320Ah LiFePO4 4S battery build with AliExpress cells, Overkill BMS and compression box

T3TRIS

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Feb 22, 2022
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Hey everyone,

I've learned quite a bit from browsing other people's posts and builds (especially from Dan-O's post) so I figured I'd share my own attempt at building a LiFePO4 battery system.

The goal is to add an auxiliary battery to our beloved 4WD 1987 Volkswagen Vanagon, which has no auxiliary battery system built-in as it was not originally a Westfalia camper. Without getting into too many details, I'm trying to design a system with an oversized capacity so that we don't have to rely on solar panels (though I'll probably install something at some point), the irony for a DIY Solar Power Forum... In any case, with 320Ah, the idea is that we'd have enough power to enjoy up to a week of camping which would drain the battery about 50%, and with a beefed up charging system (upgraded alternator + 50A DC/DC charger), the battery would recharge within the first few hours of driving. This makes it so that we don't have to think about solar panel placement when we camp.

Anyhow, so far I've built the battery compression box with the Overkill BMS using cutting boards and 1/8" aluminum pieces and other hardware. I used 5/16 threaded rods and these steel shelving rack brackets from Ace Hardware in which the 5/16 locking nut nests itself (self tightening basically). They also had these springs that are rated for 200lbs. Apparently, we're all aiming for 12PSI of pressure, which means 168lbs on each rod for a 6"x7" cell size. The cells sit on a tray that is not connected to the side pieces so that they can move independently when the cells swell and contract a little. The cells are separated by a heavy duty piece of silicone. The threaded rods are sheathed with tubing. The bus bars are flexible bus bars from eBay.

320Ah LiFePO4 Battery Compression Box Build - 1.jpg320Ah LiFePO4 Battery Compression Box Build - 2.jpg320Ah LiFePO4 Battery Compression Box Build - 3.jpg320Ah LiFePO4 Battery Compression Box Build - 4.jpg320Ah LiFePO4 Battery Compression Box Build - 5.jpg320Ah LiFePO4 Battery Compression Box Build - 6.jpg320Ah LiFePO4 Battery Compression Box Build - 7.jpg

This was the first iteration, since then I've made a few changes. I've flipped the BMS around to prevent the positive and negative wires from overlapping.
I also tested these compression springs and the first one collapsed with less that 70lbs of weight so I upgrade the hardware to 3/8 and die springs from McMaster Carr (9573K63 Blue Medium Load, 1" diameter, 1.25" long, 225lbs max load, 450lb/in spring load). I compressed the springs to around 1" or about 110/120lbs with the cells depleted.
In the photo, you can also see a couple 12W heating pads though I haven't quite figured out the details for this yet. I have a small 12V programmable thermostat for this (35mA while at rest) and plan on wiring it to trigger only when the alternator is running.
I also added a 150A block fuse directly on the cell's positive terminal.

320Ah LiFePO4 Battery with hardware updated and heating pads.jpg

I still have to figure out the heating part, how to secure the battery to the van and making a top for it to prevent shorts.
Here's the wiring that this battery will power.

T3TRIS Vanagon LiFePO4 Auxiliary Battery Wiring Diagram V2.jpg

Lastly, I'm trying to figure out how to setup the BMS parameters to stay on the conservative side of charging and discharging since this system is oversized. By default, the Overkill BMS stops charging when the battery reaches 14.6V or any one cell reaches 3.65V, and stops the discharge when the battery drops to 10V or any one cell to 2.5V. I've definitely noticed that cell #3 drops down much quicker than the others and cell #1 reaches 3.65V first (though less dramatically than cell #3 on discharge). In the middle, they are all within 0.03V though. The cells were top balanced in parallel to 3.65V until the 10A power supply was pushing less than 0.2A.
I wondered if I can set the BMS to stop charging when a cell reaches 3.5V or drops to 2.8V (and 14V/11.2V) or if that was bad idea or unnecessary. I noticed the delta between cell #3 on discharge augments quickly. By the time it reaches 2.5V, the other cells are still around 0.35V higher and within 0.15V of one another.
I've read other posts about this but haven't quite found if a "conservative" approach is beneficial, though I've seen people mention "low stress charging" or something like that.

T3TRIS 320Ah LiFePO4 Battery Overkill BMS settings.PNG

Any input on the BMS settings would be appreciated.
 

MisterSandals

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Any input on the BMS settings would be appreciated.
Cells can handle 2.5-3.65V. Your BMS should handle the safety limits and NOT be used to control charging (guessing that might be your thinking based on these settings).
What are your charge settings? They should work in conjunction with the BMS settings.
I wondered if I can set the BMS to stop charging when a cell reaches 3.5V or drops to 2.8V (and 14V/11.2V) or if that was bad idea or unnecessary.
Bad idea. Set your charging to never trip BMS.
 

T3TRIS

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Feb 22, 2022
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Cells can handle 2.5-3.65V. Your BMS should handle the safety limits and NOT be used to control charging (guessing that might be your thinking based on these settings).
What are your charge settings? They should work in conjunction with the BMS settings.

Bad idea. Set your charging to never trip BMS.
Excellent input. I had no idea that the BMS shouldn't be used for that purpose. I read the Overkill Solar BMS instructions from cover to cover, and unless I missed that or forgot about it, I don't think that is mentioned in there. I guess it makes sense now that I think about it, no need to have the safety system (the BMS in this case) do something so "active" so often. That's probably why they are "protections" settings. I'll reset the protection settings back to factory.

Charging will be handled by a Redarc DC/DC 50A charger. It has a variety of charging profiles based on battery chemistry. For LiFePO4, the max voltage is 14.5V (under the BMS's 14.6V max). I guess I'll have to check and make sure that 14.5V happens before any one cell reaches 3.65V then (which I think would be the case).

As far as discharge though, can we only rely on the human element before the BMS protections kick in? I have a battery monitor that has an alarm feature if the capacity goes under a set value, but it'd be nice to have an automated system (like the 14.5V max of the DC/DC charger) to prevent the BMS to have to do it. Though I guess that wouldn't happen all that often.
 
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T3TRIS

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Feb 22, 2022
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I've been trying to figure out the best way to secure the battery pack to our van and it hasn't been easy. I went through the trouble of making a "compression" box, so I guess I need to make sure that the compression is not inhibited by securing it to the vehicle. This is even more challenging in a tight space of course.
The best solution I've come up with so far was to make a lid with 2 panels and use that to hold the battery down.
320Ah LiFePO4 Battery installation Vanagon11.jpg
The top panel protects the battery terminals from shorts while the side panel encloses the self-heating part of the battery. The top panel is sitting on three 1" tall rubber spacers so that tightening pressure is applied to the top of the cell rather than the terminals. The fuse is visible from the outside. One hook is connected to a rod in the frame of the seat while the other side is connected to a D-ring.
320Ah LiFePO4 Battery installation Vanagon12.jpg
This should allow the walls to move back and forth should the batteries decide to swell a little, while keeping the assembly secured to the van. The rest of the components can be mounted on the triangular open space to the left of the battery pack.
I'm wondering if I should drill holes directly above the battery vent holes. Any input?
 

MisterSandals

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I'm wondering if I should drill holes directly above the battery vent holes.
Over heating is rarely a problem. I really have never gotten any battery warmth thru hundreds of charge cycles. The gaps and cracks around the edges should be more than enough for normal camper use.

Looks very nice and tidy!
 

T3TRIS

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Over heating is rarely a problem. I really have never gotten any battery warmth thru hundreds of charge cycles. The gaps and cracks around the edges should be more than enough for normal camper use.

Looks very nice and tidy!
Thanks! It's been a fun project for sure. My question about drilling holes is more for the safety vent valves on the cells themselves though. I guess people who have had catastrophic failures had their cells vent out through those valves. Should this happen to us (hopefully not obviously), I wonder if there should be an opening above each of those safety valves. I'm probably over-thinking it though as always, it can work its way out the side holes (those cutting board handles).
 

Bob B

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Or .... go with a 2P4S configuration ... using one BMS and keeping the other as a spare.
 

MisterSandals

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This is from a document in resources. I forget who did this document, but I'll look it up and edit it in.
I think you are posting in the wrong thread.

This is a show and tell thread.

Maybe you meant to post here?
 

Bob B

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I think you are posting in the wrong thread.

This is a show and tell thread.

Maybe you meant to post here?
He asked what I meant by a 2P4S configuration.
 

jpcjtrtj

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Pretty amazing build. I've used a Dewalt toolbox to house my battery (you can see it below), but sounds like that would not fit in your application. I use simple tie downs and strap to hold it to the floor of the camper. Plenty of room inside the box for the vents to blow if they have to; although I'm hoping never to experience that.

jps
 

T3TRIS

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Feb 22, 2022
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He asked what I meant by a 2P4S configuration.
Actually, I’m familiar with 2P4S configurations and that wasn’t what I was asking. What I meant was that I’m not sure how a 2P4S configuration is relevant to this discussion (I could’ve been more clear in my question I guess).
I built a 4S configuration to have a compact 320Ah 12V battery system. This design only needs 1 BMS. I don’t want 8 cells nor do I want 2 BMS’s… there’s no question about having cells in parallel or series for the setup discussed here. Either there is a misunderstanding or, as MisterSandals mentioned, you might be contributing to the wrong thread.
 

T3TRIS

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Feb 22, 2022
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Nice build!
Thanks!!
Pretty amazing build. I've used a Dewalt toolbox to house my battery (you can see it below), but sounds like that would not fit in your application. I use simple tie downs and strap to hold it to the floor of the camper. Plenty of room inside the box for the vents to blow if they have to; although I'm hoping never to experience that.

jps
That's cool! Yeah the box I have here is workable but not ideal. Securing it to the van has been its own dilemma but it's been workable so far. The whole solid toolbox approach is nice. I think I'll definitely end up using some sort of strap to hold it though, rather than this solid bar.
 
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