24v 8s lifepo4 pack for camper conversion aux power, how to connect together?

[whereswally606]

So this is a mockup of the pack as I intend to build it. the copper bars are 275mm long which is annoying as 250mm are far cheaper from china so I will have to either do the parallel and series as separate or buy locally cut more expensive ones already cut to size.

all in I need 2200mm x 15 x 3. however I have also been toying with the idea of using flat braided copper.

To act against puffing of the packs and provide some compression/structure I will be using two steel end plates and 4 1.25m threaded rods. This will then go inside an insulated box, preferable under the floor of the camper out of way of being a trip hazard and keeping the weight nice and low.

Monitoring will hopefully be using a smbs0 by dacian todea (electrodacus) but also I should finish my v4 fechter bms and can use a cell log 8s in the mean time. eventually I aim to charge using 3 or 5 200w panels either directly in parallel or via an mppt tracer inverter.

currently I am charging these very slowly in my garage using a very cheap Chinese dcdc that is suppling 3.4v. All of which are paralleled to even them out. infact only 15 are charging like this since I have another 10 on a shelf which are currently at a higher state of charge and need to be discharged and paralleled in too to balance them all out before re configuring to the pic above.

anyway any thoughts on the merits of braid versus solid bar in this application. 7 weeks to get this installed and bullet proof.

 

[billvon]

anyway any thoughts on the merits of braid versus solid bar in this application. 7 weeks to get this installed and bullet proof.

I would either go with braid or use foam separators and use solid bar. If each cell swells by only half a mm (likely) then you will see ~3mm of distortion of the battery terminals at the end of each bar.

 

[amberwolf]

Given the likelihood of swelling (regardless of compression), braid is a safer bet to not damage the terminals/casing around terminals.

On the various car conversions I've seen using these types of cells, the individual cell connections tend to be a relatively thin (but wide) bus plate with a little "hump" bend in the center, to allow expansion--not as flexible as a braid, but much better than the thick busbars.


On the TS60 cells I had, I used braid crimped to full-loop tinned copper lugs, with some automotive battery terminal grease (dielectric) to minimize the likelihood of corrosion. Mine werent' very good crimps, but I didn't use them at high current. If you're using them at high current I recommendwhatever crimper it takes to make a good cold-welded joint.

I used thick (2-3mm?) hard SS washers as wide as my lug faces under the hex bolt heads to ensure flat contact with the terminals.

FWIW, the braid I used was the tinned-copper shield from old RG75 coax., because I was cheap, but if it will carry the current you need, you could do something similar.

 

[whereswally606]

Just decided on braid, going to use these

https://www.etsy.com/uk/listing/584616732/eyelets-135575mm-100pcs-metal-copper

to help protect the braid from the screwing of the bolts.

so will order 14mm wide braid. question is do I want to double it up or no? I.e. double the current handling capacity.

https://www.ebay.co.uk/itm/123021066910
just ordered 3m of the 14mm braid.

I think maybe its enough the way I have it.



going to got this way as in the short version above since I can use more clamps and get better clamping force. Also it should be easier to construct and deconstruct should I need to isolate a given 3p section.

 

[amberwolf]

Just decided on braid, going to use these

https://www.etsy.com/uk/listing/584616732/eyelets-135575mm-100pcs-metal-copper

to help protect the braid from the screwing of the bolts.

Not sure I understand how those would be used? Punch holes in the braid, then crimp the eyelets in it? Then use washers on top sides of the eyelets to make it contact the terminal correctly and evenly with the bolts' pressure?

I think you will have too much resistance in the eyelet-to-braid crimp, and potential for corrosion, unless you have a crimper for the eyelets that will cold-weld them to the braid. (usually eyelets like those are very thin metal, so may just deform or tear around the braid instead of cold-welding to it. Also, craft-eyelets I've handled are rarely actualy made of copper, brass, etc., and are more commonly steel plated with something to look brass or copper, occasionally are actually plated with it).

I'd recommend using ring-terminals and cold-weld-crimping short segments of braid between the rings, then bolting the rings to the terminals.

 

[whereswally606]

the eyelets may not be copper but they are sold as such.

I am going to give it ago, no harm in trying and I have copper lugs as a fall back but wont be anywhere near as neat and tidy if I use them.

I have quite a bit of work to do on it this weekend to bring the spare 10 cells which I haven't yet used inline to the voltage with the 15 I have already in parallel on a slow charge.