No time right now. I will be back tonight with instructions on doing a voltage drop test.
'K I’m back.
Voltage drop test. First I think we need a little background so that you understand the test and what we are trying to accomplish. Typically resistance is checked using an ohm meter. But this is a static test not a dynamic test and can be fooled. Let’s take an example. Lets say we take a battery cable off of a car and out of the 100 strands of copper wire, we cut 99 of them clean in half. However that 100th strand is complete from end to end. If we measure resistance on this cable it will read 0 ohms. But it will not start the car. A voltage drop test is done while current if flowing, and measures the voltage lost due to bad resistance. It is the electrical equivalent of a treadmill EKG. It measures resistance dynamically under the stress of the circuit.
**What do I need?**To do this test you will need a Digital Volt Ohm Meter (DVOM) It does not have to a a Fluke 88 an inexpensive unit will work. You may need a spool of wire and some alligator clips.
OK so how do you do it? First off write the following down on a piece of paper, and take said paper with you when you do the test.
If you have to, write this on your forearm with a sharpie.
So a voltmeter measures the difference in electrical potentials. Usually we measure to ground, but not this time. This time we are going to put one voltmeter lead at one end of the wire circuit we want to measure and the other voltmeter lead at the other end of that wire. We are going to then energize the circuit and turn it on to get current flowing.
Let’s take a car for example. If I want to measure the voltage drop on the positive cable from the battery to the starter, I would put the red lead of the voltmeter on the positive terminal of the battery, and the other end on the other end of the starter cable down on the solenoid. I would have my trusty assistant crank the car, and I would observe the reading on the meter. If my battery maintains 10V during crank, but only 9 get to the starter, my voltmeter would read the 1-volt. (10-9 =1 for the math challenged)
OK, now with your boat you have a special problem. If your boat is moored in the llake, no problem, tie it to a dock, and set up to measure and have your trusty assistant turn on the trolling motor. If you are on a trailer you will have to get creative. Say a trash can full of water should do the trick. Remember you have to have current flowing, and you are trying to stress the circuit. The trolling motors turning in air does not cut it IMHO (course what the hell do I know, I thought it ran on 12V 
) Seriously you need the circuit somewhat stressed. Not full atomic power, but half speed in water at least.
But you say, I am working on a boat, and my batteries are at the other end from the trolling motor (thank OG we are allowed to use the word troll now, it would be a pain in the butt to call them under bridge dwelling motors, but I digress). Fine I understand that the batteries might not be next to the motor. Remember that wire I told you might need? Use that hook the wire to the battery terminal, and the other end of the wire to the lead from the voltmeter.
Now I am going to assume that the speed control for the trolling motor is in motor and not a separate controller. If it is a separate item, you will have to test that leg twice, from the battery to the controller, and from the controller to the motor. Do not test the controller as it is designed to drop voltage for slower speeds.
Anyway test the positive leg of the circuit. (+ battery terminal to + on motor) then test the negative leg of the circuit (motor - to battery -) Then don’t forget to test the cables between the batteries. (+ on one battery to - on the next with the motor running)
Great I got some numbers, what the !@#&& do they mean? Good question, I wish I had a good answer. :smack:
Look here is the deal, on a car starter circuit we are looking for no more than .1V drop per connection, and no more than .3 for the entire starter motor leg (Positive or negative)
You however don’t have a car, you have a boat. Not only that you haven’t offered to take me fishing yet.
Anyway with 36V to play with you can stand a little more drop. ::: Rick reaches behind him and pulls a number out of his ass::: I would say that you should have no more than 1V per leg. (no more than 1V drop on + side, and no more than 1V on the - side) Don’t forget that Voltage Drops are just like golf low score wins! If you can take a voltage drop from 1V to 0.5V you will probably notice a difference.
So you do the test and find too high a voltage drop, what do you do next? Localize it.
Let's say this is your circuit, number represent connections, --- = wire
1 = battery, 5 = motor
1-------2-------3------4------5
Measure at points 1 /5 to high?
Measure at points 1/4 OK? connections at 4 or 5 or cable between is bad.
Not OK? Problem is elsewhere, continue
Measure 1/3 OK? Problem is at 3 or 4, or the cable between is bad.
Not OK? measure at 1/2
you get my drift, continue to isolate the faults until you find the fault(s) and fix them.
But what about my fish finder? It does not use a lot of power. Yeah but is is probably not very tolerant of voltage drops. Again I pull a number out of my ass. I am guessing that the fish finder wants no more than 0.2V drop. (assuming that the ff runs on 12V) Don’t forget low score wins!
Great, how do I fix them? First off go to an auto parts store and get some Silicone Dielectric grease (sometimes called low temp grease, or tune up grease.)
Take the bad connection apart, and clean the terminals with emery cloth. Coat with silicone grease and reassemble. Retest.
I think I should have answered most of your questions, if not let me know.
NOTE: if during your testing you come up with a 0.00Volt voltage drop refer back to the piece of paper I aske you to copy down. You don’t have any current flowing. No current = no drop.
Here’s a simple way to do it:
A few weeks after you install a new battery, conduct the following test:
- Ensure vehicle has not run for at least 8 hours.
- Open hood.
- Measure battery voltage using a digital voltmeter (DVM).
- While the DVM leads are still connected to the battery terminals, have someone turn on the headlights.
- Measure the “new” battery voltage using the DVM. (This should be done within 3 or 4 seconds after the headlights are turned on.)
- Turn off headlights.
- Remove DVM leads from battery terminals.
- Close hood.
Conduct this test once a year. Over time you will be able to track the health of the battery. To help ensure the data is “good”, the conditions should be as similar as possible during each test, i.e.:
a. The ambient temperature should be about the same.
b. The headlights should have the same “on” resistance. (This is just about impossible to ensure, and thus accounts for the largest error component.)
Is this a very accurate test? No. It assumes a lot of things. But the key here is that it is very *simple * to conduct. Even then, it will likely alert the vehicle owner to a failing battery.
Rick,
Thank you. I’ve had had people tell me to do these tests, but now I actually think I ‘can’ do these tests and have it make some sense to me…
I’m fortunate that at this point of the game, I’ve already done most of the ‘fixing’ of connections I can find… (why in the h*ll do boat manufacturers insist on using cheap terminal and running them thru the bilge???) For a long time I was seeing a voltage drop each and every time a new item was turned on, running a secondary ground and higher gauge main power took care of most of that … the front fish finder is isolated enough I’m just going to run new wire to it and have it’s own dedicated circuit… it’ll read 8-10v (constantly changing) while the main fish finder on the same battery (different circuit) will read a steady 11.7-12v all the time. My main concern is the m
But I digress… as for the fishing… If you’re in KY, we’ll make it happen.
Crafter Man, thanks! I’m going to try that sort of regime on our cars to see what kinds of patterns I see. It sounds like it could be really useful if you bear in mind the limitations.
Your location matters! I bet it gets cold where you live? Cold weather makes batteries sluggish and on top of that the turning force is increased because of mollases like motor oil and so on.
But in any case batteries actually suffer from increased aging in HOT weather, sulfation mainly. It pays to use a “smart charger” like a battery tender or very light trickle like a 1 or 2 amp “dumb” charger. Arizona places a lot different demands on a battery, and I bet todays cramped modern styling makes underhood temps much higher. Tires and batteries just have to be among the most ignored items in cars.