Power tool bms

thefixer06

thefixer06

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Dec 19, 2017
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Does anyone know how any of the power tool bms works? I know when I'm using any of my battery tools once the battery gets so dead it shuts off. From what i can tell it doesn't not appear to disconnect in the battery. Or if it does it is beyond my understanding.

I would like to repurpose some of my dead 12v packs with working cells to make a portable/back up power. Nothing to fancy just enough for some cell phone charging or light when the power is out, but then shut itself down like the power tool once the voltage drops to low.

This theoretically should be possible. I just need to know what if anything I need to add to my contraption to make it work.

Thanks for the read.
 
Low voltage disconnection made by the BMS yes. Common functionality by most BMS systems
 
It depends on the manufacturer how they implement this stuff.

If they do it right, in my opinion, then there is basically nothing built into the battery, apart from a temperature sensor and a state of charge indicator. The tool monitors the voltage of the attached battery and stops working when the voltage is too low. So when you use the batteries for something else there is no BMS functionality whatsoever.
If you want to use these batteries for something else and need an automated low voltage cutoff then you can always add you own BMS.

Some manufacturers add a BMS to the battery, a bit like in laptop batteries. This is what you need in your case, in theory, but it is generally something you don't want as a customer. Makes the batteries more expensive, more difficult to repair (replace dead cells) and brings no benefit for you as a power tool customer.
 
I know in Ryobi, the bms does disconnect at high current draw; besides the normal low voltage cutout. This happens when I push too hard on the circular saw or the miter saw. And it'll do it on a freshly charged battery, too. So I know it's current draw limiting.

@DarkRaven: Ryoby, Dewalt, Makita, Milwakee, etc all have the bms in the battery pack. The only thing it really doesn't handle is the charging.
 
Yes, I know, especially Ryobi and Makita were the prime examples I was thinking of. DeWalt, don't know, and you can't buy Milwaukee stuff here, so I don't know either. Ryobi is especially infamous for selling you cheap tools and expensive batteries and Makitas BMS used to count the charging cycles, not sure if they still do. This BMS turns the battery into a brick after x cycles for no reason and you need a new BMS when you want to replace the cells. There are chinese copies of some of the major power tool BMS.

Personally, I take this as an attempt to scam me as a customer. That's why I buy my tools from Bosch, not because I think their tools are massively superior, other companies make good tools as well, especially Makita and DeWalt, but because they don't try to lock me out of their batteries and even sell them comparatively cheap. I know Hitachi didn't do any fancy stuff either in the past, not sure about today. And there are many other companies that I can't comment on, I hope there are more that don't add the BMS to the battery.

It is just so pointless because the tools survive for such a long time. Doesn't make much sense to build the BMS five times for five batteries when it would have been sufficient to build it once into the tool. It is a massive waste of resources and and money. And its only purpose in the battery is to make you spend even more money. I could rage on about this forever, even thinking about it makes me really angry.

I think the charging is done the same way on all of them, there is no charging function in the BMS anyway and the chargers aren't balance chargers either. They rely on identical cell behaviour and when the cells get old, potentially out of balance and die then the battery is just dead. I think this approach is very reasonable in this case, everything else would be overengineering. Unless you are a customer of one of the not-so-honest-in-my-opinion companies you can always make a battery for your tools that can be balanced charged on any charger, not only the original ones. I know that some people do this sort of thing.
 
Korishan said:
I know in Ryobi, the bms does disconnect at high current draw; besides the normal low voltage cutout. This happens when I push too hard on the circular saw or the miter saw. And it'll do it on a freshly charged battery, too. So I know it's current draw limiting.

@DarkRaven: Ryoby, Dewalt, Makita, Milwakee, etc all have the bms in the battery pack. The only thing it really doesn't handle is the charging.
Click to expand...

If I am understanding you correctly if I am only worried about high current and low voltage the circuitry inside the battery will do what I want without modification. As aposed to the cut off in the tool itself.

I'm not overly worried about balancing as I don't intend the packs to be cycled often. Provided I have cells that won't self discharge bringing the pack out of balance.

I am also starting this side project with the Dewalt 12v system.

DarkRaven said:
It depends on the manufacturer how they implement this stuff.

If they do it right, in my opinion, then there is basically nothing built into the battery, apart from a temperature sensor and a state of charge indicator. The tool monitors the voltage of the attached battery and stops working when the voltage is too low. So when you use the batteries for something else there is no BMS functionality whatsoever.
Click to expand...

That right there is what I am unsure of. I haven't had the stones to tear apart a working tool. I don't think twice about something that is dead already.

Are there inexpensive off the shelf low voltage cut off boards I can add or would that be a simple circuit? I am going to do some research on that now myself should the Google give produce favourablesearch results.
 
DarkRaven said:
Yes, I know, especially Ryobi and Makita were the prime examples I was thinking of. DeWalt, don't know, and you can't buy Milwaukee stuff here, so I don't know either. Ryobi is especially infamous for selling you cheap tools and expensive batteries and Makitas BMS used to count the charging cycles, not sure if they still do. This BMS turns the battery into a brick after x cycles for no reason and you need a new BMS when you want to replace the cells. There are chinese copies of some of the major power tool BMS.

Personally, I take this as an attempt to scam me as a customer. That's why I buy my tools from Bosch, not because I think their tools are massively superior, other companies make good tools as well, especially Makita and DeWalt, but because they don't try to lock me out of their batteries and even sell them comparatively cheap. I know Hitachi didn't do any fancy stuff either in the past, not sure about today. And there are many other companies that I can't comment on, I hope there are more that don't add the BMS to the battery.

It is just so pointless because the tools survive for such a long time. Doesn't make much sense to build the BMS five times for five batteries when it would have been sufficient to build it once into the tool. It is a massive waste of resources and and money. And its only purpose in the battery is to make you spend even more money. I could rage on about this forever, even thinking about it makes me really angry.

I think the charging is done the same way on all of them, there is no charging function in the BMS anyway and the chargers aren't balance chargers either. They rely on identical cell behaviour and when the cells get old, potentially out of balance and die then the battery is just dead. I think this approach is very reasonable in this case, everything else would be overengineering. Unless you are a customer of one of the not-so-honest-in-my-opinion companies you can always make a battery for your tools that can be balanced charged on any charger, not only the original ones. I know that some people do this sort of thing.
Click to expand...

Ryobi is starting to make a huge name for itself here in the US. They are making some darned good tools relatively inexpensive. I have a quite a few tools and my uncle has even more. They've worked for us for years without issue.

The reason the BMS is in the battery, and not the tool, is in the event the BMS fails. You can tell right away that the battery is no longer functioning and just replace the battery. A battery costs about $30-$50 (depending on brand). But if the drill fails, you now have to replace the whole drill and you get new sets of batteries, and you have to spend over $100 for it. At least if the BMS fails in a battery, you can keep working. This is especially great for those who use these tools for working on the job. They don't have to leave the job site to go get another drill if it fails.

The BMS does do a certain level of balancing. Not sure how much so. In the Ryobi packs and Dewalt packs I opened up, there were other wires to the BMS. Not just 2 for the Pos/Neg leads. So, I'm guessing the BMS does do some sort of work other than just overcurrent protection. You don't want a 12V or 18V pack going out of balance and blowing up because of cell went low/high voltage, and/or reversed voltage. The batteries aren't balanced on the charger because there's only 3 connections (Pos, Neg, Temp). Sometimes there's a 4th connector, but it's usually tied to Pos or Neg and is really there to help keep from using that pack in another power tool brand.
 
I agree Ryobi are not bad at all for the price.

Some power tool bms also have balance wires.

I open a LOT of power tool packs. All major brands. One of the best I've seen are metabo. Panasonic look good too.
 
Wattsup said:
I open a LOT of power tool packs. All major brands. One of the best I've seen are metabo. Panasonic look good too.
Click to expand...

Havent heard of either in the power tool department (Panasonic only in electronics)
 
Makita, Dewalt and Bosch here all have built in BMS of what I worked with. They have both voltage and current protection built in that disconnects the battery if needed.
 
Okay, I now have two new questions with a bit more detail this time. On closer inspection of my crafstman C3 battery it appears it does do it all and from what I can tell the power passes through two triacs. (I recovered batteries from another pack to repair this one. Does that count for the cell counter? I want to be up on the board so to speak and my power wall is a bigger slower project.)

image_aqxkup.jpg

However on the Dewalt 12v pack which I want to harvest the BMS from doesn't appear to have anything between the battery and tool battery connection. What I did see is the center 3 or what I thought was three is actually 6 pins. With very little on the board itself.

image_ebqxcc.jpg


image_kyanph.jpg

Could it be that the battery does the controlling, but activates a relay or triac in the tool when connected?

I don't have any dewalt tools, just some dead battries. Looks like a new tool is in my future to take apart to get to the bottom of this.

My second question. The tempture sensors, can I add some in parallel? I know in pizza ovens it works, but it ends up averaging the tempture. Would that be a bad thing if the pack is say 18 cells 3x6 but has some other probes further in the pack? I could be overcomplicating it now too.
 
An endless-sphere member called doctor bass acquires pallets of warranty-return cordless tool batteries, and resells them to the ebike community. Cordless tools use high-amp and high-capacity cells, while power-wall builders most need low cost per amp-hour...since they have plenty of room for the pack, while ebikers need high power in a tiny package.

Most of the packs he sells have one bad cell out of five or ten (18V / 5S-1P/2P being common). Imagine that a user is running their Makita drill/driver constantly, installing drywall, one screw after another. The pack will get hot. Then, he runs it down to the LVC (Low Voltage Cutoff) when hot. Battery chemicals have more resistance when cold, and...the next morning it is very cold. One cell tested fine when it was new in the factory, but...it has slightly more internal resistance than the others. When run hot to LVC and then cooled off? It sags badly when the charger tests it on start-up, and the charger throws a "bad pack" error code. The BMS then prevents the pack from running, even if you jump-start the pack with a dumb charger.

You return the pack on warranty, and get a new pack at half-price. The code reader states how many times it has been charged, and a dozen other bits of data that was recorded (which prevents the user from trading-in a reallyold pack, claiming he bought it just last month).

Companies are concerned about liability, and if one cell gets too hot, it will also throw a "sorry, but it's now disabled from use" code, out of fear it might catch on fire. They sell millions a year, so a few warranty returns are no sweat for them, as far as costs go.

The problem with asking questions about the cordless tool BMS's is that...each manufacturer has their own proprietary hardware, firmware, and software...and some are even potted (like my EGO tools).
 
bosch batteries does not have a BMS built in, so be carefull when using them in custom projects.
The Tool it self has low voltage cut off and I guess also over current protection.
Bosch does also do not balance the cells at all. They solving this by using high quality cells with very low tolerances.
In the battery pack is no electronic, just a small resistor which is used to identify the battery pack.

Most other powertools I saw so far have a typical BMS build it. Makita is really good, they include even 2 temperature sensors in the battery back.

The cheaper ones usually do not have temperature senors and are often not coated, so they don't like vibrations and high humadity
 
I just pulled some 18650's from a few Makita packs, I can send you the BMS's if you want to dissect them. Just pay postage.
 
Roland Domke: Are you sure about Bosch? Those few I have opened have had BMS. One of them a very complicated one too...
 
I think he is sure and I'm absolutely sure as well. I've said it before, no BMS in batteries from Bosch. At least the ones I know, that means 10.8V/12V and 18V green (home) and blue (professional). The tools have built in low voltage and over current protection. The batteries have just a very small PCB with almost nothing on them apart from the main positive and negative terminal and a separate connector for the temperature sensor. The blue batteries also have a state of charge indicator, but that does nothing apart from showing you the state of charge (surprise :)) when pressing the button.

Maybe you had some 36V batteries? I don't know them, maybe they are different. Bosch also makes lots of other stuff, maybe the batteries you had weren't ones for power tools but some other applications.
 
Could have been some other crap then :) I only own one smaller Lithium Based Bosch blue. The green ones i would not even touch and currently most of the gear is Makita stuff :D
 
The green ones are actually not bad at all, the green 18V series offers good tools at a reasonable price for most people. The difference between green and blue is the amount of available tools within the given voltage range. There are much more blue 18V than green 18V tools available so depending on your needs you might choose blue because there is a tool that just doesn't exist in the green series. Bosch is still expanding the green series though, recently a angle grinder and a random orbit sander was added.

The blue batteries are also the ones with the highest capacity, per cell and per battery, the green batteries get upgrades very late (or not at all). There are still no 2p batteries in green available due to weight and handling concerns. I think this is rubbish though, the blue 18V tools aren't really heavier or more difficult to handle.
With the green series Bosch sells tools cheap and the batteries are expensive, blue is the other way round. The tools are more expensive, but still not overpriced like with some other brands, while the batteries are comparatively cheap.

In the 10.8V/12V range this is an entirely different matter. The green ones are almost pointless. In some extreme cases, like the drills, the blue ones are better and cheaper in every way.

Bosch knows about these issues, the blue ones got more and more popular over time with home users and that made everything worse. You can easily buy blue Bosch tools in most DIY stores / hardware stores which was never really planned by Bosch. Some stores even offer a bigger range of blue than green tools.

If you can afford it I will always advice to buy blue 18V and blue 12V. If price is an issue and both 18V and 12V is needed, then I would think about mixing green 18V with blue 12V.
I might be biased a bit though, I don't know about the prices in other countries. Blue Bosch tools are comparatively cheap here due to the sheer number of sales I guess.
 
The blues last so much longer and the feel is so much better. Im never going back to the greens. I own one green machine still from them and i hate it. :p
The blues are 3x the price here but the greens just doesnt last. 1 year at most if you work with them meanwhile the blues just run. :D

But thats my feel about it. Im very biased to having tools that does what i want them to do when it comes to every day work

The green does of course work as well.
 
I have the "cheap" but blue bosch drill with 10.8V (3s1p and 3s2p) :D and the battery has no PCB inside.
I break always this small clips on the sides of the bigger 3s2p batteries :(. seems to be a very bad plastic.

yesterday I recieved my first Impact Driver from Parkside
feels suprisingly solid and well build than I expected :) comes with 5s1p (advertised as 20V 2Ah) Battery + Charger for 50
I've ordered a second bettary fur just 20 ...

The battery does have a BMS inside but not coated.
 
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