The Ultracortex M4 is normally powered by a recommended Lithium Polymer cell of about 500mAh capacity. That doesn’t give as much run time as most people would like, and the included 4xAA battery box is far too large to fit in the case. There are several solutions available that I’d like to cover in this post.
The M4 Dev Kit released in March of this year contains a variety of upgrade accessories for Ultracortex. Among them is the Octamount, which is a generic bracket that screws into an Ultracortex node like an Octabolt. The Octamount has mounting holes that accommodate several generic battery boxes or the OpenBCI case itself, so you can mount the board or batteries wherever you would like for a custom configuration. The Octamount hass a pass-through hole, and the Long Octamount allows the node to contain an active electrode, so you won’t have to modify your node locations.
One accessory designed to match the Octamount is the Octamount Battery Tray. Featuring the same hole spacing as the OpenBCI case (48.5mm), it can hold the 4xAA battery box that comes with the OpenBCI board. This means that if you can handle the extra weight, you can power the Ultracortex for weeks without disconnecting or recharging.
While the Octamount adds great flexibility, four AA batteries can add significant weight, so there is another solution available. A deeper frame for the OpenBCI case allows the use of an AAA pack inside the case, which still holds many times more power than the 500mAh LiPo. Packs such as these can be purchased or assembled for just a few dollars.
If, for whatever reason, you’d like to make the frame deeper, or make basic edits to any of the STL files in the M4 Dev Kit, here is a quick, single-image tutorial on how to move vertices around in Blender, the free mesh modeling and animation software.
For completeness, I’d like to add a bit about making the packs. AAA rechargeable batteries, typically NiMH type, can make a good large capacity pack. One concern in doing this is that the voltage of a NiMH cell is 1.2V. This results in a choice between a 3xAAA (3.6V) pack which will reach the OpenBCI boards 3V minimum voltage long before the pack has exhausted its capacity, or a 4xAAA (4.8V) pack which will only cut off when the cells reach 0.75 volts each, which is not good for the life of the cell (it will have fewer charge cycles). Fortunately, a set of 8 mediocre quality NiMH cells with a USB charger can be found for such an absurdly low price that you can just recharge at your leisure without much concern.
4xAAA packs can be found at electronics stores, but the chargers with compatible connectors for packs are harder to find. If you have a generic charger or can make your own connector, you might as well make the pack. Higher quality batteries cost many times more and are better suited to the AA battery box arrangement. The pack I made simply used common NiMH cells that were soldered together, with a JST-PH connector added.
I’ve offered this soldering jig for download to make things easier. After protecting the contacts (I suggest using both shrink-wrap or tape and hot-glue or epoxy), the 4xAAA pack should fit (tightly) at an angle into the case.
These higher-capacity solutions for powering the OpenBCI board should help avoid the fuss of constantly charging or replacing batteries, so you can spend more time collecting data.
Update: Additional sourcing info.
The other parts of the updated case design are include the Cover and the Snap Base, which allows the board to be easily removed without opening and removing fasteners.
A simple hobby charger capable of charging a 4.8V pack is here, but it will still need the connector replace with JST-PH as the RC hobby chargers use higher current connectors like Tamiya. Consider getting a better, more flexible charger such as the iMAX B6, which will suit all your battery charging needs when matched with proper connectors.
I will soon be offering a kit with Cover, Frame, Deep Frame, Snap Base, JST-PH connectors, Shrink wrap, Copper contacts and USB charger together. Contact me at threeformfashion at gmail for more information.
UPDATE: I’ve added variations of the battery case for Octamount that is compatible with common LiPo cell sizes.
You can download the complete Octamount/Battery holder kit to print yourself from Thingiverse Here, or you can order the LiPo holder with Octamount base from Shapeways Here.