The problem with most cheap battery testers is that they only indicate voltage under minimal load. This doesn't give a true indication of battery condition when its required to operate under a heavy load, drawing maximum current.

Ideally you need to measure voltage under minimal load and current under heavy load to assess the batteries condition.

This can be done manually with a multimeter, but its tedious. You need to hold two probes on a battery and switch between current and voltage modes which also requires you to swap leads at the meter probe socket terminals.

Wouldn't it be great if all this functionality was integrated in the more convenient battery tester case and we could get both these readings at the press of a button?

This formed the basis of my requirements for this project.

Step 1: Parts & Skills List

Parts required (available from ebay/banggood/eachbuyer/dx.com):

* cheap donor analogue battery tester (for case).
* digital DC voltage and current panel module. able to work from 12v supply and measure upto 100volts DC 10amps. When i received mines it was rated on side sticker as 40v/10a.
* 12v battery and holder
* on/off sliding switch. I prefer larger dpdt for ease of use but smalller spst or spdt will do. Or you could use another push switch as below.
* push to make load test switch
* solid core and flexi wire

skills/tools required:
* hacksaw or dremel with cutting disc
* drill
* hot glue gun
* soldering iron, able to solder wires

Step 2: Hack Donor Case

Open up donor tester case.
Remove analogue display panel and wiring.
Dremel larger opening for digital panel
Cut/drill holes for power and load test switches.
keep space reserved for 12v battery.

Step 3: Solder Wiring

Solder as per wiring diagram.

I have tried to colour code white wires wires red, green and black marker in pictures. I soldered direct to 3 pins on DMM as space was restricted for female plug.

Use thicker core wire capable of carrying heavy current from digital panel to contact terminals. i used speaker wire.

My panel meter was missing an internal current loop link so i had to solder this in order to get a current reading.

You could add a load resistor in series with load switch to limit the current, but my load switch seems to be handling the juice ok.

The digital display panel requires minimum 4.5v supply, so the internal 12v battery should last a good while.

Step 4: Assemble Components in Case

fix load test switch.
hot glue sliding switch.
hot glue 12v battery compartment.
snap/glue digital panel.
reassemble case.

Step 5: Test It!

Switch power on.
Display should light up 0.00V/0.00A
Insert test battery between case probes.
Get voltage reading eg 1.34V

Now temporarily press load test button for a few seconds and obtain current reading eg 0.3A. The voltage reading will drop when doing the load test, ignore this.

You now have a maximum voltage and maximum current reading for your battery under test.

Step 6: Analyse the Battery Readings

More than often you will have more than one battery to test.

Multiply the voltage and current readings to calculate max power (watts). You can sort batteries by strength from this power rating.
eg
batt 1: 1.4v x 3.8a = 5.32w
batt 2: 1.45v x 1.9a = 2.755w

Note batt 1 is stronger than batt2 even though voltage reading is slightly less.

I have tested duff rechargeable batteries which read 1.4v with 0 amps under load. This shows how deceptive a voltage reading on its own can be.

If you have a device using say 4 batteries, one poor battery can let the other 3 good ones down. Save money and the environment by only replacing the poor battery.

Step 7: Switch Off the Meter!

Dont forget to switch it off after testing or it will eventually drain the internal 12v batttery. Been there done that twice :-(

I suppose you could make the power switch "push to make" rather than slide, but then you would need to hold down 2 buttons.

I have since made this mod (replaced slide switch with push switch) as theres nothing worse than going to use a battery test meter which needs new batteries itself ☺

Have fun making this useful project, and save a good batteries life from landfill ☺