BMS integrated in the charging brick - nice or stupid idea?

So, my project is a little tight in space. Just like Vanarian I'm building electric rollerblades, but thought it might be useful for everyone as it would save money for everyone IF it works.

Since VESC has discharge protection, we don't need the BMS for riding, only for charging.

My idea for charging and balancing the cells is to modify a 36V power brick with a BMS and instead of the normal 2 wire cable would use a 9 wire JST plug for 8S or a 11wire for 10S

This would save a lot of money since you can use a 10A BMS instead of a 100A BMS and save a lot of space inside the board/blade.

So, am I missing something here? why wouldn't it work? Or is it just not worth it?

Sounds like you want to use an RC Charger--those already do what you want.

amberwolf said:

Sounds like you want to use an RC Charger--those already do what you want.

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Not really, RC chargers are programmable. You have to input how many cells, how many amps, balancing or quick charging etc. You have to carry the brick AND the charger.

What I'm asking is what if you get a true plug and play solution for your specific project. Just plug in the wall and into your board and wait for the green light.

or maybe is there a RC charger ready for 8S that will charge always in balancing mode for 1.5A for each cell with brick and charger in one unit with no LED display, no beeps and low cost.

I've seem quite a few people here in ES asking how to use a BMS and that they do not want to mess around with RC charges and want a more convenient solution. What's the problem with making it easier to use?

Sorry I couldn't help.

Ah, I see.

As long as the BMS you want to use is ok with being connected and disconnected repeatedly, and the connector you use is designed so that it connects things in the right order (if the BMS requires say, main power first, then the balance wires, or some other particular order of connection), you could set this up however is convenient for you.


I think this has actually been done before, here on ES, several years ago, but I can't remember by who. Pretty sure it was for an RC LiPo type pack on a bike, though. Not sure what search terms to use to find it htough.



Personally I like having the display that an RC charger has, though, with the ability to quickly check cell voltages during any stage of charging, and the wattmeter/etc information they can show. The former is the one thing I really wish the Cycle Satiator could do that it can't.


If an RC charger could be set like the Satiator can, for a specific profile, and just plug in and go for the default profile, that'd be nice.

At least some of them do come up with the last profile used, and can just be started with a couple of button pushes (I can't remmeber exactly how many, but I think it's just two on my Accucell6 to use it the same way it was last used).

amberwolf said:

Ah, I see.

As long as the BMS you want to use is ok with being connected and disconnected repeatedly, and the connector you use is designed so that it connects things in the right order (if the BMS requires say, main power first, then the balance wires, or some other particular order of connection), you could set this up however is convenient for you.

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Didn't know BMS would require to be plugged in a specific order, I mean, they are designed to be soldered to the battery.

even then, It might be worth lets say to put a 9-pin JST and a 2 wire coaxial plug that already comes with the bricks usually.

Hi! I thing its a good idea, because thats what I am thinking about for the last few days. I have a brick and Imax charger, but I want to have a brick-only charging solution that I can leave at work.
Instead of JST connector I have what is called 8pin micraphone connector:



8 pins for 6s battery is enough, even for 8s it would be enough if you use case as ground.
Charging is done through balance leads only and at 2.5 amps - no problem so far...

Hi back you could also look for a PSU?

Vanarian said:

Hi back you could also look for a PSU?

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but that doesn't balance anything

Nordle said:

Vanarian said:

Hi back you could also look for a PSU?

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but that doesn't balance anything

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A PSU to plug the BMS on!

If the BMS is not in place when discharging then I really don't see the need for it, To charge safely you will need a balance port of some sorts as the cells fall out of balance with use and age so the rc charger is the best solution in my eyes as its parameters are adjustable and it displays its output, when I turn mine off its always in the last position so I only need to hold start.
My concern would be compacting the PSU to supply decent amps remain compact and have a fan that doesnt piss off the neighbours. The BMS is bairly any more compact but its more expensive, My idea is a 300w 10s rc charger stripped down into a PSU for a all in one device mains plug to battery.

To me the discharging aspect of BMS "on the board" is useless because the ESC does manage cut off protections already. So that's one component not needed to carry of ESC is advanced enough.

Instead letting it on a power supply or charger cable will keep balancing features (including charge / discharge bleed) so cells get reset every time you charge your battery. And this also means no limited amp discharge for the batteries unless limited by ESC. And less weight too. To me that's best of both worlds.

You forget that a BMS does monitor cell level for dis/charge cutoff and not pack level like the controller. So it adds some important protection level.
Monitoring the pack level may be ok if not discharged too low and if the pack is new and never abused too much, but can be essential with weak, old or abused packs that are discharged to it's limits.

izeman said:

You forget that a BMS does monitor cell level for dis/charge cutoff and not pack level like the controller. So it adds some important protection level.
Monitoring the pack level may be ok if not discharged too low and if the pack is new and never abused too much, but can be essential with weak, old or abused packs that are discharged to it's limits.

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What would be a good voltage per cell to monitor IF we don't have the BMS with the battery? I've been using 3.3V to cutoff. So in a 4S battery would cutoff at 13.2V. So far so good, but I don't want to test hundreds of times to see if something will go wrong.

I agree that a BMS inside adds this protection level, but even comercial RC equipment don't use them. Quadcopters, airplanes, cars, etc they don't use BMS. Specially drones where weight and space is at a premium. So, do you guys think it's really necessary? Most DIY stuff here also don't use BMS for discharge protection. We just balance our cells every few quick charges as well. does this impact that much on the battery life?

@Njakts
are you using this already? What's the size of this connector? It looks big from the photos.

are there any other alternatives to a 9-pin JST?
Also,

they are GX-16 connectors, and are 16mm in diameter

Cut off depends on chemistry. There are hundreds of post with different suggestions. But for lipo w/o bms i would not go below 3.6v. Not much energy below and they start drifting very much.
Why rc toys don't use bms is quite simple: as you said: they add weight and normally those batteries are balance charged after each use. If they are broken you will notice that very easily. If they go up in fire that's no big deal but a lost rc toy. If you 20Ah battery starts to burn it will take down your house.

Ryuudan said:

are there any other alternatives to a 9-pin JST?

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Probably hundreds.

A few that have been used here on ES include

DB-xx series (like an old serial or parallel or VGA cable on computers). There are variations on those that also include at least two larger pins that could carry significantn current, for main +/- leads. A dust cap could be made for them that is relatively water resistant on the eskate end, and they're not that much bbigger than a JST (and will last a lot more interconnects; easier to secure the cable and housing so the wire can't be pulled out of hte conttacts).

Canon connectors, whihc are waterproof, though rather large, and these also include options for which pins make contact first, so you can ensure connections happen in hte correct order. (this is done with some short pins and some long). This would be nice for the eskate end, especially since it can be panel mounted.

Anderson makes a series of connectors that are in a single shell but have some large contacts and some small. Or multiple regular PP45s can be mated together in away that a keeyd connector is made, and pinned together so they cant' come apart. There are panel mounts for thse so they can be secured into the eskaate or charger box at that end if desired. But again this would be pretty big.

GX-16 connectors i like