DIY LiFEPO4 Battery Excess

[bigfoot_ev]

Let me say right away this post could be considered unreasonable excess with regards to battery capacity. I just finished installing 3.4kWh of LiFEPO4 (LFP) battery energy into my Weekender. As you will see it's actually less confusing if we talk in terms of kilo Watt hours vs Amp hours. This is because I built a 24V battery instead of a 12V battery. I believe if I had built a 12V battery it would have required 265Ah cells.

Another caveat, for those of you that regularly read the Vanagon forum this post will look very similar to many of the posts over there. And for me, especially this one:
https://www.thesamba.com/vw/forum/viewtopic.php?t=760365.

I thought it would be good to have some of this information in the Eurovan forum because I know there are some folks that would never venture over to the Vanagon forum.

As some of you know, the interior of my van is heavily modified. So, there might not be much useful information about where to cram this much energy into an unmolested Weekender. It might be possible to fit under the passenger seat but I would probably look instead under the bench seat.

So, on to building the battery. A number of years ago I built a 3.4kWh solar generator before there were 'affordable' options from Bluetti or Ecoflow or Goal Zero. My primary reason was to power a small electric heater overnight for our teardrop since running a propane heater in such a small place seemed crazy. But, I also wanted backup electricity for the house in case of a power outage. Fast forward to today and my solar generator spends more time collecting dust than powering anything.

During our last trip out I spent waaay more time chasing sun with my solar panel than I wanted because I was using the diesel heater in an on/off/on/off situation instead of running it more continuously. Why does this matter? The diesel heater uses more energy during start-up and shut-down because it heats the glow plug. When everything is heated up and running the glow plug turns off and energy usage goes way down. That's when I got the bright idea, why not repurpose my solar generator for a big ole van battery so I don't need solar panels anymore!

The solar generator was built into a Dewalt tool box. This particular model of tool box had a small tray area built into the lid which housed my various outlets, ex USB, 12V plug, and 120VAC outlet. Open the lid and I had my first AC inverter sitting on top of the LFP cells and also all the various electronic items. The first inverter died so I upgraded and moved it out of the tool box. I added a 125A Anderson connector to the solar generator and the matching connector to the new inverter. Here's a few shots as I was getting ready to disassemble the solar generator.

This last photo is all the electronic doodads I had installed to handle various inputs and outputs.

As you can see, I have 8 cells, that is the reason why I put a 24V battery in the van. These cells are rated at 135Ah and my most recent tests indicate I still have all that capacity. Most folks that DIY a LFP for there vehicle (van, overlander, etc) seem to focus on 12V which would be 4 cells. I suppose I understand why but as a point of interest all my current flow at 24V is half the current flow at 12V. This dramatically effects wire sizing, wire runs, costs, etc. But it does add a little complexity. Fortunately for me, I already dealt with all this for my solar generator.

If you look back at the picture of the 8 cells you will notice at the bottom is a thin'ish aluminum electronic item. That is the battery management system (BMS). This is a critical safety item for any LFP battery. Also notice the orange'ish tape over the cell terminals. This is Kapton tape which acts as an insulator. Well, as I was pulling the cells out of the box the BMS popped out and fell across the cell terminals creating lots of sparks and excitement. I don't know what the hell happened but it ended up frying my BMS and melting a little off the top of a couple of terminals. In the end there appears to be no harm done to the cells and I got to get a new BMS. There have been some nice improvements in BMS features, specifically active balancing for my needs. So in the end all seems well.

Once the cells were out I decided to do a top balance to get everything ready for assembly.

Next I had to plan out the new battery. I wanted the battery to be self contained and isolated. That meant to me, I wanted to be able to turn the battery off, disconnect it from the van, and pull it out. Also, since the battery will live in the van I wanted to able to use the battery during winter season. This means I needed to include heating inside the battery. During the design I had a cool space within the battery for my new BMS. Unfortunately while doing trial fittings the battery box was too large so I had to do a redesign on the fly and move the BMS to the side of the box. As some folks might be aware LFP cell compression is a hot topic on battery box design/build. My box does not compress the cells. They fit tightly, but that's about it.

I cut some plywood and glued and screwed it together. Then paint it black, just because. First I put some silicone heating pads in the bottom. I then put a sheet of aluminum on top of the heating pads to spread out the heat better.

Next, I fit the cells in. In the background is the now empty Dewalt tool box.

Here are some shots of the battery assembly. Before I bolted anything to the cell terminal I cleaned the terminal with some sandpaper wrapped around a hole saw cutout to remove oxidation. Next apply Ox-gard, then the bus bar and BMS lead on top of that. And of course, properly torque the nuts.

And this is what the new LFP battery looks like. Behind the switch the positive battery leads go directly to a 100A MRBF fuse attached to the input side of the switch. The switch output feeds the external positive terminal. The BMS feeds the external negative terminal. The 24V and 12V systems are grounded together to the van.

While designing this upgrade I did a fair amount of research and thinking on the charging system. My old CTEK DC-DC charger needed to be replaced since it didn't handle LFP battery chemistry. In the end, I decided to go with Victron because it seems to have the best integration between components and I can more easily tailor the characteristics to my needs.

For battery monitoring I have a BMV-712. I went with the Orion Tr Smart 12|24|15A for DC-DC charging. I really wish Victron had a 24V XS charger available. If I had a 12V system I would definitely go with the newer XS. I use the relay output from the BMV to control the Orion charger. This is important because I need to discharge the LFP battery sufficiently before I let the Orion charge it. The only other charger I have is a bench power supply that is CC/CV supply so that I can fine tune the charge periodically. This is to remove any memory effects should there be any. I am considering adding a Victron IP22 AC/DC charger but I don't know if I need it yet. Since the Orion isolates the starting battery from the house battery I put it up front, like the old CTEK. The charge current flows to the rear but since we're charging at 24V it's half the current.

That Orion charger is definitely larger than the CTEK!

I also changed my rear distribution to handle the 24V bus. You can see the shunt for the BMV behind the yellow covered 24V bus bar.

And here's the battery in it's resting place at the rear passenger side of the van.

Wiring Diagram

I've been doing some extensive garage testing. I've run a couple of full charge/discharge cycles and I even tested out BMV control of the Orion. All seems to be working well. We'll be heading out soon for some field testing and I'm not bringing a solar panel. I'm going to commit! I will provide updates as I use the system more.

[bigfoot_ev]

First update. We went for a weekend camp and since the weather is pretty mild we didn't really use much battery power. Needless to say the LFP battery did just fine. I was more concerned with the charging aspects.

When I got home I continued to do some charge/discharge cycles making sure I was happy with all the parameters. It became clear that the bench power supply, while more than adequate, needed to be replaced with something that didn't require such close attention while in use. One forgetful moment and I could ruin my entire battery pack. I bought a Victron IP22 24V 16A AC-DC charger. This turned out to be a great addition to my entire system. I've known about Victron for a long time and I always felt there was better value for the money elsewhere. But I have to say, I'm a convert now. I'm really impressed how the newer components integrate and the control I have over many parameters using the bluetooth app. One small issue is that the Orion Tr DC-DC is older tech and is being replaced with the Orion XS. I would gladly have paid the premium for the XS if only they sold a 24V version. It's coming I'm told.

The BMV monitor provides voltage and current sense to the IP22 charger over a local bluetooth network. I set the IP22 to end charge based on voltage and current, which is perfect. The BMV also remotely controls the Orion Tr to turn on and turn off based on overall battery state of charge. This is not perfect, but it's completely workable. I can also leave the IP22 plugged in and it doesn't trickle charge the LFP. But, it re-bulks when the battery gets to a low level and also doesn't float. Just what I wanted.

Something I discovered while doing my testing. I can run the IP22 and the Orion Tr at the same time. Here's a screen shot from the Victron app. I'm charging at over 28 amps at 27 volts. That makes for short charge times! Of course, this is a really silly way to charge normally but it was fun to discover. Also notice at the bottom of the screenshot is a section titled Relay and the state is Closed. This is the how the Orion is remotely controlled by the BMV.

I decided to mount the IP22 AC-DC charger on a shelf above the battery in the rear. This way I can still easily remove the battery. The IP22 remains hard wired into the circuit and can act as a power source with the battery removed.

And here is the new wiring diagram.

[dsh1705]

Great write up. Makes me want to change out my charging system just to get all that cool Victron stuff!
_________________
David
1705.net
95 EVC
ACU,098

[bigfoot_ev]

Haha! I guess I drank the kool-aid.

I don't think these LFP cells have seen this much exercise, ever. I need to calm down I only have about 1,980 cycles left.

[bigfoot_ev]

Just a quick update on charging.

The DC/DC charger is really pretty unsophisticated. But, it does its primary job well enough and that is to control the current flow from the alternator to the battery. Fortunately, I can control it remotely based on other parameters.

The AC/DC charger is much more sophisticated. I have configured it to stop charging based on voltage 28V (3.5V/cell) and a tail current of 3.1A. No float and no absorption. This works just as expected and just what I wanted. Something new I discovered is Storage mode. Once the battery is fully charged it goes into storage mode immediately. I set this voltage to 25.84V (3.23V/cell). I have a parasitic drag so over a number of days the battery will eventually get to 25.84V. As soon as it does the charger maintains that voltage. It will do this up to its maximum current of 16A. I thought this was pretty cool.

I also discovered that storage mode is not remembered. As soon as I unplug the charger and then plug it back in, it will go into bulk charge until 28V and 3.1A is hit. I guess that's fine now that I know how it works.

[shirk]

Great info and thanks for sharing.

Do you have a rough estimate of your total cost?

I have been looking at what options I might be able to fit into the stock Weekender under seat position.

[bigfoot_ev]

Glad you liked it. Regarding price, well it would be mostly moot since I bought my LFP cells many years ago. Prices have easily halved since then plus my system is 24V and you could save money, time, and complexity by going with 12V.

I don't know if you saw this post but it looks pretty cost effective for the weekender ($220 for 100Ah under the seat).
https://www.thesamba.com/vw/forum/viewtopic.php?p=10261265#10261265.

I would just remove the plastic box and make the whole thing even smaller. IMHO I would also consider a DC/DC charger, solar if you really need it, and possibly an AC/DC charger.

And then there is the Ecoflow, Bluetti, Goal Zero path. I seriously considered just plopping my DIY solar generator into the van and calling it done. In the end, I liked having a battery installed and all I wanted was more capacity.

[bigfoot_ev]

A small addition to my LFP battery. When I purchased the new BMS I was pretty happy to find one that had an output circuit controlled by temperature cut-off settings. The idea is that you could control heating pads when the temperature drops below a set point. Nice idea, but in execution a little lacking. The heating pads would not turn on until the BMS cuts off the battery.

What I really wanted was to start heating the batteries before they reached a temperature cut-off value. That way, they are still usable as the temps start to drop. And it keeps the BMS as my safety net. So, the solution is to add my own temperature controlled heating pads. This is a 24V version and all my bench testing looks good. I think it's time for winter camping!