Batteries in series - will both BMS protect them from over-discharging?

Batteries, Chargers, and Battery Management Systems.
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orangejuice   1 µW

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Batteries in series - will both BMS protect them from over-discharging?

Post by orangejuice » Jan 11 2020 5:31pm

Hello friends,

I wanted to put in series my e-bike battery 36v 8ah Li-Powith another 16.8v 8ah Li-Po. The smaller battery has the same properties as the original bike battery except the producer is different and the original battery 6 months older. I wasn't sure if the smaller battery might discharge a little earlier because of the slight differences. Both batteries do have a BMS installed, but I've read that some of these might not work together every time.

Is it reasonable to trust that both BMS will protect the batteries against over-discharge? Or is it better to add a volt meter to be sure?

If someone probably has a second to give a hint, I'd be so thankful.

Thanks so so much in advance

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by john61ct » Jan 11 2020 6:34pm

Cannot be determined

if you can even do that

without a lot more detail, perhaps even requires exposing the BMSs, identifying them, seeing how they're wired.

Ideally for such usage, the packs are purposefully designed and BMS selected with that intention in mind.


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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by orangejuice » Jan 11 2020 8:25pm

John thank you very much for your advice,

oh and Please forgive me in advance, I'm really such a noob. I've tried to gather some descriptions, do you think they might help or should I look for other data?

(Oh and I made a mistake I've said my extra battery was 16.8v, but it's 14.8 actually)

My Ancheer e-bike battery is 18650, 36v, 8ah, 3C rating, 24 Amp, with BMS
The Extra battery: 4S4P, 18650, 14.8 v, 8ah, 3C rating, 24 Amp, with BMS, (PCB protection circuit).
BMS of the extra battery: 24 Amps maximum

Controller: "Cutoff voltage 28/38V"


I've thought I should probably just add a voltage meter into the circuit to be on the safe side, but I've read that it might drain the energy?

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by sn0wchyld » Jan 11 2020 10:04pm

unfortnatly will need far more info than that - the issue lies in how the BMS protects the batteries - this is done (99% of the time) with a solid state switch called a MOSFET. These mosfets have a certain voltage rating that when exceeded, cause them to fail (often with a lot of smoke). unless the mosfets in the BMS can handle the extra voltage from being in series with another pack, it'll be a no go. Even if they can, there'll be some risks in that 1 pack may discharge faster than the other, so you'll be limted in how much of each pack you can utilise (as you mention). You also need to check your controller can handle the extra voltage too, if you haven't done that already.

Generally speaking you can safely parallel BMS packs of the same pack cell count/voltage - but seriesing them is generally ill advised unless thats what they're designed to do (some power tool batteries can do this for instance). If your just looking for some extra range, while utilising your 2nd smaller pack - that could be achieved with a boost converter - you wont gain any top speed but you'll gain some more range, if thats what your looking for in your proposal here.

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by amberwolf » Jan 11 2020 10:30pm

orangejuice wrote:
Jan 11 2020 5:31pm
I wanted to put in series my e-bike battery 36v 8ah Li-Powith another 16.8v 8ah Li-Po.
<snip>
I wasn't sure if the smaller battery might discharge a little earlier because of the slight differences. Both batteries do have a BMS installed, but I've read that some of these might not work together every time.

Is it reasonable to trust that both BMS will protect the batteries against over-discharge? Or is it better to add a volt meter to be sure?
it will mostly work fine.

buuut....

there is one problem you must worry about in this specific situation is that when one bms shuts off, the full pack voltage of both bateries in series is placed across it's now-off fets, as it cuts current thru teh whole series setup.

if those fets cannot handle that much voltage, they'll probably fail.

they may fail open, in which case you simply won't be able to turn it back on again.

but they may fail shorted, in which case the bms can no longer keep the output off, or turn it off in the future, so the cells that are already low or empty continue to discharge, and damage to the pack could occur.

as long as your controller has an lvc that is higher than where the bms's normally shutoff at, as a whole, then you probably wont' see this kind of issue, as long as both packs are good,


to find out if your bms fets are likely to have this problem, you have to look up the part numbers on them to see what their vds rating is.

if all fets' vds ratings are higher than the total pack voltage with both batteries in series, then yo'ure unlikley to run into this problem, and it is probably safe to series these two packs.

if any of the fets' vds ratings are lower than that total pack voltage, there is a likelihood of fet failure on the bms that shuts off first, which goes up the lower the ratings are vs the total voltage.


it's likely tath even if the fets fail the cells will still be fine, so if you blow up one bms you can just then replace *both* bmses, instead using a single one of the right type to monitor both seriesed packs as one pack.

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by orangejuice » Jan 11 2020 11:50pm

Thank you all so much for taking the time to answer so detailed john61ct and sn0wchyld and amberwolf, I appreciate it so much. I'll try to find out more on the BMS' mosfets rating, wow I honestly didn't even know they exist. Thank you again:)))

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by goatman » Jan 12 2020 1:34am

orangejuice wrote:
Jan 11 2020 8:25pm
John thank you very much for your advice,

oh and Please forgive me in advance, I'm really such a noob. I've tried to gather some descriptions, do you think they might help or should I look for other data?

(Oh and I made a mistake I've said my extra battery was 16.8v, but it's 14.8 actually)

My Ancheer e-bike battery is 18650, 36v, 8ah, 3C rating, 24 Amp, with BMS
The Extra battery: 4S4P, 18650, 14.8 v, 8ah, 3C rating, 24 Amp, with BMS, (PCB protection circuit).
BMS of the extra battery: 24 Amps maximum

Controller: "Cutoff voltage 28/38V"


I've thought I should probably just add a voltage meter into the circuit to be on the safe side, but I've read that it might drain the energy?
why is your 4s 4p battery only 14.8v? it should be 16.8v.
I don't know if your electrically inclined but you could make your bms's removable and just use them for charging. your controller you could open up and see if you have 63v caps and see what mosfets you have.

ive wrecked a lot of things messing with them but that's what I pay for my education. I would be tempted to join your packs together and just get a 14s bms. you might be able to turn up the voltage on your 36v charger to 58.8v, ive wrecked a few chargers doing that too. or make 2- 7s packs and use a 7s hobby charger instead of a bms

your 18650 cells are pretty crappy cells so its a good pack to learn with but if your not comfortable with electricity or don't have the basic tools like a dmm or soldering iron then don't try. I usually consider the battery a bomb whenever I open it up to work on it, 1 wrong move and it can go boom.

I left a watt meter on a 14s4p pack and after a month it wasn't 58v anymore it was 0 volts, wrecked

cut off voltage 28/38v is weird to me. there should be a LVC 28? but whats the 38v mean? does it mean the 36v battery is only 9s/37.8v so the high cut off is 38v. if that's the case you cant go up in voltage

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by Voltron » Jan 12 2020 1:50pm

Some controllers these days auto select between 36v and 48v.. not saying that's what he has, but the numbers are about right for a 10s or 13s battery.

For the OP, if your goal is a little more speed, I get it if you're on a budget, but just be ready for ruining one or both packs, or the controller, or the motor wires. That's the cycle I went thru anyway... You get into
the of feeling the bike really coming alive with the extra speed, but then you're having so much fun you melt thru one of the strips joining the cells. Then you solder copper wire across it, and shove insulators under all the other strips that are melting into the holders (probably because you took out the annoying fuse that kept blowing). Then you're back on it hard feeling good again... then you blow your controller from running hot and close to the voltage limit of the capacitors. Then you scrounge up another controller... and melt thru your motor wire connectors, and replace the whole run with bigger wires with no connecters to melt... etc.

That's how it went for me anyway, on the way from the first bike to the blue one.. I just wanted a liiiittle more speed! 😂
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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by methods » Jan 12 2020 2:40pm

Agreed that you have to respect the maximum voltage rating down to the component level of the individual modules if wiring them in series.

It is highly probable that it "will work".

It is also highly probable that you could see some unexpected failure. When one or the other BMS "blows open" under load you will see a spike. This spike can degrade (slowly or quickly) the primary mosfets and associated circuitry in the system.

For instance, the 18V pack has to measure voltage. To do this it uses a voltage divider that assumes it will never see over... say... 20V. This voltage divider is likely protected by diodes, TVS diodes, or some sort of snubber. These snubbers are value engineered to the max to be as small as possible. They can sink the maximum power generated by the 18V tool over the life of the 18V tool...

But -
When you blow open a much larger power path the spikes are bigger, longer... a lot more noise to crowbar. You can blow off these protection diodes, which lead to many failure modes. Once your protection circuitry is shot, you may start seeing degradation in your processor chips around the ADC inputs.

Basically - your reliability will start to fall off

So - In general - unless you want to do electrical engineering, testing, etc... most people do not hook together 3rd party "protected packs". Those that do account for it by assuring they treat the system kindly.

An example of treating the system kindly is not blowing open under full load, especially at the module level, but instead throttling the load (by crowbarring the throttle) then opening the loads only under low-load.

... Just think of current flowing thru an inductor creating a spark gap

Dont make a spark gap on 18V parts hooked to 36V parts running in a 60V system.

You can -
But it will not be a reliable system

-methods
Increasing battery voltage and controller current limit will result in a non linear experience

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by john61ct » Jan 12 2020 8:45pm

A better approach might be

sub-packs get no at-that-level BMS while in use ( discharging hooked up in series )

Pack-level high voltage LVC yes.

And maybe a 14S BMS that gets plugged into the balance leads when the two 7S sub-packs are wired in series.

If you think it necessary.

Balancing obviously handled by the charging process separately at 7S, and maybe a 26V HVC as protection in case the charger's regulation fails.


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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by docw009 » Jan 13 2020 2:30am

I occasionally do what the OP has proposed, except I am putting a 3S pack in series with a 10S pack to get 48 volts for my dual voltage ebikes. I consider a pack failure to be a rare event. Never seen one, So I thought it would be unlikely to ever get both pack voltages across an open MOSFET.

Now I am wrong about failure being unlikely. The BMS may trigger because of overcurrent or voltage sag. Then my series combo will expose the BMS deivices to higher voltage. In my case that's 55 volts from both packs. Not a problem for the 10S, but maybe the 3S could blow up. Well if it does, no big loss. The 3S cost me about 20 bucks to make myself, so I will look at the devices if it ever blows up.

The other thing to consider is whether the OP's Ancheer 36V bike controller can take 14S.He needs to at least look at the controller's filter capacitors to see if they are 50V or 63V.

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by orangejuice » Jan 15 2020 1:55am

Hey everyone, thank you again so much for your advice,

I've come to the thought that I may just try it with both BMS and treat the system as kind as possible, as you have suggested. And if one BMS does happen to fail, I'll replace it with one 14S BMS for both batteries. Please wish me luck, that only the BMS will be damaged and not the battery:))

I'll try to charge both batteries separately and have bought a 16.8v battery charger for the small battery today and I'll always try to have an eye on the voltage meter to shut off the motor before the batteries lvc is reached.

Luckily the controllers capacitors seem to be rated at 63v and it's MOSFETS at 68v.

Again I have to thank you all so much again <3

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Re: Batteries in series - will both BMS protect them from over-discharging?

Post by john61ct » Jan 15 2020 8:54am

Try to measure as much as you can and report back here so everyone can learn more

please

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