You really don't NEED a BMS with cheap lipos

ESK8 Electronics
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I keep seeing people getting confused with BMS setups. Building a new board and trying to understand why they need a BMS, which BMS?, will it be for charging and discharge?, or just charging? How do they wire it? etc etc.

You really don’t need one in most cases.

My board charges directly from a £14 lipo charging brick from ebay (2 packs of 6s 15000mah each). You can get them for 8s, 10s, 12s… I use this http://www.ebay.co.uk/itm/21-6V-22-2V-2A-DC-25-2V-Three-stages-Lithium-Battery-Charger-14500-14650-18650-/182209541446?hash=item2a6c88f546:g:0XQAAOSwNKRXjIVP

It means I can install a 3.5mm power jack on my board and simply plug in the charger and walk away. When it’s charged the LED on the charger will go from red to green.

I’ve been using lipos for years and years, well before eelctric skateboards. Unless you have a bad pack, or you regularly run your packs below 3.2 volts per cell they will stay mostly in balance. By mostly, I mean close enough it makes zero difference. If you have a bad pack then no amount of BMS balancing will make it good. If a pack has a lazy cell i.e. all at 4.2v and one at 3.95v , it will never really recover.

Set your ESC to cut off before your batteries get to 3.2v (at rest) and you can’t flatten your lipos. On most ESCS choose the middle to cautious cut off setting. If your pack doesn’t seem to run for long before cut off, measure the voltage at rest, and if it’s ok, reduce the cut off voltage, then retry. This is the key. Over discharging packs kills them. You don’t need a BMS to stop this happening, you need to use the correct low voltage cut off on your ESC.

When you charge packs, they equalise automatically. Once near full they’ll all end up at 4.2v per cell. If you have a duff cell then you can of course run into trouble as it lags way behind, but I’ve not had this happen on probably 40 packs I currenty have in use (heli’s quads, cars, esk8).

With some packs (heli/quad) I “treat” them to a balance charge once or twice a year, but no more. It takes ages, and makes near to no difference. They only real benefit is it will show if you have a duff cell.

For the low cost of lipos it’s really not worth obsessing about balancing.

Sure, if you have spent a lot if time and money making a 18650 slim pack, I’d probably install a BMS too - to protect your investment, and you are using MUCH lower Mah cells than big lipo packs, so you run a higher risk of knackering them all if one starts going down as it will go from full to empty much quicker. Plus it can be a fun project. But I’m seeing lots of people building boards with cheap lipos being told they need a BMS. You just don’t.

As a case in point , I’ve just metered my two 6S Turnigy Multistar (cheap) 15000mah packs (normally connected in parallel but unplugged for measuring voltage). Used all last year, never balanced, always charged as above.

Pack 1: 4.01 4.02 4.00 4.02 3.99 4.01 Pack 2: 3.95 3.95 3.98 4.05 4.04 4.05

Pack 1 is near perfect, Pack 2 is fine, not quite the quality of pack 1 but easily good enough - a max of 0.1v cell difference. Those packs have been sitting unused in my garage since last summer. All the cells will equalise under charge. And because I don’t run my packs under 20% of their capacity, it doesn’t matter that I’ve got a 0.1v cell difference.

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If you want to go this way, fine. But you really shouldn’t be giving this kind of advice especially since there are people on this forum who are just starting out and have little or no experience with Lipo cells. You say its ok to charge without balancing. But what if you have a couple bad cells. So your no balance charger keeps charging until it overcharges and ruins the cells that where good. Or even worse causes a fire. True, you don’t have to use a bms. Using a bms is about extra protection and convenience. However if your not going to use a bms, then you should use a hobby charger and balance your Lipos every charge for maximum safety and performance.

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You’ve ignored what I’ve said. It isn’t bad advice. It’s not “going this way”. It’s why I posted this thread. There is a lot of unnecessary fear over charging.

You can “what if” all day but the reality is that uneven cells balance themselves pretty well . If they are massively out you’ll know when you receive them and test them, from new.

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You’ve been using Lipo cells for years, but have you put hundreds of hours on said lipo packs? Sure to start they will be “perfect” but they will all have varying internal resistances etc. They will charge and discharge differently every so slightly eventually potentially killing cells. There’s a reason hoverboards are banned, which is an extreme example with poor quality cells but its what would happen in the worst case scenario. Discharging AND overcharging is what can damage cells as well as heat. The biggest problem with not using a BMS when charging is that while the charger may see 42V that doesn’t mean each cell is at 4.2V some could be 4.3V which will continuously degrade the cells until issues occur. And they dont automatically equalize when you charge them…

In the end its 40 bucks for a decent BMS, its worth the expense, especially considering it will extend the life of the battery.

Finally I’d more likely not put a BMS on quality 18650’s than lipo packs as a personal note.

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I didn’t ignore what you said. But I feel strongly enough about this subject that I had to disagree with you. Safety needs to be a priority. That means using quality Lipos and balancing them regularly.

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It is dangerous to say you do not need a BMS. Have you seen a LiPo fire? I’ve seen tens, most of them under controlled experiments but some accidental fires at RC airfields. It’s one thing for it to happen on an RC airplane that you fly in a controlled environment, it’s a completely different thing if it happens on a skateboard under your feet in a populated street or as someone in this forum already experienced in their apartment.

Most are likely a cause of stupid mistakes by people. You cannot possibly believe that everyone will do things properly when setting up their skateboards for the first time specially if they have no experience with electronics or rc toys.

A VESC with bad settings could easily be the cause of a LiPo fire. A BMS does not have any settigs to mess up and will save a battery when the VESC would be the cause of a failure.

A wall charger like yours does not balance cells and cells do not balance automatically when charging, that is completely false. If cells are in parallel, they will balance each other, if cells are in series they will not. That is the issue, if you have a bad cell and your wall wart charges to 25.2V for a 6S, then the bad cell will be overcharged while the others stay below 4.2V. As you keep cycling the cell, the problem will get worse and worse, at the low end the bad cell will be over discharged and at the high end the bad cell will overcharged. This unbalance will eventually drag the other cells out of balance and from there it’s only a matter of time before they blow.

A BMS prevents this by balancing your bad cell every time you charge and prevents it from quickly deteriorating. Now, a real BMS monitors cell voltages individually and cuts off the pack when any one cell reaches the low voltage cutoff. I do not think the BMS most people here use (the one from batterysupports) does this. It only monitors pack low voltage so in this case your cell will still deteriorate but much slower than if it wasn’t being protected at the high end.

Balance charging literally takes an extra 15minutes on a good charger with fast balance settings.

I agree that a BMS is not required if you use quality packs (which if you buy the cheapest from hobbyking you are not), set up your vesc properly and nothing goes wrong.

Do you know how FETs usually fail? (FETs are the switches on your VESC) They usually fail closed which means a direct external short to your batteries. A BMS could save your battery in this case.

And it’s not only about defective electronics. What happens when you hit a curb wrong or a rock or go through a puddle and your wires or ESC or anything down the line of your battery shorts? A BMS could prevent a fire.

There are plenty more reasons to use a BMS if you do not have a balance charger or monitor your cells. Do you need one? No. Will you regret not having one when your VESC shits on you or when you get a defective pack or when you get a charger that tends to overcharge your pack constantly? Absolutely.

So yeah, everyone here should advice others to get a BMS. Your post is like saying you dont really need a helmet to ride, all you need to do is not fall on your head ever.

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I have never used a BMS. just careful battery voltage monitoring. you don’t need one.

however, if you are not an experienced lipo user a BMS is a bit of “idiot proofing” so to speak.

TLDR: BMS not needed but recommended.

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oh, and one more thing. a lot of people around here seem to thing that a BMS is full of magical unicorns that will save their battery from everything. its not. BMS can and do fail.

everything you have said here can just as easily kill your BMS. they are still made of FETs and resistors after all…

the bottom line here is that lipos need to be looked after… BMS or not.

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Yes, BMSs can also fail, but what are the odds of your VESC failing and your BMS failing?

As one user here experienced, he burned his VESC and his vedder switch saved him. Then for some unknown reason he decided to turn the board back on and his switch failed which led to a battery fire. A BMS, even a cheap one would add an extra layer of protection.

Properly designed BMSs do not suffer these issues as they are set to cut off current well below their rated capacity. I’ve tested this many times at work :slight_smile: (My job is fun)

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I wish to have simple and easier way to charge my lipo, if without using BMS, can it be using laptop style charger charge directly to Lipo?

I am BMS is for charging purpose. Make it more convinient and simple.
I have less knowledge or experiance about lipo or li ion battery, and there is tons of thread in here talk about BMS. so i just follow the commend way of here.

Beside using BMS, any other way ?

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Besides a bms, a good hobby balance charger is actually better for charging than a bms. As mentioned already the short protection that a bms has is a huge plus. I had a short once at the charge port on my board while the bms was on and it instantly shut the circuit down so nothing was damaged and the batteries didn’t even get warm or loose any noticeable voltage.

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I have a cheap balance charger using when i haven install BMS. It take sometime to charge the battery and feel abit troublesome to charge. Last two day i just installl my BMS. Everything find.

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I’ve had 2 external E-switches fail on me both times it was stuck in on position. The first one failed due to heat. the 2nd failed in the first 5 min of operation. The built in E-switch on my Bestech BMS is still working perfectly after months of use. Funny part is the E-switches cost almost as much as the BMS!

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I totally agree with you on the voltage monitoring. I have my voltage meter showing through the top of my deck and keep a close eye on pack voltage even with a BMS.

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I mean, you guys would chew up anyone who showed up here and posted a thread about “You really don’t NEED a helmet with a cheap longboard” We have a sticky thread about it.

In that same manner, you would get chewed up at an rc airfield if you showed up with a laptop charger and no charging protection to charge Lipos, you most likely would get kicked out! I don’t know why you guys insist in risking yourselves and others like that. If you want to do it, fine. You have been warned but don’t tell other people to make unsafe decisions based on you not having accidents.

You dont need a seatbelt or airbags to drive your car, you dont need a helmet or a bms or a fuse to ride an electric skateboard/bike, you dont need a fire extinguisher in your home. You don’t need a pilot since we have autopilots that can take off, fly and land automatically. You dont need a reserve parachute to skydive. I can think of sooo many things that are similarly important, lives are at stake. Take your chances with your own, not someone elses.

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Amen to that!

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A hobby balance charger is nothing more than an external BMS, it’s almost the exact same circuit as the BMS’s that people put on their boards.

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A BMS can’t balance cells that are drastically out of balance. That’s why cells need to be equally charged before they are connected to a BMS. This is usually not an issue if using new cells or new Lipo packs because they come pre balanced within 1 tenth of a volt. But if your building a pack of used or mixed cells, or replacing cells or a pack on a used build then you will need a hobby charger to balance them prior to assembly. Maybe they both balance the same, I can’t say for sure but with a hobby charger you can isolate cells or packs for individual balancing during maintenance. So really, it’s good to have a hobby charger on hand even if you use a BMS.

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Ish. Hobby balance chargers usually have the ability to discharge up to 1A on balance leads. The way they balance in principle is the same, all cells get charged but power is dissipated through a resistor on the cells that are fully charged. A BMS usually doesn’t have this kind of current capacity (its more like in the order of tens of milliamps) in their balance circuit and that is why they recommend pre-balancing when you assemble a new pack. Otherwise, you would take an absurd amount of time balancing.

Now, real good and expensive BMS have gates on each balance connection and can charge/discharge through them. Making them a lot faster and efficient since now you only charge the cells that need charging instead of the whole pack.

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@RogerD please explain to me how uneven cells “auto” balance with your brick laptop charger.