Voltage indicator for 1–7S lithium battery packs

I sometimes order small meters for battery pack builds, and I finally got around to testing them—and writing a mini-review while I was at it.
A look at the build, a few tests, and some conclusions. Hopefully it’ll be useful.


Unfortunately, shipping from the store is paid, so I ordered several at once to make up for it.
When I placed the order, the seller seemed to have only four versions: 1S, 2S, 3S, and 4S. Now they also have 6S and 7S, but oddly there’s no 5S version for sale. I suspect it’ll show up soon.

I gave most of the meters to a friend, but kept one of each for myself.
Each meter comes in its own bag. The only difference is a label with Chinese markings and the voltage range it measures.
1S - 3.3-4.3 V
2S - 6.6-8.4 V
3S - 11.1-12.6 V
4S - 13.2-16.8 V

There’s also a marking for the glow color (presumably), but the seller only offers them in one version.


If you buy several different versions, it’s best to mark them right away, since they have no markings or visible differences of their own.


There’s a spot for a button on one side of the board, probably to turn the indicator on, but the board has neither a button nor any of the associated components.


When the indicators arrived, I was a little surprised by their size. For some reason, I expected them to be smaller, especially knowing how sellers in Chinese stores like to take photos.
The indicator itself measures 31.5x20 mm; overall dimensions are 43.5x20x9.5 mm, and the distance between the mounting holes is 36 mm.


To avoid mixing up the indicators, I had to mark each one with a marker.


Overall build quality is mediocre: there are flux residues, the soldering is so-so, and on some boards the indicator is soldered on crooked relative to the board itself.


The circuit is fairly simple. There’s no supply voltage regulator, so brightness depends on the supply voltage. It has a reference voltage source based on an adjustable TL431 shunt regulator, as well as reverse-polarity protection.
I couldn’t identify the chip that handles the measurements. At first I thought it was an LM339 quad comparator, but its outputs are on pins 1, 2, 13, and 14, while the chip on the board under review has outputs on pins 1, 7, 8, and 14.

Below are photos of two boards, 1S and 4S, so you can see how they differ.
1. Resistors that supply power to the indicator segments (R1-R5).
2. Resistor R9.

All the other components are identical on every board.
The resistor supplying the TL431 has the same value on all boards, so current consumption depends on the input voltage.


The indicator has five segments: one common battery-shaped symbol and four charge-level segments (which is why I thought it used an LM339). However, there’s also a version with five charge-level segments; I came across one on Taobao.
What’s more, there are many different indicator colors.


The indicator dimensions on the board in this review and the one shown above are very similar: 30.8x17.8 mm versus 31.5x20 mm for the board under review.


Now for a few tests.
The indicator on the board under review glows in two colors: the battery symbol is red, and the segments are blue. The battery symbol consists of six LEDs connected in parallel.

Brightness is adequate, but on the lowest-voltage version it depends heavily on the supply voltage. That’s to be expected, though; the others are much more stable.
There’s also a minor complication: since the glow colors are blue and red, it’s best to use a neutral filter.
Here are four examples below:
1. No filter
2. Green filter: all segments are visible, but brightness drops significantly, and the LEDs illuminating the battery symbol become more noticeable.
3. Red filter: only the battery symbol is visible
4. Blue filter: the segments are clearly visible, but the battery symbol is almost invisible.


Measurements—first, current consumption.
Below are the results for four of the five modes: battery symbol only, plus one segment, plus two segments, and plus four segments. I didn’t post a photo with three segments, but I think you can assume it would fall between the third and fourth photos.
In all the photos with the segments lit, current was measured immediately after they were switched on.
1-4, 1S
5-8, 2S
9-12, 3S
13-16, 4S

You can see that the current keeps increasing, even though the resistors supplying the segment LEDs have different values. This happens because the resistor supplying the TL431 is the same on all the boards. If you need to reduce the current draw, you can increase the value of this resistor (R14) proportionally. For example, use 2 kΩ for the 2S board.


Now for the voltage thresholds at which the segments turn on. A quick aside: there is either no hysteresis or very little, so the lowest-voltage version can sometimes show "brightness flicker," even though I increased the voltage in 10 mV steps during testing.

I also converted the display voltage thresholds to the equivalent voltage per battery cell for each meter version. Here are the results:
1S.......2S.......3S.......4S
3.35 - 3.36 - 3.43 - 3.37
3.57 - 3.53 - 3.64 - 3.57
3.72 - 3.70 - 3.81 - 3.76
3.92 - 3.90 - 4.03 - 3.97

The results vary a little, but the overall picture is quite clear: the measurement range is about 3.4–4.0 V, which roughly corresponds to a nearly discharged and a fully charged battery. A lithium battery's voltage usually drops sharply from 4.2 to 4 V, then declines relatively gradually to 3.3–3.4 V, before dropping more sharply again. I'd say the indicator covers roughly the 15–90% range.


Later, I found a couple more versions of simpler meters.
For example, a waterproof one — link.


And a version "with digits" — link


A reader from France sent me a circuit for this meter. It was originally configured for a 4S pack. Many thanks to him :)



After inspecting and testing them, I can say that these indicators work well and are useful, but I do have a few comments:
1. The individual LEDs in the battery symbol are visible
2. Current draw increases noticeably as voltage rises; this can be fixed by replacing resistor R14
3. There is no power button.

A note on the last point. Since there is no button to turn the indicator on "in software," the only way to do so is to apply power. But there's usually no reason to leave it on all the time, and a typical small button has relatively high resistance, so the measurement would depend heavily on how hard you press it and how long the button lasts.

Otherwise, it's a useful and, in my opinion, inexpensive device. The wide range of versions makes it suitable for different devices, such as a cordless drill.

That's all from me. I hope you found the review useful. As always, I'm looking forward to your questions and comments.