BT-168 Battery Tester Review and Teardown

A while ago, I came across the BT-168 Battery Tester. It seems to be cheap and widely available. Because I wanted to see what’s inside, I decided to order one from an online seller, and managed to get it in a week.

BT-168 Battery Tester?

According to the seller’s description, the BT-168 battery tester can indicate the health status of almost all 1.5V cells and 9V batteries. Test contacts for the 9V battery are located on the side while positive test contact for the 1.5V cell is a red sliding terminal (rather annoyingly, not a spring-loaded one).

There’s no battery inside the battery tester as it draws power from the cell under test.

The dial of the analog moving coil meter is clear and easy to read. Because it is not spring loaded the needle will fluctuate if the test contacts are not tight enough!

The battery tester is tacked together with two small screws, and those two screws is all it takes to get into the inside electronics.

Rather shockingly, the electronics of the battery tester has only four regular resistors, apart from the analog moving coil meter. The meter’s sensitivity isn’t marked anywhere but I found that its resistance is around 280Ω (more soon).

Below is the crude schematic of the BT-168 battery tester (drawn by me)!

The “load” that the tester circuit puts on the 1.5V cell under test is around 3.9Ω, and it’s around 208Ω for the 9V battery. Therefore, the 1.5V load current is around 385mA (seems strange), while the 9V load current is around 43mA.

In my random tests, I noticed that the meter provides a full-scale deflection (FSD) when it receives 400mV at its terminals. So, it might be a 1.4mA FSD meter (it’s something that externally looks like a voltmeter, but it’d be nice to count it as a milliampere meter).

Jumping to a quick conclusion, the BT-168 battery tester is an inexpensive device that can roughly estimate the health status of 1.5V cells and 9V batteries. Don’t expect great accuracy or reliability from a crude tester like this.

Reuse of mA/VU meters (Build Your Own Battery Testers)!

As said above, the analog meter used in the BT-168 battery tester is in fact a mA meter. You can find similar relatively abundantly moving-coil galvanometers as VU meters in many audio instruments.

Electrically it’s mainly characterized by the coil resistance and the full-scale deflection current (sensitivity). An example combination is 600Ω/500uA. The corresponding full scale deflection voltage on the coil, according to the Ohm’s law, is therefore 300mV.

Such a simplistic device (with d’Arsonval meter movement) can serve as a voltmeter or ammeter. Its accuracy is often low, and the coil parameters aren’t optimal for either voltmeter or ammeter. However, the cost factor (sometimes even salvageable for free) makes it a tempting solution in many low-end/hobby-level readout applications.

You can of course make your own simple battery tester from components in your junk box for testing common batteries under load with a “VU meter” you already own, even though it may not have a datasheet. You simply need to do some calculations!

The schematic is simple – a couple of resistors are used to select the range for the meter, and a load switch is used to apply the load resistor. Recall, batteries should normally be tested under a load because a no-load voltage test could read normal, but drop significantly when the battery is under load.

Probably the only thing that may need explanation is the values ​​of the resistors R1 & RP1 on this schematic. Simply, it all depends on what kind of meter you use to show the battery level.

For example, let say you’ve a VU meter (milliammeter) with a sensitivity of 2mA. Using Ohm’s law, you can calculate that to get voltage of 2V (for 1.5V test) to deflect the needle to the maximum range of the milliammeter (2mA) you need a 1KΩ resistor in series with the meter (2.0V/0.002A = 1000Ω).

Depending on what resistors you might have handy, you can select something like a 470Ω fixed resistor (R1) and a 1KΩ multiturn trimpot (RP1). This will give you a range of 470-1470Ω, so plug in a dc source with known output voltage and adjust the trimpot until you get the needle where you want it.

Note, the tester draws about 150mA (excluding the display part) because of its 10Ω load resistor (R2) on the 1.5V input. The 10Ω/0.5W load resistor is enough to bleed off surface charge and provide a more accurate indication of remaining battery power compared to reading the open state voltage.

Now see, you can take apart the VU meter by removing two pieces of used adhesive tape to secure its casing. And then carefully remove the dial to replace it with a piece of paper with a custom-made battery level scale on it (the dial drawing needs some serious homework).

Of course, this do-it-yourself battery tester too is basically a d’Arsonval meter with an appropriate voltage divider, and a load resistor to allow for the bleeding of surface voltage from the battery to see the voltage under load. The exact value of the resistors is probably not critical as the dial is relative and small movement errors are unlikely to be noticed by most end users (naturally, the is not exactly linear with difference respect to voltage, and it’s probably related to spring tension across the dial).

While on the topic, an unknown VU meter (and/or any galvanometer-based meter in general) can be scoped using a source of steady low voltage dc and a suitable potentiometer in series. This allows you to employ a VU meter, stripped from its mother circuit, in your new project, as done above!

Closing Thoughts…

Honestly, I’m glad to have the opportunity to see the internal electronics of a popular battery tester. By gathering the cheapo battery tester, and tearing it apart, I managed to trace out its schematic, learned the maths behind and evaluated the characteristics of the delicate moving coil meter. Further, I was also able to share some thoughts on adapting the idea by reusing a salvaged milliampere meter. For a few bucks, I think it’s worth doing!

Well, I decided not to stop there. I just got another battery tester, this time, the digital version BT-168D. I’m currently testing the device to get some hacking ideas. I’ll write more about it in the next few weeks. So, stay tuned!

Leave a Comment