All posts tagged power

My next project is to be a hackable, wearable LED toy. I’m designing it to be self-contained, with a lithium cell, charging circuit, and boost regulator all on the same board. To that end, I figured the easiest pathway to follow to make it easily hackable would be to use an ATMega32U4 MCU, and have it present over USB in the same way the Arduino Leonardo does.

Since I’d already measured the average power usage of the prototype at around 100mA, I figured I’d start with the power system.

Panacea denied

This kills the cell.

This kills the cell.

I began my search looking for an all-in-one solution which could charge the battery, provide boost regulation to 5V for the LEDs, and provide an undervoltage lockout during discharge. While the lithium cell I’ve chosen in the interim has a protection PCB built-in (as will any future cell), said protection circuits are designed solely to save the battery from catastrophic failure modes — swelling, leaking, bursting into flames — but not from any mistreatment which will “only” shorten its cycle life by a large factor. And unless there’s something I’ve grossly misunderstood about protection circuits, I figure having a second UVLO can only add to the safety margin.

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Joke's on them, I was listening to VNV Nation!

Joke’s on them, I was listening to VNV Nation!

The past week for me had the singular focus of assembling the housing for the Tripper Trapper’s power system. It encloses a set of deep-cycle batteries, a busbar each for the battery bank and load distribution, a circuit breaker panel, and a solar charge controller.

Since material in the US is generally only available in English sizes (for some reason, the U.S. saw fit to overthrow their oppressors, but not to abandon England’s ancient system of units), I’ve had to adapt to using inches and feet. It’s odd, but manageable. The box is 21.5″ x 20.5″ x 17″, and its structure is made entirely of scrap material donated by other burners in the community.

Pictured: battery-O-fail, storage busbar, breaker rail.

Pictured: battery-O-fail, storage busbar, breaker rail.

I took some pictures, since watching glue dry is 99.444439% (±0.555561%) as much fun as watching paint dry, and I figured I ought to share that fun.

Wiring the battery array was much more fun. I read what I could find online about using batteries in a parallel configuration, and what I found was that for ideal performance, each battery should see equal series resistance. People have a funny way of making that look hard on the Internet; maybe they just haven’t heard of busbars.

Diagram of how I wired the busbar

Left: 2P configuration with equal series resistance. Centre: 4P configuration with equal series resistance, based on what I’d seen online. Right: 8P configuration used in the Powerhaus.

It looks a little haphazard, but I wanted to keep the layout somewhat symmetrical, so I put the charge and load terminals in the middle. If you trace the current path from the charge controller, through any battery, and back to the charge controller, you’ll see that whichever path you take, you go through the same resistance: the leads to the busbar, four ring terminals, 6 terminal-spacings worth of busbar, two spade terminals, and a constant amount of battery cable (since I kept them the same length). Same goes for the load, which means that this array is kept balanced by design. Since the batteries are AGM lead-acids, there is no need to apply an equaliastion charge, or to balance the cells electrically like in lithium-ion batteries.

This part of the project gave me a lot to research, and I now know a lot more about power systems as a result. The Tripper Trapper has turned out to be the most rewarding thing I’ve built to date.

Dare to fail

Dare to fail

In the early stages of designing the Tripper Trapper, I decided to use 5V addressable LED strip for two reasons: firstly, the mad resolution of 1-LED pixels which would look nice up-close, and secondly because the newer, faster, flicker-free four-wire APA-102C LEDs that I wanted to use didn’t (and still don’t) have a 12V variant.

So I set about finding some point-of-load (POL) DC-DC converters to convert the lead-acid battery voltage (between 10.5V and 14.4V 13.8V) to the 5V needed for the LEDs. I found some quality converters which were rated for a maximum of 14V at 10A continuous output, but would require the fabrication of a PCB with either sockets for these or solder pads for these. Being lazy[1], I kept looking and found some prepackaged BEC modules which HobbyKing were selling for $14.

Bargain, right?

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