HD clip on ammeter

[Shep]

Looking for a Clip on ammeter to measure the starter current draw on my boats engine ( a perkins 3 cyl 3HD diesel engine)

Most automotive units seem to be very light weight, i would think I would need a minimum of 500-0-500 preferably 750-0-750.


I've got a problem with the starting system on the boat, the battery voltage is dropping to 5-6v when I try to start the engine, it's a new battery (which I think may be faulty, if i jump another battery across it all starts OK) the only other thing that could drag the starter battery voltage down that low could be a dead short in the starter (which jumping an extra 660ah battery pack across it is managing to hide the short) or a seized engine (which it isn't)

That only leaves the starter or battery I think, any ideas?

Shep

 

[Slowdown]

You've deduced the problem yourself. If it all works ok with another battery then it has to be the new battery that's at fault.

What's the open circuit voltage on the battery? Are you sure it's fully charged? etc etc etc....

 

[Shep]

Fully charged, V good sg readings, brand new battery. 12.6v+ drops to 5-6v when cranking, I think its the battery but the guy who sells them says it could be the starter and is asking what the current draw is, i have no idea how to measure this without a clip on ammeter, when i did this for a living i had an inline shunt and used a voltmeter but to be honest i've forgotten how to do that

Shep

 

[Deleted account W]

Got one that measures AC current 6 Amp to 300 Amps. I presume your dealing with DC though.

 

[Shep]

Got one that measures AC current 6 Amp to 300 Amps. I presume your dealing with DC though.

Yep DC, its normal cold crank draw is supposed to be 3-400 amps, with a real cold engine it would easily get to 500 ish so it really need to measure higher than that, if I could find a 500-0-500 i could warm the engine before testing that would give me an idea.

I still think its the battery though or possibly the starter, ho hum i used to be ind.......

Shep

 

[Slowdown]

Yea, but.....

You said in your first post that with another battery it all works as expected.

With this other battery there are no problems, yes?



So the only thing you replace is the battery and it doesn't work.

So I can't see how it's anything other than the new battery. Either the new battery is duff or it's not powerful enough. Can the new battery provide 500A???

 

[Greg Masters]

Most decent garages and battery shops will have a high-discharge battery tester - a gizmo that will draw starter-motor sized current from the battery whilst it measures the voltage. Something like this:

or this older style item:

Get them to test your battery.

Alternatively, crank your boat with your battery fully charged and record the voltage drop at regular intervals. Borrow a known good battery of similar size and do the same thing. If the results are significantly different, you'll know that your battery isn't up to it.

Finally, is your battery big enough? I would imagine an old diesel engine will draw around 750 amps when cranking. If the battery can't deliver that because it's either too small or the wrong type of battery, it'll still fail even though the battery might be in good condition.

Greg

 

[Shep]

Yea, but.....

You said in your first post that with another battery it all works as expected.

With this other battery there are no problems, yes?



So the only thing you replace is the battery and it doesn't work.

So I can't see how it's anything other than the new battery. Either the new battery is duff or it's not powerful enough. Can the new battery provide 500A???

It should be able to, for a short while at least, it's a 110ah CF battery wet lead acid type, and a battery of similar spec has started the engine for the last 15 years or so. Huge cables as thick as my thumb carry the current, so there will be no problem there.

If there was a partial short in the starter field windings, armature or indeed the solenoid ( its a pre-engaged starter) the standard single battery may not cope but adding the whole service bank (a further 660ah capability) may be enough even if the starter has a partial short.

Shep

 

[Shep]

Most decent garages and battery shops will have a high-discharge battery tester - a gizmo that will draw starter-motor sized current from the battery whilst it measures the voltage. Something like this:

or this older style item:


Get them to test your battery.

Alternatively, crank your boat with your battery fully charged and record the voltage drop at regular intervals. Borrow a known good battery of similar size and do the same thing. If the results are significantly different, you'll know that your battery isn't up to it.

Finally, is your battery big enough? I would imagine an old diesel engine will draw around 750 amps when cranking. If the battery can't deliver that because it's either too small or the wrong type of battery, it'll still fail even though the battery might be in good condition.

Greg

That older style battery tester was exactly what I used to use to check the batteries i used to work on, I also had an individual cell tester for knife cells on electris forktrucks. All had to returned when i left the job

The battery is a fekker to get in and out, it is a combined marine quality starter/service battery designed to do the job and to be honest i'd rather make sure its the battery before I get it changed (i bought the battery from a marina shop and to replace it he will have to get another in, so i need to make sure the starter is ok before i give him the hassle)

Cheers for the ideas

Shep

 

[Greg Masters]

That older style battery tester was exactly what I used to use to check the batteries i used to work on,

It's for sale on eBay - item 110225388948

Greg