DIY Lithium Packs, Proposal and Discussion

[luthj]

Good gosh $253 just for shipping 4 cells???

That's air freight for 45lbs of lithium. If you opt for ocean freight is $50-80 cheaper. I believe registered shippers can ship air ship LFP cells without hazmat (up to a certain weight).

If you compare the total cost per AH, even at 240$ per 4EA, its still fantastic value.

 

[Hillbilly Heaven]

I was hoping shipping would be cheaper per unit if I ordered 2 (8 cells), Nope doubled it. I would settle for sea freight. I don't deny it is a good price per AH.

 

[luthj]

If you contact the seller you can request ocean freight (30-50 days from order I think). I paid ~$940USD for 8 cells delivered. Shipping breaks would probably require moving to 32+ or more cells on a pallet. For weight and safety reasons you can't typically package more than 50lbs of lithium batteries to a box.

 

[john61ct]

Absolutely yes need to use compression plates & strapping

True for all LFP prismatics afaik

 

[luthj]

Agreed, this battery maker lists a force spec. In this case 300kg load on the pack for optimal cycle life.

 

[luthj]

Mmmm, Box.

 

[luthj]

I can find the spec later, but they list cycle life with and without restraining fixture. plus why specify plus/minus? I don't think I need the full preload to start with, just a fixture that can take that. The cells will expand 0.5-1 mm after a few hundred cycles, which should create the preload? I have an email to EVE support for clarification.

 

[luthj]

Here is the somewhat vague details on fixture vs non fixture.




Looking at RV tank heating pads, this pad looks promising. Two of them will wrap the entire vertical perimeter of the cells. ~160W total, and they have an internal thermostat 45-68F approx. I would still control them from the BMS, but the built in thermostat provides a nice second layer of protection against an overheat due to failed thermister.

https://www.amazon.com/Facon-Holdin...ds=tank+heating+pad+12v&qid=1593693905&sr=8-5

 

[luthj]

The cells, or entire fixture ?
Might think a heatingpad would object to 300kg squishing upon it.

From what I understand, most of the effect is due to high C rates causing anode expansion later in the cycle life (2500+ cycles). Restraining the cells prevents this. I don't think I will go to 300kg though, just restrain them with maybe half that.

With regards to the heating pads, My thought is to make the end plates from 1/4 " aluminum. Then wrap the heating pad over the plate.



I am not sure how I will insulate the box. It may be easier to insulate the exterior, but that presents other issues. I need to strap down the pack, so Maybe just 2 nylon straps to the bottom, and have the insulation internal? Trying to keep my labor as low as reasonable, and make assembly less hazardous. Playing with metal around a pack with exposed terminals has some risk. I have vaporized a couple wrenches in years past...

 

[Hillbilly Heaven]

I think what that spec is saying is the fixture must resist 300kg force placed against the wall. Like was said that isn't too definitive. Is that kg/mm2 or kg/cm2??

If the heater covers the whole container or the container is insulated after heater wrap, I would make the container out of some 1/4" aluminum. That should be thick enough to contain the expansion and not enough to conduct the heat to air, but provides a good heat conductor to the battery.

 

[john61ct]

No need to wrap the pad around the battery

Pad goes underneath, a shallow drawer would make it easy to fix / replace leaving the bank in place.

Just insulate the box well, relative to how extreme the ambient cold.

But ideally let it breathe well in hot conditions too, maybe slide-in foam panels for the sides, leave bottom and lid in place.

 

[luthj]

My plan is to assembly the pack on the bench. That will consist of buss bars, BMS leads, and the compression plates. The box will be mounted in the van through the floor, and insulated with 1/2 or 3/4" foam. The pack will then drop into the box, and strap/clamp down with some yet to be determined method. There will be an air gap between the pack and box by necessity. I am also considering a plexiglass cover for the top as well, just for belt and suspenders.

A pad on the bottom works, but I am trying to avoid more expensive custom sized pads. The 7x25" ones are a good fit. Depending on how the thermostat works I can wire them in series for 1/4 the power. Having 160W of heating is about 3x what I need for deep winter conditions. That gives me enough overhead to let the cells cool to ambient and warm them up in reasonable time. My plan current is to have a box vent fan (60mm probably). The BMS can support multiple thermisters, so if the external temp is lower than the pack temp, and the pack is above 25C, it will ventilate the pack.

With regards to the 300kgf figure, that would indicate 300kgf across the entire face of the cell, as opposed to pressure. Which yields about 12psi on that face. Which passes a sanity check, and would definitely be possible if the cells are experiencing expansion internally.

 

[Wyuna]

I might have missed it,, but did you work out which BMS you were going to use.

 

[luthj]

I might have missed it,, but did you work out which BMS you were going to use.

I have a couple I am considering. Pending some details back from tech support.

I will probably go with this option. Its a bit pricey, but has lots of user configurable options. I can use a OBDII to bluetooth adapter to view vital states from a smartphone. Not terribly advanced, but does the job.

Orion Jr 2 BMS | Orion Li-Ion Battery Management System

www.orionbms.com

 

[Jman99]

I have given up on the BMS, the compact off the shelf cheapies with MOSFETS look pretty good to me, they look to be alot more reliable than I thought. Not much to go wrong and if oversized I believe they might last quite a while. Add in a victron battery protect & I thick thats plenty for me.