4.1V 4s2p - bms with adjustable top and bottom cutoff

Batteries, Chargers, and Battery Management Systems.
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4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by purduephotog » Aug 22 2018 4:36pm

I did my searching and found much debate on Max voltage to charge to but not the means to accomplish it safely.

I'm looking to have a 4s pack of EV batteries for a power wheels car. I'd like to not exceed 4.1v and keep the cutoff to around 3.0v

Are there circuits available that I can adjust to those limits by varying the sense resistors or changing a program?

I'd like it if the bms also has control for charging but that isn't critical.

If there's a topic that I missed on searching I'd be most appreciated if you'd point me to it.

Thank you


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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by purduephotog » Aug 23 2018 8:06am

Yup! But they don't seem to provide the lower Max voltage charge.

I swear I saw someone doing this with a power wall, so maybe that's my search next.

I take it out is uncommon to cutoff the charge for bikes looked this?

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by fechter » Aug 23 2018 8:34am

purduephotog wrote:
Aug 23 2018 8:06am
Yup! But they don't seem to provide the lower Max voltage charge.

I swear I saw someone doing this with a power wall, so maybe that's my search next.

I take it out is uncommon to cutoff the charge for bikes looked this?
There are some BMS units that have programmable limits, but they are expensive.

What most people do is simply turn down the charger voltage a little. You could set your charger for 16.4v to get what you want.
The BMS is there to protect the cells in the event the charger voltage is too high or the cells get way out of balance. It's the charger that determines the charge voltage. The downside is the BMS won't really be balancing unless you charge to 4.2v/cell. With only 4s, it's easy to periodically check the balance with a voltmeter. If you see it getting out of balance, then run a charge cycle to the higher voltage.
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by eee291 » Aug 23 2018 9:32am

You can programm these BMS models to stop the charge at any Voltage you like. Or am I not getting something here?

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by fechter » Aug 23 2018 2:40pm

eee291 wrote:
Aug 23 2018 9:32am
You can programm these BMS models to stop the charge at any Voltage you like. Or am I not getting something here?
Not those models. They are fixed. And even if they were programmable you would not want the cells to reach the point where the BMS cuts off on a regular basis. The charger should be set to slightly below that point.
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by eee291 » Aug 23 2018 2:46pm

I don't think it's that big of a deal if you rely on the BMS to stop the Charger. I've been doing it for years with a Bestech BMS.

If I can't rely on the BMS then why even put it on my Battery? Just to generate heat?

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bms with adjustable top and bottom cutoff

Post by billvon » Aug 23 2018 3:08pm

purduephotog wrote:
Aug 22 2018 4:36pm
I'm looking to have a 4s pack of EV batteries for a power wheels car. I'd like to not exceed 4.1v and keep the cutoff to around 3.0v
OK a few misconceptions here.

A BMS cuts off voltage if cell/pack voltages exceed the safe range. It does NOT terminate charge, nor is it set to the same levels. It is there only for protection. For example, a common cutoff voltage for a BMS is 4.3 volts per cell. The charger limits to 4.2 volts per cell, and if any exceed a voltage above that (i.e. an unbalanced battery) then the BMS protects that cell from an overvoltage by cutting off charge completely.

In addition, all a BMS can do is turn a battery on or off. If you set it to 4.1 volts and you terminate charge when you hit that voltage, you are only going to get about a 50% charge - because you've eliminated the entire constant-voltage portion of the charge.

Quick summary - BMS protects battery from damage; charger sets end voltage.
If I can't rely on the BMS then why even put it on my Battery? Just to generate heat?
To protect the battery. The reason the BMS is there is so that a single failure (charger overvoltage, controller short) does not cause a fire. (Some BMSes also have additional functionality, like balancing or charge accumulation.)

This is common in other applications. All sport skydivers in the US, for example, use a reserve parachute as well as a main. Is the main reliable? Definitely; many skydivers go thousands of jumps between malfunctions. But there's still a very good reason for that reserve, because "thousands of jumps" is not the same as "forever."
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by eee291 » Aug 23 2018 3:57pm

Sorry if I sounded like a newb back there, but I know what a BMS is.


At 4.1V the Battery is at like 90%+ not 50%. And that would be perfect for most, since they love the idea of extending the Battery life 3-fold or whatever by not charging fully.




What I meant if you have the Bluetooth BMS that I linked above (3 of which aren't reprogrammable totally overlooked that) :oops:

But this one is:https://www.lithiumbatterypcb.com/produ ... 2-2-2-2-3/

Let's imagine this scenario for a sec.
If I like the idea of only charging to 4.1V (which I don't personally, but others do) and my charger isn't reprogrammable to a lower cut-off Voltage, everyone would just tell me to get another charger or put Diodes in series or whatever.

BUT I just happened to have a smart BMS installed on my Battery pack. So I just reprogram it to cut the charger at 4.1V per cell. Most people would lose their minds over this. They say stuff like I would have to be an idiot to rely on the BMS to stop the charger. To that I say, LOL what's the point of having a BMS then, if I can't rely on it?



I have a small UPS battery, made from used cheap EV cells that gets charged daily by a dumb solar voltage. The (4.2V/Cell) customized Bestech BMS I put on there has protected this Battery from overvoltage for years. I don't even feel a noticeable capacity loss yet, I will do a capacity test sometime in the future.

I find it funny because if I test that my BMS works, why would I not be able to rely on it daily? Are you guys that scared of the BMS just failing all of a sudden?

I have kept the BMS away from all the elements and it is still serving me to this day, of course I am cautious and have Battery located in a fireproof place.

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by amberwolf » Aug 24 2018 3:08am

eee291 wrote:
Aug 23 2018 3:57pm
Are you guys that scared of the BMS just failing all of a sudden?
Well, it's not likely to fail a little bit at a time. Usually what happens is the FETs fail, and when they do they tend to fail shorted.

If they do that, then the BMS can't cut off teh charge.

If the voltage of the charge input is a lot higher than the battery can safely take, and the BMS can't cut it off when the cells reach their max safe voltage, then if the overcharge is sufficiently high, the cells may ignite (regardless of chemistry).

Additionally, if the FETs do fail shorted, they aren't a dead short--they're a "low resistance" short. If the charge current is high enough, the heat generated inside the failed FET(s) can be high enough to start a fire either with the FET plastic or the PCB, or with materials near or enclosing the BMS.


So the problem is not being afraid the BMS will fail...it's that if it *does* fail then there is no second line of protection against an overcharge fire.

The chances of a fire may be small...but the consequences of such a fire can be disastrous, so it's much safer to assume the worst case could happen, and then try to design the system to do what it can to prevent it from being possible to reach that stage.

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by fechter » Aug 24 2018 8:06am

I didn't see the programmable one. If you have one of those, then I don't see a problem with letting it cut the charge early as long as the charger is set at safe (higher) level. The idea is you want to have redundant safety features. If the BMS fails, the charger voltage is still in the safe range. If the charger fails, the BMS will cut it off.

The Chinese BMS units don't have the best quality control in production so always a good idea to thoroughly test the function when it is first installed. If everything tests OK, then it should be reliable for many, many years if not abused.
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by purduephotog » Aug 24 2018 8:31pm

Thanks all.

I'm probably convoluting the smart charger and bms system.

What I want is a smart balance charger with bms to raise the cells to 4.1v. maybe 4.15v.

Looks like that system doesn't exist... So back to the standards and figuring out how to lower stuff.

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by ScottHW » Aug 31 2018 2:30am

I'm interested in understanding this same issue (although for a larger 16s14p battery pack)

Is the BMS responsible to cutoff charging at the desired voltage (e.g. 4.1 V) ?
Or should that be the job of the charger?

Can the BMS serve as a low-voltage cutoff (e.g. 2.9 V)?

More than just safety, avoiding fires, and keeping the cells from being totally destroyed, I want to explore how to keep the cells in a voltage range that can extend life cycles, preserve capacity, recognizing that would mean each individual charge/discharge cycle would have lower stored and usable energy (Wh).

Appreciate the discussion, and will keep watching, reading and learning.

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by flippy » Aug 31 2018 3:45am

the BMS is a PROTECTION. period, end of sentence.

the charge dictates the end voltage the battery tops out at.

if the charger goes too high then what is set on the BMS the BMs will trigger a fail and cut off power to prevent damage to the battery.
it is NOT a charge regulator, that is the job of the charger. the only thing the bms usually does is keeping the pack balanced during charging, nothing more and it only becomes active when something goes wrong.

a BMS should be viewed as the rpm limiter on your car. it prevents damage to the engine but it does not mean you can drive all the time on the limiter to use as a cruise control.
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by kdog » Aug 31 2018 4:26am

And given the amount of dud BMSs I've had/seen recently, I would never solely trust them to cut off charging. Not sure if I can trust them to do anything for that matter! :roll:

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by eMark » Feb 08 2020 2:23pm

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by Schlafmutze » Feb 08 2020 3:36pm

Get an off the shelf 4.2v BMS and just charge to 4.1v, cells do stay balanced prett well and you only really need to charge up to 4.2v to balance them every now and then.

I've got a 16P20S 18650 pack on my ebike that I only charge to 4.0v/cell, I've used it like this since last summer when I last ran it full 4.2v to balance.

So almost a year of every day use charging once a week and my cells have remained within 10mv of each other.

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by obcd » Feb 08 2020 6:29pm

A BMS should not only protect a battery against over charging, but also against over discharging. Finally, it should also protect the battery against over current.
And, there are balancing BMS and non balancing BMS.
IMHO, a bluetooth BMS is worth it's money as it allows to inspect the voltage of every cell group. If a cell in a parallel group starts to fail, that group will discharge faster than the others. It will also get a higher voltage after a period of charging compared to other groups. As a bonus, they usually are adjustable.

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by eMark » Feb 09 2020 10:09am

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by flippy » Feb 09 2020 10:59am

eMark wrote:
Feb 09 2020 10:09am
flippy wrote:
Aug 31 2018 3:45am
the only thing the bms usually does is keeping the pack balanced during charging, nothing more and it only becomes active when something goes wrong.
Is this only true with a more expensive bluetooth BMS ? In other words does a bluetooth BMS actually balance the cells over the complete charging process instead of just at the very end as is the case with all inexpensive BMSs ?
yes, cheap bms don do this. they onyl start balancing when the voltage goes BEYOND 4.2V, it does nothing below that.
ideally you want balancing to start around 4.05~4.1V wich is only possible with a programable bms like the ones listed in the bms thread.
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by eMark » Feb 09 2020 11:19am

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by fechter » Feb 10 2020 2:55pm

eMark wrote:
Feb 09 2020 11:19am

Thanks for your reply. The following question previously asked on another thread being the vast majority of 18650 battery packs use inexpensive/cheap BMSs, but no answer. Again, asked two times on this thread, yet no answer ... WHY ???

Is there any advantage or downside to leaving the charger plugged into a battery for a few more hours (or as long as 8-10 hrs) after green light comes on to allow extra time for a $30-$40 BMS to balance the cells of questionable quality (Chinese) with an aging battery ? What about only once a month for as long as 8-10 hrs with a DIY battery with name brand Grade A cells ?

What about a programmable bluetooth BMS when it comes to any advantage of leaving the charger plugged in a few extra hours after the green charge light comes on at say 4.1v to allow a programmable BMS to further balance the cells?
[/list]
Why? because they are cheaper so it saves money. They also have fewer things to go wrong compared to the bluetooth programmable ones and they work well enough.

When the green light comes on most chargers, it indicates the end of charge has been detected and the output is turned off, so it won't make any difference how long it stays plugged in after that.

If using a "dumb" charger like a Meanwell, then you can get the pack to balance completely by leaving it on for a while.

The real test is to use the pack for a while, then check the balance by measuring all the cell groups. I have some packs that have gone for two years without balancing and are still within a few mV.
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by flippy » Feb 11 2020 2:07am

the green light only indicates that the current has dropped below a certain level, usually a couple hundred mA depending on the power of the charger.

there are hardly any chargers that actually disconnect the output. only "smart" chargers that have data/control lines going to the bms are able to do that properly.
the "dumb smart" chargers you see sometimes (a lot are on ali) have a pulse type test on the output so it continues to pulse power into the battery after charging as it is testing the output.
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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by eMark » Feb 11 2020 7:21am

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Re: 4.1V 4s2p - bms with adjustable top and bottom cutoff

Post by john61ct » Feb 11 2020 1:57pm

There are literally dozens of categories of devices that balance cells, and some categories have hundreds and hundreds of devices, each with their own parameters and behaviours.

You can try to come up with generalizations, but really you need to know

A. what balancing strategy **you want**, and

B. how a given device operates, specifically

so you can judge how it fits those needs.

viewtopic.php?t=104641#p1529141

The definition of the various **protective** functions a BMS might handle varies widely by user, designer and model.

The balancing functions are completely separate, and plenty of "non protective BMS" only do those, completely separate Balancer devices, regardless of a protective BMS being used or not.

And balancing does not need to happen during charging, in fact only top-balancing does, and even then not necessarily.

If you can't tell when balancing is needed, or when it is finished, that is IMO a pretty idiotic design and to be avoided.

I feel the same way about the inability to adjust the start-balance setpoint.

And faster (higher) balance current is always a good thing.

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