Dummy C Cell (ER26500) Battery Eliminator - with optional PCB bay v4

Version 4 - bigger cable channel (5mm) + lid to close the PCB with voltage stabiliser.

A dummy C-size cell that lets a 3.6 V battery-powered device run off an external supply instead of eating a primary lithium cell.

The body copies the envelope of a Fanso ER26500H (Li-SOCl2, C / R14, 3.6 V, 26.2 x 50 mm), so it drops into any holder that takes a C cell.

How it works

The body is the cell's envelope, 0.4 mm under on diameter and 0.3 mm short, so it slides into the holder and the spring still closes. The poles are two ordinary M5 washers, 5.3 / 15.04 / 1.35 mm, pressed into a pocket in each end face:

- the "+" washer stands 0.8 mm proud, standing in for the cell's positive nub,

- the "-" washer sits 0.15 mm proud, so the metal and not the plastic rim meets the spring.

Each lead is soldered into its washer's centre hole from behind; the blob drops into a 6 x 1.5 mm recess so the washer still seats flat. From there the lead runs in a channel in the pocket floor, out to a 4 x 3.5 mm groove that runs the full length of the body, and leaves through a notch in the "-" end face.

Two versions

- dummy-plain.stl - plain body: washer contacts and wire groove only.

- dummy.stl - the same body with a 17 x 23 mm pocket for a small supply board (a buck converter, an LDO, whatever makes your 3.6 V). It opens on the side opposite the wire groove and is 17.4 mm deep across the board's full width, as deep as the barrel allows with a 1.5 mm web left to the groove. The board's leads leave through its two short (17 mm) edges into a channel at each end of the pocket, which meets the same longitudinal groove.

Both come from the same OpenSCAD file, one parameter apart.

Parametric

model.scad is included and fully parameterised: cell diameter and length, washer dimensions, how far each pole stands proud, wire and groove sizes, which end the wires exit, and the whole board pocket. Change cell_d / cell_h and you get a dummy AA, D or 18650 instead. Asserts catch the combinations that would leave a washer pocket too shallow to locate the washer, or a board pocket that runs into a pole.

Print settings

PETG, 0.20 mm, 3 perimeters, 20 % infill. Standing on the "-" face, "+" pocket up. Add ~0.1 mm elephant-foot compensation, the "-" washer pocket is on the bed and closes up without it.

dummy-plain needs no supports: every cut is either top-down or vertical. dummy has one bridge, the roof over the 17.4 mm board pocket. Either paint supports onto it or accept the sag and scrape it out before the board goes in.

Assembly

1. Strip ~4 mm, tin, bend 90 degrees, solder the lead into the washer's centre hole from the back.

2. Lay the lead in the floor channel, press the washer into its pocket, a drop of CA glue under the rim. "+" is the proud washer.

3. (dummy) Slide the board into the side pocket, leads out of its short edges into the end channels, then round into the groove.

4. All leads down the groove and out of the notch. Tape or heat-shrink over the groove keeps them in, and holds the board in - the pocket has no latch.

5. Polarity: supply "+" to the proud washer. Check your holder's marking first.

Safety

You are replacing a primary lithium cell with a supply you provide. Get the polarity right, keep the supply at the cell's nominal voltage (3.6 V here, not 5 V), and do not leave the original cell in the circuit alongside it.