Posts: 2754
Location: Mauston, Wisconsin | I also have a 12/24V on my boat. Having been involved in UPS/battery industry and use of batteries for deep cycle applications for the past 25 years, I'd suggest using the trolling motor at the higher voltage (24V) and lower power setting (1-3) versus using the lower voltage (12V) and a higher setting (4-5) for the majority of your usage.
The problem with the 12V setting is that you are seriously depleting one battery because of the higher current draw. Then when you switch to the 24V mode you already have used one half of your battery power in that 12V setting. If you use the 24V mode you will see significantly better performance from your batteries because of the lower current draw. FYI- I only use 12V when I really want to sneak around in shallow water.
OK, I'll say this one more time here on MF for those whom are not familar with lead-acid batteries and their design/discharge characteristics. Many folks think about an individual battery (jar) as the equivalent to a tank of gas, i.e., there is a linear relationship between capacity and consumption rate.
This is an absolutely false assumption for a battery, F. ex. if I have a 5 gal gas tank and consume the gas at 1 gallon per/hour, I would expect the tank should last ~5 hours. Likewise, if I consume the gas at 2 gallon per/hour the tank should last ~ 2.5 hrs.
The problem with lead-acid batteries is the capacity versus consumption rate is nonlinear. If I have a marine battery rated at 180min reserve capacity at 25A discharge rate. That battery will last for ~ 180 minutes @25A (~3 hours@ ~300W). Now if I change the discharge rate to 50A (600W), most folks would expect that battery to last for 90 minutes (1.5 hrs). That's false, that same battery will only last ~ 60 minutes (~1 hour) @ 600W. Likewise, if I drop the discharge rate to 12.5A (150W) that same battery will now last ~ 540 minutes (~9 hours).
Now lets look at a 12/24V system with two (2) 180 min reserve marine batteries in series. The math would seem to say that 180+180 = 360 miinutes @ 25A. Let's assume I deplete one battery using the 12V mode @ 25A discharge (300W) setting. So I get ~ 180 minutes(3 hours) @ 300W from battery number one. Now I switch to 24V mode to get more juice at the same power level/thrust (300W). What happens? Well first, battery number one is empty, that leaves just battery number two to supply the total power. The trolling motor is now demanding the same power (~300W) from both batteries. But only battery number two can supply the power so I get another 180 minutes (3 hours) or a total of six hours.
If I had simply operated the trolling motor at the same power level/thrust (300W)using the 24V @12.5A setting I would get ~ 9 hours.
The rule of three's for lead-acid batteries- "Cut the load in half you get ~ 3 times the runtime. Double the load you get ~ 1/3 the runtime."
When it comes to batteries- don't expect miracles! You have to take care of them and they will perform (much like a wife). However, just like a wife, they won't live forever!
Al
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