Can you ever have too much battery capacity on hand? I think not…
After the 25Ah battery pack was complete, I started to consider another use case scenario where a smaller battery with less capacity, smaller form factor, and lighter weight might be handy to have on hand.
With simpler requirements, I got to work on this next one.
I started out with a Goal Zero Yeti 150 case that came to me as an empty enclosure that was used as an in-store display unit; it was devoid of any meaningful guts. There was no battery, no control board, and the connectors had nothing attached. The size and carrying handle looked interesting, and I was sure the front panel could be reworked into something suitable for ham radio.
The original Yeti 150 would have come with a 12Ah sealed lead acid battery, and it had a small inverter built in, a lighter socket, and a couple of USB ports.
I looked for cells that were the conventional cylindrical cells, again LiFePo4, with the expectation that this will result in a smaller, lighter pack. The cells are 3.2V 32700, meaning they are 32mm in diameter and 700mm long. Also important to me is that they have screw terminals on the ends. These are 4mm threads. I wanted to avoid having to get a spot welder to work with flat top cells. Maybe for a future project…
The cells I bought from a popular Chinese market took an extremely long amount of time to arrive. The best response the seller could offer about the duration and lack of tracking updates was “be patient.”
I was also a bit skeptical of their claim of 12.8Ah, but the price was reasonable even for a 5Ah capacity. Clearly, I’ll have test these and post my results.
To group the cells into a brick, I found a pattern on Thingiverse for battery holders. I printed enough for the 8 cells I have.
Battery Management System
The BMS I selected for this build is a pretty basic one with no interface or management capabilities. It is a 4S 12V 100A board. The 100A rating may be optimistic, but I only intend to run a radio and similar loads.
Next came the battery interconnects. For my other battery pack, I fabricated rigid buss bars from copper tubing, but that material is too thick and too difficult to work with in this configuration, so I bought a roll of copper plumber’s strap; it is 1/2″ wide and 22 gauge thick. I cut short lengths, bent them to shape for clearance, and connected the cells.
CAUTION and Warnings
At this point it is critical to pay attention to polarities and cell orientations.
With every jumper, I measured the voltage, added the next one, and measured again. Every cell you add on adds 3.2V to the total – 3.2V, 6.4V, 9.6V, and finally 12.8V.
An error here can easily cause a nasty outcome that may involve your local fire department. So BE CAREFUL, measure twice. You’ve been warned.
Hooking it up
This is also when I connected the BMS sense harness to the correct battery junction points. Note that I didn’t plug this into the BMS yet. Dabs of hot glue are handy to hold these light wires in place.
The BMS was a simple circuit board with drilled solder pads for the battery connections. I made up, cut, and dressed some short wires which I soldered directly to the BMS pads. These are the P- and B- terminals. I hot-glued the BMS to the side of the battery ‘brick’ and then connected the sense leads.
I measured the battery voltages once again after they had been connected together for a while. The voltages were consistent cell to cell, so it was time to connect to a charging power source. Similar to my other battery pack, I charge these with 14.6V. I monitored charge voltage and current throughout the charging period. The charge current dropped to .02A after a while, so I was certain they were fully charged. I disconnected it from the charger, and after a short rest period, it measures 13.2V.
The original Goal Zero faceplate has nothing I can easily reuse for my application. It sits neatly in the opening with a small flange to hold it in place when the the top cover is screwed down.Rather than try to adapt and modify it somehow, I thought that starting from scratch with a new panel made the most sense. I went looking on line to find a blank or similar faceplate, but had no luck. So I’d have to design and build my own. I sketched it up on paper and started taking measurements.
So now I’m into a whole new area: 3D design.
On advice of a few friends, I explored OpenSCAD and Fusion 360. After hours with Fusion 360, I had nothing even resembling this panel, and a serious headache.
So I tried OpenSCAD.
This opened interesting possibilities and looked promising…
The basic shape and geometry was good, but it still needed some refinements like radiusing corners, ensuring the angles and clearances are right, and so on. Days later, I printed a draft copy of only the very top outer layer to check the fit in the groove. More drawing adjustments, another draft print, test the fit. Rinse, repeat. Finally I had a good fit.
Then I added the hole pattern for a Powerpole connector, a 12V meter, and a switch. I even added my callsign to the panel.
If anyone is interested in this panel, a no-hole version and a 3-hole version with no callsign is available on Thingiverse. I’d be happy to modify the STL file to add your name or callsign. Email me at my QRZ.com address.
The wiring was simple on this battery pack, with the output of the BMS going to a switch, a voltmeter, and Powerpole connectors. Here is the back side. Note: I’ll be adding the second Powerpole connector shortly.
And finally, the finished product is charged and ready for field service.
The entire package weighs in at 5.4 pounds, and with the extendable handle, it is very easy to carry and pack around.