Balance charger vs BMS for 12S5P Lithium Ion pack

Hey guys, so I’m in the process of of soldering up my 12S5P pack made up of 18650 batteries.

I’ve read that BMSes don’t actually work very well for balancing over time since the resistors they use to create drain are so small. I’ve also read that they frequently burn out and you have no way of knowing without periodically testing your pack to see if it is balanced.

Do any of you guys use a balance charger for a 12S battery pack? If so could you recommend one? Are my BMS concerns unfounded? What BMS do you guys recommend, I’m not worried about the price of the BMS, the battery is one of the most expensive parts of this board as well as the most dangerous and I’d rather protect it well.

Another question I have is, do you think it is necessary to monitor voltage on each of the 12 parallel packs or is it sufficient to just monitor the voltage of the pack as a whole if I’m not draining it close to the cut off voltage.

I will probably only drain it to around 39.2 volts which is 3.3V per pack.

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Your battery seems awesome! That will be BIG battery! :slight_smile: I cannot advise you which BMS to choose. Neither can I tell you if BMS’s is safe. All I can tell is, that using a balance charger is not as nice/easy/cool as charging through the BMS with a laptop style charger. I think that the convenience of BMS’s charging oppurtunities is enough reason to use them. Especially if you are charging a 12s battery. Balance chargers for 12s batteries are expensive! Of course you could charge the battery as 2 6s batteries, but that makes the whole charging process even more inconvenient… I hope that helps

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Yeah that is what I’m noticing looking around. The 12S balance chargers are really expensive. It is a big battery, I was contemplating doing 12S4P but it’s too late now :smile:. My board is big, and I’ve yet to make the enclosure but it should all fit.

Do you have any advice on a decent laptop style charger for a standard 12S BMS? I’ve found one that looks good. It’s rated up to 80A continuous and my battery is going to be 75A continuous max.

I can only recommend ‘‘meanwell’’ chargers. Though You should get one with adjustabale voltage. Im not sure was it as easy to get it for 12s but you might still look in the forum…

Which rating of charger you took? 80A might be 80W… which would be quite weak for your battery setup! I would advise to get about 150-200w charger… You probably will want to charge at around 5A, so ~50V x 5A = 250W

How much capacity will your pack have? I would advise that you can get a charger that is able to charge at half amps of your battery’s capacity… this way you wont need to wait more than a couple of hours till it finishes charging… More like 2h for 0.5c charge rate…

So if you got 12.5Ah pack (2.5ah x 5), you might want to get about 6A capable charger. This way, if pack is really depleted it should be able to fully charge it in about 2 hours… If you get a charger which is capable of only 2A, then you might need even 6hours for fully depleted pack. 4A might be your best shot! (~3h charge time)

Though you can dismiss my advice as Im interested in being able to charge batteries fast… It might also not be that good, since you probably will need a bms which is capable of such high charge rate and not always you might want to charge the batteries that fast…

There is this ‘‘Raphael Chang’’ bms… if you are willing to shell out more than 100usd for sophisticated / user configurable bms, it might be the best on you can get… though it is still experimental and the first beta units will be only sent out in the coming weeks…

You can check it but it might be a ‘‘heavy read’’ if you are new to battery stuff.

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The capacity will be 15AH, 3000mAh x 5. Capable of 75A continuous discharge. From what I can see there are BMS capable of 80A discharge 10A charge for around $80-$100USD. The main issue with fast charging that I can think of is the ability of the BMS to apply drain to the parallel packs decreases as the charging amperage increases due to the small resistors. They seem to decrease the available current by something like 5mA-50mA, so if you’re charging at 2000mA that is a bigger difference than charging at 10000mA.

That is why I wanted a charger rather than a BMS. They have much greater capability to vary the charging current (and that’s why they require fans to stay cool, they dissipate the energy off as heat).

I am against meanwell power supplies/chargers. We have gone through 5 at work. Unless you have them on a UPS they will blow in a few months.

We originally bought 2 and they both blew, we got them replaced through warranty and they both blew after a few months, got them replaced again and placed them on battery UPS and surge protectors, they survived for a year or so until one of our interns decided to plug one in straight to the wall and it blew after 15 minutes.

So yeah. Don’t go with meanwell if you can avoid it imo

I’ve heard of using ups units + voltage booster “modules”’ How well does this combination work? @PXSS

I heard this idea from electric bike builders as they.need chargers in 1kw+ range.and I believe some of these’ units were rated at about 1.5kw

@radium in your case I might consider going with dual 20A version of Turnigy reaktors. Wont be as nice as “integrated” bulk charging with bms but charging speed might be really sufficient. Of course u will also need beefy psu for this charger…

350ma balancing current capable Charging 2x6s 15 000mah, at 7.5A or so, should probably yield 2+ hours charge time.

Actually from this it looks like you could go with 10A version but you have to check the specs then… Also consider fan noise might be an issue when charging at higher amps… I cannot actually be in the same room sometimes when my little 80w turnigy charger kick in its noisy fan.

As - wont be able to recommend much else. I would perhaps.look at to drive the.cost.down./ improve other.areas of it. But if u that"s fine

I suggest that you contact They sell a lot of different BMS units. To buy from them you have to contact them. You need to buy at least two units but if you ask politely for a sample you might be able to buy a single. And yes, you dont need the discharge capabilities if you are using the VESC since you can limit the amps there instead. I have not yet got a problem with a laptop style power supply though… You may also be able to buy your power supply through bestech.

Has anyone ever thought of installing a small balance charger in their board to act as a BMS for charging?

That charger is quite small and has drain capability of 200mA per cell which is quite a bit more than what I’ve seen from any of the other lithium BMS I’ve seen around. But if installed on the board it would provide the convenience of laptop style charging. I would need two for my setup and I think I’m just going to electrically split my cells with a loop key that keeps them in series for operation and parallel for charging so I can use 2x 6S chargers for my 12S cell. I will probably use external chargers though.

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I actually considered doing the same awhile ago!

They are quite small and can be fitted onto the board!

Though, for faster charging I would actually consider mounting 2 of these, if the space allows it!

Otherwise you will probably need to leave your board for quite a while, till it finishes charging!

Great, if you are visiting someone and just have to keep it somewhere for more than a couple hours to fully charge!

Some charging time speculation / calculations

At ~2A and 6s, it looks like you would need something like 7+ hours to charge your battery, if it was really empty…

So, in an hour you would get approx 2 Ah, so the minimum would probably be to charge about 2-3 hours before the next ride…

Half that with two chargers :slight_smile:

If you’re going to buy two of those at $15/ea plus shipping, just get a 12S BMS. It won’t be much more expensive.

Then you can skip the series/parallel loop key

jmasta the problem with BMS is that they’re shit and you have no way to know when they’re working or not without checking your cells are in balance. The tiny resistors they use on them only apply like 5mA-50mA of drain tops. That’s almost nothing when you’re charging at say 5,000mA a pack.

With 5P in each pack that’s only 1A per battery. So let’s say the BMS is capable of applying drain at 50mA(which I doubt it is), that’s 1% relative to the charging amperage. If your batteries are unbalanced by more than 1%, the BMS is going to require more than 1 cycle to re balance them. Heaven forbid they’re more like 10% out of balance that’s 10 charging cycles to re balance your pack.

In the meantime, assuming your BMS is working it will prevent the depleted pack from being excessively drained by shutting your battery down when that pack reaches 0%, but you’ll still have capacity sitting in the rest of the pack.

A lot of people one this forum seem to be wiring their BMS so that it is active only during charging and not discharging though. In that case the BMS wouldn’t even prevent that one pack from being discharged excessively.

That’s why I’m going to use a cell logger as well as balance chargers instead of a BMS.

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Cool. Just trying to help out

I have cell voltage monitors built into my deck, so that is not a problem for me. One flip of a DPST switch and I can quickly check to see if they are balanced

@jmasta Wher did you get that cell voltage monitor?, I’m looking for one.

What balancer charger are you using?

I’m using a Turnigy reaktor 2x 300w charger. It’s 2 6S capable chargers off 1 power supply. I wound up breaking the pack into 2x 6S packs wired in series when the board is running. They can charge simultaneously with this charger which is nice.

I don’t mind taking my enclosure off to charge, especially since the battery pack was the most expensive single component of the board. This way I know the pack is healthy and being charged correctly.

If you don’t want to do 2x 6S packs or similar for anything above 6S, the only other reasonable option is to use a 12S BMS and laptop charger. 12S chargers do exist but they’re insanely expensive.

I went with the reaktor 2x 300w solution because although it’s slightly more expensive up front, I can use it to charge all my boards(building a second right now) and batteries. And since I had to buy at least 2 12S BMS from the source I found and they were almost $100 each after shipping it wasn’t really that much more expensive.

The power supply I’m using to power the reaktor is just an old 1200W desktop power supply that I tuned to be appropriate for the reaktor. I just happened to have it laying around.

If anyone happens to be reading this, I seriously over specced my battery. I built a 12S 15Amp Hour pack. I don’t really regret it, but I use maybe 50% charge on my longer rides. Unless you’re commuting and need 25+ mile range I’d recommend a smaller battery :). I’d rather have a 12S 8Ah battery that weighed half as much for most rides and that’s actually what I wind up using a lot, is 3x 4S Zippys. My riding style is still pretty conservative and I find myself coasting a lot.

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I have a 10S4P 18650 Li-Ion battery, several packs are unbalanced even though I use BMS only for charging.

I’m looking for an adjustable charger to set the voltage to 41V and balance the whole pack but I have only found enormous/expensive chargers, I found no ajustable portable chargers at all.

Your BMS might have fried. Do you have a multimeter? If it’s working it should re-balance the pack over time. The resistors on the BMS are pretty small and can only apply a relatively small amount of drain to each pack. Your best bet might be to get another BMS and replace the one that’s on there.

Or alternatively do what I did and break the pack into 2x 5S.

I used a second bms since I thought the one o had before was bad, but it was exactly the same. You need to use a variable charger with adjustable current to be able ti balance the pack.