Voltage drop when glowplugs on

[Stu Jackson]

If you measure the positive and negative lead voltage drops, then add them together, this will tell you how much the battery is sagging vs. just the wire...

I'm not getting something. How do you measure positive and negative leads?  I measure voltage by connecting the positive and negative terminals to my volt meter.

Measure the voltage at one end of the positive wire and then at the other end.  Do the same for the negative.  Add 'em up.

Since the starter is bolted to the engine, that creates its ground, so there is no negative to measure.

[KWKloeber]

If you measure the positive and negative lead voltage drops, then add them together, this will tell you how much the battery is sagging vs. just the wire...

I'm not getting something. How do you measure positive and negative leads?  I measure voltage by connecting the positive and negative terminals to my volt meter.

Mark if you get a long test wire, put one end on, say, the battery + terminal, put your meter between the test wire, and, say, the solenoid B post, You are then directly measuring the V difference between those points, and the reading is a direct measurement of the V drop on the positive cable.

kk

[Jon W]

Are the glow plugs or starter energized when this measurement is being made?

[mainesail]

Are the glow plugs or starter energized when this measurement is being made?

You can't have voltage drop without current. In order to do this for cranking a meter that can capture fast transient peaks will be best.

[mainesail]

If you measure the positive and negative lead voltage drops, then add them together, this will tell you how much the battery is sagging vs. just the wire...

I'm not getting something. How do you measure positive and negative leads?  I measure voltage by connecting the positive and negative terminals to my volt meter.

Measure the voltage at one end of the positive wire and then at the other end.  Do the same for the negative.  Add 'em up.

Since the starter is bolted to the engine, that creates its ground, so there is no negative to measure.

Stu,

Confusing...?  Did Ohm's law change while I was asleep??    Your first line in green is correct. The second line is where the confusion lies.. The circuit is between the battery and starter. Whether the engine is at Earth potential or not has no bearing on the DC circuits voltage drop and yes the negative side needs to be measured... You can't have a one-leg positive only DC circuit. 

There is still voltage drop between the batt neg and the starter motor thus you can't ignore the batt neg cable because it is part of the 12V circuit for the starter motor. Best to place measure for VD is directly onto the metal case of starter motor and to batt neg to get the actual neg side of the circuits voltage drop....

When measuring voltage drop the most accurate method is to measure both the positive leg and the negative leg. This helps to isolate where the problem may lie. Connect the DVM voltage sense leads to the battery + terminal and then the other lead to the starter post. With zero load you should read 00.00V because it takes current passing through the wire to create a voltage drop..


No you set the DVM to peak capture and hit the glow plugs or starter motor and measure the voltage drop on that leg. It may look something like this. If it looks like this with a 40A glow plug load you have issues.


You now repeat this same test only on the negative side of the circuit between battery - & the starter motor case. For the glow plugs you can also locally sense at the glow plug busbar and engine block close to a plug or directly on the glow plug body.

[Jon W]

Are the glow plugs or starter energized when this measurement is being made?

You can't have voltage drop without current. In order to do this for cranking a meter that can capture fast transient peaks will be best.

I learned that in my post a couple months ago and is why I asked. It may be implied, but I didn't see energizing anything in these responses, just measure between points.

[KWKloeber]

If you measure the positive and negative lead voltage drops, then add them together, this will tell you how much the battery is sagging vs. just the wire...

I'm not getting something. How do you measure positive and negative leads?  I measure voltage by connecting the positive and negative terminals to my volt meter.

Another way rather than the direct measurement I posted, would be - measure terminal to terminal at battery, and then measure solenoid B post to starter frame.  The difference between the two would be the total loss in the (positive + negative) cables and connections.  To determine them separately you need to bypass one or the other and jumper a test wire as I described. 

As RC described, whatever you are running (in your case you asked about drop from glow plugs) will cause "X" voltage drop, and a different load (say starter) would cause "Y" voltage drop.

k

[mark_53]

No you set the DVM to peak capture and hit the glow plugs or starter motor and measure the voltage drop on that leg. It may look something like this. If it looks like this with a 40A glow plug load you have issues.

Thanks for the explanation and pictorial view. Not sure what you meant by the above statement. Looks like you left a meter reading off.  I suspect it's a DVM meter capable of graphic view of current over time. Would the voltage drop be the difference between the high and low points?  I only have a DVM that gives one number.

[mainesail]

If you measure the positive and negative lead voltage drops, then add them together, this will tell you how much the battery is sagging vs. just the wire...

I'm not getting something. How do you measure positive and negative leads?  I measure voltage by connecting the positive and negative terminals to my volt meter.

Another way rather than the direct measurement I posted, would be - measure terminal to terminal at battery, and then measure solenoid B post to starter frame.  The difference between the two would be the total loss in the (positive + negative) cables and connections.  To determine them separately you need to bypass one or the other and jumper a test wire as I described. 

As RC described, whatever you are running (in your case you asked about drop from glow plugs) will cause "X" voltage drop, and a different load (say starter) would cause "Y" voltage drop.

k

That will tell you the combination of battery sag and wire drop but it does not really isolate where the issue is just that there is an issue.. For in-rush type loads ideally should be done simultaneously with two DVM's being used, one at the battery end and one at the starter or glow plug end. Based on what I don't see aboard boats, I suspect you'd be hard pressed to get most boaters to pony up for just one decent DVM but two is, well........... If not done simultaneously the battery is now a bit weaker on the second go and the readings will be a bit off a "fresh start". This can cause owners to assume there is more of an issue than there may actually be. 

Ideally we want to find or isolate the issue and the battery can be load tested separately. If the positive wire is dropping .72V and the neg is dropping 1.45V we now where to begin chasing the issues.

[mainesail]

No you set the DVM to peak capture and hit the glow plugs or starter motor and measure the voltage drop on that leg. It may look something like this. If it looks like this with a 40A glow plug load you have issues.

Thanks for the explanation and pictorial view. Not sure what you meant by the above statement. Looks like you left a meter reading off.  I suspect it's a DVM meter capable of graphic view of current over time. Would the voltage drop be the difference between the high and low points?  I only have a DVM that gives one number.

We are looking at voltage drop along a length of conductor using the above test procedure. A good quality DVM will capture and hold fast voltage transients and you can select Peak High Voltage, Peak Low Voltage or an Average. Some of them are fast enough to capture blazing fast solenoid transients. Inexpensive DVM's may not have this feature and you would need to monitor the screen while holding glow to determine max volt drop. For starter max drop you would close seacock, pull stop lever and crank for 10+ seconds or until you have good data.

The high point would be 0.00V or and unloaded circuit. The peak low might be -0.89V at peak current load...

[KWKloeber]

No you set the DVM to peak capture and hit the glow plugs or starter motor and measure the voltage drop on that leg. It may look something like this. If it looks like this with a 40A glow plug load you have issues.

Thanks for the explanation and pictorial view. Not sure what you meant by the above statement. Looks like you left a meter reading off.  I suspect it's a DVM meter capable of graphic view of current over time. Would the voltage drop be the difference between the high and low points?  I only have a DVM that gives one number.

We are looking at voltage drop along a length of conductor using the above test procedure. A good quality DVM will capture and hold fast voltage transients and you can select Peak High Voltage, Peak Low Voltage or an Average. Some of them are fast enough to capture blazing fast solenoid transients. Inexpensive DVM's may not have this feature and you would need to monitor the screen while holding glow to determine max volt drop. For starter max drop you would close seacock, pull stop lever and crank for 10+ seconds or until you have good data.

The high point would be 0.00V or and unloaded circuit. The peak low might be -0.89V at peak current load...

I should have mentioned that you needed to monitor the battery also.
I suppose using only one meter you might measure the sag, measure the cable V loss, measure the sag.  And for the middle test average and subtract the two battery sags.  With all that, it's just as easy to simply use a jumper test wire with the direct method!