Is there a max voltage you can feed a Li-Ion pack while charging?

[atarijedi]

Okay, I have a BMS that will cut off power from the power supply when the cells hit a certain programmed voltage (I'll be setting it at 4.1V).

What is the max voltage you can feed a pack? Lets say it's a 22S pack, meaning 92.4V when charged to 4.2V, can you feed it 100V? Does anyone know?

I haven't been able to find any info about power supply voltages when it comes to charging packs, because most pack voltages aren't the same as power supply voltages (12/24/48/60/96/100/etc...).

Obviously, it's best to get as close to pack voltage as possible, just curious if anyone knows a technical fact about this?

 

[999zip999]

Playing with fire. Yes you cab but you can not ever be able to pull the plug in time very often. As the charging curve to capacity to voltage will overwhelm you and fire will happen. Trust the bms for cutoff why ? Just get a series of meanwells or a bms charger if you want to roll the dice.

 

[flippy]

you can set the voltage to 7000 volt if you wanted to. the voltage will simply not rise to that voltage as the charger is in constant current mode and the voltage is basically capped to control the amps to the set limit. but as you reach 4.2V the charger simply does not stop the constant current mode and keeps dumping power into the battery eventually destroying your house.

there is not a single reason whatsoever to set the voltage to anyting higher then 4.2V per cell. charging will not go faster unless you increase the current and murder your expensive ass battery with huge charge currents.


sidenote:

if you rely on the BMS to cut the battery off the charger in order to save your house from burning down you are a dumbass.

 

[billvon]

there is not a single reason whatsoever to set the voltage to anyting higher then 4.2V per cell. charging will not go faster unless you increase the current and murder your expensive ass battery with huge charge currents.

Agreed.

Side note there - some chargers do increase the voltage at the output of the charger beyond 4.2VPC to compensate for I2R losses. However this does NOT increase the voltage at the cells beyond 4.2 volts; it just increases the voltage at the output so that during high rate charging, the charger will put out (for example) 4.3 volts per cell, knowing that the wiring between the charger and the battery will drop an additional .1 volt per cell.

And that isn't something you should do unless you know _exactly_ what you are doing.

 

[amberwolf]

What is the max voltage you can feed a pack?

You could theoretically feed it any voltage up to whatever the electronics (FETs, etc) in the BMS could handle, as long as the BMS is guaranteed to cut off at the correct voltage, and not allow the cells to overcharge.

However, any failure of the BMS to stop charging could cause cells to overcharge, with more risk the more unbalanced the pack happens to be at that time.

What happens after that could be as simple as premature cell aging, or as bad as a huge fire.

I haven't been able to find any info about power supply voltages when it comes to charging packs, because most pack voltages aren't the same as power supply voltages (12/24/48/60/96/100/etc...).

Why not just use a charger for that size and chemistry pack? Then it would be set for the right voltage, and probably somewhat adjustable to fine tune it if you needed to. I don't have any links to one, but I've seen them linked and discussed here on ES, so they are available for your voltage range.

Or why not use an adjustable voltage / adjustable current power supply? (See the many threads about Meanwell HLG / ELG -A version PSUs, for example).

If you just use a typical common power supply that happens to have about the right voltage, it probably will not have the right kind of current limiting, and won't be usable to charge a battery (it'll just shut off when more current is drawn than it can handle, like when the battery is low).

 

[Sunder]

You really need to get the right charger for your pack, not get a "close enough" power supply.

All chargers designed as such are a CCCV design. They limit current (the CC part of the design) until the internal resistance of the pack is the limiting factor, then supply constant voltage (the CV part)

However, cheap power supplies are usually designed as CV only. They use "hiccup" mode instead. In this case, if the current that the battery can take is bigger than the power supply can give, it will stop, or hiccup for a second and then try again. Since the battery demand for current hasn't changed, it will just constantly hiccup.

If you do get a quality CCCV power supply of too high a voltage, then relying on the BMS alone to cut charging is dangerous. Many components fail closed which means if they fail rather than create a safe open circuit, they will just let power through.

If for some reason you can't get a properly designed charger, your second best option is to lower the effective voltage coming out of the charger using diodes. That way, if the diodes fail, you still have the BMS. If the BMS fails, you still have the diodes.

 

[atarijedi]

Playing with fire. Yes you cab but you can not ever be able to pull the plug in time very often. As the charging curve to capacity to voltage will overwhelm you and fire will happen. Trust the bms for cutoff why ? Just get a series of meanwells or a bms charger if you want to roll the dice.

What's the difference between a power supply and a bms, and a bms charger? And why would I be pulling any plugs when the BMS will cut the power for me?

power supply > bms controlled contactor > battery pack + bms

 

[amberwolf]

We explained why in the previous posts, like here
https://endless-sphere.com/forums/viewtopic.php?f=14&t=95009&p=1391883#p1391543
and offered other options.

If you don't accept the explanations, you can get a nice high voltage PSU and try it out, and see what happens when the BMS fails to shut off (if it does, it may show you no sign that it has).

Hopefully the BMS will never fail, but if it does, there's nothing to stop the PSU from (severely) overcharging cells.

 

[atarijedi]

you can set the voltage to 7000 volt if you wanted to. the voltage will simply not rise to that voltage as the charger is in constant current mode and the voltage is basically capped to control the amps to the set limit. but as you reach 4.2V the charger simply does not stop the constant current mode and keeps dumping power into the battery eventually destroying your house.

there is not a single reason whatsoever to set the voltage to anyting higher then 4.2V per cell. charging will not go faster unless you increase the current and murder your expensive ass battery with huge charge currents.


sidenote:

if you rely on the BMS to cut the battery off the charger in order to save your house from burning down you are a dumbass.

The BMS is a part of the "charger". There is no specific charger, there is a power supply connected to the battery pack via a bms controlled contactor on the positive, and a shunt on the negative for current measuring by the BMS, as well as all the individual cell wires going from the pack to the BMS so it can measure individual cell voltages, as well as balance.

So is doing it this way going to end up in burning down the house?

 

[amberwolf]

The BMS is a part of the "charger". There is no specific charger, there is a power supply connected to the battery pack via a bms controlled contactor on the positive, and a shunt on the negative for current measuring by the BMS, as well as all the individual cell wires going from the pack to the BMS so it can measure individual cell voltages, as well as balance

So, do you mean you are using an RC charger or similar?

Those are the only off-the-shelf ones I know of that charge and monitor cell-level voltages.

Unless you have built something that uses BMS that is only connected during charging, and is boxed together with the charger itself.

If you're doing something different, attaching a schematic or other drawing or set of pictures clearly showing the setup of exactly what you already have would be helpful.

 

[amberwolf]

So is doing it this way going to end up in burning down the house?

That depends on how much excess voltage your charger can supply to the pack when the BMS fails to stop it from doing so, and how unbalanced the pack happens to be at that time.

If circumstances conspire to allow a cell or cells to overcharge far enough, there's certainly a chance it could start a fire.

How much of a chance? Depends on the cells, quality, age, and how far overcharged.

I wouldn't risk it, becuase there's zero reason to do it, since adjustable chargers and power supplies abound.

 

[atarijedi]

The BMS is a part of the "charger". There is no specific charger, there is a power supply connected to the battery pack via a bms controlled contactor on the positive, and a shunt on the negative for current measuring by the BMS, as well as all the individual cell wires going from the pack to the BMS so it can measure individual cell voltages, as well as balance

So, do you mean you are using an RC charger or similar?

Those are the only off-the-shelf ones I know of that charge and monitor cell-level voltages.

Unless you have built something that uses BMS that is only connected during charging, and is boxed together with the charger itself.

If you're doing something different, attaching a schematic or other drawing or set of pictures clearly showing the setup of exactly what you already have would be helpful.

There is a quick diagram of how it is setup. The power supply would be connected before the charge contactor for positive, and after the shunt for negative. "Controller" is the motor controller.

I don't have an RC charger, just an intelligent BMS. I can set the overcharge protection voltage to anything between 3.90V and 4.30V +-0.01V, and overcharge release voltage to anything between 3.80V and 4.25V +-0.01V. This is for LiPO and Li-Ion chemistries, for LiFePO4 and LiTO it has different values. It can also balance between the cells when it both, or either, charge mode and discharge mode.

I need to connect a power supply to it, in order to charge. I want to buy a power supply I can use at home, connected to the 220V dryer socket, and that I can also use at Level2 car chargers (I already bought a J1772 to 220V adapter w/ pilot control, etc...).

I am guessing I will need an adjustable power supply, both voltage and current. And adjust them based on the final pack voltage (90.2V) and for current, whatever the total 1C rate (or whatever the max rate is for the batteries) is for the pack.

I know electronics, I just don't know anything about batteries. Hence my question.

Edit: I forgot to add, when in "charge" mode, the discharge contactor is open, and when in "discharge" mode, the charge contactor is open.

 

[atarijedi]

So is doing it this way going to end up in burning down the house?

That depends on how much excess voltage your charger can supply to the pack when the BMS fails to stop it from doing so, and how unbalanced the pack happens to be at that time.

If circumstances conspire to allow a cell or cells to overcharge far enough, there's certainly a chance it could start a fire.

How much of a chance? Depends on the cells, quality, age, and how far overcharged.

I wouldn't risk it, becuase there's zero reason to do it, since adjustable chargers and power supplies abound.

The BMS can balance the cells while charging and discharging. It will also cut power from the power supply to the batteries when in an overcharge condition, or disconnect the batteries from the "load" when in an undercharge condition.

 

[flippy]

The BMS is a part of the "charger". There is no specific charger, there is a power supply connected to the battery pack via a bms controlled contactor on the positive, and a shunt on the negative for current measuring by the BMS, as well as all the individual cell wires going from the pack to the BMS so it can measure individual cell voltages, as well as balance.
So is doing it this way going to end up in burning down the house?

No, and then even more no.

the BMS is NOT a part of the charger, it is a PROTECTION system that does nothing until things go out of spec. that means it will trigger only when damage is already happening.
it is the job of the charger to stay within the spec of the battery you are connecting it to. a charger is NOT a power supply. a power supply only has CV, a charger has CC and CV.

what will happen is that you charger (if set too high) will contine to charge above 4.2V and eventually around 4.25 it will trigger the BMS to cut power. after a few seconds the voltage has dropped and the BMS releases again putting full power of the charger into the battery again wich will trigger the BMS again after a few seconds. this hiccup behaviour will repeat constantly until you have killed your pack.

also: you cant use the battery in this condition. it;s impossibe to drain without bypassing the BMS.

The BMS can balance the cells while charging and discharging. It will also cut power from the power supply to the batteries when in an overcharge condition, or disconnect the batteries from the "load" when in an undercharge condition.

no, there is no BMS on the market that can balance while discharging.


dont be dumb and take the hint of half a dozen people here and do it proper. first learn the difference between a power supply and a charger. aka: CV and CC.

 

[999zip999]

Most BMS balances at the end or top of charge to and during HVC. And it has LVC when first of the goes to low. Yes just one. Do not run your battery too low voltage cutoff as a rule

 

[atarijedi]

No, and then even more no.

the BMS is NOT a part of the charger, it is a PROTECTION system that does nothing until things go out of spec. that means it will trigger only when damage is already happening.
it is the job of the charger to stay within the spec of the battery you are connecting it to. a charger is NOT a power supply. a power supply only has CV, a charger has CC and CV.

what will happen is that you charger (if set too high) will contine to charge above 4.2V and eventually around 4.25 it will trigger the BMS to cut power. after a few seconds the voltage has dropped and the BMS releases again putting full power of the charger into the battery again wich will trigger the BMS again after a few seconds. this hiccup behaviour will repeat constantly until you have killed your pack.

First off, I know the difference between a power supply and a charger, and it isn't just one has CV and one has CV and CC. I have a 2 bench top power supplies, 60V/10A, sitting not 15ft from me. They are both CV and CC. They are not chargers, are they? Of course not.

I have been looking at adjustable power supplies for my build that have both adjustable voltage regulation and adjustable current regulation, aka CV and CC. They are not chargers. The Meanwell PSUs that amberwolf suggested, they have CV and CC, they are not chargers.

That said, the BMS is a part of my charging setup. You can set it up so that it cuts power at 3.9V if you wanted. So I'm not sure what you are on about, since I clearly stated that the OCP would be set at 4.1V

The current tapering, and knowing the battery is fully charged is what makes a charger a charger. It is what sets my BMS apart. I can manually set current limits, but it doesn't happen automatically, so no tapering happens. It will only know that the battery is fully charged when the battery sits between OCP and OCR, but inevitably the battery will drop past OCR. Not sure how long that would take, depending on the settings (say 4.1V and 3.8V).

Giving the battery full current (say 0.5C or 1C) for the entire length of the charge, according to BU, wouldn't kill the battery. It would cause stress, lower the amount of full charge, and diminish the number of cycles though. It's called a full saturation charge. It's what a lot of people seem to do here, when they use just a PSU like a meanwell with adjustable voltage and current.

also: you cant use the battery in this condition. it;s impossibe to drain without bypassing the BMS.

Sorry, I'm not sure what you mean by "in this condition"?

no, there is no BMS on the market that can balance while discharging.

The manual states that this BMS can, but I presume it simply means during discharge "mode", when the charge contactor is open, and the discharge contactor is closed.

dont be dumb and take the hint of half a dozen people here and do it proper.

You mean like using an adjustable voltage and adjustable current power supply? A meanwell PSU? That proper? It's still not a charger.

 

[atarijedi]

Most BMS balances at the end or top of charge to and during HVC. And it has LVC when first of the goes to low. Yes just one. Do not run your battery too low voltage cutoff as a rule

Yes, this BMS says it can balance during charging or discharging, which I assume to mean charge mode or discharge mode, but not specifically while actually charging or discharging.

 

[999zip999]

As said earlier the BMS only balances at top of charge. Probably set to 4.2- 4.30v to bleed down ( active ) and your charger is set barely barely barely above so the BMS can bleed down the first high cell then next high sale that's right to they hit your 20th cell if you run your 20s pack

 

[Sunder]

The current tapering, and knowing the battery is fully charged is what makes a charger a charger. It is what sets my BMS apart. I can manually set current limits, but it doesn't happen automatically, so no tapering happens.

Very few "chargers" current taper. If you find one, it's usually in something REALLY expensive like Tesla, who want to extend the life of their batteries to 8 years, while still having a fast charge rate.

Otherwise, high quality lead acid chargers have current tapers to prevent acid boiling. Lithium chargers are more often than not nothing more than CCCV power supplies with a nicely printed label.

There is a degree of natural current tapering anyway. Let's use your example.

If you had a 22S pack. It's a medium quality pack, let's say 110mohm internal resistance (5mohm per cell)

If you were charging to 4.2v per cell, with a 92.4 volt charger, the absolute max that you could charge at, once all cells reach 4.1a is 20amps.

Once you reach 4.15v per cell, the max you could charge at would be 10 amps.

However, if you had a 100v charger, and cut power at 92.4v, you could pump in nearly 70A all the way until instant cut. This is one reason why you would not want to use an over powered charger and simply cut instantly once the battery is full - Lithium batteries take more damage when being fast charged at empty and full, than when it is in the bulk absorption phase.

If you've ever driven an electric car, try to regen when the battery is "nearly" full - I.e. empty enough to take a charge, but won't take much of it, because doing so at such a high rate would damage the battery.