Building a 37V/1kWh Lithium Solar Power Toolbox for $300

If you were living under a rock, you may have missed that there was an eclipse in 2017.  And, predictions for traffic were that it would be awful.  So, for a long duration camping trip, I decided to build myself a “Power Toolbox” out of some scrap batteries I had laying around, a few components, a solar panel, and an afternoon.

This is a companion discussion topic for the original entry at

(Comments from Blogger)

2018-02-12 by Aaron

I think I have 1 or 2 of those exact adjustable buck circuits. Or the same thing from a differently named Chinese company. Handy little thing that can handle a fair amount of power.

Question on the lithium packs though, did you tape them down or something? With other metal, angular edges in there, while they shouldn’t be penetrated, in enough force (car crash, etc) I can see a cell or two getting punctured, or at least dented. Also, the battery packs, I’m assuming you have a 10s charge PCB already on it and not relying on the MingHe charge controller for that?

Also, all those exposed edges from the ring connectors, I twitched for a second when I saw them, almost seems like there’s potential for an accidental short there, even if it it ends up being OK with the breaker on the batteries. You still have the solar panel on there, unless you’ve got another fuse/breaker on the solar side as well. I guess after accidentally shorting a 12v SLA and having it burn through 14ga wire (and my finger a bit when I tried to pull anderson connectors apart), I’m a bit paranoid about having correctly sized fuses between any source of power and everything else.

2018-02-12 by Russell Graves

Yes, the lithium packs are held into place with my favorite double sided foam tape. They won’t go anywhere without an awful lot of force, though in a suitably severe car crash, any battery pack is a risk.

I don’t have a balancing BMS on the packs because the cells are self balancing (I’ve documented this elsewhere on the blog). A quirk of the spinel LiMn chemistry is that they will stay well balanced without external balancing, and this works nicely (after hundreds of cycles, these packs are still within 0.01-0.02V between the highest and lowest cell group). The MingHe units I’ve tested are actually quite good in terms of voltage accuracy, so I don’t worry about them that much. They make surprisingly good hardware. I just don’t like that this unit is sold as a MPPT unit when it’s clearly not.

Some better insulation would be reasonable, though I’m simply not that concerned about it. Everything in the box is well secured, and this was an afternoon build for my needs, not anything I’d consider selling as a commercial project.

2018-02-13 by Aaron

Well, you didn’t mention you had LiMn cells in those! I assume a more regular lithium chemistry :stuck_out_tongue:

2018-02-13 by Russell Graves

It’s a fair assumption, but nearly every lithium pack I’ve worked with in this blog is LiMn, because that’s what the older BionX packs are.