12V Batt Charger Meter Swap

I have a Shumacher SE1510MA automotive battery charger and the analog ammeter on the charger has quit. The charger has a 100A start assist and I suspect maybe that feature did in the analog gauge.

So I purchased a digital display 0-100A ammeter from Amazon (see link below) that also has a voltage readout, and I'm wondering if I can just install the digital ammeter in place of the analog or would I need to install a shunt too? There's no shunt inside the existing (super basic) analog meter, and I can't find a wiring diagram for the charger to see if there's one elsewhere in the charger, so I wanted to see if anyone had an idea?

https://www.amazon.com/gp/product/B00C7MJ5LE/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

I took the charger apart and the ammeter itself is the exact same one as in this video below and had the exact same melting happening. I notice his meter also has the 100A start assist and believe that probably sends too much current through the meter when used and thus melts where the contacts enter the plastic on the meter housing?

https://www.youtube.com/watch?v=SqfP-6ehySQ

I am pretty sure the shunt is what goes between the 2 rivets that have melted into the plastic. Notice on the bottom of the amazon link there is a couple items listed that others have purchased when purchasing the same digital meter you are looking at.

Thanks for the reply. I thought the same thing at first, but there's no resistance between the two rivets/connection points on the meter. The shunt shown in the Amazon link is pretty large, which I'd assume it would have to be for a 100 Amp start assist.

Others on Amazon with this charger have had the same issue, and I'm guessing they're the ones who have used the start assist feature. If the unit is only used as a 2 or 15 Amp charger, I think the meter probably doesn't overheat and melt and continues to function.

The start assist in a charger this small is designed for 10 second attempts with a minute or 2 of cool time. The 1 pictured on the amazon link states 100 amp 75 millivolts. That means with 100 amps going through it, there will be .075 volts dropped across it. There is resistance across your shunt. You just do not have a meter that will read it. If there was 1 ohm of resistance you would only get 12 amps of current through to a battery.

Makes sense. In the diagrams for this meter, it says <10A doesn't require an external shunt, so it seems to have a 10A shunt built in? If that's the case, the internal shunt would have to be removed in order to use an external shunt for higher current measuring? I don't think that the "instructions" cover that.

I guess I could wire it in w/o a shunt and try it on the 2A setting to see if I get a read out.

Without a shunt, the meter will peg out. It is looking for 75 millivolts to give it full swing. You will have 12 volts across the meter movement. It will go full swing instantly. The meter reads voltage drop across the shunt.

Right, that's understood a shunt is needed. It just seems odd that the directions on the Amazon page say that a shunt isn't needed for currents less than 10 amps, which would suggest an internal shunt is already present in the meter.

Also, given the size of the 100A shunt shown as an accessory on that Amazon page, it's no wonder the original cheap analog meter that came in the charger melted (if indeed the shunt is inside the meter). It doesn't look like it could handle 100 amps for 5 seconds, never mind 10.

It'll be great if this digital meter works for this charger. I didn't realize it comes from China, so it's going to take a while to get here.

Thanks for all the assistance. I decided to go with this digital meter below, shows both voltage and current and comes with a 100A shunt.

Replacement Meter and Shunt

Installed it in the charger today and it seems to work well. When compared to my DVM, the voltage meter is spot on, but I think the current meter is reading .7 A too low. That's assuming the current meter in the DVM is accurate. Might try putting a 14 Ohm resistor across the clamps of the charger and see if it reads around an amp, and calibrate it if not.