Negative BMS switch

Hi guys, this is my first custom battery build with a bms which has its pros and cons. One of the con is a cheap bms like I am using can be quite unpredictable and sometimes dangerous. Therefore, I would like to cut power to the bms when I dont need it to do any balancing or when my battery is in storage. My initial thought was to just have a switch inline with the negative wire which in my mind I thought would cut power. Would this actually cut power or would the other wires still be interacting with the bms since they have their own negative when connected in certain ways?
If you have any ideas please share, wiring isn’t that hard so I will definitely be open to re-wiring if I need to.

Yes, this is a 6s battery. I am very light

Also no, I’m not using the bms to do any output current. It is wired as a charge only bms.

I saw that thread which is why I had the idea of just cutting power to the bms when I dont need it.

Honestly it’s just much safer to forego the BMS, get a good charger, and balance charge every time. I currently have a 5 Kw power system in my van and use a small balancer every now and again to balance the cells. Mind you I Keep the capacity locked at 20%-80%

Sure it’s slightly more hassle to charge but it’s literally one more plug per battery. Plus less things going wrong during a ride.

If you really must have a BMS please don’t skimp out on quality.

2 Likes

I understand its safer but I basically want to just have it wired in like a balancer that I can turn on when I need it. Over 99 percent of the time I want it so it will be off and completely disconnected from the battery. I was just wondering if disconnecting the ground will basically stop the bms from receiving power so that it’s not as dangerous. Also, when I’m riding I will switch the bms off because I’m using my esc for low voltage control and its connected as a charge only bms anyway. I would rather have a bms that I can turn off than a balancer because I will have a small flip up part on my board where I can charge it, turn the board on, or turn on the bms. If I used a balancer I would have to have wires dangling our or I would have to completely open up the board to balence it. So I was just wondering if disconnecting the negative lead would actually do anything?

I figure you can but I don’t think a switch would be the safest option. Maybe just a small connector would be fine. You just don’t want a lot of resistance on the balance wires.

Hi.

Please do your research. Make sure you have studied li-ion batteries extensively before designing and building your own battery.

These batteries are very dangerous. My 14s14p e-foil battery is over 2KW of power. 42 A/hrs. These batteries demand respect.

The picture you posted of a basic BMS is not nearly adequate enough to prevent you from exploding your battery, or burning your house down.

Please correct me if I’m wrong, but the BMS pictured is not a smart BMS. It will not have any capability to stop the charger on your “not smart” charger if one of the cells is malfunctioning, or over 4.2v. That type of basic BMS can only balance by discharging the high cell at a rate of less then 2amps. Meanwhile you are charging the pack at 10-20amps. This is extremely dangerous.

If you don’t want a smart charger (balance charger) and want to charge with a power supply then that charging method needs to charge through a “smart BMS”. The reason for this is as I described above, if something is wrong the BMS must be able to stop the charge. However, don’t be fooled that a smart BMS is safe! It’s not. The vast majority of them do not have any short circuit protection.

There is nothing you can do to prevent a Smart BMS from malfunctioning and causing your battery to spontaneously combust. The best way to avoid that is to buy a very expensive name brand BMS like Lift or Flightboard or Onewheel uses. But, even those regularly fail causing the battery to shut down (better then a fire). I personally know people who have had to send the battery back to Lift and Fliteboard. And my smart BMS failed in two of my Onewheels, my evolve skateboard, and an electric bike.

I have removed all of my BMSs. They are dangerous and a band-aid solution.

In my opinion the only reason BMSs exist is to protect companies selling these products from getting sued by customers hooking up chargers wrong and blowing them selves up!

Just my 2¢. Please be careful! And always charge, transport, and store battery in fireproof enclosure.

:call_me_hand:

1 Like

I have built batteries before and I understand they are dangerous with or without a bms. The only batteries I have built are without a bms and just have a balence lead to charge and use a balence board. Therefore, bms’s are still somewhat new to me. The main reason why I wanted to go with a bms instead of just a balence lead was so I dont have to pull this heavy ass battery out of the board each time and re secure it every ride. If you really think there is absolutely no way to cut power to the bms I will change it to just a balence lead but I would really like to have it all self contained.

I’ve decided just to cut out the bms and stick with the trusty balence lead. I already have basically everything to convert it I just need a 6s charger because I currently only have a 4s smart charger. I’m sure I can also make a pass through for the balence lead.

Hi.

Just use a waterproof plug with a screw cap for the balance leads.

This is what I use. It’s easy to waterproof.



US $5.27 20%OFF | WS16 Series 2 3 4 5 6 7 8 9 pin Aviation Metal Circular Cable Connector LED Power Waterproof Connectors IP67

.Yes Chinese cheap BMS can burn and cause the actual damage but without a BMS there are lots things can go wrong! your motor locks, you packs burn, your charger goes wrong your pack burns, one cell wears out sooner and drops the voltage faster, heats up below 3v and your pack burns. Running a kwh pack without a reliable BMS is a huge risk.

3 Likes

Yes!

All good points. I agree with them all. But, most of these risks have been mitigated in my battery design.

  1. motor locks: My pack is fused with 200a fuse on main battery wires (6awg). This would just blow the fuse.
  2. charger malfunction: This is always a risk. But, the risk is only while you have the charger connected. Use a quality charger and always charge in a fireproof enclosure, and keep an eye on it. A smart BMS on the other hand is always hooked up, and could spontaneously combust at any time.
  3. One cell goes bad: my cells are all individually fused. This type of cell failure will just blow the fuse wire on that individual cell, and the pack will continue to function normally. You will notice the problem next time you charge, as that cell group will take 3ah less then the other groups.

The biggest risk to running a pack without a BMS is human error. If you make a mistake, there is nothing to protect you. For example, if you hook up the charger cable to the battery on the left side, but accidentally hook the balance leads to the pack on the right side the charger might charge the battery untill it explodes!

Never leave your battery unattended unless it is in a fireproof enclosure.

:call_me_hand: