The battery charger is a tough one. This seems to be a scarcely served market. You need a battery charger that can safely and effectively charge the batteries, and a power supply that can consistently power the charger. If you happen to find those two capabilities in one product, awesome! I wasn’t able to, but it seems possible that someone somewhere is making a high performance battery charger with an included power supply
I ended up turning to the RC Racing market segment for a good battery charger – I’d love to hear if there’s a better or cheaper alternative. The key at this point is parallelization. The charging process takes a long time, and acts on a single module at a time, so 28 * a long time ~= 28 long-times. Other than just being impractical, the closer this can be done to completely in unison, the easier the next steps are. To that end, as many battery chargers as possible is ideal.
One that comes recommended on PriusChat is the Hitec X4.
It has a handful of convenient features, the most important for this application being an automatic charge/discharge cycle with configurable charge/discharge rates and limits. Additionally, it has 4 ports! That’s a boon for this application because it drops the price per charger significantly. Unfortunately they’re still relatively expensive, I paid $117.98 each and bought 2 units, which means 8 chargers for 30$ a piece.
As part of this project, I reached out to Hitec (super responsive!) to try and get more information about their charger. It turns out the product is just a sticker over top the SkyRC Quattro B6 80W. I reached out to SkyRC to see if I could get more information and received this pseudo-English in response:
Thank you for your inquiry.
As the charger monitor is compatible with SkyRC old items which discontinued or will discontinue, we have no plan to make update to it.
You can still find the Quattro for sale various places, but I don’t see an updated product on SkyRC’s website, so I can’t link that product since it’s been discontinued.
The cheaper model (the one I got) only runs off 11-15 VDC (think car battery, or perhaps more realistically a car battery charger). You *could* run it off a car battery, and if you read through the PriusChat forum there are people who did that, but it strikes me as less than ideal. I’d hate to kill the small battery by accidentally discharging it too much or something. The expensive model runs off of the mains in Canada (120 VAC), but can also run off 11 – 15 VDC. When I was buying, it was almost 100$ more expensive for the AC model, so I figured I could do DIY that part and save a little money. Here’s the AC version, looks almost identical.
I thought it’d be extra cool to collect the data while charging, so I bought the USB adapter:
Theoretically, this would allow me to talk to the charger with a computer. This turned out to barely be the case. First of all, the interface is per-charger-module. That means to collect one run off of one unit, one would need 4 adapters and perhaps more importantly, 4 USB ports connected to 4 instances of the client software. On top of this, the charger can be programmed to cycle charges and discharges. Unfortunately, it disconnects at the end of each cycle. You would have to babysit the machine the whole time if you wanted anything useful out of it. I looked at the graph from one cycle, then gave up on the unit for the rest of the project.