Difference between an Ammeter for 12V Car Battery Charger and a plain DC Ammeter

HolyHarp

Joined Jun 23, 2018
11
I tried replacing the Ø2" ammeter on a 20-year-old adjustable-amperage Chinese battery charger. After trying it in different situations, I found that the ammeter is special for charging batteries. It will give a reading when charging a battery. It will not give readings when a DC motor (pulling 4.5 amperes) is connected to the charger (while a DC ampere panel meter will give a reading). Can someone please explain this or give a link to a good explanation? And, what are the actual specifications for the type of meter I need for measuring the DC motor's current draw? How can I make sure I don't get a battery charger ammeter?

MrChips

Joined Oct 2, 2009
28,153
It could be the internal resistance of the meter.

In theory, ammeters should have a very low internal resistance. This is achieved by having a low value shunt resistance placed across the terminals of a sensitive meter movement.

Get a DMM and measure the resistance across the meter.

Dodgydave

Joined Jun 22, 2012
10,698
Can you post pictures of both ammeters ?

HolyHarp

Joined Jun 23, 2018
11

The meter on the left is the old one from the battery charger; the resistance across the terminals is 40 ohms.
The meter on the right has resistance across the terminals of 10 ohms.

Dodgydave

Joined Jun 22, 2012
10,698
Looks like the Old meter is for AC ! it has a ~ under the A.

Joined Feb 20, 2016
4,268
Somehow, I think 40 Ohms is way too high for an amp meter. So is 10 Ohms for that matter. And looking at the windings on the meter, I think your readings are in error.
And I agree with Dodgydave above. The original meter is an AC one so it is probably connected between the transformer ad the rectifier. A DC meter will not indicate there as each half cycle will try to move the meter in opposite directions so the result, due to the meter inertia, is no movement.
Try the DC meter in line with the motor to see how that goes. You will have to put the AC meter back in circuit too to complete the circuit I reckon.
Can you draw a diagram of your setup, including the meter placement in the charger circuit?

AlbertHall

Joined Jun 4, 2014
12,202
As the scale on the left meter is non-linear it is probably a moving iron meter and so would respond to either AC or DC. If it was connected in a place with AC current then replacing it with a DC only meter isn't going to work.

ian field

Joined Oct 27, 2012
6,536
I tried replacing the Ø2" ammeter on a 20-year-old adjustable-amperage Chinese battery charger. After trying it in different situations, I found that the ammeter is special for charging batteries. It will give a reading when charging a battery. It will not give readings when a DC motor (pulling 4.5 amperes) is connected to the charger (while a DC ampere panel meter will give a reading). Can someone please explain this or give a link to a good explanation? And, what are the actual specifications for the type of meter I need for measuring the DC motor's current draw? How can I make sure I don't get a battery charger ammeter?
Moving iron meters were most popular for cheap battery chargers, they're not particularly accurate. One of my very ancient and very rusty Halfords chargers has an admiralty bulkhead mounting moving coil meter on it, never had any problems with it.

These days, I mostly use discount store "smart" chargers. Generally the cheaper type are good for charging a pretty much flat car battery in a reasonable time. More expensive types take good care of your battery - but a full charge can be a long wait.

HolyHarp

Joined Jun 23, 2018
11
I remeasured the moving iron (old and new) and regular ammeters using an analog ohm meter and they all came in at less than 1 ohm. For some reason the moving iron ammeter works well for batteries being charged, but not for DC motors. I checked out Wikipedia's Ammeter Article. It seems very comprehensive. Here are a couple photos of the charger. You might be able to see that the ammeter is in the 12V rectified (half wave?) DC output circuit. (How does one break the "link" formatting in this forum?). I'm going to look for a Ø2" regular ammeter - there don't seem to be any in Udon Thani.

Moderators note : corrected link

Last edited by a moderator:

Alec_t

Joined Sep 17, 2013
13,337
The 'Tx' button at top right of the Reply box removes formatting.