battery question...

raider

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i have an extra 24 series marine battery i'm gonna hook up in parallel to an existing 27 series starting battery in the boat... i now have 2 graphs wired to 2 30's for the trolling motor, and am gonna add a powered anchor winch, so i have to do some wiring anyway... my plan is to move all the 12 volt stuff to the starting batt(s)...

here's my question - i should be fine to run the 24 and 27 in parallel to run the ignition and graphs and winch off of them right??? also, does it matter which battery is in what position or which one gets hooked up to for the cranking vs graphs???

i've heard not to run different size batts in series, but i don't think it would matter in parallel... anyone else done this???

thanks...
 


Wild and Free

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Thing to remember about mixing and matching batteries no matter if it's in series where it doubles the voltage or in parallel were it doubles the amperage your system is only going to be as good as the weakest and smallest battery in the system.
 
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Motor Mouse

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Well, current will flow equally from each battery regardless of which battery you hook the load to. That is as long as the voltage drop across the jumpers you use to tie the batteries in parallel have negligible voltage drop. I suggest using 6 guage copper stranded wire and keep the length as short as possibe. + to+ and - to -.
Your ampacity will be 2x your smallest battery, not totally true but close enough. Wild and Free is right on in his writing that the most important thing is that both batteries are equally good or healthy in their life cycle. Your system will only be as good as your weakest battery regardless of if that is the 24 or 27 group size. The weaker battery as it discharges will draw the good battery down. Also never mix battery types ..... lead acid goes with lead acid and so on.
 

SDMF

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I'd isolate the 24 as your dedicated "electronics" battery and the 27 as the dedicated starting battery. Run a single channel 10amp onboard for the electronics battery, your big motor will keep the starting battery charged.
 


raider

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thanks guys... i was thinking i could run them together and increase my amp hours and safety factor without having to add another charger...

is there a formula on what the combined reserve would be by hooking 2 up???

mm, you wrote this - "Your ampacity will be 2x your smallest battery, not totally true but close enough" - is that another name for total amp hours???

thanks guys...
 

Motor Mouse

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Yes your amp hour rating will basically be the amp hour sum of the two batteries divided by two. That is if both batteries are still capable of full charge. In parallel your amperage capability sums and your voltage stays the same. Your setup will work fine if both batteries are good it will take twice as long to charge the two batteries in parallel than a single battery. Just like the current supplied to load comes from each battery equally, a similar situation happens when charging. The current splits but in the case of charging the current to each battery can be different depending on internal resistance and other factors. If your batteries are good do it.
 

SDMF

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thanks guys... i was thinking i could run them together and increase my amp hours and safety factor without having to add another charger...

If you're going to try and charge both batteries off of one bank of a multi-bank charger you have to remember that you essentially cut amps in 1/2 on that bank. An 8-10 amp bank would be basically putting 4-5 in each battery and you won't get a full charge if just plugged in overnight. Could make for some irritating sunday afternoons if you're ready to head home and the boat won't start.
 

LBrandt

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My Alumacraft came with one 27 series in the front for trolling motor. After 5 years I replaced with 2 29 series. If I am out for the weekend I just have a little trickle charger that I hook up over night and I am good to go for the whole weekend.
 


Motor Mouse

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Hey! I screwed up. Yes your amp hour rating will basically be the amp hour sum of the two batteries divided by two
thats not right .... it is the sum but not divided by two. Your total amp hour will be about two times your group 24. Sorry
 

raider

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If you're going to try and charge both batteries off of one bank of a multi-bank charger you have to remember that you essentially cut amps in 1/2 on that bank. An 8-10 amp bank would be basically putting 4-5 in each battery and you won't get a full charge if just plugged in overnight. Could make for some irritating sunday afternoons if you're ready to head home and the boat won't start.


i haven't used a charger on my starting battery in years... i watch the voltage on the graph and always have jumper cables with me as well... was hoping that by adding the other battery i wouldn't have to worry about running the graphs and anchor on the pair... i usually run a ways to fish and am hoping the alt on the 150 will take care of the starters... i'll start that way this spring and add if i need to...

- - - Updated - - -

Hey! I screwed up. Yes your amp hour rating will basically be the amp hour sum of the two batteries divided by two
thats not right .... it is the sum but not divided by two. Your total amp hour will be about two times your group 24. Sorry

that's what i was thinking... thanks!!!
 

weedy1

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I had a Lowrance depth finder hooked in to my starting battery circuit and ignition switch. I had trouble with the depth finder turning off when I used the ignition switch to start my boat motor. Good friend informed me that problem was caused by the short term minor voltage drop that occurred when the motor starter drew on the 12 volt (size 24) circuit. I fixed the problem by running a separate set of wires to one of my trolling motor batteries ( 2 size 27's in series for my 24 volt trolling motor). I also added a switch in my dash to the new wiring to make sure I didn't have a hot open circuit when the depth finder was unplugged. Worked like a charm!
 

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