Wednesday, April 2, 2014

Making a cheap Chinese battery tester more accurate and useful

A few months ago I was ordering some stuff on EvilBay from a vendor and noticed, for only $5 or so, what looked like a nice, little battery tester with an LCD readout showing voltage.

"Only $5 - I'll get one."

Well, it arrived and I stuck a new 1.5 volt battery on it and it read a bit low - around 1.45 volts.  Grabbing a real voltmeter, I saw that the battery was really closer to 1.56 volts when connected to the battery tester so I put it on the workbench and ramped the voltage up and down, finding out that only around 1.35 volts or so was it actually correct - above or below this, it departed radically from where it should have been.  After all, what's the point of having two digits to the right of the decimal point if the first one isn't even likely to be correct?

The battery tester after modification - reading the correct cell voltage on
a fresh AA cell!
Click on the image for a larger version.
Now, I know what to expect for $5 or so, but I decided to take this as a technical challenge - even though it might smack a bit of "turd polishing":  "After all", I though,  "How hard could it be to fix this?"  So I decided to reverse-engineer this thing.

Popping the unit open I could see that it was built not too unlike those cheap Harbor Freight voltmeters:  A section of voltage dividers that feed into a black glob of epoxy hiding a die-mounted chip that does the magic that drives the LCD.  What this meant was that I needed only to identify the power supply and signal input lines for the DVM chip and work backwards.

Alternately powering the circuit from the 1.5 volt and the 9 volt battery test inputs, dangerously wielding a voltmeter and scribbling on a piece of paper I soon had an idea of the "lay of the land" and worked backwards from there, coming up with the following diagram:

A partial schematic of the "front end" portion of the battery tester using the silkscreen parts designation from the circuit board with a facsimile of the original circuit in the upper diagram and the added modifications to make it accurately read 1.5 volt batteries in the lower diagram   While the resistor values in the original section could be easily read and/or measured, I did not bother doing so with the capacitors and a few of the other components.  I'm pretty sure that this diagram is at least "mostly" correct!
Click on the image for a larger version.

A quick analysis of the diagram above will reveal the problem:  Diode V4.  (No, I don't know why they used "V" do indicate diodes - that's what the silkscreen on the board says...) What the designers apparently did was to "fudge" the scaling values that the 0.6 volt or so drop of V4 would  come out about right at "nominal" battery voltages.

At this point I decided to do what the designers should have done in the first place:  Properly measure the 1.5 volt input and one of the ways to do this was to use an analog switch.  Fortunately, there are some other signals available, such as "Q2" - a "blocking" type voltage converter (think "Joule Thief" - Google it!) that produces 6-12 volts from a 1.5 volt cell that is then regulated downwards again to allow the DVM chip to be powered properly - and also provides at least a small load with which to test the cell.  Since this is used only when a 1.5 volt cell is connected, its output could also be used as a signal to indicate when the "1.5 volt mode" is to be used.

In the diagram above, a TC4S66F chip is used which is essentially one quarter of a CD4066 quad analog SPST switch.  While there is plenty of room in the case for a full-sized 14 pin DIP and a small piece of prototype board within the case TC4S66F chip as there is a large enough area of ground to which it could be easily attached as can be seen in the picture below.   

  • The TC4S66F may be discontinued at the time of this update (3/15).  While there may be similar devices by other manufacturers, an entire CD4066 - either in full-sized DIP format or SMD - could be used instead, using just one of its four gates, or all four gates in parallel.

The way it works it this:
  • The TC4S66F chip is powered from the same supply input as the 3 volt regulator which could be either the 9 volt battery or the 6-12 volt switching converter powered by the 1.5 volt cell.
  • If the voltage source is a 1.5 volt cell, Q2 is up-converting and a voltage is present between "V4" and V3 (see lower diagram) which closes the SPST switch within the TC4S66F.  With the TC4S66F's switch closed, the 1.5 volt input is connected to the resistive divider of the DVM chip via Rb, which is used to calibrate the input.  Rb also servers to protect the TC4S66F in the event that the 1.5 volt cell is connected backwards by limiting the amount of current that could flow into its input protection diode as well as to (somewhat) limit the amount of ESD discharge current that could occur.
  • If the voltage source is the 9 volt battery, Q2 is not active and the TC4S66F's switch is open, with the DVM voltage source coming solely from the scaled 9 volt input.  (Ra is used to make sure that Ca, the filter capacitor on the output of the voltage convert is fully discharged.)
The interior of the modified battery tester showing the added components connected with the #30 wire-wrap wire.  The added components were later stabilized with RTV (a.k.a. "silicone") sealant.
Click on the image for a larger version.

The results:

With the modifications complete I find that the accuracy in the 9 volt range is within about 20 millivolts and that the accuracy in the 1.5 volt range is within about 5 millivolts - plenty good enough for about any practical purpose! 

Was it worth the trouble?  Probably not, but it was still a fun project and an interesting exercise.

The one thing that makes me nervous is that the regulator chip's maximum voltage is on the order of 13 volts - and with a fresh 1.5 volt cell - which can output 1.65 volts in some cases - the Q2 converter can output slightly more than 13 volts!

(It was only $5 anyway, right?)

Whatever you do, do NOT test Lithium-Ion cells with one of these testers or you will likely blow it up, modified or not!
The output of the voltage converter (Q2) on the "1.5 volt" input is roughly proportional to the input and connecting such a voltage would far exceed the rating of Q1, the regulator and likely Q2 itself - not to mention the fact that the display will read, at most, "1.99" volts in this mode, anyway!

This page stolen from


  1. I found the TC4S66F on Ebay. Would I need any other parts to do the mod?


    1. Hi Robert,

      The parts additional to the original meter are in the lower half of the diagram and they include resistors Ra, Rb and Ca, re-using V4.

      As noted, one could also used 1/4th of a normal CD4066 CMOS gate - which is probably easier to wire to than the small TC4S66F, but with a different pin-out, of course. (One would tie the un-used control lines to either V+ or GND.)

    2. Would the modification also allow accurate readings for a 18650 3.7v lithium battery?

    3. Probably not.

      At the 1.5 volt connections the voltage is boosted to 5-12 volt internally. Unless some sort of regulation is added to the input of the boost converter, it would probably blow the converter and/or regulator. In theory, this could be as simple as a simple emitter-follower type of regulator, but that would make it useless for "normal" 1.5 volt cells as the added voltage drop would prevent it from functioning down to the 0.9 volts needed to test "weak" Alkaline/Carbon cells - or possibly even "good" ones.

      There are probably better testers for LiIon cells, anyway.

  2. Hey KA7OEI, can you please help me. I used the wrong battery (I have attached a picture) and I think I I burned something because now the tester is not working. Can you see from the picture which part do you think I need to replace and where I can buy it.

  3. From the picture I cannot quite see which component is damaged.

    Based on the "partial" schematic diagram one could infer the following:

    - If it works with a 9-volt battery, the meter/display chip is probably OK, as is regulator Q1.
    - If it works with a 9 volt battery but not with a 1.5 volt cell, Q2 has probably been damaged.
    - If it works with neither, connect a 9-volt battery and check the output of Q1 to see if it is outputting approx. 3 volts: If it is, the meter/display chip is likely damaged and the unit is a loss. If it is not, it MAY be that Q1 has been damaged AND/OR the meter/display chip is damaged and shorting out the power supply.

    Q2 is a standard NPN transistor, the part number being noted in the diagram, above. Q1 is a 3.0 volt regulator, but based on the fact that the one on my tester and your seem to be different part numbers it would seem that a number of different types may be used, provided that it can handle at least 12-15 volts input - the amount of voltage produced by the Q2 circuit.

    (Off-hand I don't know what type of regulator it is since I don't have my tester handy/open to look at.)

    The above, partial diagram - even though it shows the modified circuit - should give the general idea of how the Q1 and Q2 circuits work together and where one should measure the various voltages.

    1. Thank you so much for your answer! I tried all the things above but still not working I guess the problem is with the display chip. I will order a new one :-). BTW do you know which battery from the three ruined it? Thanks again.

  4. Upon looking at your picture more closely I can definitely tell which one damaged the meter: The "Camelion" brand A23 battery - which is actually a 12 volt battery of the sort often used for remote controls.

    The "A23" battery can put out about 10 times the expected voltage of a normal AA/AAA cell, so that is definitely your culprit!

    In theory, it probably would have been safe to connect it across the 9 volt terminals, but that is not easy to do, nor would it be one's first inclination without having first (closely) looked at the labels.

  5. Hi,

    I have bought one of this battery tester, and also found it not very accurate... A quick web search makes me land here. Many thanks for your reverse engineering :-). This saves me a lot of time. So I have tried to understand and modify the BT-168D bought for 3euros including delivery cost from China on Ebay...
    The problem was actually the diode V4, but I have searched another solution than using an analog gate. The problem is the voltage drop of V4 diode, so the goal is to use a perfect diode instead. With a small MOSFET (2N7000 in discret for example), it can easily be achieved thanks to the voltage converter which give a high 12V voltage when 1V5 battery is connected. this 12V so can directly drive the gate of MOSFET, using the same V4,V3,Ca and Ra as your design. In fact just the analog gate can be replaced by a discret MOSEFT to build the ideal diode. Then it is easier to find a MOSEFT than an analog gate and also easier to wire it. The voltmeter seems to need about 26,2kohm for good accuracy, so I use Rb=8k2 and I add also a 1k serie resistor for 9V measuring which also with R7//(R10+R6) give about 26k. At end I realize that I could have change R10/R7/R6 to have about 26K2 for both 1V5 and 9V input. In 1V5 we have 8K2+(20k//(150k+30k))=26k2 and in 9V 1k+(30k//(150k+20k))=26k5. Here is some picture of my changes :


  6. Thanks for that.

    I might comment that the pictures are probably not visible to anyone other than those with whom you specifically wish to share, but the description is likely enough to get the general idea.

    1. I was impressed with the nice $5 tester but also found it +/-10% error and not trimmable, frustrating for a digital tester. Wasn't going to go deeper but then saw ur schem- thnx! The diode V4 is culprit and don't know why they used it! Remove it & replace with 6.2k and remove R10 which eliminates interaction with 9v input. add 220k in parallel w/ R7 to bring it to 26.4K and its done!
      For testing Lion, get another unit and rewire +9v term to slider; of course u wont be able to test 1.5v anymore but i cant find another tester for $5. And i bought another thinking it might be more accurate. Unsolder wire from +9v term and slider wire from ckt bd and solder to each other; the tiny flexible wire should last longer than any wire i had. UNFORTUNATELY 18650 wont fit! But with rotary tool u can open up the -side and rebend the -term to gain extra 6-7mm.
      Final touch would be a more realistic test load of 100ma. this means a 40 ohm but .4 watt-i used 3x 120 ohm 1/4w for Lion unit soldered to ground with free end as tie point for the +wires. For the 1.5v unit, add a 22 ohm from ground to where V4 anode was.
      Don't solder to SMD R's and C's as they seem to die when i did!

    2. I have amended the article with the warning NOT to try to test ANYTHING other than 9 volt batteries or 1.5 volt cells, so it is probably a good thing that you haven't been able to fit a 18650 in there!

      The reason for this is that, as noted in the article, that the 1.5 volts is boosted to 6-12 volts with a converter circuit, but if a 3-4 volt lithium cell were used this voltage would be in the 20 volt area and likely blow things up. Testing from the 9 volt terminals may work as long as the minimum voltage for the display/voltmeter circuit is maintained, having passed through the voltage regulator - but I don't know what that "minimum" voltage at which calibration is maintained.

      As for the more realistic load, I thought about that, too, but didn't do it, considering the way that I actually use the tester: If I find a cell that is at 1.45 volts or higher, I'm quite sure that it is good. Since I don't keep partially-discharged alkalines kicking around (I put them in the device and they stay there until they are dead - verified by noting that they are below 1.3 volts with the tester) I use this tester more as a sorting/verification tool.

      Having said that, a 40 ohm (or so) load resistor is easy and a good way to go if that fits your "work flow" in testing cells.

      Finally, I haven't had any trouble soldering the SMD C's and R's, but I always use a temperature-controlled soldering iron (cheap these days), solder with "component friendly" flux, and keep soldering time to a minimum and in that way find them to be quite durable.

      Thanks for the post and good luck!



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