Mercane Battery pull down (part 1)

Warning: don't do this if you don't have a clue

For some time I've felt that my 500W single motor Mercane Widewheel may not be "fully up to scratch" and the teltale to me was the display battery voltage dropping from 54.4 to 52.2 immediately after unplugging it from the charger. Further evidence was the abrupt change in charge rate (1.8 amps still getting sucked) when some particular (seemingly arbitary) voltage was reached. Note how at about the 10second mark in this video it suddenly drops to 0.00Amps (and the charger light goes green).

This suggested to me a failure of the BMS in some way ... (actually I am still not sure if its a fault or a design failure of the BMS, but I digress).

So to answer the question I decided to pull the battery out and dissect it to examine the internals. Warning, if you do this yourself be fucking careful, cut shallow and cut along the corners carefully. If you need a pry tool (I did) make it NON Conductive (an old spatula came in handy, had to get a kitchen utensil in there somehow right?). In short I found the following:

• the Mercane battery is very well made and well sealed
• the wiring connections are excellent and very well put together and protected
• the critical connections seem well sealed (perhaps not the controller itself)
• again I'm impressed with the incredible webbed stiffening around the main board

And so with the belly pan off you can see that the battery is well sealed (even more than you think here) and you can see that the connectors are actually similar to ones that are used on the Zero 9 (which hang out in the open and so one can assume are pretty water resistant), but in the Mercane they're actually shielded more from water by being inside and under the board (which is solid cast alloy). XT-60 connectors (which use gold plated terminals) are found embedded in that foam.

so removing the battery was as simple as unplugging that and removing the charger port at the front (also XT-60)

To get the battery out I needed to move the controller there too, so I could get my fingers under the battery and rip off the velcro tabs (vibration made them well held I must say).

which is as good a time to remind anyone who has an engineering eye just how bloody strong the Mercane chassis is, look at the size and thickness of the cast alloy chassis

and just how dense is that strengthened section where the stem bolts into!

So next it was to slice into the battery compartment. This is actually (inside out) first covered in sheets of fibreglass then fully shrink wrapped

nicely quality control marked ... 

 then silicone sealed at the joints even into the protrusion holes. Very impressive.

So, finally I get to the battery (or array of cells) pairing back the fiberglass sheath with my spatula (yeah - reddit approved kitchen utensil)

and finally we can see where the BMS is ...

This is where I find that (when I prise it off the sticky stuff) that its actually a well made bit of gear even if its programming may be suspect. It has 6 FETs in it while the one I bought has only 4.

fortunately I bought two (of different types) because (surprise) there is no wiring standard for these fucking things, and the even though the plug is a perfect match if I were to plug that red one in, then it would blow up because +ve is on the opposite side of the plug wire loom direction. These are the two BMS I bought 

and the one on the right will plug right into the existing harness and need a little soldering (and I think its amenable to regen braking too).

Measurements

So now that I've got it open heart and on the slab I can take measurements and I didn't really like what I found:

Battery #	V
1	4.19
2	3.97
3	4.18
4	4.21
5	3.86
6	3.98
7	4.06
8	4.01
9	4.02
10	4.2
11	3.93
12	3.97
13	3.95
Sum	52.53

The pack was clearly unbalanced, and while the mathematical average seems ideal (to a Wanger) the devil is as always in the details.  52V would be an average of 4V per cell, which if they were all at that level it would be "ideal" for those Wangers who want to somehow preserve their battery life till well after their scoot is dead for other reasons. Basically 3.5V is getting pretty close to "don't take it there" and under load its pretty clear that some of these cells will quite likely buckle down to that, especially...

the one just above 3.75V, where the amount it can fall before likely triggering a BMS shutdown for low voltage (which has happened twice) is not far at all (while others would still have plenty of capacity). Further the ones at top charge of very close to 4.2V (one is over), which probably triggered the BMS to shut off the charging, as observed in that video, while there was still clearly some more charging to do on cells.

I anticipate this is why (pretty much probably from new because range has not changed) that my scoot has had less range than I thought it should (see here). I will be interested to see if my scoot has better range after this.

This underscores why you DO NOT want to be regularly "saving your battery" by unplugging (or turning off with a timer) your charger before it gets to some some magical number.

So I don't know if this is a malfunction of the BMS or if the BMS is a simpler one from times gone by when balance charging (as observed on the Mercane Dual motor) here:

Current state

So now I'm doing individual "parallel bundle cell" charging to bring up those which are down with my dedicated LiPo charger. I've given all the lower cells about an hours charge at 3A to bring the pack into balance.

As this is still happening (I've got a few to do still) I'm planning to simply re-balance the pack and will then re-wrap and re-install and see how it goes before deciding if I need to replace the BMS or not.

Side Note

I found that the nomenclature on the battery had INR in the name on it:

this makes me wonder if this is the newer chemistry with Nickel in it?

INR - NMC - Lithium manganese nickel

The reigning champ of the 18650 vaping world. This chemistry adds nickel to the IMR chemistry above, making it a "hybrid" chemistry. It combines the safety and low resistance of manganese and the high energy of nickel.

The resulting battery chemistry gives you a reasonably high capacity and a high discharge current. Importantly for vapers, the chemistry is very stable, meaning that you don't need expensive built-in protective circuits. 

There is extensive innovation within this chemistry as well. Sony, Samsung, and LG are all developing next-gen INR batteries with different ratios of manganese, nickel, and cobalt. 

which is very interesting.

Conclusion

I might give it a few test runs and see how long it takes to get out of balance again.

I'll keep you posted

PS: job done and charging is now behaving properly see my post here.
PPS: no job wasn't done, follow links from there.