# I want to Add a voltmeter to auto battery charger

#### Richard C

Joined Dec 20, 2008
10
Greetings, I want to add a voltmeter to my battery charger. The charger has an amp meter but it is not easy to tell if contact is complete or if the battery is taking a charge. What do you think?

#### thatoneguy

Joined Feb 19, 2009
6,349
Car Battery chargers are generally constant voltage/current limited chargers, so unless the terminal voltage of the battery is measured before connecting the charger, it wouldn't give much information, other than the Battery Charger is working.

The Ammeter would tell you more than a voltmeter as far as connections go. The voltage will be relatively constant compared to current draw.

If the battery doesn't draw any current when connected, yet isn't charged, check connections, grounds, etc.

If you want to add one for something to do, that's entirely possible too. Both LCD and "needle" analog meters are available, calibrated for voltage, from a few suppliers, as well as on ebay.

Adding the meter with a switch that disables the charging circuit would allow you to see the voltage before and after the charger was turned on, even though it is connected. You would need to switch the charger off earlier in the circuit than at the clamps, as not many affordable switches can handle the current output of most chargers (> 200A in starting scenarios)

#### Richard C

Joined Dec 20, 2008
10
Greetings, The amp meter is: 1-100 so its hard to see 3 or 4 amps. As the battery begans to take several amps it becomes clear it is working. This is an Associated Charger and I love it but I would like something to indicate good connection and/or charging.

#### Richard C

Joined Dec 20, 2008
10
RE: I'll keep that in mind [digital 1-100 amp meter]. I would rather not bite that bullet if there is another way.

#### thatoneguy

Joined Feb 19, 2009
6,349
Does the charger have an easy/direct switch so that the charger can be connected to the battery, with power on, but not charging? This is the biggest Question, and presents other opportunities.

A panel meter is $20-$40 for outdoor use, an Ammeter is essentially a voltmeter with a very low resistance shunt, and a different label on the scale in the case of an analog, or different scale in a digital version.

Simple Soldering/Wire Only option #2. (digital Ammeter was Number 1)

There is a cheap ($12) multimeter at Radio Shack that could be modified to run from a small regulated voltage tapped from the charger's main power. Connect the multimeter leads to the + and - wires inside the charger, and leave it on the 20V range. Cut a window the size of the LCD display, fill window with plexiglass and border, then somehow mount the meter so it is easily visible through the "window". The orignal batteries could be optionally used, and switched in with a Dual Pole, Dual Throw switch to measure voltage of what the clamps are on, without power applied to the charger. Changing those batteries would be very difficult though. If you are somewhat knowledgeable with electronics, or want to learn, this could also be done with radio shack parts and some soldering. The first option here is no more difficult than anything listed above, possibly simpler. 1) Single LED with a 1k resistor in series will light up when there is voltage on clamps. Would only be useful if charging current were able to be turned off easily, while leads are connected. Cost:$2, including a nice bezel mount LED holder.

2) Same as #1 for requirements, but slightly more useful. This is an "Expanded Voltmeter", showing good detail on only a small range. Use a LM339 quad comparator, and set them to light 4 LEDs based on 0.6V increments by using diodes in the reference chain, from 12V, 12.6V, 13.2V, 13.8V. If connected to a battery, and not charging, anywhere between 1 and two LEDs would light (12, 12.6) depending on battery charge. With charger on, all four would be lit, however, that wouldn't tell you if the battery is connected or not, only what the voltage is at the clamps, which would be 13.8V whether a battery is present or not. Use a Zener Diode with current limiting resistor to keep voltage spikes from destroying the circuit. Cost: $6 from radio shack, including LEDs, quad comparator, 2 zener diodes (one for reference, one for regulator) 3) Similar to #2, this is an "expanded scale Ammeter". Requires a high wattage, very low resistance, 1% tolerance resistor. 1/4 Ohm or less resistor. With a 0.25 Ohm resistor, there would be 1/4V per amp dropped across the resistor. As you are only concerned about the first 5 Amps or so, some protection/bypass would be needed once the voltage drop exceeds 2.5V (10A). Use three comparators for the first 3 levels (1, 2, and 3 Amps), and the last comparator (over 5 Amps) to latch a high current SCR (or more than one in parallel) that would bypass the circuit so it wouldn't blow up when high currents are present. Zener diodes aren't capable of the current levels that would need to be bypassed in a shunt arrangement. Cost:$?? Unsure until a safe design is drawn and simulated.