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Tips on special care for vehicle mounted systems?

Martijn

New Member
Joined
May 6, 2021
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235
Hi all,

After just having read a thread in the 'up in smoke' forum, I wonder what you're tips are when creating a LiFePO4 battery for a truck camper.

Here's what I currently have in mind:
  • no specific 'compression'
  • possibly still used threaded rods for fixation using front and back plates OR build a plywood case that holds the cells, tape together with fiber tape
  • use flexible (cable) busbars
  • use isolation between cells
  • use paste on cell terminals
  • use a torque wrench on the terminals
  • put the case on rubber mounts (like these)
Suggestions, thoughts, anything else?

With regards,

Martijn
 
I would at least double the insulation between the cells, and don't forget the bottoms.
I'd only use a metal enclosure to keep any fires contained.
Tie everything down, especially the BMS wiring, and try to think of any potential short circuit paths. I'd even put the BMS wires in some braded insulation to keep the chafing to a minimum.

I didn't worry too much about the cells themselves bursting into flames, but I am worried about a short that starts a fire and leads to much bigger issues. If you have ever witnessed an RV fire, you will NEVER forget it. They burn up incredibly quick so even seconds count.

I spent a lot of time thinking and reading on this topic for our RV and ended up just buying the server rack style packs from Signature Solar.
 
Hello J,

Thank you for your suggestions.

I don't think I can use a metal enclosure, but could use special flame retardant panels on the inside of the electrical compartment (under a seat/bed). There will be a fire alarm in there. However, now that I think of it, because of several appliances in that compartment (inverter, mppt charger), there will also be venting holes.

And yes, tying down cables and protecting those with additional materials is on the list too. I also have an hydraulic crimping tool in order to create proper connections for the cable lugs.
 
If you don't implement some form of fixture, the cells could move around, which would be bad. I have a compression fixture. It's nothing more than 3/4" plywood ends and four 1/4" threaded rods. It keeps the cells locked together and at the same time provides the minimal compression that EVE has recommended. I use solid bus bars between the cell terminals and simple screws (no lock washer or thread locker) into the threaded terminals.

I used no isolation between the cells. If I were to redo the batteries I would use a thin sheet of flexible cutting board, but only for isolating the voltage potential (which hasn't been a problem for me).

My connections have stayed nice and tight despite going down some very rough roads. The battery fixtures are attached to the floor to ensure there is zero movement of the battery.

Don't make it any harder/complicated than it needs to be.
 
If you don't implement some form of fixture, the cells could move around, which would be bad. I have a compression fixture. It's nothing more than 3/4" plywood
Hello Jim,

You don't think the cells, as a pack, should be placed on rubber mounts? Would you consider it a nice addition?

Of course, they will be fixed together, haven't yet decided on how exactly. What do you consider to be the 'minimal compression' to be reached with your 4 rods? Did you use a torque wrench for that, if so, on what setting?

Reason I'm asking, is I got this from the document available here, and it says "can be done using fibre tape", but also gives a torque specification for which I have no idea how to apply. I guess this also depends on your configuration (eg 4 cells vs 8 cells in a row).
1655307480263.png
 
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Hello Jim,

Of course, they will be fixed together, haven't yet decided on how exactly. What do you consider to be the 'minimal compression' to be reached with your 4 rods? Did you use a torque wrench for that, if so, on what setting?

You don't think the cells, as a pack, should be placed on rubber mounts? Would you consider it a nice addition?

I did use a torque wrench, but it was a 1/4" digital torque wrench and I likely had it set on inch pounds, not foot pounds. It wasn't much. Just enough to get it snug and to ensure that all four nuts were are the same torque. Maybe 12 inch/pounds? It's been a while since I did it.

My experience with mounting the batteries says that they don't need additional shock absorbing mounts. If you did that, where would you place the rubber mounts? On the end plates? That would leave the cells unsupported on the bottom. If your fixture wasn't sufficiently tight then the cells would move.
 
My experience with mounting the batteries says that they don't need additional shock absorbing mounts. If you did that, where would you place the rubber mounts? On the end plates? That would leave the cells unsupported on the bottom. If your fixture wasn't sufficiently tight then the cells would move.
Besides end plates, there would be a bottom plate, the rubber mounts would be screwed into that plate and then the bottom of the camper box.

So the cells would be clamped, the bottom plate would attach the battery as a whole to the floor, but still lessen vibration.
 
Besides end plates, there would be a bottom plate, the rubber mounts would be screwed into that plate and then the bottom of the camper box.

So the cells would be clamped, the bottom plate would attach the battery as a whole to the floor, but still lessen vibration.

It's certainly not wrong to pursue that solution. It's overkill though. Given the scarcity of space in a slide-in camper, anything you can do to minimize the space that a component takes up is well worth it.
 
IMO you're worrying about the wrong things.
The fixture isn't critical, just a way to assemble the cells into a battery and hold them, and a way to secure them.
The risk of fire danger isn't with the cells, they're very stable, but with poor wiring, terminals, connections, or configuration.
If you're going to build a DIY battery you'll need some specialty tools - primarily a way to crimp lugs, a DMM, a charge source for top-balancing, and some basic handtools for building your jig and fastening it down.

Wire sizing, fusing, and layout leave a lot of leeway for how to get it done, so it pays to develop a plan and publish it here in your build thread, get advice or ask for help. With proper wire sizing and fusing and good assembly techniques you'll have a solid setup, plus the knowledge of what went into it and how it was built so any troubleshooting is much more straightforward.

Also assessing your actual usage and needs, and sizing your setup to accommodate, is sometimes easier said than done. I find many over-complicate this, and actually think they'll run their mobile setup like a house. The more you use, the more complicated and expensive, and the more danger there is for making mistakes or having failures. There's a lot of value in keeping it simple.

In my case I already knew what my usage was, we've been dry camping in our motorhome for the past seventeen years and not much has changed. We got by all that time with a pair of 6V golf cart batteries in series for a 12V bank - about 100Wh usable. So a 230A lifepo4 battery gave us twice the usable amp hours for half the size and a third the weight. But all systems were updated to accommodate it, and moved inside for a climate-controlled environment in lieu of the chassis mounted external battery tray.

My battery fixture is simply screwed down to the plywood subfloor of the coach - it's solid. It has proper wire sizes and fuses, and feeds run to new and existing buses. An inverter was added, and solar panels and charger, as well as a dc to dc charger in lieu of direct alternator charging from the chassis side. The cells do set on a 1/8" rubber sheet, and the fixture compression rods were simply snugged up to hold it and prevent any battery puffing from charging and discharging.

Here's my finished battery project, with relocated converter/charger and 120V inverter. The battery pack is simply 4ea 230A prismatic cells, with a ClassT fuse and marine type main disconnect. Feed wiring is 2awg - my system BMS is rated for 120A max, so everything is sized to suit.



Here are my replies to your actual questions:

  • no specific 'compression' - use a fixture to hold the cells and prevent them from puffing. Something as simple as threaded rods is fine.
  • possibly still used threaded rods for fixation using front and back plates OR build a plywood case that holds the cells, tape together with fiber tape - I would not use fiber tape, but the rest makes sense. Plywood is fine, as is StarBoard, what I used.
  • use flexible (cable) busbars - Not necessary in a fixture.
  • use isolation between cells - something as simple as thin polyethylene cutting mats.
  • use paste on cell terminals - Yes, ox guard or similar, and avoid mixed metals as much as possible. For example, no copper to aluminum.
  • use a torque wrench on the terminals - yes, you don't want to strip out your terminals, or have them come loose. Nylocks and/or split ring or bite-ring washers where appropriate.
  • put the case on rubber mounts (like these) - did not take the link, but rubber isolation isn't critical.
 
IMO you're worrying about the wrong things.
The fixture isn't critical, just a way to assemble the cells into a battery and hold them, and a way to secure them.
The risk of fire danger isn't with the cells, they're very stable, but with poor wiring, terminals, connections, or configuration.
If you're going to build a DIY battery you'll need some specialty tools - primarily a way to crimp lugs, a DMM, a charge source for top-balancing, and some basic handtools for building your jig and fastening it down.
Chris, thank you for your reply. I've assembled the 8 cells (24V, 280Ah) before, with knowledge from this forum. I have a crimper, multimeter, power supply and so on. I guess that's why I no longer worry about that part of the build.

Thank you for sharing your experience, much appreciated. I will still use the flexible busbars though. ;)
 
I did use a torque wrench, but it was a 1/4" digital torque wrench and I likely had it set on inch pounds, not foot pounds. It wasn't much. Just enough to get it snug and to ensure that all four nuts were are the same torque. Maybe 12 inch/pounds? It's been a while since I did it.
Ah, I found a converter for kgf.cm to torque. Seems 160 kgf.cm is about 138 pound inch or 15.7 newton meter.

And from your post, seems you have to apply this torque to all rods involved. That works, thanks.
 
If you don't implement some form of fixture, the cells could move around, which would be bad. I have a compression fixture. It's nothing more than 3/4" plywood ends and four 1/4" threaded rods. It keeps the cells locked together and at the same time provides the minimal compression that EVE has recommended. I use solid bus bars between the cell terminals and simple screws (no lock washer or thread locker) into the threaded terminals.

I used no isolation between the cells. If I were to redo the batteries I would use a thin sheet of flexible cutting board, but only for isolating the voltage potential (which hasn't been a problem for me).

My connections have stayed nice and tight despite going down some very rough roads. The battery fixtures are attached to the floor to ensure there is zero movement of the battery.

Don't make it any harder/complicated than it needs to be.
1657789547364.png
I've used Phenolic paper as insulator, could do 2 sheets. I've done the rods and for slight compression/fixture, tried moving the cells, but they won't move at all. But as the cells aren't perfectly flat, on the top/bottom, there's still some room though. But they look 'tied down' quite enough.

I've tried looking for actual compression numbers and foot pounds on the rods, but this is way too complicated ;)
 
View attachment 102521
I've used Phenolic paper as insulator, could do 2 sheets. I've done the rods and for slight compression/fixture, tried moving the cells, but they won't move at all. But as the cells aren't perfectly flat, on the top/bottom, there's still some room though. But they look 'tied down' quite enough.

I've tried looking for actual compression numbers and foot pounds on the rods, but this is way too complicated ;)
Easier to just use four 160lb die springs for a single stack.
 

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Just no. The springs are really extraneous, and you can't gauge the compression anyway.
What do you mean? You most certainly can, you have the compression rate of the spring. The ones I use are 160 lbs compressed to .63”, this puts me at 640lbs so just below 12psi. The pressure will go up and down slightly with SOC but we’ll within the recommendation to not go above 17psi.

If you just go with a hard case or threaded rod with nuts how do you know you are not over compressing your cells above 17psi at 100% SOC? Or you just don’t care?
 
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