Batteries again

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Don Dalgity3

Dec 27, 2000
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I'm running a 12 volt system on my trolling motor, but carry 2 batteries
Can I connect the 2 batteries ..pos-to-pos. and neg-to-neg and not harm the batteries.
Also when charging, will both charge at the same time if I connect only to one, these batteries are both deep cycle, but different brands..does it matter
Any help on this will be greatly appreciated
to answer your questions,

Yes, you can hook them together that way.

Yes you can charge both at the same time, ( but it will take quite a bit longer to do.

And NO, it shouldn't matter as long as they are both deep cycles.

Now for my opinion, The are a few problems when hooking two batteries together. One is the extra time it takes to charge them, and the other is that you will not get as much run time out of them hooked together as you would if you ran one down and then swithced to the other. Two batteries hooked together will only be as good as the weakest battery. I mean that the good battery will try to charge up the weaker one and thus you loose run time. I would invest in a switch that allows you to use one then the other. and when you hook up the charger, you can set the switch to hook them both up to charge them at the same time.(I beleive Perko makes one.)

A dual bank or triple bank charger would be better, but the switch would be a whole lot cheaper.. .

Oh, boy..... Now I'm asking for it....... Smallest guy here dares to disagree (but only slightly!) with BIG John...... Sure am glad he's such a kind, friendly sort! (And 2,000 miles away!)

Don -

John is correct in answering Yes, Yes and No......

But both batteries do not have to be deep cycle to be run / charged together - only the same voltage.

After a day of use, charging two batteries together won't take any longer than just one. You are only going to draw so much power in a given period; two batteries just gives you more to draw from. The length of time is determined by the rate at which your charger can replace the power used.

You will get just as much time out of two hooked together as you would from them individually. Actually, if you do get to the point where you have run them that far down, it may actually SEEM that they are lasting longer.

You are right to a point. But when two batteries are hooked together they do become one larger battery. But they do not double your time you can use them. you will get a longer service life out of two batteries hooked together only if you compare it to how long a single battery would have lasted. But it will be shorter then if you would have just used one of the batteries then the other.

Batteries Equalize themselves when hooked together. So two Batteries that are hooked together will only be as strong as the weakest battery. Because when the weak battery starts to go dead, the strong battery will have to not only do it's work but also the work of the weak battery. Just like when you try to jump start a car whith a dead battery. By therory, hooking your car battery up to it is all you need to do to make the dead battery good. But sometime you will still need to let your cars altinator charge the dead battery for awhile before you can turn the dead cars motor over. that is because the weaker battery is drawing the amps in a attempt to recharge itself. So, instead of getting all 650 Cranking amps to the starter, you only get about half or less.

Now for the charging. Scott is correct in that amps used will determine how long the batteries will need to be charged. but compared to a single battery that is dead. a double battery will take longer to charge back up to full power. What I mean is that if you had a dead group 27 battery. It would take 6-8 hours to recharge compleately with a 10amp charger. Now take two group 27 batteries and hook them together. If they were dead it would take 12-14 hours to get them back up to full power. Because the two batteries would be sharing the 10amps. What it comes down to is whether you have the time to recharge 1 or both batteries before you need them again. That is why I recomend a charge with more than one bank. that way you can charge both batteries at the same time with both batteries getting 10amps.

As far as the batteries being the same type, Let me clear that up too. If you are just trying to charge them at the same time, then it doesn't matter which type they are as long as they are both 12volt like Scott said. But if you plan on using them hooked together they should be of the same type and size. Due to the Equalizing, if you hook a smaller battery up to a larger one, the smaller weaker battery will discharge faster making the larger one have to work harder to copensate. thus shortening its usable work time. Rmember, Deep cycle batteries are different then Cranking batteries. Cranking batteries have high Cranking amps but relitavly low reserve power. so if you try to use a Cranking battery and a deep cycle together. the Cranking battery will discharge faster then the deep cycle, making the deep cycle do more work.

I hope it all makes sence.


Thanks guys, but maybe I'll wait and upgrade to a 24 volt system next year , with a dual bank charger...LOL
I do appreciate your info..and I did get a response to my question 30 minutes after I posted...again thank you
Ifin your ever bassin in Ontario, Canada or just wanna fish look me up...

Don't hook a deep cycle and a starting battery together unless it is just a temporary measure.

Here's where it goes wrong: Starting batteries do not stand up to being deeply depleted. The thin lead plates that make a starting battery good at starting a motor deteroriate quickly when deelply discharged (deep cycle batteries have thicker lead plates). Eventually, one of the cells in the starting battery will fail and both batteries will behave as if they will not hold a good charge.

Not only should the batteries be the same type and capacity, but ideally, they should be about the same age and pattern of use. A weaker battery will make the entire battery bank behave weak.
John -

If disagreeing with you is what it takes to get you and your family back here in Missouri where you're appreciated - then that's what I'm gonna have to do! (Or maybe I'll just have to come out there to California!)

Sounds to me like we're all saying the same thing - just in different ways!

I'm so charged up I can hear my hair cracklin'!!......Then again,...maybe it's the caffeine! Good info guys....thanks!

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