Overkill Solar 4S 120a BMS Order OPTION Recomendations?

HammerToe

Solar Enthusiast
Joined
Apr 11, 2021
Messages
35
I am on the verge of ‘pulling the trigger’ and placing an order for one of Overkill Solar’s 4S 120a LiFePo4 BMS’s.

The BMS comes standard with a Bluetooth communication module so I should be able to use the associated android app to communicate with it to both monitor the status of the battery I plan on building up and configure various parameters to my tastes. (If I’m correct, I would have to pay a few bucks extra to upgrade to a Professional version of this software because, though I understand it, the free version only comes with the Monitoring functions enabled, but that’s not a problem).

I also plan on including the ‘optional USB module’ in my order because, as I understand things currently, this option and nothing over and above this option is required if I want to use ‘the’ Windows PC Software available to interface with the BMS. (Please inform me if I’m wrong).

But I have a few questions I would appreciate if someone could answer for me before I finalize and submit my order so that I don’t make some decisions I might regret later. (It’s always nice to listen to people who already been there before you and done things THEY regretted).

Briefly, here are the two general areas I would welcome some advice and suggestions on:

  • Should I go with ‘only’ the 12 inch cable length option or should I ‘pop for’ the 22 inch option? (“Does size matter? Currently I am leaning towards going with the 12 inch soldered on cable option. If I didn’t plan on ALSO going with an option to have Crimped Copper Lugs included as well, I probably would pay the extra cost for the 22 inch cables and just cut them to length BUT I really don’t want to have to install my own Copper lugs because I don’t have any decent crimper tool, let alone have to worry about figuring out what lugs to buy and use. To me, it is worth the extra money to have Overkill install them.

  • Regarding the ‘Crimped Copper Lugs’: Suggestion as to whether I should go with ¼ inch or 5/16 inch or 3/8 inch lugs? I am inclined to go with the 5/16 inch lugs as a decent compromise but would be interested in suggestions from people who have already built their own batteries.
So, with regards to the cable lengths I should go with: I don’t plan on trying to mount/attach the BMS directly to the sides of the cells but I do plan to mount the BMS ‘very close to’/’in parallel with’ – ‘final orientation to be decided’ so in people’s experience, you think I would be happier with 12 inches or 22 inches. (I know what my girlfriend would tell me…, but I would like to hear from you).

With regards to the ‘hole Diameter’ of the lugs I should select:

Should I go with ¼ inch or 5/16 inch or 3/8 inch?

If I recall correctly, the terminals on the cells use a 6 mm screw or post whatever and so even ¼-inch lugs should be sufficient to fit over the post. (6 mm = .236”). BUT:

On the other hand, I was just thinking that there might be ‘trade-offs’/some implications. I can think of at least two POSSIBLE factors: depending upon the overall size – length and width – of the ‘flat part’ listen to me of the lug which makes contact with the cell terminal ‘underneath’ – if that does not also simultaneously increase as the hole size enlarges, then you would be giving up some surface area underneath where the lug ‘meets whatever’ the battery it is connected to – i.e. – listen to me which would result in a little bit of increased resistance. (Not sure if this would happen and/or how significant it might be and do not feel like trying to try to dig up the data and estimated) but anyway, it is a thought that I had.

And second, on the subject, depending upon how much space there might be between the ‘tapered end’ of the ‘socket’ of the lug into which the wires are inserted and crimped, if that might – because of the smaller lug used – e.g. say ¼ inch – MIGHT that pose some ‘interference problems’ adding additional wires with terminals to the post in question? (I plan to use M6 threaded posts (‘setscrews’ I think they’re called) so that might not be a problem because I could just use some washers to create some extra horizontal space for additional lugs but just thought I’d throw it out there is a thought).

So, bottom line:

  • 12 inch (“I’m leaning”) or 22 inch wires ‘soldered on’?
  • 5/16 inch (“I’m leaning”) or a different hole diameter?
Opinions and recommendations solicited.

Thanks, and ‘Cheers’.
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
8,002
Location
Somewhere South of Denver
Some of your questions come down to an answer of, you should be doing your own crimps. If you're doing a full scale install, there will be many wires that need to be crimped. Buying pre-crimped wires means that you know exactly how long each wire needs to be. Until you get all the components in place - and even with them in place - it is very difficult to determine wire length. Too much wire is OK, right? Not really. That extra wire has to go somewhere and it can be challenging to arrange the wire in a way to make that happen.

1/4" on the battery is OK if the terminals are M6. But the other end coming out of the BMS depends on what you're connecting it to. A shunt is likely to be 3/8". A common bus bar could be 5/16".

I found it difficult to work with the three 10 gauge wires when connecting them to the battery terminal. I tried putting lugs on all three wires for the first battery. It's not very neat and may require a taller stud/screw to fit them all. On the second battery I put all three cables into a single 4 gauge (I think) lug. That was much more manageable and it looked better.

If I was doing it again, I would order the BMS with the threaded terminals instead of the soldered wires.
 

HammerToe

Solar Enthusiast
Joined
Apr 11, 2021
Messages
35
I wonder why that is... I've been playing around on the specific web page on their site for their 4S 12V 120a BMS AND, while some configurations (such as if you want one with 24 inch wires instead of 12 inch wires, or want one with M6 threaded terminals, the site does indicate that they are 'BACKORDERED' but right now they seem to both have stock of ( according to their webpage) the 12 inch wire version AND say something about 'they can ship within a few days'. I think in the end, I will probably wind up trying to place an order for the one with the 12 inch wires AND use the $10 discount currently available for the month of June to pay to have some lugs crimped on and throw in the USB module just because it looks like I might actually be able to get my hands on one of these things. They are very hard to come by. I think I probably would prefer one with the M6 terminals but, 'only God knows' when I might be able to get my hands on one of those things. Everything is in a mess right now because of covid - chip shortages and disruption the supply chains and so on. I THINK I may have finally managed to 'close in order' for one of these units from China with a Bluetooth adapter ( the 12 inch 8 gauge wire one) but I couldn't get the USB adapter. Took days and days and days because of communications problems trying to find out exactly what they had and did not have – in the end, and hopefully it works and they can actually ship me something, I told them that I would be willing to take just the BMS with a Bluetooth adapter which they indicated they seem to have stock of and could ship to me. Again, things are in such a mess right now/in short supply that I figured - what the hell? If I wind up with two of these things, well, at least I got a spare lying around and if I really want to unload it, I'm pretty darn sure I can sell (likely the China one) for what I paid for it on the Internet.
 

time2roll

Photon Sorcerer
Joined
Mar 20, 2021
Messages
3,887
I have the OK BMS 12v 120 amp with the pair of 6mm bolts. I really wanted to avoid the 3 to 1 crimp and still ended up with a 2 to 1 crimp. Next battery will have a bus across the terminals for a single wire feed. The fewer crimps the better and these multi crimps are awkward to get the twist and position correct in a tight space.

As far as terminal holes... I find the terminal itself is often the same with just a different size hole. I used some #10 hole lugs and drilled them to 6mm to match exactly to the battery and BMS. You could probably drill to 1/4, 3/8, 5/16 or whatever. This is only 100 amps not 4/0 and 300+ amps. All my stuff runs cool BTW.
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
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Location
Somewhere South of Denver
I have the OK BMS 12v 120 amp with the pair of 6mm bolts. I really wanted to avoid the 3 to 1 crimp and still ended up with a 2 to 1 crimp. Next battery will have a bus across the terminals for a single wire feed. The fewer crimps the better and these multi crimps are awkward to get the twist and position correct in a tight space.

That's a good idea with the bus across the terminals. I'm in complete agreement with you on the multi crimp issues.

As far as terminal holes... I find the terminal itself is often the same with just a different size hole. I used some #10 hole lugs and drilled them to 6mm to match exactly to the battery and BMS. You could probably drill to 1/4, 3/8, 5/16 or whatever. This is only 100 amps not 4/0 and 300+ amps. All my stuff runs cool BTW.

I disagree on drilling out lug holes. It's very easy for the lug to get away from you when drilling. The lug has to be secured down very well to ensure that doesn't happen. Drill before the crimp is done. I used a vise on my drill press to hold the lug. Now, I just buy the right size lugs.
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
8,002
Location
Somewhere South of Denver
Who has good lugs with metric holes? Yes I prefer the right stuff.

One of the few metric holes I had to deal with was on my battery terminal. I think most everything else was inch measurements. The M6 battery terminal is close enough to 1/4" that I didn't notice the difference.
 
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