12v Battery Usage with Trolling Motor

[Bibz]

Hi Everyone,

I'm new to the forum and to inflatable boats and I have a couple questions regarding battery usage.

My setup is as follows:
- Seahawk 4 inflatable boat: modified with plywood floor and seats
- 30lb Minn Kota Trolling Motor
- Nautilus Deep Cycle Battery: Group 27, 705 MCA, 175 min Reserve Capacity, 90 Amp Hour
- NOCO Genius GC015 12V Eyelet Battery Indicator

I took out the Seahawk for the first time yesterday to go fishing and it was great - both the boat and the fishing! I was out on the water for about 3.5hrs and the trolling motor was at speed 1 for about 95% of the time. My battery was fully charged before I left the house to the lake (1.5hrs away) however, when I plugged my indicator into the battery it was displaying that I had 75% battery power left. It was at 75% power for majority of the time on the water but near the end it went down to 50% then to 25% when I had the motor at speed 5 going back to the dock. When I got home, before I plugged my charger to the battery I wanted to see how much juice was left so I attached the indicator and it was displaying I had 75% battery power left?

My questions are:
- Why was it showing I had 75% power left?
- How do I know how much battery power I have left overall?

I understand that the motor will draw more power when at higher speeds, battling against wind, going through weedy areas etc. which is why the indicator was showing I had less power at speed 5 but the indicator never went back up past 50% when I was at speed 1 so I assumed I was running out of juice.

The battery indicator I have has 4 displays - 100%, 75%, 50% and 25%.

Appreciate the help and input!

 

[JimS123]

I've only ever had one Minn Kota, so I'm no expert. But on mine the % display was just about worthless.

Get a volt meter and check the battery's voltage. Download a voltage vs. % charge graph and tape it to the side of the battery. That'll tell you what you have withing a couple percent.

 

[trey22]

If there is a load on the battery (trolling motor running or fish finder turned on or powering a radio, etc. ), the indicator will always give a false reading. You have to have zero load on the battery (nothing sucking juice) for an indicator to be semi-accurate.

Edit: I have a 100ah battery and 45lb thrust trolling motor on a 12' 6" inflatable and after spending a day on the water (8 hours-ish), trolling at least 75% of the time at various speeds, I'll have around 25% or less battery when all is said and done. I've never run dry. I'd love to get another 100ah battery as a spare that I can leave in my truck while on the water. Not cheap!

 

[Romans5.8]

If you can, figure out the amp draw at various speeds for your trolling motor (If you have a meter that can read amps; awesome! If not, google around. Or even search YouTube. I found videos of people testing their trolling motors and noting the amp draw).

I keep a list of amp draw at each of the 5 speeds of my Minnkota 55lb trolling motor, and a list of voltages for my type of battery (flooded lead acid). One of the issue with the meters is that unless they're programmable, they are pre-set to be 'universal'. Which works about as well as a one-size-fits-all shoe. AGM, Gel, FLA, SLA, etc battery types all behave differently as they discharge. Thus the voltage at various charge states is going to be different.

The most accurate reading is the one before you put it on the charger, after it cools down a bit and has rested on the way home without a load. (Technically, 24 hours of rest is the ideal time to check voltage for capacity. But that's also enabling sulfation to set in. Always get it on the charger ASAP.) On the water, the most accurate reading is the one with the motor off. If you wanna be REALLY accurate, you can find charts for your type of battery (Google suggests yours is a flooded lead acid battery. Not sealed/maintenance free, AGM, or gel. If it is, keep in mind some maintenance is necessary occasionally) at various temperatures and use a temperature probe. Temps affect voltage, too. So you can get a very good idea, if you know the battery temperature, of what your state of charge is. Technically, the MOST accurate test of a flooded lead acid battery involves using a hygrometer on the electrolye solution itself and noting the specific gravity. That is asking for trouble out on the water though and totally unnecessary.

All that said, a ballpark estimate is probably good enough. As simple as the voltage with the motor off, compared to a chart. For example, 12.25v is about 50% for a standard flooded lead acid battery.

Regarding the amp draw, you can also estimate that and have a ballpark. For example, if you know that at speed 1 pulls about 9 amps, and you have a 90Ah battery, then that gives you 10 hours of theoretical capacity. You should never pull that much, though. Aim for using 50-85% of the nominal capacity of the battery. 85% of 90Ah is 76.5Ah. Or 8.5 hours at speed 1 (assuming it draws 9amps at speed one of course). Keep in mind non-variable speed trolling motors (i.e., one with speed 1-5) are capable of drawing a battery down below it's minimum charge (because they can operate on less than 10-11 volts, whereas electronically controlled trolling motors will shut down). So it's a great idea to keep track of this stuff to avoid damaging the battery.

If you want something that gives you an at-a-glance look, they can be a bit pricier, but there are meters out there that measure and tally amp-hours used. They actually read the amps as the trolling motor is drawing them, and simply adds it up as time progresses. So, in that example, you don't need to worry about voltage or speed or anything (though it's still a good idea to keep an eye on voltage. As the battery starts heading towards 12.0v, you're getting low). If you've got 76.5Ah of usable capacity, then your goal is to keep that number below 76.5 before you get back to the dock/ramp. Those meters will be agnostic to load and will give you the advantage of being able to glance down at it while you're fishing and have a pretty good idea of what the battery is doing. The other HUGE advantage of those is that you can keep an eye on battery health. If you periodically compare voltage, you'll be able to tell when the battery is losing capacity. For example, if you only drew 20Ah out of the battery but it's reading 12.2v (less than half full), then the capacity of the battery has diminished.

NOTE: I'm a total noobie when it comes to boats, too. But I do know a thing or two about batteries.