After showing how the ANENG 613 can be opened to install a lithium ion cell, I thought I would take a minute to show you that every unit doesn't have to be opened in order to make a conversion.
Here is my DMM tweezers. Doesn't have good screen contrast on standard rechargeables or even half-used AAA cells. There is no on-off button, so that's a bit wasteful of primary batteries waiting for the auto power off (Insert Jeopardy song here)
This unit also has a relatively high "off" current draw of 15.6 uA, called the quiescent current. This means the installed batteries are being drained more than typical when the unit is off. For comparison, the ANENG M118A only consumes 0.45 uA when off.
There is also an issue of weight for a set of SMD tweezers. Too heavy, and it's unmanageable to use being bottom-heavy. Lithium cells have an outstanding capacity to weight ratio that I want to take advantage of.
And we all know primary cells can leak, especially alkalines.
All things considered, this unit is a good candidate for conversion. Let's do it without voiding the warranty.
I used the same components as the ANENG 613 conversion. However, I made everything compact enough to fit inside the battery bay. I don't remember what the donor lithium ion cell came out of. I recently tested it for capacity of 225 mA*h, a little less than half of the capacity of the cells I put in the ANENG 613.
The purpose of the R3 increase is to reduce the charge current, to appropriately match the lithium polymer cell being used. The hidden-from-view double-stick foam tape between cell and charging board is not only attachment but also thermal isolation, keeping the TP4056 from heating up the cell. Most of the heat the chip creates gets driven into the PCB layers, we want it to be dissipated away from the cell out the top. By the way, this charge board also comes with a USB-C connector, but it’s too tall for this application. Stick with the micro b version for lowest height
I removed the LiPo's protection circuit this time, because the charging board already has that. This reduced the number of wires, too, because they were spaced the same as the bottom side of the charging board Bat+ and Bat- solder pads.
The XC 6206 voltage regulator was pre-tested at 5 uA quiescent current, so the total is about 20.6 uA in theory. Doing the math, the cell should keep it's charge for over a year in standby. Putting the device on a small SOT board with the SMD capacitors turned out really well, sorry I didn't snap a picture before heat shrinking it. Here's the circuit.
There is plenty of room for a longer cell in the bay, but this is the size I had on hand. I can be certain things won't shift around with this foam. The back fits on nicely without pressure on the sandwich I made.
If I want to go back, I just pull the assembly back out and restore the crossover terminal. I can easily transfer this to another device if I wish. Just have to insulate the one terminal before removing the second, to avoid shorting things out.
The key here is everything was done inside the battery bay, no modifications to the UNI-T. I didn't even need to remove the crossover terminal, but I liked removing one potential source of shorting.
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