Battery gauge

So I bought this 10-bar LED battery gauge off Ebay for my 36# thrust Minn Kota. Last weekend, after about half an hour, the darned thing was on the third-lowest bar!

(I'd used the same group 29 battery about a month ago on a 45# Minn Kota, and played around for over an hour using only 25% of the battery.)

So I got off the pond early, came home, and hooked up my Schumacher charger. It said I had 80% reserve left!

Any ideas what I'm dealing with here?

 

Too many unknowns.

Did you wire the LED bar gauge in directly - if so it may be measuring across a dirty battery terminal - giving you a reading which includes the 'voltage drop'.

you need to calibrate it against a range of known battery states - it could be that it shows zero at 60% battery.

A voltmeter shows a real number - more use for diagnostics.

 

Thanks, Andy. The wire ends were all new, but I don't think I cleaned the battery terminal. (I use the smaller threaded one.) I don't see any provision for calibrating it--it's a sealed unit.

I use a solenoid controlled by a kill switch, and the red wire from the gauge is connected to the large "hot" terminal on the solenoid. The black gauge wire goes to a bus bar connected directly to the battery. I figure I can't get much closer than that to the battery.

I just found a wire with a loose terminal....

 

A battery tester is the only way to make sure.

 

Most people don't have any idea how much of an issue even minor corrosion is on terminals.

You're probably aware of the equation for electrical power;-

P=IV

Power is Current x Voltage.

There is another equation when you're talking about Resistance which can be rearranged to give;-

Voltage = Current x Resistance V=IR

and this can be substituted for V in the power equation to give

P= I x (IR) or I squared R


I believe one of your other posts mentioned a current of 33 amps

Square that and it gets to be big numbers

P= 1089 x R

Now if all of your dirty terminals add up to 1 Ohm resistance you've just lost 1089 watts of useable power, 1 kW is 1.34 Horsepower

So that is like a loss of 1.4 Hp - Whats the Hp rating of your Minn Kota?

Can you imagine what happens if you have that same resistance on the starting circuit of a big diesel drawing 230 Amps?
52,900 watts of power lost and that can be the difference between turning the motor over quick enough to start or not.

That power has to go somewhere = heat. It would be heating up that loose terminal and as the metal gets warm, it expands, as it expands the joint gets even looser and the power loss increases...


Clean all the terminals, check they're tight and then cover with some spray on corrosion preventing electric grease.



I didn't mean your LED bar graph needs adjusting, just that you need a set of (mental or written) numbers which give real world meaning to 10 bars, 5 bars 2 bars etc.
Does 1 bar give you enough of a reserve to get to shore without having to paddle or do you head in at 3 or 2?

 

Oh and BTW, on any boat electrics, the above is why I never just 'Crimp' a terminal

Crimp then solder - it really is worth the extra effort.

 

All I know is what I read. Apparently discharging a battery below 30% can damage it, and three bars is supposed to represent 30% reserve, so at three bars I instructed my intrepid test pilot to head for shore. It may be a good idea to install an actual voltmeter in addition to the "gauge". I presume I could connect a voltmeter to the same points as my battery gauge.

Those are indeed some big numbers, Andy. I've given up trying to convert pounds of thrust to horsepower but a corroded terminal plus a loose wire could indeed explain why I couldn't hit 3 knots while drawing 33 amps. I've cleaned, recrimped, and tightened and hope to test it again on Sunday. My battery testing equipment is pretty primitive but I'll take it along.

Y'know, I swear I read that marine terminals should not be soldered. I was prepared to do so until I read that somewhere. Figured it one of the galvanic mysteries or something. This whole project is an experiment (I was pleased that the darn thing floated right-side-up!) so I'll add soldering to my list of over-winter mods and enhancements.