Technique - Converting a Dymo Letratag to use 18650 cells
All electronic parts I keep in familiar drawer cabinets need to be distinguishable in one way or another. For this I use a Dymo labelwriter for years if not decades already. In the beginning this was still a Dymo when Esselte was still the brand, but now I use a Letratag.
Powering the thing...
The Esselte Dymo used the rather impractical number of 6 batteries in the equally impractical AAA dimension to power the device. The Letratag uses AA's and (fortunately) only needs 4.
That seems like a leap forward, but truth is they don't last very long and when using rechargeables the device already starts warning for low batteries after just a few prints. The prints also aren't fully black and this improves significantly when using alkalines.
Ordinariy alkalines were not high on my list but considering the downsides of rechargeable AA's, which were necessary elsewhere, those were not an option. But I've got quite some 18650 lithium cells here and there, retrieved from all kinds of batterypacks. A quick fit to measure showed that I most likely would be able to fit two 18650's in the battery compartment that usually holds the 4 AA's, as especially on the side where the lid clicked into place there was quite some room to spare.
Fear not when seeing this picture of two completely unwrapped 18650's: those are dummies, left over from one of my videobatteries.
The saw would have to come out for this one, but without very weird interventions inside, even the lid would be able to be closed afterwards.
The voltage...
But... 4 fresh AA alkalines have a voltage of about 6-6,5 Volt, while two fully charged 18650's will deliver 8,4 Volt. That didn't sound like a healthy option.
Lowering this voltage with a diode is no use, as the voltage drop provided by a diode is dependent on the current running through it and hardly any current would flow when the device would be turned off. Even with the device turned on but not printing, current drawn would still be rather insignificant for the average diode.
At Mouser, I searched for a nice, unisolated DC/DC converter, as the thing won't be connected with anything else anyway. Upon starting the conversion I noticed the diodes at the DC-jack at the bottom of the thing, where 9 Volt DC was stated as a maximum input voltage. Thus I search and bought the converter for nothing, because this meant I could connect the cells directly without any converter in between.
Opening it up
How pleasant to open this thing: removing ten well accessible screws enables you to open both halves of the device without any part shooting out by spring force or whatever.
However, it is a good idea to decouple and remove the lid that covers the cartridge by removing the spring from the hinge.
After opening I could remove the old battery contacts. I will partially reuse to connect the 18650's, holders unfortunately are just a tad too thick to make them fit.
As the 18650's are so called flat-top cells (there's no protruding bit on the positive terminal of the cell) I had to raise a contact and I did so with some nickelstrip.
Hacking and sawing (Well... hacking...)
After sawing and milling away in the case of the Letratag, a quick testfit. That fits nicely. The PCBs with contacts for the cells were attached to the case with some hot glue. I usually don't like hot glue (I like to screw things down) but the PCBs in this case have almost nowhere to go should they come loose.
At the same time the side where the lid of the batterycompartment meets the case provided a large flat piece of plastic, where the PCB could be mounted well. After milling flat a few of the standoffs where screws meet I could also use a nice blob of glue to get the PCB on the other side mounted.
Between the 9 volt supply and the electronics a diode was found as well, so just to be sure, I also added one between the cells and the device.
And done
And everything assembled. Two screws almost completely in hte middle of the battery compartment had to be sacrificed, but in reality you fortunately don't notice this.
This conversion isn't mothers finest, but it is really functional. Two UR18650SW cells now take care of the power supply and this works a lot better. Besides that I can fully charge them within 3 hours should they be depleted. And the lid can even be closed like before...