I got a lot of help from people on this forum, so sharing back in case it helps anyone else.
Before we get started - if cost is your primary concern, I don't believe it's possible to do it cheaper than Bluetti or Jackery or the others. I went back and forth a few times but I decided I wanted to build, because then I would be in control of the quality of the components that mattered to me, be able to fix it if it breaks etc.
Specs:
Fits inside a Plano Rustrictor "Large With Tray" Field/Ammo Box 15" x 8" x 10", $25 on Amazon.
Contains 4 x Ampere Time 12V 6Ah LiFePo4 Batteries in parallel making 12V 24Ah, or 288Wh. These were on special on Amazon at $29.99 each, but they are back at $39.99 now.
The batteries connect together using Wago 221-415 lever nuts and then via an Anderson Powerpole connection to make an easy disconnection, to a fuse box. From there, individual fused connections to the front panel connections and to the solar controller.
The front panel connections are Anderson Powerpoles panel mounted for solar and battery connections, a cigarette lighter socket and a USB C / USB A with voltmeter. The two solar pairs are wired together just behind the panel. I only have one panel but I wasn't going to pay $20+ for Powerwerx's single Powerpole panel mount connector. Unfortunately its not easy to connect two different solar panels; the upper and lower pair are mated into a 2x2 block with no gap between.
Cheap solar controller to get me started, and no provision for AC or car charging yet but my plan is once I know what I want to use I can simply connect it to the battery Powerpoles. Or work out how to fit it inside the box if I decide to.
I cut sections out of the tray that came with the Plano box and use that to separate the batteries from the electronics. Under the divider the batteries are surrounded by foam pieces, and on top there's some space to breathe. It will get hot (not so much with a PWM controller but an MPPT would get hotter) so I'll see how it goes and either add vents or a fan, or just crack the lid when I'm charging.
If I ever decide to add more batteries, there is space to do so but I would have to lose the tray and build something new to mount the fuse box and the solar controller. I made a thin board that rests on the lip where the tray sits, and there is enough height for the solar controller when the lid closes, but there would be zero air space so not ideal. I think I'll be OK with 288Wh so I won't solve this unless I find I need to later.
My main use will be a fridge, for which I will set the low voltage cutoff, and otherwise it's going to be small stuff like charging devices so right now I don't think I'll fit a low voltage cutoff inside the box.
Weight as currently built = 12 lb 3 oz.
Future enhancements:
I've ordered a TR16 (also known as AiLi) coulomb counter and display which will fit in the empty space below the Powerpole connectors. $21 from Aliexpress.
I will probably add some white paint to try to keep the temperature down.
I struggled with the non-splashproof nature of the Powerpole connectors, so my plan for that is to fashion some bungs out of Sugru to protect them at least when they are not in use.
Before we get started - if cost is your primary concern, I don't believe it's possible to do it cheaper than Bluetti or Jackery or the others. I went back and forth a few times but I decided I wanted to build, because then I would be in control of the quality of the components that mattered to me, be able to fix it if it breaks etc.
Specs:
Fits inside a Plano Rustrictor "Large With Tray" Field/Ammo Box 15" x 8" x 10", $25 on Amazon.
Contains 4 x Ampere Time 12V 6Ah LiFePo4 Batteries in parallel making 12V 24Ah, or 288Wh. These were on special on Amazon at $29.99 each, but they are back at $39.99 now.
The batteries connect together using Wago 221-415 lever nuts and then via an Anderson Powerpole connection to make an easy disconnection, to a fuse box. From there, individual fused connections to the front panel connections and to the solar controller.
The front panel connections are Anderson Powerpoles panel mounted for solar and battery connections, a cigarette lighter socket and a USB C / USB A with voltmeter. The two solar pairs are wired together just behind the panel. I only have one panel but I wasn't going to pay $20+ for Powerwerx's single Powerpole panel mount connector. Unfortunately its not easy to connect two different solar panels; the upper and lower pair are mated into a 2x2 block with no gap between.
Cheap solar controller to get me started, and no provision for AC or car charging yet but my plan is once I know what I want to use I can simply connect it to the battery Powerpoles. Or work out how to fit it inside the box if I decide to.
I cut sections out of the tray that came with the Plano box and use that to separate the batteries from the electronics. Under the divider the batteries are surrounded by foam pieces, and on top there's some space to breathe. It will get hot (not so much with a PWM controller but an MPPT would get hotter) so I'll see how it goes and either add vents or a fan, or just crack the lid when I'm charging.
If I ever decide to add more batteries, there is space to do so but I would have to lose the tray and build something new to mount the fuse box and the solar controller. I made a thin board that rests on the lip where the tray sits, and there is enough height for the solar controller when the lid closes, but there would be zero air space so not ideal. I think I'll be OK with 288Wh so I won't solve this unless I find I need to later.
My main use will be a fridge, for which I will set the low voltage cutoff, and otherwise it's going to be small stuff like charging devices so right now I don't think I'll fit a low voltage cutoff inside the box.
Weight as currently built = 12 lb 3 oz.
Future enhancements:
I've ordered a TR16 (also known as AiLi) coulomb counter and display which will fit in the empty space below the Powerpole connectors. $21 from Aliexpress.
I will probably add some white paint to try to keep the temperature down.
I struggled with the non-splashproof nature of the Powerpole connectors, so my plan for that is to fashion some bungs out of Sugru to protect them at least when they are not in use.
