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!

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