tanoshimini
Solar Enthusiast
- Joined
- Mar 28, 2021
- Messages
- 110
First, a picture:

Now, some explanation:
We’re building a house up in Maine on a large, undeveloped lot (100+ acres, 50+ hectares). The site we chose to build on is well back from the road and our driveway is about 950’ (~300m) along it’s length. I was quoted an exorbitant price by the local utility to put in poles, string them, etc... all just for the privilege of paying a monthly bill and still suffering the occasional power cut. So, solar and batteries it will be.
I looked at Tesla’s Powerwall, but at north of $550/kwh, it’s just too spendy for the kind of sizes we’re looking for — and now, you can’t buy them without their solar tie-in. Shiny, pretty and new... but very, very spendy. Similar offerings from other companies had similar issues. We’re also going to need a system (or portion of it) up and in place on the property before we build the house in order to run tools and equipment. So, DIY it will be.
I’ve had a bit of experience (at a smaller scale) with this after putting 4 salvaged modules from a Model S into our sailboat some five years back (24v system), so it wasn’t a completely out-there idea for us to roll our own. That system is still functioning wonderfully, and I think it has a lot to do with the fact that we used the batteries lightly. While our small family lived on the boat for many years, the batteries almost never saw a DoD of more than 30% and we didn’t charge them to more than 4.1v. So, with this new setup, we’re going to want to try and oversize that, too, for all the same reasons.
Nowadays, the secondary market for lithium batteries is much more of a “thing” than it was when we did our boat up. More choices... so what to choose? I thought about buying an entire salvaged pack (or packs) from the Model S/X, but still couldn’t hit a price point I wanted... and rewiring their internals for different voltages is not something that I wanted to contemplate. The boat project was 24v, so the modules were “more or less” drop-in without much modification, but we had some problems with some DC equipment not liking the slightly lower voltages of the 6s arrangement of the modules (7s would have been better, but all of that was worked out eventually.) Doubling up modules to 12s leaves the voltage pretty light for a 48v system, with people seeming to favor 13s or 14s packs for that voltage range... and 48v seems like the way to go.
Batteries!
So, design goals:
Next, I’d need to work out where to put these things. My first thought was to store them the way they were built for, which was to use a server-rack of some kind. This would give me a sort of tall-ish profile, and I’d still need to insulate them from the cold... It didn’t seem like it was going to be workable without building a structure around them and that just ups the complexity of, well, everything and does nothing for ?. I tossed around a lot of ideas for various kinds of metal boxes and went back and forth for weeks, and then I happened to drive past a construction site and was struck by an epiphany: a jobsite box. I grabbed a tape measure and lit off to see what the internal dimensions of these things worked out to, since they come in a number of sizes from different sources.
After visiting several different stores and checking out a number of brands, I managed to find a 48x24x28” (~1220x610x710mm) one that would fit 8 units between the locking mechanisms with ⅛” (3mm) to spare, and room for 2” (~50mm) of foam insulation all the way around. I had been planning on using blocks of 7, but after reviewing the spec-sheets on the inverters on my boat (a pair of Victron Quattro 24/5000/120v), I noticed that I could go higher than 24v with them. I wondered if I could do the same with their 48v cousins, and sure enough, they’re rated to handle 66v. Eight of these rack units is 16s, so 60v nominal with a high voltage cut off of 4.1v/cell... that’s 65.6v. It’d all be within spec.
So, 6 jobsite toolboxes for 48 batteries, organized in blocks of 8, with 2 cells each. At $319/ea, the boxes aren’t too spendy... they’re completely reusable / resellable, heavy-gauge steel, designed for the outdoors, can be locked securely, are moveable by any machine with pallet forks and come in a lovely shade of blue. I had to hit four stores in the area to source them all. Watch out, though, not every one on offer was a gem... there were some I saw with dents or really ugly welds, etc. I left those and grabbed just the prettiest ones. We’ll put a concrete pad next to the house, and they’ll be two rows of three of them, looking all lovely and neatly spaced.
Running the numbers... with a large inverter at full continuous load, pulling 12kw from all of these batteries, that’d be 12000w / ~60v = 200a, split over 6 big batteries is ~33a to each, and each cell (of the 16 in each box) is made up of 7 trays of 4 pouches, so 33a / 28 pouches is ~1.2a per pouch, 4.8a per tray. The pouches were originally rated at 15Ah and 3C so even at full-load, that’s a sleepy 1.2a / 15Ah = ~0.08C. I’ll size the wiring as if the load will be two inverters at full tilt, even though the vast majority of the time we’ll be seeing a tiny fraction of that — wire losses should be very low. If my math is anywhere close to right, this will be a lovely, slow-paced retirement home for these old soldiers — may they live a long time.
In order to keep these things modular, I decided on using (genuine) Anderson SB175 connectors to transfer power. They’re rated at 280a wire-to-wire, and 200a wire-to-busbar and designed for 10,000 no-load connect/disconnect cycles. Conveniently, these job boxes have a well-placed 3” (75mm) cutout at the back for power cables, and the SB175’s are small enough to fit (2-1/4”, 55mm) through this opening. For now, I’m just going to pass them through, but these connectors are designed so that they can be hard-mounted. If I’m clever about it, I should be able to make the connector flush-mount with the outside of the box for a clean look, as well to avoid any protrusions that could get scraped off while moving the batteries around. Maybe 3d-print a nice weather-resistant cover plate.
More pictures!









Now, some explanation:
We’re building a house up in Maine on a large, undeveloped lot (100+ acres, 50+ hectares). The site we chose to build on is well back from the road and our driveway is about 950’ (~300m) along it’s length. I was quoted an exorbitant price by the local utility to put in poles, string them, etc... all just for the privilege of paying a monthly bill and still suffering the occasional power cut. So, solar and batteries it will be.
I looked at Tesla’s Powerwall, but at north of $550/kwh, it’s just too spendy for the kind of sizes we’re looking for — and now, you can’t buy them without their solar tie-in. Shiny, pretty and new... but very, very spendy. Similar offerings from other companies had similar issues. We’re also going to need a system (or portion of it) up and in place on the property before we build the house in order to run tools and equipment. So, DIY it will be.
I’ve had a bit of experience (at a smaller scale) with this after putting 4 salvaged modules from a Model S into our sailboat some five years back (24v system), so it wasn’t a completely out-there idea for us to roll our own. That system is still functioning wonderfully, and I think it has a lot to do with the fact that we used the batteries lightly. While our small family lived on the boat for many years, the batteries almost never saw a DoD of more than 30% and we didn’t charge them to more than 4.1v. So, with this new setup, we’re going to want to try and oversize that, too, for all the same reasons.
Nowadays, the secondary market for lithium batteries is much more of a “thing” than it was when we did our boat up. More choices... so what to choose? I thought about buying an entire salvaged pack (or packs) from the Model S/X, but still couldn’t hit a price point I wanted... and rewiring their internals for different voltages is not something that I wanted to contemplate. The boat project was 24v, so the modules were “more or less” drop-in without much modification, but we had some problems with some DC equipment not liking the slightly lower voltages of the 6s arrangement of the modules (7s would have been better, but all of that was worked out eventually.) Doubling up modules to 12s leaves the voltage pretty light for a 48v system, with people seeming to favor 13s or 14s packs for that voltage range... and 48v seems like the way to go.
Batteries!
So, design goals:
- 48v or higher, to keep the wire size down or to run cooler on the same mm² wire.
- Oversized for the need, to keep the DoD shallow, cycling slower. Targeting 100kwh.
- Cheap, so that I can afford to oversize the pack. Under $100/kwh would be ideal.
- Outdoor, secure storage, because ? and ?.
- 5 year service life, hopefully more.
- Reusable parts, because change happens.
- Modular, if possible, so that parts can be taken offline for maintenance or repair without disrupting life too much.
Next, I’d need to work out where to put these things. My first thought was to store them the way they were built for, which was to use a server-rack of some kind. This would give me a sort of tall-ish profile, and I’d still need to insulate them from the cold... It didn’t seem like it was going to be workable without building a structure around them and that just ups the complexity of, well, everything and does nothing for ?. I tossed around a lot of ideas for various kinds of metal boxes and went back and forth for weeks, and then I happened to drive past a construction site and was struck by an epiphany: a jobsite box. I grabbed a tape measure and lit off to see what the internal dimensions of these things worked out to, since they come in a number of sizes from different sources.
After visiting several different stores and checking out a number of brands, I managed to find a 48x24x28” (~1220x610x710mm) one that would fit 8 units between the locking mechanisms with ⅛” (3mm) to spare, and room for 2” (~50mm) of foam insulation all the way around. I had been planning on using blocks of 7, but after reviewing the spec-sheets on the inverters on my boat (a pair of Victron Quattro 24/5000/120v), I noticed that I could go higher than 24v with them. I wondered if I could do the same with their 48v cousins, and sure enough, they’re rated to handle 66v. Eight of these rack units is 16s, so 60v nominal with a high voltage cut off of 4.1v/cell... that’s 65.6v. It’d all be within spec.
So, 6 jobsite toolboxes for 48 batteries, organized in blocks of 8, with 2 cells each. At $319/ea, the boxes aren’t too spendy... they’re completely reusable / resellable, heavy-gauge steel, designed for the outdoors, can be locked securely, are moveable by any machine with pallet forks and come in a lovely shade of blue. I had to hit four stores in the area to source them all. Watch out, though, not every one on offer was a gem... there were some I saw with dents or really ugly welds, etc. I left those and grabbed just the prettiest ones. We’ll put a concrete pad next to the house, and they’ll be two rows of three of them, looking all lovely and neatly spaced.
Running the numbers... with a large inverter at full continuous load, pulling 12kw from all of these batteries, that’d be 12000w / ~60v = 200a, split over 6 big batteries is ~33a to each, and each cell (of the 16 in each box) is made up of 7 trays of 4 pouches, so 33a / 28 pouches is ~1.2a per pouch, 4.8a per tray. The pouches were originally rated at 15Ah and 3C so even at full-load, that’s a sleepy 1.2a / 15Ah = ~0.08C. I’ll size the wiring as if the load will be two inverters at full tilt, even though the vast majority of the time we’ll be seeing a tiny fraction of that — wire losses should be very low. If my math is anywhere close to right, this will be a lovely, slow-paced retirement home for these old soldiers — may they live a long time.
In order to keep these things modular, I decided on using (genuine) Anderson SB175 connectors to transfer power. They’re rated at 280a wire-to-wire, and 200a wire-to-busbar and designed for 10,000 no-load connect/disconnect cycles. Conveniently, these job boxes have a well-placed 3” (75mm) cutout at the back for power cables, and the SB175’s are small enough to fit (2-1/4”, 55mm) through this opening. For now, I’m just going to pass them through, but these connectors are designed so that they can be hard-mounted. If I’m clever about it, I should be able to make the connector flush-mount with the outside of the box for a clean look, as well to avoid any protrusions that could get scraped off while moving the batteries around. Maybe 3d-print a nice weather-resistant cover plate.
More pictures!







