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How do you guys know how much juice is left for your trolling motor battery?  Is a battery capacity meter useful?

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I push a button on the trolling motor and it shows me in increments of zero to 4 bars, 

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Any decent battery box will have built in circuit breakers and a built in meter. I just press a button on the top of the box and a 5-bar meter lights up showing me the charge level left.

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I've installed them a couple of times on previous boats but found I seldom paid any attention to them.  They are useful when your motor does not seem to be performing up to speed and you want to make a quick check of the batteries.  Since you are normally on the bow when running the TM, and the meters mounted on the bow, you can make a quick check without have to stop fishing.  It also lets you see what they are doing under a load with the TM running.  That's hard to do with a hand held voltmeter.  If you have one that lets you select which battery you want to check, you can go through the batteries and make sure they are discharging evenly, which is a good indication of a battery going bad if they are not.  I've had my current boat for 15 years and never installed one, even though I have two of them sitting in a box in the garage (I used to work on electric fork lifts and kept them in my spare parts). 

 

I'm just going to run my batteries until they start loosing power or have finished fishing.  If they seem to be loosing power too soon, I'm going to use my DVM to trouble shoot the problem.  After 50 years of doing this stuff, I kind of have an idea of when things are not right, and with a degree in electronics, I tend to trust a good DVM to trouble shoot any possible problems.

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I could be wrong, but my understanding of batteries is that the voltage is not a reliable indicator of capacity.  Doesn't the voltage stay pretty much at 12v until it's just about depleted?

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I installed a cheapo 12v meter that has 5 status bars to my TM battery. My motorguide and boat don't have one already. It's served its purpose perfectly once when my TM stopped all of the sudden. I open up the hatch and pushed the battery meter button and it was at 4 bars, so I knew I had power. I tightened up the connections and away I went. It's worth the $10 and super easy installation.

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You posted for TM battery, are you running one, two, or three TM batteries for 12V, 24V or 36V?  If you are running just one battery, most any voltage meter will work.  If you are running more than one battery as for 24V or 36V TM, you need one of the panels that has a switch so you can select battery A or B, to check voltage on, or C if 36V.  It's also advisable to have a switch, button, or something to disconnect the meter when not in use.  As long as the meter is connected across it is drawing current, which will discharge the battery while the boat is not being used.

 

As for battery voltage, no the voltage does not stay pretty much the same, however, on a cheap, funky meter it may look that way. Flooded cell, cranking batteries are generally at approx. 12.6VDC when fully charged and deep cycle TM batteries are around 12.8VDC when fully charged.  However this can change based on the manufacture and type electrolyte they used.

As a general rule of thumb, the battery will drop approx. 0.1VDC for each 10% of discharge. Understand, this is not exact but it gives you a general idea as to the state of charge of the battery, so if the deep cycle TM battery is 12.8VDC fully charged, it will be a fully discharged battery at 11.8VDC., which by the way, damages the battery to discharge it that far.  You should NEVER discharge a battery below 80% discharge and for the longest life, don't discharge one below 50%. 

Also, when you first disconnect the battery from a charger or turn the charge off, the battery may be showing over 13VDC.  This is a false surface charge, and the battery will drop to it's true charge voltage within a few minutes of use, or after it has sat for a day or two. No lead acid 12V battery has a fully charged voltage over 13VDC, at least not any you will ever deal with.

 

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I only have 1 TM battery, although I've been considering adding another in parallel.

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Batteries in parallel are all going to read as one battery and their voltage is going to be the same, so you don't have to be concerned with switching the meter between them.  Just a button or switch to take your reading and disconnect from them when not needed.

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The only battery meter I have ever used on my bass boats is cranking OB charging amp/V meter. My onboard TM battery charger tells me what condition the TM batteries are before I go fishing, nothing anyone can do to charge TM low batteries while fishing.

Never had a battery issue on the water because I take care them off the water.

Tom

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I'm a knob watcher. 

 

I have a little Minn Kota meter and seldom use it. Very seldom. I have 3 Schumacher Ship n Shore chargers in the garage that read voltage, but are automatic so I set and forget them for the most part until they say they're done.

 

For the past 6 years I've had a Maxxum 70 on a heavy Grizzly 1648 and my meter is the knob on the pedal. As the day progresses the knob works its way up from 20 or so to 30 or 40 usually. The boat will make 4 mph set to 100 hauling 2 200-pounders, a cooler, and too much gear, and a little more if we use the ancient Minn Kota Turbo-something-or-other on the transom. It's mostly there for backing up to retrieve lures and loading the trailer.

 

My mentor has an early Maxxum 70 on his even older 1648 Polar Kraft and watching the knob works for him. 

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More than one of my buddies have meters wired into their bass boats.  External led display and they can monitor the charge rate, battery discharge rate, life remaining and about 10 other things.  They have an app on their phones that will give them the read outs as well.  

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On 8/5/2019 at 1:12 PM, billmac said:

I could be wrong, but my understanding of batteries is that the voltage is not a reliable indicator of capacity.  Doesn't the voltage stay pretty much at 12v until it's just about depleted?

It needs to be read under a load to be reliable.  Therefore a meter you could see as you used it would be helpful.  My trolling motor has a 4 bar system built into the front of it that I find myself checking often. 

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