Making good use of The One True Battery (except it's a cell) #
As you might have gathered, I like to make videos of various projects, but I'm a small-time video producer and thus can't justify spending much money on fancy camera equipment. As such, an old Canon EOS M does the bulk of the heavy lifting. It's fine, but battery life is poor and there's no in-camera charging, and to change the battery you have to take it off the tripod and lose your shot.
Which is annoying.
So, whereas any sensible person would just save up some cash and spend it on a camera that can charge its own battery, I invested many many hours and a fair chunk (probably even enough to buy a nicer camera) of cash designing and building a battery grip that can be fed freshly charged 18650s without ever having to power down the camera or take it off its mount.
And here it is!
So, let's have a closer look at how it works. Embedded in the grip is a board with a synchronous boost converter (measured at about 91% efficiency at 1A output load) to take the ~3.8V from the active cell and turn it into 8.4V, as this camera takes a 2S pack. It has a small, low-power microcontroller (STM32L0 series) that is monitoring the voltages of the two slots, and manages switchover when the active one runs low (assuming there is another sufficiently-charged one in the other slot).
Most of the time it's just scanning them with the ADC, but if one of the cells suddenly disappears it is caught by the comparators - the result being that you can accidentally remove the active cell, and it will switch over to the other one fast enough to keep the output on and the camera doesn't even notice that anything happened.
Inputs are switched using a pair of back-to-back NFETs each. It's got a small OLED display to tell you the state of the cells, plus this is repeated in a basic fashion by blinking red and green LEDs next to the cell slots so you can see what's up from across the room. It even has a piezo speaker in it so if the active cell is on its way out and there is no reserve, it can beep at you to signal impending doom.
There are a couple of buttons to switch in and out of standby mode and change what's displayed on the OLED, and there's an auxilliary output that can cope with running a small accessory such as a mic pre or a small ring light, perhaps even a field monitor. There's even a temperature sensor on the cell bay in case the load causes one of the cells to heat up.
In order to get the power into the camera, I converted a clapped-out old battery into a connector block.
These are available off the shelf for powering the camera from a wall wart, but those have you plugging a cable in through a little flappy bung in the battery compartment door, and I didn't want a semi-permanent wire running through that hole as it would get in the way of accessing the SD card (not to mention looking extremely unprofessional).
So, since this is how I roll, I drilled a hole in the side of the camera:
Luckily for me the battery bay is mounted right up against the right hand side of the camera body, with no electronics to the side of it:
The battery bay itself can easily be cut in-situ (I drilled the hole and then carved the square hole with a scalpel) but the front metal cover is in the way and I didn't fancy getting metal shavings in the workings, so I took that off so I could work on it elsewhere and mask off any bits of it that might trap aluminium dust.
With all that done, it just remained to package all of that into something that at least vaguely resembles a camera grip, and is as small as I can manage. I made a plate that replaces the right-hand end cover of the camera, picking up its original two screws and also tying into the strap peg on that side:
The rest of the grip then mates to that end plate, and attaches to it with a load of screws. It also picks up on the original tripod mount, and the frame at the bottom features a facsimile of a 323 quick release clip.
All told, the housing is made of 10 3D-printed parts held together with 20 small screws of various sizes. I'm particularly proud of the slide-latch-flip-up cell compartment lids, it took a lot of fiddling to find something that worked okay, was possible to print, and wasn't too bulky.
So there you go.