What is it???

[Eaglewood]

Thank you everyone. I think you are on the right track. I wonder if I could use it to measure the current in a 12v lead acid battery? The battery charges to 13.6v but doesn’t seem to have any current capacity. I have a Simpson 372 ohm meter and a Fluke clamp multimeter.

[BuffaloJohn]

Thank you everyone. I think you are on the right track. I wonder if I could use it to measure the current in a 12v lead acid battery? The battery charges to 13.6v but doesn’t seem to have any current capacity. I have a Simpson 372 ohm meter and a Fluke clamp multimeter.

If you know the resistance of the shunt, you can measure the voltage across the shunt and derive the current:

V=I * R

If you have two, you can calculate the third.

In looking at the Simpson 372 ohm meter specs, I see that you should be able to measure down to fractions of an ohm, but I couldn't find any specs other that +/-3% of full scale and this being the low end of a 2 decade log scale, short of having reference resistors, I don't have a clue if it would work. If it were me, I would stick to voltages, those meters generally have more digits and a better chance at getting enough resolution.

But, you asked about a 12v battery that doesn't have any current capacity. 13.6 is the float level of a 12v wet cell lead acid battery but charged it should get above 14v. If your charger can't get it higher than 13.6v (you can put a voltmeter on a charging batter and it won't harm anything - neither charger nor battery nor meter), tHen you charger might be shot. If it charges to above 14v and then quickly drops after the charger is removed, that is usaully a bad battery.

If it should have much more current, then it is possible the battery is almost dead (internal resistance, internal bridging, broken plate(s), electrolyte problems, and on and on), but without much more detail... This is not a case where Miracle Max can revive the battery with a magic pill:

Could you measure the current capacity of the battery with this device? Probably. To do so, you need a load and the shunt and the voltmeter.

I would take the battery to a battery store and have them test it...

[Eaglewood]

Thanks for the tutoring. If I put the device on the positive post of a battery in the vehicle and measured the voltage from the body to ground with starter or lights (load)and used ohms law what would I expect to see? Thank you everyone for your patience with me as I don’t often understand logic.
Dick

[BuffaloJohn]

If I put the device on the positive post of a battery in the vehicle and measured the voltage from the body to ground with starter or lights (load)and used ohms law what would I expect to see?

The current through R1 is the same as the current through R2. So we also know:

Ir1 = Ir2
Vbattery=Vbulb + Vshunt
Vbulb=Ir1 * R1
Vshunt=Ir2 * R2

We think R1 > R2 (by how much? maybe 10 to 1000 or more times higher)

If we measure Vbulb and Vshunt we will see the ratio of how much R1 is greater than R2. If a meter is inserted to measure current through R1, we can calculate R1.

If we don't have a current meter (has to be DC, not a clamp on AC meter which is what you said you had) we can insert smaller bulb or a resistor of known value. Or if we can measure resistance (you have the ohm meter), you can meausre the bulb while the circuit is active (yes, you can do that as long as you only cross the bulb) and now we know R1 and can calculate Ir1. (Why when it is active? because if the bulb heats up, resistance can change)

Once we know that, we can calculate R2 because we can read Vshunt and Ir1=Ir2.

I wouldn't use the starter or lights of the vehicle until you have figured out what Rshunt is. If it is milliohms (which it should be), then you can use Rshunt to measure vehicle operating values (such as starter current or power consumed by the lights).

[Eaglewood]

Thank you very much. I a very happy that you could explain what I needed to know. I will be able to sort out this thing and maybe make use of a tool that my dad gave me 55 years ago and I had no idea what it was for.
Dick

[ductape]

Not discounting any of the suggestions here, because they are quite good. Just a few thoughts.
The piece looks like an attachment to a lead-acid battery with the starter cable on the tall post and accessory connections on the other. If it is for that type of battery, aluminum or similar alloys really aren't the ideal material due to galvanic reaction. Being that old and showing no signs of corrosion is interesting.
That aside, there is also no way to clamp it to that style of terminal unless it is for the type of battery with screws through the top of the posts, like deep cycle and/or marine. That may explain why it has the brass piece going to the smaller screw. Path of least resistance etc. Deep cycle batteries last longer with a slow charge.
The conversation leaned toward load testing of batteries. The tools to do it aren't terribly expensive and are very useful.
It may also not be for anything electrical at all and part of an old drafting pantograph or something. Hard to say. Best of luck with your quest for the answer.

[Philip Davies]

I had to smile when I read it had been in your toolbox for FIFTY years, it looks very clean. And perhaps you have one of those large, multi-drawer boxes. I have been acquiring for 50 years, including many boxes, and there are several items that a) never used or b) purpose unknown. All dusty. Also lots of tools which I MAY use some day. I hope.