$140 400wh LiFePO4 DIY

[hour]

Or as low as $125, I just bought another 40 cells at a better price, to build a clone of this pack. I'll take pictures along the way for that one.

I decided I'd had enough of the lead acid world after building quite a few ammo can and toolbox battery projects for charging things at camp. They were heavy and didn't take kindly to being forgotten during the winter, and it was always a drag knowing I shouldn't draw them down more than 50% (at most) if I cared at all about longevity.

I've been familiar with the 18650 3.7v battery for quite some time, but knew that there was no perfect way to assemble a 12v pack for my uses. With three in series your voltage is low, four in series your voltage is high. The major appeal is energy density, but at the tradeoff of safety and inevitable boost or buck contraptions. The other barrier to entry was figuring out how to assemble them.

So I concluded that any battery I built would be using LiFePO4 cells for their safety, and excellent match for all my 12v gear (3.2v, 4 in series). But 18650 cells in this flavor are pretty weak, say 1500mah versus 3000mah from 3.7v sony/lg/panasonic offerings. And all these solderless battery building block kits were 18650 only. Doh.

Then I found batteryblocs.com which has many things to choose from in the 26650 variant. Enough blabbing, here's what I purchased.

(4) 10P batteryblocs to hold a total of 40 26650 cells - $36
(40) K2 26650 3200mah lifepo4 cells / 10.24wh - $85-100

• Male and female XT60 connector
• 5-pin balance harness

Of course buying the bottom two items results in having several leftovers for future projects, but if you go to a local hobby store you can get single pairs of XT60s and a balance harness.
Why XT60? I like my [insert connector here] A lot of RC stuff uses XT60, including my charger, so it was the path of least resistance.

To support the hobby in general, as I intend to make several more of these in the future, I also purchased:
(1) ISDT Q6 Pro 14 amp charger/balancer - $50
(1) Cellspy8 cell monitor/balancer - $14

and stuff I already owned:
Soldering iron, solder, heat shrink tubing assortment pack, digital multimeter, bench DC power supply, beer

Since I'll be building a clone of this pack in a couple of days and taking photos for a step-by-step, I'll just quickly cover exactly what I did when both packages arrived on my doorstep last night.

First, I checked all 40 cells with a multimeter. These are new cells and all were at 3.24-3.27 volts, great. I took groups of 10 and built four 10P packs using the Batterybloc kits. These kits include all the ring terminals you need for charge/discharge lead and for connecting to the five wires from the balance harnesses I purchased, so I crimped, soldered, and heat shrunk those as I went along. Then I connected the four packs in series and tightened all the bolts. Plugged in my cellspy8 and voila, a 13.something volt battery. I then replaced the cellspy8 with the ISDT charger/balancer and charged it up.

Total build time: 20 minutes

I have a [not great, not terrible] LiFePO4 BMS/Balance board coming from China which will be here some day. It cost $14, but you can buy cheap ones for $2 each. Just be mindful of the current ratings and user reviews.

Terrible pics because I haven't mounted the four 10P packs together and didn't want to move it from my test area, aka my living room floor. The batterybloc kits accept threaded rod so that's easy to remedy. If I had a big roll of kapton tape I probably would have just used that.

Battery will be going in a toolbox and I predict another $30 or so on fuse block and panel mount cigarette lighter ports.

 

[hour]

Also worth nothing, you could scale up the batterybloc kits to create whatever you want. They also have a kit for creating a lead acid 12v replacement using 120 26650 cells.

120 cells @ $2.12 a cell ($85 for 40 if you can find that ebay deal) = $255 + kit ($130) + (probably a nicer BMS for this scale - $80) equals $465

$465 gets you a 96ah/1,228 watt hour 12v battery, a little less than half the price of a battleborn 100ah.

Edit as john61ct has pointed out, you could also just buy four 3.2v prismatic cells and create a 12v battery out of that. I think the lowest I've seen for new CALB cells is ~$125 for 90ah.

 

[john61ct]

Pretty sure you're mixing chemistries there.

If longevity is important to you, the only quality maker of LFP cells in 18650 size I know of is A123, and personally I'd use their 26650 size to reduce cell count.

K2's cells are apparently not in the same quality ballpark. I wouldn't touch no-name generic, and of course be very careful about counterfeits if ordering direct from China through Ali or eBay, you really want a trusted supplier in your local jurisdiction.

The time and difficulty involved in assembling a reasonably sized House bank (100-400Ah @12V?) robust enough for mobile use is "not trivial".

Going to much larger Ah-per-cell CALB, GBS, Sinopoly, Winston etc prismatic cells is not just quicker easier and safer, but most likely cheaper, and much greater longevity.

And much easier to monitor / maintain / protect / balance at a per-cell level.

 

[john61ct]

There is no such thing as 3.7V LFP, they are nominal 3.2, often called 3.3V.

There are dozens of different LI chemistries at that 3.7 voltage, most Lipo type are LCO, lots of 18650 size are NMC, LMN or NMA.

Due to much higher risk of "sudden thermal runaway events" (boom bad) I personally would only play with those out in the back shed, never in my living space, much less in a small mobile one.

LFP is the only non-lead chemistry safe enough for that IMO.

 

[john61ct]

Ciggie ports are a dangerous abortion, inherent in the design, only suitable for temporary low-amp connections, IMO 4-5A maybe 8A for the locking version in Blue Sea ports.

BMW/ Hella/ Merit/ Powerlet "Euro-style DIN" (ISO 4165) style is very robust.

But for high amps, standardize on Anderson plugs

Can get or make adapters for guests, temporary use of devices with standard ciggie plugs.

 

[hour]

Pretty sure you're mixing chemistries there.

If longevity is important to you, the only quality maker of LFP cells in 18650 size I know of is A123, and personally I'd use their 26650 size to reduce cell count.

K2's cells are apparently not in the same quality ballpark. I wouldn't touch no-name generic, and of course be very careful about counterfeits if ordering direct from China through Ali or eBay, you really want a trusted supplier in your local jurisdiction.

The time and difficulty involved in assembling a reasonably sized House bank (100-400Ah @12V?) robust enough for mobile use is "not trivial".

Going to much larger Ah-per-cell CALB, GBS, Sinopoly, Winston etc prismatic cells is not just quicker easier and safer, but most likely cheaper, and much greater longevity.

And much easier to monitor / maintain / protect / balance at a per-cell level.

What chemistry did I mix? Also I have no intentions of using 18650s for anything. The K2 cells were from BatteryHookup, and I don't have any reason to not trust them as a supplier. As far as the cells themselves, as long as they're in the ballpark of advertised capacity I'm happy. That's to be determined today.

You're right though, for a 12v pack in the 100ah range I'd also opt for prismatic. You can see my prismatic build in one of the photos above.

There is no such thing as 3.7V LFP, they are nominal 3.2, often called 3.3V.

I must have boofed a sentence, I thought I clearly stated that I was using 3.2v lifepo4 cells and why I opted to NOT use the 3.7v offerings of the lithium family.

 

[hour]

Ciggie ports are a dangerous abortion, inherent in the design, only suitable for temporary low-amp connections, IMO 4-5A maybe 8A for the locking version in Blue Sea ports.

BMW/ Hella/ Merit/ Powerlet "Euro-style DIN" (ISO 4165) style is very robust.

But for high amps, standardize on Anderson plugs

Can get or make adapters for guests, temporary use of devices with standard ciggie plugs.

Of course, and it depends on what you're doing with the pack. I waffled on stating cigarette lighter port because really, I'm just going to use the panel mount round "cigarette lighter port style" USB offerings on whatever toolbox this goes in to - at least one with USB PD. I'll probably throw a cigarette lighter port on still, for air mattress pumps... and yes I use marine locking style ones and have a few blue seas laying around.

While I could probably run a pretty nice sized inverter off of this pack, I don't have much of a use for 120v anymore, so it wasn't a consideration.

 

[john61ct]

The BatteryBloc idea looks genius if

resistance is low
connections very secure
and all holds up well to shocks / vibrations

Great find!

Here's its genesis, great learning forum for non-LFP chemistries BTW

Modular Rebuildable Repairable 18650 battery. - Page 3 - Endless Sphere

 

[john61ct]

I must have boofed a sentence

NP, just for those less knowledgeable can be confusing, most think "lithium ion" is a specific battery type rather than just an umbrella term.

Even in the LI batt techie forums, they'll often discuss many different chemistries together as if they're interchangeable, publish detailed performance charts on a new cell without bothering to find out.

 

[hour]

The BatteryBloc idea looks genius if

resistance is low
connections very secure
and all holds up well to shocks / vibrations

Great find!

Here's its genesis, great learning forum for non-LFP chemistries BTW

Modular Rebuildable Repairable 18650 battery. - Page 3 - Endless Sphere

I need to remove the slight curve from one of my series connector bars, but otherwise the plates themselves are held on very well by the nylon nuts and bolts. While the blocks do lock in to each other, I think a lot of the structural integrity comes from adding the threaded rod through all the parallel packs and I might whip up end plates from aluminum plate or something.

If you're curious, the maker of the batteryblocs does tests at the end of each build video

 

[john61ct]

Looks like a great project!

For under say 80-Ah blocks, nice to be able to scale the Ah capacity (and size / weight) up and down like that.

You mentioned putting it into a toolbox, do you mean as part of another project, or just as the outer protection for your portable powerpak?

Nice thing with LFP too is Peukert factor is so close to 1.0, no real compelling reason to build one big monolithic House bank.

Each 12V pack can be treated as either standalone / portable, or get "merged" / rotated back into the mother bank.

Bring a backpack into Starbucks to recharge one unit at a time inconspicuously, or put your portable solar panels out in the sunlit clearing while the van's parked in the shady forest, take a few units to power the electric dinghy for a shore run, etc.

 

[john61ct]

If you're curious, the maker of the batteryblocs does tests at the end of each build video

More than curious, pretty chuffed, good stuff, grateful for you bringing this BatteryBloc system to my attention!

 

[john61ct]

All kind of ideas. . .

The espar-based "tent heater" box, a watermaker, a 12V fridge, even a 24V winch or 48V aircon unit could be made portable / standalone / self-sufficient with its own integrated power source, then when not in active use the battery modules returned to the main House bank.

Replace the Starter motor even, just rotate the modules so they roughly wear evenly. . .

 

[hour]

I’m more excited that you’re excited, I thought you had blown the whole concept to pieces in your first few posts.

I have a large dewalt ds400 box that I put a stereo in to, with 8” subwoofer and 6.5” speakers. It was powered by an 18ah lead acid battery that has since failed.

So this pack or its clone that I build next week will live inside the boombox-which also has a bunch of options for charging phones or blowing up a friend’s air mattress. Then i’ll put the second pack in a much smaller toolbox or ammo can for taking inside tents or in the camper (boombox is large and heavy)

I also have an espar style diesel parking heater that i built in to a dewalt ds300 (build is on here in camping equipment subforum) so i’m onboard with your thought.

 

[john61ct]

I thought you had blown the whole concept to pieces in your first few posts

Sorry about that, not my intention at all, just nit picking really, and clarifying where noobs might make assumptions or get the wrong idea.

Nice idea for a DIY boom box! Been lusting after a KEF LS50 Wireless system would lend itself to something like that for mobile use.

And yes, using such small cells for a 30-50Ah LFP pack is fine, so much easier to source than the prismatics. Crazy how limited distribution is for those in NA, just hate dealing with direct from China in case warranty service is needed, shipping back costs too much.