First Time Making Battery Pack...

[Bedmountain]

Hi all

After some negative experiences with Chinese made battery packs I've decided to make my own.

I have very limited electrical experience but I've read all I can find about making packs and I think I'm nearly ready to go.

I'm making a 14s8p to run my MP5 using Samsung 25R's

I've drawn out my cell configuration and the location of my nickel strips and BMS wires.

I would be hugely grateful if someone could have a quick sanity check of the layout to make sure I'm not doing anything stupid as once I get started I will be using this diagram like a bible....

Also can someone advise on the right size BMS to use (30A/40A) and can anyone recommend a good quality one that they have used?

Many Thanks

[Bikemad]

Hi Ben,

Your diagram appears to be correct, but there will be a lot of double (and a few triple) joints on the nickel strips where they overlap which will probably need to be spot welded separately in two (or three) stages, or you may end up just welding the links to each other without achieving good penetration/connections onto the cells themselves.
 
I would suggest attaching the more important series links first and the parallel links second.

Presumably you will assemble the pack completely using a hot melt glue gun to hold all the cells together before welding on any of the individual connector strips?

And don't forget to take plenty of photos so that we can see how the assembly process progresses.

Alan
 

[Bedmountain]

Brilliant - thank you Alan

Your diagram appears to be correct, but there will be a lot of double (and a few triple) joints on the nickel strips where they overlap which will probably need to be spot welded separately in two (or three) stages, or you may end up just welding the links to each other without achieving good penetration/connections onto the cells themselves.

yep - will definitely do this.

I would suggest attaching the more important series links first and the parallel links second.

good idea.

Presumably you will assemble the pack completely using a hot melt glue gun to hold all the cells together before welding on any of the individual connector strips?

yep - will glue the cells together then use heat resistant tape over all the connections on each side.

And don't forget to take plenty of photos so that we can see how the assembly process progresses.

Will do - this is going to be a lengthy project though - I'm currently in the process of making a spot welder (http://www.instructables.com/id/DIY-Arduino-Battery-Spot-Welder/) before I can actually start the battery build. I'm determined to make this pack with as few items coming direct from China as possible...

Do you have a view on the BMS size? MP5 is set to max continuous 25A I believe so I guess 30A BMS is ok? I read somewhere that it is better to overspec if possible so I have been looking at 40A versions. I don't really know what to look for to determine what constitutes a 'good' BMS... Anyone have any experience with this that they can share?...

[Hastings]

I can recommend the Ping batteries BMS . I have used earlier v2,5  and that is now improved http://www.pingbattery.com/blog/whats-new-on-v5-lifepo4-bms-battery-management-system/

I would over dimension the BMS slightly in terms of max load and hat dissipation i.e make sure it is cooled as that is what has . killed some of mine and some of my buddies BMSes

I know of your adversity to Chinese products but there are  huge differences between producers especially in choice of materials and in customer support. Everything breaks and you will need support : Ping Li message to me on these occations has been  : Dont worry we fix it ! and he did.

[Bikemad]

I can recommend the Ping batteries BMS . I have used earlier v2,5  and that is now improved http://www.pingbattery.com/blog/whats-new-on-v5-lifepo4-bms-battery-management-system/

Ben is using Samsung 25R lithium-ion cells for his battery, but the link you posted is for the lower voltage LiFePO₄ cells:

We sell V5 LiFePO4 BMS (Battery Management System) separately, but there's no warranty for BMS only.

Ben, your 8 parallel Samsung 25R cells should theoretically be able to output 160A continuous (which may be more than the nickel connecting stips can withstand), so I would look for a BMS with a 60A continuous output, as this should be more efficient, having less voltage drop (and generating less heat) running at 25A than a 30A BMS would.

Alan
 

[Bedmountain]

Ben, your 8 parallel Samsung 25R cells should theoretically be able to output 160A continuous (which may be more than the nickel connecting stips can withstand), so I would look for a BMS with a 60A continuous output, as this should be more efficient, having less voltage drop (and generating less heat) running at 25A than a 30A BMS would.

Thanks Alan

I've struggled to find any European options for the BMS. There are a handful of 14s 60A versions on Aliexpress...

https://www.aliexpress.com/item/14s-51-8v-60A-bms-li-ion-e-bike-battery-bms-pcm-battery-protection-board-polymer/32748539695.html?spm=2114.01010208.3.18.v1RLZa&ws_ab_test=searchweb0_0,searchweb201602_1_116_10065_117_10068_114_115_10069_113_10084_10083_10017_10080_10082_10081_10060_10061_10062_10056_10055_10054_10059_10078_10079_10073_10070_421_420_10052_10053_10050_10051,searchweb201603_9&btsid=5079906f-3c0f-4b4d-8e5a-0b046088ffd3

Does this one seem viable? The 2.5v minimum individual cell discharge cut off seems low but I guess I can set the LVC in the controller to 42v to achieve a 3v cut off?

[Bikemad]

Hi Ben,

I think that BMS should be up to the task, it will even provide enough power for a dual Pie setup should you decide that you need more power in the future. 

Alan
 

[Bedmountain]

Well - 6 weeks on and I think I've finally made a working version of the tool I need to make the pack...

3rd time lucky (only tested on a dozen welds so far) I've managed to build this: http://www.instructables.com/id/DIY-Arduino-Battery-Spot-Welder

here's mine...





Now I can actually order the cells and start the build - still waiting on the BMS though (ordered 6 weeks ago...)

[Bikemad]

I made it but I'm struggling to get decent welds...

I get a spark and the welding tips get stuck to the nickel (0.15mm) but no weld to the cell is made.

I'm using soldering iron tips as probes so maybe that's the problem although I've seen them used on some of the builds?

Any ideas how I can troubleshoot this? I'm not entirely sure that turning the weld pulse time up is actually making any difference so maybe that's not working right?

Ben, was the lack of weld penetration due to a low battery voltage, low current output, or a problem with the programming etc.?

Alan
 

[Bedmountain]

Ben, was the lack of weld penetration due to a low battery voltage, low current output, or a problem with the programming etc.?

Hi Alan - those comments were based on the 2nd time I built this. That build did 7 or 8 weak welds then the probes became 'live' without pressing the foot switch - did one very strong pulse that pretty much destroyed the nickel strip - some smoke came from the pcb then the Arduino went off and never came back on...

I'm afraid I don't have the skills to troubleshoot this when it goes wrong and there doesn't seem to be a lot of help out there, for this build, for someone of my level of expertise
For this 3rd build I used thicker probe wires (4 gauge) and 5mm diameter solid copper probes but it's just as likely the previous failures were down to dodgy soldering or some other error in the build process.

I'm using a 45aH car battery for the juice which was recommended by the author of the instructable.

I'm going to test this build for at least 50-100 welds before I shell out for the new cells as I'm not sure I can go through the process of building it again and there doesn't seem to be any Chinese options available that will run off a UK domestic mains supply.

What's everyone else using to build packs in the UK?...
Is there a welder out there that I haven't come across?

[Bikemad]

Is there a welder out there that I haven't come across?

There are plenty of spot welders available from AliExpress and on eBay, many of which will run off 240V like this one:



Unfortunately, these are likely to cost a lot more than your DIY spot welder.

Alan
 

[Bedmountain]

Yeah - I've looked at these...

The ones with the fixed probes wouldn't reach into the middle rows of a triangle pack.

The only one I can find with hand held probes requires an RCB above 60A. I've read quite a few reviews from people that have been unable to use it with a domestic supply:



As for being more expensive... That would depend on how much one has spent on replacement bits and car batteries and the like ...
I don't think I'd be too far off the price of a Chinese welder if i'd kept a count.

[Bedmountain]

OK - so I finally managed to scrape up the funds to by the cells - so it's time to get on with this build...

112 Samsung 25R's purchased from:
https://eu.nkon.nl/rechargeable/18650-size/samsung-18650-inr18650-25r.html
Took 4 days to arrive in the UK and of course no nasty custom charge surprises.

I ran a voltmeter over each cell to check initial voltage and all cells measured 3.51V. Happy days...




Pure nickel strip 8mm x 0.15mm purchased on Ebay:
http://www.ebay.co.uk/itm/281919785894?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Only a week or so to arrive from China which is impressive.
I took a piece, gave it a good scratching and soaked in salt water for 24 hours. Showed no sign of rusting so I'm happy to say it's not nickel plated steel...

An hour or so with the hot glue gun (and 18 sticks of glue later) I have my triangularish pack. I designed it to fit inside a plywood case that I made for my last battery as I'm too lazy to make another one.




Now for the nickel strip. Welding with my homemade spot welder:
http://www.instructables.com/id/DIY-Arduino-Battery-Spot-Welder/
I just had a quick look at this and I can see the build instructions have been improved considerably since I struggled through it...




The welding seems to be going pretty well and the spot welder seems to be holding up ok. Just another 3,000 or so more welds and I'll be done...

[Bedmountain]

Welding now complete (with only a few minor 'incidents'...). I decided to double up the nickel strips for all the series connections.



A bit of foam round the edge and the BMS taped in position and all sense wires taped on to mark lengths:



I decided to pre-solder my sense wires to a short nickel strip then weld the strip in place to avoid any heat on the cells



Sense and positive/negative wires attached and all the nickel taped over with kapton tape



a bit more foam...



a bit of dodgy heatshrinking



and it's ready to go back on the bike:



Massive thanks to Bikemad (Alan) for all his assistance. I wouldn't have got very far without his help - in fact I wouldn't have even attempted the project if it wasn't for him...