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  #11  
Old 07-27-2012, 03:52 PM
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Default Re: Detachable BMS; should it work?

PStechPaul,
cells are cool, no heat at all. We are charging at modest rates, 6 amps for 60ah cells.
We are thinking about upgrading the wires. Right now we use thick wires for + & -, and thin wires for each individual cell conected to the BMS. I don't have the size number here, unfortunately. Lenght is about 2 feet (maybe a bit more) from cell to BMS conection.

I don't have much info on the BMS, we bought it at evassemble. There a few specs there:
http://www.evassemble.com/index.php?...products_id=46

major,
yes, this one or a clone. We bought it from Ali-express.
Connection is very unreliable, we are going to solder a new one on it soon.

skooler,
hi Mike, yep the project is moving fast this summer, thanks.
The Sinopolys are behaving well (granted, we are not pushing them hard yet) and the old motor seems to cope well with the load, but the little Kelly KIM we bought is probably too weak for it.
Well, that's life. As I told you, our budget was small
I should post a video soon, on AEVA.

What you say about the wiring sounds right, so I guess we will have to do some re-wiring there. However, there is still a lot of work to do on the rest of the vehicle so I guess we will check the BMS later. I should have calculated the voltage fall looking at amsp anf the wire sizes before connecting all the mess!
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  #12  
Old 08-11-2013, 09:30 AM
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Default Re: Detachable BMS; should it work?

Hi there; after all those months, the BMS problem has evolved and we finally removed it from the charger. I am posting our observations here in case anybody founds them useful.

As the bad balancing persisted, we tried to experiment adding more wire to the suspicious cells, to see if it was a problem of voltage falling due to thin wires...it did not change results at all. Same unbalancing.

A few month laters, the bad balancing mismatch grew, letting half of the battery empty. It was time to try something new, so we went for bottom balancing.
We removed and discarded the BMS, discharged all cell to 2.75V, and charged until the first one hit 3.5V. It worked ok, all cells were pretty close to 3.4V, so we have built a 3.5V detector for 3 cells and set a relay for charge end.

We have been running the package BMS-less for a few weeks now, and all cells seem good. The pack charges correctly, end of charge is not premature, all cells being close to full charge.
The pack also discharges nicely, cells are always 0.00 to 0.01V from each other.
This pack of Sinopolys has already a lot of work hours on it and is behaving wonderfully without the balancer, so it's pretty clear the BMS was ******* it!

I have read the threads about "bottom balancing yes or no", here and in the endless sphere forums. Arguments from both side are pretty convincing, but in absence of cheap and reliable BMS, I am joining the bottom balance side, it works perfectly!

In defence of the BMS, though, I must say we found a possible cause of troubles. When we installed the BMS and the charger in the same box we observed the voltmeter and ammeter behaved poorly, showing unstable readings. After some weeks both of them died. We were puzzled, and I suspected of PWM. Maybe the meters were not ready for a PWM source...
Recently we discovered the cause: we installed the charger without its aluminium box, and therefore let it's induction field hit the nearby circuits. It seems to be a strong induction field, as it caused havoc with the meters.
The BMS was also close, so maybe it was also affected by this problem.

Last edited by peskanov; 08-11-2013 at 09:33 AM.
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  #13  
Old 08-11-2013, 03:08 PM
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Good to here it all worked out well I have run a couple of those 60ah sinopolys to empty before and they have come back absolutely fine a couple of times now!

Any updated videos of the quad?

Cheers,

Mike
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Mazda RX8, Soliton Jr, 24KWH Sinopoly LiFePO4, Kostov 11alpha

http://www.diyelectriccar.com/forums...iii-61556.html
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  #14  
Old 08-12-2013, 02:25 AM
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Default Re: Detachable BMS; should it work?

Hi Mike,
we are usually very careful about not discharging the battery. In the last year the lowest we went was 3.1V. But it's good to know the cells can survive a full discharge!

Yes, I want to shoot a video; the Quad is finished and has a nice Curtis 1236 now (bought used). I also have access to a cool gimbal camera perfect for that kind of movies.
Eventually I will do it, but now I am too busy working in my car conversion. It seems I got the EV fever and it will not le me go...
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