Building 100 AH LiFePo4 Battery Box

Hello everyone,

I started acquiring parts for this project just over a year ago, but what with three days off due to snow days (!) I finally decided to get started on assembling some pieces together.

Technically, I started assembly about a month or so ago when I got my 100 AH LiFePo4 batter cells--four of them.  The first step was to balance the cells.  That meant checking each individual cell voltage to see if there was any discrepancy.  Three of the cells read 2.999 to 3.000 volts, which is about perfect for shipment.  But one cell read 2.990 volts.  This might not sound like much, but over time it will affect the lifespan of the batter pack.  So I connected all four cell in parallel for a week and checked again--and the one cell had gone up to about 2.994 volts.  So I connected them back and let them sit--for a month (I got involved in other projects).  After I checked, all four cells are now reading 2.998-2.999.

After I decided to start assembling, I got to cutting out placement holes in the ammo can and put in the basic exterior components.

I will attach some pictures soon.

Eric


Edited to add pictures

Hello Eric,

I highly recommend to get a Battery Management System that can balance the cells.
It will protect against over-charging and -discharging and balance your cells when charging is complete.

Lead-Acid batteries handle over-charging without problems, they just get warm.
LiFePo4 cells will get damaged.

Sebastian,

Absolutely, I have a BMS in the parts list.

Okay, good.

The USB chargers are nice.
I want some on my (2.5KWh) battery bank as well, but it has 16 cells in series and I have yet to find a USB-PD (Power delivery) module that supports voltages of 60V (58V when charging stops).

Sebastian,

I have plans on some day making something much larger--perhaps a 300 AH 24 Volt system.  This would be sized like a large rolling tool case and it would have a good AC inverter attached.

In the meantime, I saw this build on a YouTube video and I like the compact nature but still having a relatively high capacity.  Also, I was wanting to build the battery itself instead of buying the actual battery outright.  This piqued my interest as I could have a virtually unlimited supply of electrical charge for all sorts of small electrical devices in a fairly compact case--and these ammo cans are really nice to work in.

After this is built, I want to acquire a set of solar panels that can charge this in a reasonable amount of time.

Also, for the record, this was supposed to be an 80 AH system as per the build on YouTube but the 80 AH cells were not available.  The next size up was 105 AH which is what I got.  They look like they will still fit so that is what I will be working with.  And to boot, the 105 AH cells were actually slightly less expensive than the 80 AH cells!

Eric

For the outputs I  included a bunch of USB--Six USB type A and three USB type C.  On the lower right I have a cluster of four Anderson Power Pole connectors.  On the lower left I have an XT60 connector which I plan to use as the main charging port.

On the back side I decided to include two 3 Watt lights so that I could use the box as a light source.  I might put lights on the top so that I could shine the light up on the ceiling as an area light, but we will see how crowded things get inside.

The whole system will be circuit broken at 30 amps so that I can keep things well under the standard "C" rating for the battery cells.


Eric

One component that I have that I have not hooked up yet is a Hall Effect charge indicator.  This may be a bit superfluous as the BMS will already accomplish most of this and even connect via Bluetooth to my phone, but I like having an actual gauge on the box itself.  I will probably attach this on the top of the case, but at this point I don't know exactly where as my options are limited.  Its not impossible that I will place it at the rear in the vicinity of the light switches.  Of course, if I do this, this affects the future placement of any lights that I may place on top.  Also, I don't want to weaken the lid any more than I have to.  An alternative placement might be somewhere on the side of the box which at this point has no perforation as of yet.

Eric  

I would probably not go into that effort of adding a coulomb meter and just use a digital voltmeter that uses very little power, since you already have the same functionality in the BMS.
That way you can still get a rough idea of the state of charge, but it is not quite as complex (and expensive).

Sebastian,

Interesting thought.  Technically, the coulomb meter is for measuring state-of-charge which is challenging in lithium batteries as the discharge profile is so flat that voltage is not a reliable indicator of charge level.  I am not actually certain that the BMS has that specific functionality, but I can check in easily enough.  I already have the coulomb meter and it was not at all expensive so that part doesn’t bother me.  I do, however, have reservations about cutting unnecessary holes into the case.

I will look into things and get back to you.

Eric

Ok, it’s been a busy couple of weeks so I have not gotten much done on my battery box yet.  Also I have been waiting for a couple of odds and ends to come in.  I will not have a ton of space once I put in the battery pack and that makes putting in a fuse box difficult.  So instead, in an attempt to minimize lots of extraneous wires in a limited space, I plan to make all of my connections connect to a two posts (one positive, one negative), and then each post will connect to main power.  There will still be some internal fuses and circuit breakers, but these will be inline, automotive types that I will deliberately wire for easy access.  I decided that since I was using a post connection, it had better be the best connection that I can get so I ordered a pair of copper bolts to use as the main posts.  All connections will connect at these bolts before connecting to main power.  I also ordered a pack of copper washers to place between ring connectors in case the connectors can’t make a solid connection to each other.  

Here is a picture of my copper bolts/main terminals.



Eric

I got up early this morning and started wiring the internal connections in the box.  Most of the wiring is now done, but the exact placement may change just a little bit in an attempt to tidy things up.  Overall, I tried to make all of my positive lines all connect on the right side to a common terminal—a copper bolt.  And likewise, the negative connections line up on the left to another copper bolt.  Each bolt then connects to the main circuit of the battery.

Here are some pictures:

Eric

Ok, I will try to explain what is going on in this (mess) picture!

Up near the front I have all of the front port connections, notably USB ports, but also Anderson Power Pole and XT60 connectors.  The XT60 will be the main charging point.  All of those positive connections are routed to the right, turn back, and loop back up to the front to get an automotive style fuse.  From there they lead to a common connection point which is the copper bolt.  At the moment I have not yet decided how I want to connect the bolt to the box.  I have thought about putting a little bracket in and then bolting it to the side, but that would leave a live electrical connection on the outside.

At any rate, the top wire then runs through a long loop and goes through a 30 amp in-line circuit breaker.  From there the line goes to the center switch (rated to 30 amps).  The  line coming out of the switch then connects to the battery positive (at the moment, it is not in place).

The other side of the battery (negative side) gets more complicated.  The Battery Management System (BMS) sits on the right side of the battery positive.  There are a series of thin wires that connect the BMS to each battery cell.  This is so that the BMS can read the exact voltage of each cell and charge each cell to get them individually back up to full charge.  

In the back side of the BMS, there are two main cables.  The one on the left connects to the main switch.  The one on the right connects to the main loops around and connects the main negative terminal.  That negative terminal then has several connections that branch out to outside electrical connections.

At the moment, this is all roughed in, a very loose fit.  All of blue painters tape is strictly temporary and will eventually come out.  Its purpose is to temporarily hold things in place while I cut wires to length.

The orientation of the BMS is not as I had originally intended as the little BMS cell monitoring cables are terribly short.  In fact. For a moment I wondered if I even had enough length to attach at all.  I eventually flipped the BMS around and there is plenty of cable length, but now the main power cables from the BMS were two short and pointing in the wrong direction.  Fortunately, lengthening the power cables is easy and with a little bit of maneuvering I had things in place.

I still have a few things to tidy up.  I need to shorten some cables and tie other cables together to make things look better, but I think it is coming along.

Thoughts?

Eric

Yours almost looks professional.
Instead of lengthening the power cables, you could extend the sensing wires. Less expensive and doesn't add (almost any) weight.

I am just using standard solid wiring (probably 12 AWG), which should be fine for the peak load of 27A and which will liquefy at 200A, acting as a fuse.

Sebastian,  

I wish I could lengthen my sensing wires.  I was a little surprised when I found just how short they were.

And thank for the almost-professional comment.  Right now I think the internals are a bit messy.  Now before I finish I will clean this up, but I am still making adjustments.

By the way, what do you think about the copper bolt-as-common-terminal idea?  And, how would you attach the bolt?

Eric

The copper bold is a nice touch, but I think a steel bolt would suffice. The washer part of the terminals have more than enough surface area to conduct the current up and down the stack.
As to mounting it, if you don't want it floating around, maybe a large patch of double sided tape holding a block of wood that acts as a mounting point for the bold? It could even have space for the bold-head countersunk on the backside.

Sebastian,

Not a bad idea for the bolt mount.  I might try something like that.

Eric

Doing this project really added to my collection of small electrical connectors.  Keeping track of all of these connectors was becoming difficult to impossible so I took steps to organize my things.  This really is a necessity for me as I get tools that seem to grow legs and walk off on their own.  A part of this is my general propensity to be perfectly happy in a disorganized environment—until I need something and can’t find it!  Another reason is that each member of my family has at times used my tools but then put them wherever they thought appropriate.

My wandering tools, along with a scattering of miscellaneous connectors made it necessary to corral all of these items—which to me is like herding cats!  But my solution was three-fold.  First and foremost, I purchased several 5.56 magazine plastic ammo cans, the same type that serves as the housing for my battery box.  I like this size as it is big enough to stash it bunch of stuff without being so big that I will lose tools in them.  Further, the construction is surprisingly sturdy for being plastic, has two sturdy locking clasps and even has a water resistant seal in the lid.  They cost $40 on Amazon (and everywhere else I looked), but I consider that worth it if I can avoid losing tools.

Secondly, I picked up a little tray organizer on a random trip to Menards one day.  This was great for storing large numbers of item that came in small numbers (such as there being one tray portion devoted to switches—two of them).  Finally, I have also purchased a collection of plastic jars with screw on lids for items that might come in the hundreds (ring terminals).  

All in all, I have really cut down the clutter of my work station.  I especially like the ammo cans as they can really hold quite a lot, but are both highly portable (they have good handles) and they take up little space when not used as they are stackable!

Thought I would share.


Eric

MAKE SURE TO INSULATE BETWEEN THOSE CELLS! Any movement between any of them will cause that thin blue layer to rub through and then the cells short in-between themselves. Yep I found out the hard way and it cost me! They also have to be compressed or they will start expanding with use and cause other shorting issues, again learned through experience!  

Larry,

Will do.  Thanks for the advice.

Eric

Ok, progress!!

At this point I can get basic functionality out of my front panels.  The wiring is starting to get crowded but it is workable.

I still need to install a master breaker which will be 30 amps.  That is a fairly simple addition to what I already have in place.

In addition to the main circuit breaker, I also need to install the lights and set up and configure the BMS.  I also need to do an initial charge.

But at the moment, I have basic circuitry working.

Eric