Posts: 2753
Location: Mauston, Wisconsin | wkeegobbler - 12/23/2008 8:56 PM
Maniac,
So you are saying I would be just fine if I didn't have another Trojan battery? I can't have two of them because they both won't fit in my storage area...too tall for it. BUT.....I tried an interstate battery from my Travel trailer and hooked everything up today and the trolling motor ran...so that is good FOR ME 
So it sounds like I can just get another TM battery that will fit and as long as I charge them separately...which I was plannning on doing anyway...I should be fine?
Yes - You will be fine. Another "EM" trick for a tight space is to use a valve-regulated lead-acid battery (VRLA), more commonly known as absorbent glass mat (AGM) or gell cell or no-maintenance, and to install them on their side. They can actually be operated in any posistion except upside down, i.e., there is no free electrolyte to spill. But I would also put a 1/2 " rubber shock/vibration pad under the battery to lessen the shock/vibration.
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.
Recharging - the right size charger is important in recharging your battery. It takes longer to recharge a battery that has been discharged at a low rate versus one that has been discharged at a high rate. If we roughly calulate the watt seconds of energy removed for each of the examples above, we can see that we have removed more energy from the battery with the lower discharge rate.
25A x 12V = 300W 300W x 180 minutes x 60 sec = 3, 240,000 watt/sec
12.5A x 12V = 150W 150W x 540 minutes x 60 sec = 4,860,000 watt/sec
This means recharge time required is also variable based on how you discharged the battery. How big of a charger do you need - generally the charger should have an ampere rating (A) that is from 5% to 20% of the battery ampere/hour rating. F.ex. a 100AH battery would require a 5A to 20A charger for proper recharge. Don't go smaller or don't go larger. Either would likely damage the battery. 100AH and 80AH are very common battery raings for trolling motor's. I have two (2) 100AH batteries and I use two (2) 15A stand-alone battery chargers to recharge. them.
As Shep said, paralleling 3 batteries will really boost the runtime. But also it was pointed out that you need to increase the charging power. You can use a single charger or you can use three separate chargers or a three bank charger to recharge them, i.e., each battery needs to get recharged within a reasonable period of time, typically 8-24 hrs. My batteries are usually fully recharged after ~12 hrs.
Merry Chrismas!
Al
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