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Review by Phil Prosser
DH30
MAX
Li-ion Battery Welder
It is a simple idea, and it should work well. How did it go so wrong?
I
was asked to review one of the
Li-ion battery based welders that
are cropping up on internet sites of
late.
Having just finished the Capacitor
Discharge Spot Welder (March & April
2022; siliconchip.com.au/Series/379),
my reaction was: why not? This might
be a cost-effective alternative. So I proceeded enthusiastically.
The prices of these welders seem to
reflect the capacity of the battery used,
which in practice consist of one or two
cells paralleled inside the welder. That
translates to costs broadly in the range
of $50-100.
As noted in the CD Welder article,
one challenge battery-based welders
face is getting enough energy into the
weld quickly enough. So to be fair in
this review, I chose a welder at the high
end, the DH30 MAX, which claimed to
have a 10.6Ah battery for about $100
plus shipping.
After a relatively long wait (a bit
over a month), it turned up, and I must
say it both looked and felt the part.
The case is 150 × 28 × 80mm and
has substantial heft. It is an aluminium
extrusion, and it is clearly packed full
of batteries and stuff.
Using it
Plugging the welding cables in was
a delight. I wish I knew where I could
buy these connectors as they are great
(shown below), and I would have been
tempted to try fitting some to our CD
Welder.
I was initially bemused at how they
were insulating that connector from
the front panel (it is an unclad PCB).
I will get into that more shortly. Also
in the pack was a length of 0.12mm
“nickel” strip and a USB charging
cable.
I had a prototype milliohm meter
sitting on my bench (to be described
in an upcoming issue), and I used it
to quickly determine that the leads
have a resistance of 1.5mW. This is
consistent with 300mm-long 10 gauge
(8mm2) leads. I noted that these leads
are inconveniently short, even on the
first weld. Checking the maths, though,
they need to be short for the welder
to work.
The user interface is colourful but
fiddly. It took me a little while to get
it to do what I wanted.
With the pack fully charged, I was
off to the workshop and ran a couple
of test welds on flat AA cells.
Three welds in, and everything went
pear-shaped. After the third weld,
“magic smoke” started erupting from
the welder case! With some concern
about the device catching fire, I moved
outside.
A “minor” setback
With a large coffee to calm my
nerves, I reassured the wife that the
house would clear of the acrid smoke.
This was not going to plan!
The DH30 MAX welder comes
with the batteries installed into an
aluminium enclosure. It has a rated
welding output of 4.2V at 650A. Note
that the charging port is USB Type-C.
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Silicon Chip
Australia's electronics magazine
siliconchip.com.au
►
The DHT30 MAX welder uses a 0.91inch OLED screen.
I channelled the Serviceman and
took to the case with screwdrivers and
pliers. Having extricated the PCBs and
lithium-ion cells from the box without
shorting anything dangerous, the trail
of smoke and cinders was something
of a dead giveaway to the fault.
Ignoring the minor fact that it blew
up for the moment, I will provide
some comments on the construction
of the unit. The cells look the part for
10Ah, weigh enough and have very
wide tinned connections to the ‘power
board’. The actual part numbers have
all been wiped off, but without running a capacity test, I assume they are
up to the task.
The controller PCB has a microcontroller with the top ground off, USB
Type-A and Type-C connectors, the
front panel control switch and OLED
and two capacitive switches that use
springs from the PCB to the display
panel. The construction looks OK, if
not excellent. This unit can double as
a USB phone charger when not welding, which is handy.
The ‘power board’ connects to the
two Li-ion cells, the ‘control board’
via a header, the front panel and the
welding lead connectors. It has four
4N03LR8 power Mosfets rated at 30V,
240A.
They are quite appropriate for this
job, though I would have been tempted
to use more of them. The PCB layout
has footprints for six smaller devices;
I would rather see them all present,
given the currents involved.
There are a lot of vias on the power
PCB, and for the most part, both sides
of the board have large copper fills
carrying the current with vias connecting between them on the top and
bottom layers.
The left side of the PCB has VBAT
running up to the output connector.
The right-hand side of the PCB carries GND, the battery negative terminal. This connects to all the Mosfet
source pins, with the drain on the tabs
connected to the “Out-” connector on
the front panel.
This switching method is the same
concept used in my CD Welder, but on
a baby scale. Take note of those vias
running right down the right-hand side
of the PCB; they are connected to GND.
Repairs required
I found that the PCB trace for VBAT
had overheated and fused. The Mosfets
were OK, as was the controller. All in
all, it is a credible design except for
the catastrophically narrow length of
track on the left-hand side trying to
carry 600A or so.
After scraping the charred material and solder mask away, I soldered
three lengths of copper braid (solder
wick) over this section. Solder wick is
nice and flat and can carry an awful
lot of current.
While doing this, I also noticed that
the Mosfets were barely soldered to the
The welder ‘blew’ up on both sides of the PCB, marked with red arrows.
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siliconchip.com.au
Australia's electronics magazine
August 2022 61
board. So I soldered down the floating pins while muttering many a salty
oath along the lines of “you were so
close to getting this right; what were
you thinking?”
With the sort of conviction that
you can only have when something is
about to go wrong, I started reassembling the unit. Halfway through the
reassembly, I had a minor conniption.
The only thing stopping a dead short
across those beefy batteries was the
solder resist on the PCB!
Remember those vias (shown
directly above)? They are actually
inside the slots in the extrusion! My
muttering turned to the question: “Are
you for real? This will burn my house
down!”
To fix this, I took my trusty Dremel
and ground back the VBAT fill up to
the solder braid I had added, giving
a gap of about 0.5mm between the
case extrusion and the VBAT trace. I
allowed the GND side to touch the case
since that would no longer be harmful.
some more measurements and got the
following readings:
• 2.5mW from the positive battery
tab to the tip of the positive probe
• 1.5mW from the drain of the Mosfets to the tip of the negative probe
• the Mosfet specification is 0.79mW
each with VGS = 4.5V, or about 0.2mW
for four in parallel
• the resistance from the negative
battery tab to the Mosfet sources is
about 1mW
This gives a total of 5.5mW or so,
resulting in 650A into a short circuit.
This jibes with the spec on the box.
For a 200ms pulse, this would be in
the region of 400J. The problem is
that little of that goes into the workpiece, as that is counted as 0W in this
calculation.
Basically, the workpiece needs to
have a resistance of at least 5.5mW
between the probe tips to get even half
of that energy into it, and ideally considerably more for it to take the bulk
of the energy.
During tests, the leads got quite
warm after half a dozen welds or so, as
did the tips. As you can see in the photos, while I made reasonable welds,
there was significant heating around
the weld spot.
Is it worth it?
I guess the main question is: can you
use it to make good welds? A decent
weld to an AA cell is shown opposite.
I did that in gear 13. I found that was
the minimum to get a reliable weld
that would not pull off easily.
But that put a lot of heat into the battery. I welded three times in succession on one battery and literally melted
the plastic insulation. So you need to
be very careful using this welder!
So, in summary, does it work? Yes,
Back to the review
With that done and everything buttoned back up, it was back to the task at
hand: reviewing this comedy of errors.
I took a more gingerly approach,
starting with a 3W resistor and testing
the welder in “gear 1” through “gear
20”. These equate to power levels,
which are implemented by variable
pulse widths of 26ms to 300ms (see
Scope 1).
Using levels up to about gear 8-10
gave unreliable welds with the strip
they provided. I achieved decent
welds in gears 11-13. The welds were
OK, but because of the 200ms weld
time, things got really hot making
them.
To check if this made sense, I did
62
Silicon Chip
Scope 1: a scope grab of the output at the “gear 11” setting. I found this gave OK
welds; the pulse width is 182ms. There are a couple of short pulses at the start,
which are present on all settings.
Australia's electronics magazine
siliconchip.com.au
this device will weld after significant
repairs.
Is it reliable? For welding, I would
give a qualified answer. It can weld,
but puts a lot of heat into your workpiece. So it depends on your application. I would be very cautious using it
to weld anything very sensitive, like
Li-ion cells.
Will it remain reliable? This device
is marginal. Other similar devices
could be better. The Mosfets are OK on
spec; however, the manufacturing had
several serious flaws. Also, an increase
of a milliohm or two in battery impedance would severely impact weld quality, and that could easily happen over
time or with use.
I would recommend the DH30 MAX
only to technically confident people
willing to check it thoroughly before
use, and only if you intend to undertake small/non-professional jobs.
There is a world of difference
between this and my CD Welder
design, in terms of weld repeatability
and heat in the workpiece. Granted,
there is a significant price difference.
I’d like to comment on how I think
that the design flaws in this unit came
about. My day job is in engineering in
Defence, where “engineering governance” is an integral part of life. It is
tedious, but it is there for good reasons!
This device has all the hallmarks of
a design that was originally very good,
relatively simple and fit for purpose
in its original embodiment. Looking
at the problems I found, my guess is:
• The packaging was changed and,
in this process, somebody neglected
to check the VBAT and GND trace
clearances to the case extrusion slots.
I imagine this was done by a different
person than the original designer, and
they didn’t even think to check.
• The PCB manufacturing was
cost-optimised, perhaps too much
so. Six devices were reduced to four.
Looking at the solder mask, it actually extends under the four Mosfet
source tabs! It is hard to see the original designer finding this acceptable,
but I doubt they reviewed this change.
• The PCB uses lightweight copper foil. It is much cheaper to use this
than heavier (eg, 2oz) copper – again, a
change that I suspect occurred in manufacturing without return to design
and qualification.
• Someone had reworked all the
Mosfets, but only fixed soldering on
two of the five pins. This procedure
would never get past a review or the
original designer; doing it right would
take a second or two extra! Also, the
need for rework is indicative of a
deeper manufacturing problem.
All these faults could be fixed at
marginal or nil cost. They might even
get away with the lightweight foil with
a better PCB layout.
As it stands, each of these faults
could lead to catastrophic failure.
Therefore, I recommend that you
avoid purchasing this particular unit
(and be wary of other similar units)
unless you will personally open it up
and check that it is safe to use before
powering it up.
I must also admit that I have a bit
of concern that one of these could go
up in smoke during transportation,
depending on how it is handled, given
the proximity of those ‘live’ vias to the
SC
metal case.
Improved SMD Test Tweezers
Complete Kit for $35
Includes everything pictured (now
comes with tips!), except the
lithium button cell.
●
●
●
●
●
●
Resistance measurement: 10W to 1MW
Capacitance measurements: ~10pF to 150μF
Diode measurements: polarity & forward voltage, up to about 3V
Compact OLED display readout with variable orientation
Runs from a single lithium coin cell, ~five years of standby life
Can measure components in-circuit under some circumstances
siliconchip.com.au
SC5934: $35 + postage
siliconchip.com.au/Shop/20/5934
Australia's electronics magazine
August 2022 63
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