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| I imagine this has been discussed a couple times but I haven't seen it/found it.
Would I get more amp-hours by running two 12-volt batteries in parallel or by running one down at a time? I can't think of any reason that running them in parallel would give you more work, but saw it mentioned in another thread so thought I'd ask.
Currently I always have two trolling motor batteries on board anyway if I'm fishing for more than a few hours in good winds. I run one down then swap them out. But if running them simultaneously is actually better, I'll get set up to do that.
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Posts: 4269
Location: Ashland WI | Two in parallel is much better.
I learned that from a site member. Hopefully he responds and can explain it. Something about two batteries halves the amp draw to each one which greatly increases run time. |
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Location: Elk River, MN | Also you're putting less strain on each battery, ultimately increasing their life |
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Posts: 1828
| 14ledo81 - 5/25/2016 5:02 PM
Two in parallel is much better.
I learned that from a site member. Hopefully he responds and can explain it. Something about two batteries halves the amp draw to each one which greatly increases run time.
Thanks. Hopefully the guy sees this and explains. Yes, running them parallel means each pushes half the current as if being used solo, but that doesn't explain why you'd get more run time than by running one down, and then the other.
Espy - 5/25/2016 5:19 PM
Also you're putting less strain on each battery, ultimately increasing their life
Does a battery have a longer overall life if drained [and charged] slowly? That would make it worth it. |
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Posts: 1209
| On a nice day with parallel only discharging to 70% each would be less wear on both compared to one at 40%. But I would guess it would be more like 30% on one. As the battery dies you go from power 3 to say 7... Heat is a waste of power and I bet one battery trying to run at 7 on the troller makes more heat. Also I don't think it is a linear graph for how batteries power depletes its charge but more of an exponentially growing graph . I see no reason not to parallel them, Save you the 2 minutes on the water switching the cables at least. No wing nuts in a boat.
Edited by Fishysam 5/26/2016 8:01 AM
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| Fishysam - 5/26/2016 8:00 AM
As the battery dies you go from power 3 to say 7... Heat is a waste of power and I bet one battery trying to run at 7 on the troller makes more heat.
This makes sense. As the voltage drops in a battery, the current has to increase to meet the same power demand. More current means more heat loss, so then the battery is running less efficiently.
So running two in parallel maxes the amount of time that the batteries can be more efficient, i.e. work at the high end of their voltage curve.
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| Nailed it in better words than my own. |
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Posts: 1828
| I actually did a little more research and thinking too...
A battery gets its maximum amp-hours with low current draws. So to put it simply but quantified, if a battery is rated for 100 amp-hours, you might get 50 hours with a 2-amp draw, and maybe 33 hours at 3 amps, but at 10 amps you won't get the 10 hours that you might assume. and at 20 amps, much less than 5 hours.
And that's aside from the efficiency considerations related to heat loss.
So parallel is the way to go. |
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Location: Ashland WI | I found it. I cut and pasted (and saved) this when I read it.
Multiple batteries
With respect to multiple battery's and paralleling them. If you have multiple batteries for your 12V trolling motor, the best method of use is to parallel them. This will give you maximum runtime. I said this before in another battery question thread.
f.ex. If you have two (2) batteries that are marine rated @180 minutes. That rating means those batteries can discharge 25A for ~180 minutes before they are considered as discharged and require recharging.
Now if you use them individually by swapping them you will get a total of ~ 360 minutes. However, if you parallel them you will get ~ 1080 minutes. I know it sounds crazy, but it's absolutely true. It's a phenomena of the electro-chemistry of a lead-acid battery. Many folks think of a battery as synonymous to a gas tank i.e., where you have fixed quanity of fuel. Well that's really not true.
With a lead-acid battery the amount of energy/power available for use is dependent upon the rate of use. The lower the rate, the more energy that is available. The available fuel qty changes with rate of use even though the battery doesn't change it's physical size.
The rule of thumb for any lead-acid battery: If you double the load-the runtime will be 1/3 of the original. Conversely if you cut the load in half the runtime will triple.
If we are discharging the battery at 25A we get 180 minutes. However, when in parallel with another battery of the same AH rating. The load on each is cut to 1/2 or 12.5A. Shazam! So with two batteries in parallel you get 3 x the original runtime 3 x 180 x 2 = 1080 minutes.
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| 14ledo81 - 5/26/2016 3:19 PM
I found it. I cut and pasted (and saved) this when I read it.
Multiple batteries
With respect to multiple battery's and paralleling them. If you have multiple batteries for your 12V trolling motor, the best method of use is to parallel them. This will give you maximum runtime. I said this before in another battery question thread.
f.ex. If you have two (2) batteries that are marine rated @180 minutes. That rating means those batteries can discharge 25A for ~180 minutes before they are considered as discharged and require recharging.
Now if you use them individually by swapping them you will get a total of ~ 360 minutes. However, if you parallel them you will get ~ 1080 minutes. I know it sounds crazy, but it's absolutely true. It's a phenomena of the electro-chemistry of a lead-acid battery. Many folks think of a battery as synonymous to a gas tank i.e., where you have fixed quanity of fuel. Well that's really not true.
With a lead-acid battery the amount of energy/power available for use is dependent upon the rate of use. The lower the rate, the more energy that is available. The available fuel qty changes with rate of use even though the battery doesn't change it's physical size.
The rule of thumb for any lead-acid battery: If you double the load-the runtime will be 1/3 of the original. Conversely if you cut the load in half the runtime will triple.
If we are discharging the battery at 25A we get 180 minutes. However, when in parallel with another battery of the same AH rating. The load on each is cut to 1/2 or 12.5A. Shazam! So with two batteries in parallel you get 3 x the original runtime 3 x 180 x 2 = 1080 minutes.
does the same apply to other batts ? AGM etc...
thanks very informative ! |
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Location: Mauston, Wisconsin | There is a "rule of thumb" for deep cycle batteries - If you double the capacity, you will get ~ 3x the runtime. Conversely if you double the load the runtime will be 1/3rd. Sounds funny but batteries are funny too.
Parallel them, you will get more runtime!
Have fun!
Al |
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maxing amp-hours with 12V trolling motor