Electronic Building Blocks
By Julian Edgar
Quick and easy construction
I
Battery capacity tester
f you use batteries in any
devices, here is a tool you really
must have on your workbench. It’s a
battery capacity tester, a brilliant piece
of test gear that ticks all the boxes.
Cheap? Yes, at round £6 delivered, available from Banggood (ID:
1557935). Versatile? Yes, it can work
with batteries having any voltage
from 1-15V and any chemistry. You
can also set the test discharge current (just use different-value external
resistors) and you can also set the endpoint voltage.
Before we look at the device in detail, what use can be made of it? Well,
have you ever wondered if it’s worth
buying expensive brand-name batteries rather than the so-much-cheaper
supermarket own-brand variety? Just
buy a few examples of each and then
test their respective capacities with
this module.
Are you doubtful if all rechargeable
18650 cells meet their stated capacity? Buy one that has big claims and
test it.
And, just the other day I designed
a circuit that drew 100mA from a 9V
(PP3) battery. I felt that was probably a bit high for the size of battery
– but exactly how long would it last
at this current draw? Hard to estimate
isn’t it – especially when in this application, the battery output can fall
as low as 5V and the circuit will still
operate. But using the battery capacity tester, we can test a typical PP3
battery at this current draw, setting
the endpoint to 5V. Remember also,
that battery capacity is dependent on
current draw. That is, as discharge
increases, capacity decreases (Peukert’s law – see Wikipedia). So, having
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Great results on a low budget
the connections to the battery under
test, and the RL connections are for an
external resistor that forms the load.
(The connections for the resistor are
not polarised, unless you are using
an electronic load.)
The device is powered at a nominal
5V via the micro USB socket. Ensure
that the power supply won’t turn off
after a while (eg, from a laptop or PC
going into sleep mode) as the module
must remain powered throughout the
whole test.
Three pushbuttons are provided
on the front. These are labelled (−),
(+) and OK.
Fig.1. The battery capacity tester (bottom)
is supplied with three high-power resistors
to act as loads. The tester calculates the
battery’s amp-hour capacity, and during
the test shows real-time battery voltage and
current draw. Any battery from 1-15V can
be tested at loads of up to 3A.
the ability to test a battery under the
actual discharge rate that will be
experienced when the circuit is in
operation is very useful. Finally, as
the tester also records time, we can
specify what the battery life would
be in this application, expressed in
minutes of continuous use.
The module
The unit is a very compact 54 × 41 ×
17mm. It comes in a neat clear plastic
box with a micro USB socket at one
end (no cable supplied) and a 4-connector terminal strip at the other end.
The terminal strip is labelled IN
(+ and −) and RL (+ and −). IN is for
In use
First, determine what load current
you wish to use in the test – this
will determine the specs of the resistor. Note that current is limited to a
maximum of 3A. Ohm’s law can be
used to determine the required resistor value: resistance (Ω) = voltage
(V) divided by current (A). For example, if you wish to discharge a 3V
battery at 100mA (0.1A), you would
use a resistor with a nominal value of
30Ω. Also keep in mind that the resistor dissipates power. The resistor
voltage (V) multiplied by its current
(A) gives the power (W), so in this
example we are dissipating only 3 ×
0.1 = 0.3W – a 30Ω, 1W wirewound
resistor will be fine.
The module is supplied with three
resistors (two 7.5Ω, 5W, one 6Ω, 50W).
Note that at anywhere near their rated
power dissipation, these resistors will
get very hot. It’s best to de-rate them
substantially and if you’re not able to
do that, place them on a china plate or
similar so they cannot burn anything!
Practical Electronics | August | 2020
Fig.2. Testing two AA cells. Three 10Ω resistors are being used to
give a nominal 100mA load. Towards the end of the test, the display
shows 0.179Ah. Power is supplied via the micro-USB cable at right.
Fig.3. Is it worth buying expensive brand-name batteries? This tester
can easily find out – I was surprised at its results!
In many uses, you will need resistor values other than
these. However, high-power resistors are now available
very cheaply online. Incidentally, there’s no requirement
to use a resistor as the load; for example, you could also
use a filament bulb or an electric motor.
After you have connected the battery and load, power-up
the tester. The voltage of the battery is then displayed on
the LED to two decimal places. By pressing the (−) button, we can then set the endpoint voltage, to one decimal
place. (The tester can also automatically set the endpoint
voltage, based on the starting voltage of the fully charged
battery and presumably using some internal look-up values that guess at battery chemistry. However, it seems to
me that it’s best if the endpoint voltage is set manually,
taking into account the battery type and proposed use.
This auto endpoint function can be disabled.) Once the
endpoint voltage is set, press OK.
Note that the Maximum test values are 999.9Ah and 9999
minutes (just under 7 days!).
Display
The battery test then starts, and the LED display cycles
through the following values:
The set endpoint voltage (with an ‘E’ leading figure for
‘endpoint’)
Battery voltage
Current draw
The amp-hours that have been drawn
The time (minutes) that the test has been operating
Each of these is indicated by a flashing LED that lights
next to an appropriate notation on the board (eg, V, A, Ah).
There are some additional functional settings (eg disabling the display from cycling through the different
values) but note that none of these further settings allow calibration of the current or voltage measurements.
However, in use, I found the voltage and current readings
quite accurate.
The tester can also display error codes. These are:
Err1 Battery voltage is higher than 15V
Err2 Battery voltage is lower than the endpoint voltage
Err3 Load discharge current is too high (presumably this
is determined by the battery voltage immediately
sagging below the set endpoint)
Err4 Current over 3.1A
Err5 Current sampling or output transistor defective (eg,
through reverse-polarity connection)
Practical Electronics | August | 2020
Example testing
I first tested some brand-new, low-cost supermarket brand
AA alkaline cells – two in series, as they are often used.
I set the current draw to a nominal 100mA via a 30Ω resistor and the endpoint to 2.8V (ie, 1.4V per cell) – and
sat back. I must admit that I found the process quite fascinating (perhaps I am easily entertained), as I watched
the battery voltage falling towards the endpoint voltage.
Exactly 129 minutes later, the test automatically finished
when battery voltage dropped below 2.8V. (Ending of the
test is indicated by the LED display flashing. The data can
then be cycled through by pressing the (+) or (−) buttons.
Don’t press ‘OK’ or you lose the data as the test restarts.)
And the capacity of the new two-cell battery at 100mA
current draw? As you’d expect with the above figures, just
over 0.2Ah. (Note though that the actual current draw of
the resistor of course varies with battery voltage, so it’s
not just a case of taking into account the time and nominal current draw – the tester is more accurate than that.)
Salvage those cells!
Hmm, so what about some salvaged AA cells? (I often
collect batteries that others have thrown away. Typically,
about half the batteries in ‘recycle’ containers are still quite
useable, especially for items in use only occasionally.)
This time I started with two ‘brand name’ cells having
a series voltage of 2.9V (ie, 1.45V per cell). If I were to
use these, I’d put them in a device where they could be
nearly completely discharged before the device ceased to
operate. I therefore dropped the endpoint voltage to 2.6V
(ie, 1.3V per cell). Test current was as before – a nominal
100mA. And the results? Nearly one-and-a-half hours of
use and a measured capacity of 0.13Ah. Not bad for salvaged batteries!
Summary
This is an excellent product – especially considering its
versatility and very low cost. True, it is limited to 15V
and 3A, but that still makes it suitable for most batteries
in general use. If you don’t have a selection of high-current resistors on hand, buy them at the same time as you
buy the tester, and then you’ll have a set-up that will be
very effective.
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