For the price of a mid-range Android phone, [Kenneth Finnegan] turned a 50 caliber ammo can into a 50 amp-hour portable power supply. The battery pack uses four 3.5 V LiFePO4 cells wired in series to achieve a nominal 12 V supply that stands in for a traditional lead-acid battery. The angel of second-hand purchases was smiling on this project as the cells were acquired on eBay in unused condition, complete with bus bars and mounting spacers. All it took to fit them in the case was to grind off the spacers’ dovetails on the outer edges.
There are many benefits to Lithium Iron Phosphate chemistry over traditional lead acid and [Kenneth] spells that out in his discussion of the battery management system at work here. While the newer technology has a much better discharge curve than lead-acid, there’s a frightening amount of power density there if these batteries were to have a catastrophic failure. That’s why there are Battery Management Systems and the one in use here is capable of monitoring all four cells individually which explains the small-gauge wires in the image above. It can balance all of the cells to make sure one doesn’t get more juice than the others, and can disconnect the system if trouble is a-brewin’.
Connections for the battery are routed to terminals mounted in the side of the ammo can. Since there are two contacts for each of the positive/negative connections [Kenneth] needed to split the connection when wiring things up. This is where a very interesting wire-splicing technique comes into play. Here you can see he has mated the three wires together by wrapping them with some fine-gauge wire before they get a thorough solder and shrink wrap treatment.
What does one do with so much power in a nice luggable package? We’d wager that this makes a perfect power source for a mobile amateur radio repeater.
I wrap bare AWG26 on drill bit for splicing thick wires too. Mine is round and more uniform, so the heat shrink joint looks a bit better. :)
The “interesting wire-splicing technique” is also a standard the NASA uses:
https://workmanship.nasa.gov/lib/insp/2%20books/links/sections/407%20Splices.html
Greetings !
Thank you VERY much for posting the NASA wire splicing technique link ! I also use basically, the same approach when splicing heavy gauge wires. It is nice to have the NASA guides at hand for future projects ! I always manage to learn some very useful information by reading the Hackaday posts of interest to me !
take care, Mark, Amateur Radio Station WN3SIX
And that method has been in use since the dawn of time!!
Great idea, nice craftsmanship ! I have several different size ammo cans as well, they lend themselves nicely to portable projects. I use a .50 cal ammo can to house a small, 8 watt AM/FM 10 meter amateur radio transceiver and 70 watt RF power amplifier. I have mounted external SO-239 RF antenna and DC power input connector which provide connectivity to the outside world. It makes for a great, portable, 70 watt 10 meter amateur radio station for portable and emergency use. It is watertight and may even survive an EMP blast ! Lol ! I was wondering how much the batteries may have cost ? I, too, am quite interested in assembling just such a nice, portable 12 VDC power supply ! Thanks for posting your project ! Mark.. Amateur Radio Station WN3SIX
Greetings !
Thank you VERY much for posting the NASA wire splicing technique link ! I also use basically, the same approach when splicing heavy gauge wires. It is nice to have the NASA guides at hand for future projects ! I always manage to learn some very useful information by reading the Hackaday posts of interest to me !
take care, Mark, Amateur Radio Station WN3SIX
The amount of energy stored doesn’t really matter as much for lifepo4, they have a pretty calm failure mode, mostly just expelling gas that you don’t want in a confined space. Not really any fire or exploding.
Why not use a deep discharge 100ah VRLA for the same size? I have one and it’s not bigger than the ammo box
Two reasons: weight and lifetime. Of course there is also a drawback: cost.
I use a 100Ah LiFePO4 myself for solar power storage for camping (festivals, mostly without power connection) and in my garden house.
I think the LiFePO4 weights around 14kg, while a 74Ah lead acid (car) battery weighs alredy over 20kg.
The better voltage stability of the Li chemistry is also a bonus. While a lead acid sits at 12V to 12,5V during discharge, the LiFePo is over 13V for most of it’s discharge depth. That means around 10% more usable energy from the solar panel.
The cost difference between lithium and sealed lead acid isn’t much nowadays. (Conventional lead acid still has a big cost advantage.)
Why no love for lead-gel battery?
Lead-gel batteries are proven, require little to no maintainance and are though. They can widstand comparable high short-circuit currents for a limited time. By contrast, Lithium batteries are like bombs. Huge energy that gets freed in heat and flames.. A Year ago or so the A380 plane had big issues with fancy new lithium batteries, when the involved people tried to replace traditional batteries, in order to save weight..
They’re lead-acids with the acid suspended in a gel instead of demineralized water.
So only advantage is that you can flip them around and there’d be no spill.
Still stupid heavy due to poor power density.
LiFePO4 do not produce flammable or explosive gasses, even when failing under SC conditions.
Has anyone here tried the SLA battery cases that hold 18650 size cells? I just saw them on eBay and wonder what kind of performance they have.
Metal box is just about right for all that “frightening amount of power density”. Box might glow cherry-red for awhile.
Had a “sealed” lead acid battery in an ammo can for a portable model rocket launcher power supply and launch controller. “Sealed” apparently is a relative term, as corrision associated with any acid based battery destroyed the wiring and launch controller. Unlike an alkaline cell, the battery appeared pristine, except for noted corrosion of anything metal. The can itself was fine – the paint worked well and is probably designed to resist attack from acid released of decaying smokless powder (just conjecture).
Also, ammo cans are sealed. Any emissions of gas will be traped well; that is until they exceet the seal strength or the box strength. Both are appreciable and can lead to explosive release of gasses – not sure if this impacts LIFEPO, but I’d sure recommend a safety vent like a burst disk. (The terminals may provide a sneak path for any gas buildup, too).
During overcharge the battery may off-gas some water, even for a sealed cell. This will be corrosive. Maybe if you set your charger to 14.0V absorption instead of 14.5-15.5V or so it will be much better. Battery will live much longer too.
LiFePO4 is a safe chemistry. The real hazard is a dead short cause burns. Probably not enough voltage for an arc flash. That being said it should be fused for current. I would suggest a 200A or 500A quick blow automotive fuse. It has to be sized because car starters can take 500+ amps.
That is it wont catch fire like LiNMC
For safety, fusing limits should be the lowest rating of your connections. i.e. battery maximum discharge rate, wire gauge or connectors.
Great build. Does anyone have more information on the terminals on the outside of the box (name, manufacturer, etc)?
Anderson PowerPole the most common connector amon ham radio users.
Those are 45A Anderson PowerPole connectors in their standard 2×2 panel mount frame. Red/black Powerpole is one of the amateur radio standards for 12VDC in the field.