Which BMS should I go for?

[dannydev17o]

Hi everybody,
I've been working on a new 36V battery for my Kugoo S1 kickscooter, the "official" battery is of laughably poor quality so I decided to build a 10,5Ah one out of LG MJ1 cells, only thing I need now is a new BMS since the other was ruined by my bad soldering iron (and skills), but I've never bought one before and I'd like to be 100% sure instead of buying the first 3€ BMS I find on AliExpress, the maximum load I've seen from the display is 12A but I'm not sure if it's important since this setup only allows a common port type (B- and P-).

I'd really like to install a smart one to allow checking the battery through a Bluetooth app and set a maximum of 80% SoC to improve the life-cycle, sadly this scooter is very restrictive space-wise so the maximum size for the BMS is 36x65mm and I don't know if there are any models within that size that offer a diagnostic port, I didn't find anything so far on Ali so I thought some of you can redirect me to the right model and brand.

I'm also planning to add a 20A fuse to the positive wire and install a remote switch for anti-theft purposes, this one I found claims a 6mA consumption but I'd be glad if any of you could share their experiences with these safety-wise.

Please feel free to correct me if I missed something.

 

[amberwolf]

the maximum load I've seen from the display is 12A but I'm not sure if it's important since this setup only allows a common port type (B- and P-).

The type of BMS charge/discharge port isn't relevant to the current required by the device powered by it. You can usually get the same current capability in either type of BMS. I'd recommend one that can handle more than your device requires so it isn't stressed and has characteristics good enough to reduce heating from the current flow. If the max you read is 12A, and the system is reliably measuring that, then a 20A BMS is probably sufficient (most of them allow for short spikes above that if necessary). With that rating, the charging port will probably be high enough current for whatever charging rate you will use, but I'd recommend verifying that in it's specs before buying.

I'd really like to install a smart one to allow checking the battery through a Bluetooth app and set a maximum of 80% SoC to improve the life-cycle, sadly this scooter is very restrictive space-wise so the maximum size for the BMS is 36x65mm and I don't know if there are any models within that size that offer a diagnostic port, I didn't find anything so far on Ali so I thought some of you can redirect me to the right model and brand.

I don't personally know of one that small, but someone else might.

BTW, some of the really small BMSes don't have a balancing function. If your desired unit does not specifically say it has this, it probably doesn't, and you will have to periodically test for balance and rebalance the pack manually if necessary to keep it able to supply as much of it's capacity as possible. (if the cells you use are all perfectly matched in characteristics, it could take a long time before they age enough to become different enough to have balance issues, especially if you are only using a portion of capacity each time before recharging, and not charging to full voltage. But most cells supplied by sellers to DIYers are not tested and matched, so they differ from each other by some amount (not usually much if they're all from the same box(es) of cells from the manufacturer, but it depends on their origin, etc).

I'm also planning to add a 20A fuse to the positive wire

If the max the system ever draws is 12A, 20A should be enough; keep in mind a fuse is there to protect the wiring/etc past the battery, so if a short occurs beyond the fuse the fuse will blow and prevent a fire/etc. It has to be sized to prevent blowing from startup surges, etc., but small enough that it will fail before wiring overheats from overcurrents like shorts.

and install a remote switch for anti-theft purposes, this one I found claims a 6mA consumption but I'd be glad if any of you could share their experiences with these safety-wise.

I don't have experience with those remote relays, but the consumption sounds about right for powering a reciever and MCU.

Thoughts about pack maintenance: If you have a 10.5Ah pack you are charging to only 80% of it's capacity, that's 8.4Ah available. 8400 / 6 = 1400 hours, which is about two months. The BMS also uses some power even in standby, so if you left the system sitting unused with perfectly balanced cells for that long after having fully charged it, the battery would then be completely discharged, and further sitting without recharging could damage the cells from the BMS drain. (some are powered by all cells, some only by the most negative one or few).

If the cells are imbalanced, some will empty sooner than that. If the pack was not full when put away, it will all empty sooner than that.

Since a scooter that size is so easy to pick up and steal anyway, I'd go with just a key of some type. This can be a keyswitch mounted almost anywhere on the scooter, wired to cut off a signal required to operate it (throttle, power to a display or the controller MCU, etc). Or it can be a small connector wired in the same way, with multiple pins and the jumper only between two of them in the matching plug that you can remove and keep with you. Or even a small microswitch inside the unit behind a slot that you slide a card into to activate (like a treadmill, etc).

It could also give you more room for the BMS you'd like to install, if the space the relay would have gone in is in the same volume the BMS would go in.

 

[dannydev17o]

@amberwolf
Hey, sorry for the late reply.
Thank you so much for your exhaustive info, do you also know of any way (Arduino?) to eventually link my smart TP-Link Tapo socket to my battery's SoC so that once it reaches the fitting voltage for an 80% capacity (judging from here it's 39.6V) it automatically shuts down the charger? Could it be a good alternative solution to a smart BMS cut-off function?

 

[amberwolf]

I don't know any details, but:

--if the data about your battery's SoC is available in some part of your system,
--(or you build or buy a device that monitors and stores this data)
--and you can program an MCU of whatever type to read that data,
--and your Tapo can be controlled via external commands from that MCU

then you could do whatever you need it to that it is capable of, during charge.

Keep in mind that unless your pack-level SoC monitoring device that provide the data to the MCU *also* monitors per-cell voltages, you can still have imbalanced cells and overcharge them if the imbalance is severe enough (and even if overcharge never happens, you still have undercharging of other cells, and lowered capacity because of the differences in cell capabilities).


These setups don't do anything about the discharging limits, but if you can handle that in the controller /. ESC / etc end, they don't have to.