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berfish

Two Batteries on a 12V system

20 posts in this topic

I have a 55lbs. 12 Volt Minnkota Maxxum bowmount on my boat. I have a brand new battery last year. I noticed last year when I was camping that my battery wouldn't last as long as I want it to. Is it possible to hook two deep cycles to my motor as long as it is not ran to be 24V? I am pretty sure it is but would it extend my battery life? Anyone else doing this?

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Yes it can be done - you hook the two batteries in parallel = 12V system with 2 12 V batteries. You need a couple of jumper wires - depending on distance 12 awg would be enough if they're close together, further apart go 8 awg (6 feet or more). Positive from battery 1 to positive of battery 2 to positive of trolling motor (or whatever). Negative battery 1 to negative battery 2 to negative on trolling motor. You will definately get a longer life, not sure if it is twice as long but it is longer. I used this system on my old boat and it worked well. Good luck. Paul

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Yes it will work. I'm doing it now and it works great. Just hook the + to + and - to -

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I agree with the others, putting in a 2nd battery wired in parallel will just about double your run time. You want both batteries to be roughly the same age and conditon, or the weaker battery will draw down the stronger battery.

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Good point PJ - I forgot to mention that but if possible, by two of the same batteries. Paul

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Can you charge in parallel at home as well? Or would you need to unhook and charge each battery individually? I've been curious about joining a few batteries.

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You can charge them with one charger and the batteries hooked together but your charging time will also be doubled.

If one battery is weaker than the other the charger may boil the better battery while it tries to fully charge the weaker battery.

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I agree with that also, you can charge them in parallel, and may run into those issues if your batteries are not in the same condition.

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Quote:
If one battery is weaker than the other the charger may boil the better battery while it tries to fully charge the weaker battery.

I'm not sure I agree with this statement completely. With two batteries connected in parallel, they are no longer really two batteries unless you're jumper wire is inadequately sized and causing excessive resistance. With both together they balance each other out. If one battery developes a short-circuit, both batteries will discharge. Both batteries see the load together and both batteries are delivering charge.

Think of it this way. Supppose you have two 50-gal. drums connected together witha a 2" pipe between them.

If you connected a 1/2" water line to one of them and began to drain the water, the water won't just drain from the one barrel until empty and then start draining from the other. It would drain from both barrels simultaneously for all practical purposes only being limited by the flow capacity of the 2" pipe between them. If the other barrel developed a gaping hole, both barrels would drain. If you attempt to refill one barrel, both will be refilled together.

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I've done this for 10 plus years on the same set of batteries.Need a good automatic charger so you don't overcharge the batteries..

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I don't agree with your 50 gallon drum anology Bob. The 50 gallon drums are of equal size and at the same level.

With one battery being weaker than the other the excessive resistance is in the battery itself not in the jumper between them.

It would be like one of the 50 gallon drums was older and was rusting on the inside. The rust is partialy blocking the oulet. When you are drainig the barrels, the newer one will drain faster because the rust is resticting the flow of water out of the older barrel. When you are filling the barrels the newer one will fill faster because of the same restiction and will overflow before the old one is full.

I'm not a physicist, don't play one on TV and I didn't stay at a Holiday Inn Express last night so I may be way off, but it makes sense to me.

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I ran two batteries in paralell for a few years on my 55lbs minnkota before switching to a 24v motor. Worked great!

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Sweet! I will have to check it out. My battery is new two years ago. I was thinking of getting a cheaper priced battery from Wal-Mart. But after reading that everyone who has done this used two of the exact same batteries I might have to get another one of my more expensive ones. Thanks guys!

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For all practical purposes and in simplest terms:

If when you say "weak" you are referring to one battery that is not fully charged then the result will be a total power somewhere between.

Connect two 12v batteries of different capacities together and you combine their collective capacities. For example, connect a 80AH and a 105AH together (30gal. drum to a 50 gal. drum) and you have about 185AH capacity. When you put a load on the batteries, you will draw power from both batteries simultaneously less the effect of the resistance of the wire between.

If both batteries are of the same capacity and not damaged and you connect one battery that is not fully charged to one that is 1/2 charged, the partially charged one will draw what it can from the other one. The result will be that they will balance out at about 3/4 charge each. Sort of like having one barrel 1/2 full and the other full and then connecting them together. The fluid will flow to balance them out.

If when you say "weak" battery you are referring to one that is damaged and partially shorted. A battery in this condition will not fully charge if at all and will be a load on the other. If it can't hold a charge it will drain them both until dead.

If one battery is damaged as in partially shorted, it will deplete the other one. The total power of the two batteries is only as good as the weakest link. I had this happen with my truck. One battery developed an internal short-circuit and destroyed both because it became a load on the other until they were both dead. That's the risk you take when using batteries in parallel. A "weak" battery in a parallel system is like one of the barrels getting a hole and the water draining. It will drain both unless you disconnect them.

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Now that is an explanation that even I can understand... wink

Which leaves me with this question. I have a 2 bank charging system hooked to my boat battery's as prescribed by the manufacture. Works great.

Now if I paralleled/series the two battery's as one,(continued 12 volt service) do I need to reconfigure the way I have my charger unit leads hooked to the battery's?

Thanks...

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If you leave it as parallel (12V), then yes... you can leave the way your batteries are charged right now.

If you wire it as series (24V), then no... unless you have a 24V charger, you'll need a 2 bank charger.

marine_man

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Thanks marine_man. That was quick.

I'm learning. If I can only remember the difference between parallel and series. That's why I continue to read. Parallel is what I desire and will continue to use for now.

Thanks...

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The other option is to leave the batteries completely seperate and have a plug for each one to plug the trolling motor into. This is how I ran all last summer and it worked great.

The advantage to this is the batteries do not need to be matched and you can tell if one of them is bad/weak. Physical location does not matter also. This was the predominate reason I did not connect my batteries in parallel, one was near the bow and the other was near the transom.

The disadvantage is you need to have 2 chargers, a 2 bank charger, or charge one battery and then move the charger over to charge the second.

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Thanks for the info FunFish.

Currently my 2 battery's are located in the boat manufacturer stern compartment, side by side. I have a stern mount Vantage 12 volt trolling motor that I have never used much and find that I may be using it more. The battery for the trolling motor gets very little use if any and I have often entertained paralleling the 2 together. I guess I was intrigued by some of the reply's here and maybe even a little motivated to make it a reality if it indeed does give me longer lasting power.

Sorry to have high jacked this thread but I find the information from all very refreshing if not educating...

Thanks....

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