BMS, how to make your wheel safer

Most unicycles use bicycles’ BMS (Battery Management System) which has a overdischarge cut-off circuit (T1) to prevent the battery from discharging under the LiIon recommanded voltage. It’s a feature usefull for off-the-shelf bicycles’ batteries but for monocycles, it’s
1) unecessary since the mainboard deals quite well with voltage warning
2) utterly, incredibly stupid since a cutoff by the BMS results in a faceplant for the rider. In other words, the wheel’s designer prefers to protect the battery by hurting the user!!! Many many users have been harmed, especially when the wheel is cold (under 10°C), since the batteries’ internal resistance increases and triggers the cut-off more easily.
Of course, not all wheels are affected by such incredibly stupid “feature”. AFAIK, Solowheel, Ninebot, Inmotion, ie reputable brands don’t cut off. Gotway has early versions that cut-off but its recent BMSs don’t, since the cut-off circuit has been removed.
All others do, if not proven otherwise. So they are dangerous since the probability of cut-off will increase with cold weather coming and higher internal resistance due to natural battery aging.

For prospective unicycle buyers, insist to have a safe BMS, ie without the cut-off. With enough pressure from users, the Shenzen genious will end up repairing this horrible blunder.

For those stuck with an unsafe wheel, the BMS cut-off circuit MUST be shunted. When touching the battery pack, if you can feel under the shrinkwrap film the big mosfet transistors, it means the BMS must be shunted.

The idea is to connect the battery’s 0V (B-) directly to the power output (P-), bypassing the mosfets T1 responsible for the stupid cutoff : see the dotted green line in the schematics below. (B-) and (P-) are now standard markings on most BMS boards, so any BMS can be modified by laymen based on this principle.

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You can see immediately the improvement by testing the wheel with strong accelerations, no more cut-off, ever !
Enjoy and have a safe ride.
Below are some examples of BMS shunt. Some remarks :
– no need to disconnect the battery during the operation
– for Airwheels, only dismount the half housing on the side of the battery (the side without the control panel).
– T1 is usually composed of 2 or 3 or even more Mosfets, so it’s distinctive from X1 usually made of just one mosfet.
– On most boards’ layouts, T1 is marked as Q1, X1 is marked as Q2

 

P.S. For more pictures and photos source credit of modders, see my original thread here : http://trottinetteselectriques.heberg-forum.fr/sutra13862_solution-probleme-bms.html#13862

 

TG BMS. The T1 transistors can be felt through the plastic wrap.
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The plastic is scapeled around T1. T1 is made of 3 paralleled MOSFET transistors.
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MOSFET’s drain & source pins are shunted by a solder blob. Warning, avoid shunting X1 Mosfet.
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The plastic is closed by electrician tape. A smaller opening can be made, mine is unecessarily big.
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TG BMS, with CMS Mosfet
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Dolphin D5 BMS. 3 mostfets T1 are shunted. X1 at extreme right is NOT shunted. T1 and X1 are detected by measuring the mosfet’ gate voltage (pin 1) : paralled mosfets have the same gate voltage..
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Airwheel X8 BMS. T1 and X1 are not easily distinguishable. When in doubt, shunt the two middle Mosfets.
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Recent version Gotway BMS : no T1 circuit => no shunt to do. It’s a safe BMS.
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Aliexpress generic battery BMS (Q1=T1 ; Q2=X1)
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Another Airwheel BMS, shunt directly B- to P- instead of shunting the mosfets
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Another generic wheel BMS. Shunt B- to P-

bms_generic.jpg

 

SML160 BMS. Don’t bother with shunting the mosfets. Shunt B- to P- instead.
sml160-bms.jpg

Firewheel BMS. B- shunted to P- by a big wire. Route the wire like in the picture to avoid adding thickness to the battery pack. Note that the B- wire here is red instead of the usual black color code for negative pole, an occurrence of lack of care (other Firewheel BMSs have the right color, ie black) which can be very misleading.
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10 thoughts on “BMS, how to make your wheel safer

    1. hobby16 Post author

      Rockwheels have never been modified.
      If you post good quality pictures of its bms, I can help you shunt it.

  1. Richard

    You make a bold statement with how the protection in the BMS is “stupid”. I think a good look in the mirror is in order. You are essentially claiming that you, as an individual, knows much more than all of the electrical engineers that have designed various BMS systems with this protection in place.

    I think this is a terrible idea. The batteries have this shutdown in them to protect the batteries from over-current situations. This leads to overheating of the cells, shortened lifespan of the pack and worse case irreparable damage of a cell that could then cause a fire when charging.

    With this mod you are making a conscious decision to reduce one risk and create another. I have a better idea…get a battery pack that has a discharge rate rating that you won’t exceed! Sure, it will be more expensive but it will be safer for you, the rider. And you don’t have to do this risky bypass.

    Reply
    1. hobby16 Post author

      ” You are essentially claiming that you, as an individual, knows much more than all of the electrical engineers that have designed various BMS systems with this protection in place. ”

      @Richard,
      I never “claimed” that!
      And not “all electrical engineers” are stupid or make stupid things. Some are bright, Solowheel and Gotway for example do NOT have this bloody protection in their BMS and that’s why their wheels are safe.
      As to your “better idea”, well, no offense but it’s just… an idea. You have no evidence it’s a better idea, right ?

    2. Keith

      Wow, @richard, now that is an arrogant reply and even more ill thought out “solution”.
      Hobby16 explained clearly that these BMS are made for electric bikes etc. where protecting the battery makes great sense. To use these same BMS “off the shelf” is a piece of ill thought out cost cutting by a few manufacturers, who should know better. It is not a well thought out engineering solution being ignored!
      As hobby16 also explained, the wheels control board will (and indeed HAS TO) manage high current and/or low voltage by beeping/ tilting back to stop the rider going any further or faster.

      As for getting a larger battery, yes if you have an addition €300 or more to spend and if you have room to actually fit a battery of at least twice the capacity into the wheel, as that is the only why it’s going to have a high enough discharge rate.

      On the other hand, in the real world, disabling the entirely pointless and unsafe output protection is the safest and most cost effective way as hobby16 has very well explained.

      If you @Richard, had ever had a faceplant on an electric unicycle you would know that hobby16 is spot on with his advice.

  2. Richard

    “As to your “better idea”, well, no offense but it’s just… an idea. You have no evidence it’s a better idea, right ?”

    Sorry, I think you misunderstood what I was saying. A battery pack will have a BMS that limits the discharge rate and voltage to meet the specs of the battery with some margin of safety. If you select a battery pack with a very high rating for constant current then you will not ever hit the BMS shutoff limit and therefore the shunting becomes unnecessary.

    A higher current rating can be achieved by: 1) having better individual cells with higher current carrying capability or 2) by having more cells in parallel or a combination of both. Some EU manufacturers have multiple battery packs with each having cells in parallel which results in essentially a 16S4P setup. Current that a BMS sees will be 1/2 of a EU with an individual pack. And the current the BMS sees will be 1/4 of what a BM sees with a 16S1P cheap EU battery pack.

    I’m not guessing at this being a better solution. It is a fact that it is an effective way to distribute current load. It is also much safer than bypassing the safety features of the BMS.

    I get why this is being done though and it sucks to have a BMS that drops a rider on their face. But this is the fault of the manufacturer for selecting these battery packs to use to keep costs low. People will buy the cheaper EU to save money and then complain about the limitations they have. We should instead focus on the manufacturers and education of people on proper EU/battery selection rather than showing them how to try to get more from less and create a potential safety hazard.

    BTW, I think your knowledge and research of the BMS systems is phenomenal. I would love to pick your brain on if you know of very specific BMS knowledge in regards to the ICs being used for various BMS’s. I am looking at charging my Ninebot at a higher current than the stock charger and need to make sure I don’t exceed any limitations of the BMS or cells.
    http://forum.electricunicycle.org/topic/2247-decrease-charging-time-5a-high-current-charger-mod

    Reply
  3. hobby16 Post author

    “If you select a battery pack with a very high rating for constant current then you will not ever hit the BMS shutoff limit and therefore the shunting becomes unnecessary.”

    @Richard,
    That’s what you think, but it’s untrue !!! I know at least three wheelers with massive and high quality batteries ( 680Wh on Firewheel, 680 Wh on Gotway with the first & faulty BMS, yeah all of them are 16S4P from reputable manufacturer, Panasonic, Samsung, Sony ) who have faceplanted because of the BMS sudden power cut. You are fixated on batteries internal resistance, but it is only one factor among many. Sorry to insist but it’s not a problem of battery choice but of stupid design. A cutoff can happen anytime anywhere, depending on cold temperatures, high acceleration (e.g. when the wheel gets you out of an unexpected road hole, happens to me all the time), low charge or even a faulty cell (imagine a cutoff faceplanting you, to protect… the battery, crazy, right ?).

    Your safety is the name of the game so please, don’t mix 2 different things : for a monowheel’s BMS, it’s ok to 1) have a short-circuit protection (ie overcurrent protection, only triggered when over 50, 60A, which means never, it’s more implemented for regulatory reasons than rational reasons) , but it’s NOT ok, at all, to have 2) an undervoltage protection. The latter is ok on bikes but on monowheels, it’s what should be qualified as utterly, horribly stupid.

    On stupid BMS, both 1) and 2) are implemented ! it’s very easy to disable 2) and keep 1) … if you know the schematics, but you don’t, hence my hack.

    Of course, shunting means trading for the hypothetical, preventable and near-zero risk of a battery fire, burning the house, cats and mother in law, against the horribly high risk of faceplanting and bodily harm. Any rational person knows it but would consider it a good deal. Basic cost/benefit risk assessment.

    Reply
  4. hobby16 Post author

    ” I am looking at charging my Ninebot at a higher current than the stock charger and need to make sure I don’t exceed any limitations of the BMS or cells.”

    No BMS is protected against current overcharge (only against current overdischarge), and I doubt you’ll find any. So you won’t have any problem charging at 5A because the wheel does it all the time when braking, it’s just a little hard on the cells of moderate capacities (< 160Wh) but on 260Wh batteries or beefier, no problem (even if the Ninebot’s Lemo connector is not quite good for high currents imo).
    I do it often on my Firewheel by connecting 2 chargers in parallel to charge at 4A. I have 4 chargers so I could have charged even at 6A or 8A but the internal charge wires (beetwen the charge connector and the bms) are too thin to take the risk.

    Reply
  5. Julio Map

    I understand that this is an old post.
    But, could you please recover the pictures. I can’t see the “generic AliExpress battery” nor some others as Airwheel x8…

    The proposal seems quite interesting to me. As far as I have understood, the BMS are associated to the battery (i.e.: are sold with the battery, and form a pack with the battery). The battery manufacturer, doesn’t know in advance what uses his battery packs will have, and certainly cutting abruptly the power delivery may end up with the driver hitting the floor. Even when the battery will of course have a longer life span. Our unicycles already cut the power in a soft way by lifting the pedals.

    The only issue you will not be protected against is one cell over discharging… The unicycle will not cut, the BMS will not cut, and you will end with a broken battery pack. But I prefer to live 😉

    Reply

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