Posts: 4269
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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