharge evenly and they won't charge correctly.

You can do it and believe you've accomplished something but it's not going to work well in the long run.

Indeed!

[canadianllama]

Ok, so if I use the 1bat 2bat dial and have them separate then it should be ok, i guess that makes sense. Thanks

[deckofficer]

You have to remember that a car cranking battery uses many thin plates, thus a lot of surface area for high starting amperage. A deep cycle battery uses thicker plates and will tolerate 50% DOD (depth of discharge). If you cycle a car's starting battery to 50% DOD, you will be lucky to get 25 cycles vs the 400~500 cycles of a good quality deep cycle battery.

[eyschulman]

For what its worth the industry standard advice is hook up only same type and same age batteries(no mixing) That said we are dealing with sailors boaters and other free spirits here.

[Kettlewell]

In my experience you can add together batteries to get a larger bank, even if they are of different vintages and types, as long as you aren't mixing lead acid with gel or AGM batteries. Sure, a starting battery added to a deep cycle battery, and of different vintages, isn't the ideal way to go, but in practice there won't be a huge catastrophe or anything. In fact, I have found that some starting batteries seem to last just fine in moderate deep-cycle use. One advantage for cruisers of using traditional lead acid batteries is that you can do this with reasonable results when one of your expensive deep-cycle batteries dies and there is no way to get another one. There are car and truck batteries everywhere in the world there are cars and trucks, but marine batteries not so much. It can be handy to purchase some battery to keep you going until you can once again go back to the optimum arrangement.

[canadianllama]

Yeah i own a bare bones Grampion from the 70s, 26ft long. And i do everything I can on the cheap else I wouldn't be able to do it, I had a spare battery fro m a car that died and it's only 2yrs old, so I was wondering
1) will it wreak my marine battery or do something bad to my system

2) is it better as a single or as a bank.

I have a depth sounder and plugging cell phones and basically that's it - including a few cabin lights. So sounds like it should be ok to use as I don't really care to much about it.
Thanks for all the input

[rwidman]

Originally Posted by Kettlewell

In my experience you can add together batteries to get a larger bank, even if they are of different vintages and types, as long as you aren't mixing lead acid with gel or AGM batteries. Sure, a starting battery added to a deep cycle battery, and of different vintages, isn't the ideal way to go, but in practice there won't be a huge catastrophe or anything. .............

There won't be smoke, but it's less than ideal. The weaker battery will continually discharge the stronger battery and neither will charge correctly. If it's an emergency to start the boat to get to port, fine. If you set out to do this thinking it's an improvement, it's not.