Use powertool 6s BMS for 5s battery

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bpm5cm

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Hello, I'm fairly new to batteries and BMS and all that, but after watching a few videos and reading a few articles, I thought I'd try my hand at converting dead NiCd batteries to Lithium. I know I need 5 18650 cells to replace an 18V battery and I know I need a BMS to make sure the voltage doesn't drop too low. I bought a LiPo balancing charger that will work sufficiently. Then on my search for a good BMS setup, I found someone discussing the Lowe's Kobalt 24v batteries. Then I found another reference at this website. For $10 I purchased the 1.5 Ah battery and tore it apart. I was wondering (and hoping) if it was possible to repurpose the 6s BMS for a 5s setup? If I could get the cells (plus an extra) and the BMS for $10 ($20 for 2.0 Ah), I'd be pretty happy. It seems like a lot of the 5s BMS cost $2-10 on their own, so this would make this a very useful endeavor for multiple packs.

I tried to simply remove one cell from the circuit to see what happened and the charge indicator lights indicated it was overly drained, a voltmeter and the charger both indicated 20v at the connection pins, but when trying to discharge, it would not go above .3 A, which based on what I've read about some types of BMS, limiting current is one method of "disconnecting." So essentially it didn't work.
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You could try to connect one cell to two ports of the BMS. This method is used by some manufacturers to use the same parts on different products / batteries. This essentially fools the BMS as it creates a "fake" reading for the sixth cell. But being set up for 6S it will still detect a low voltage across all cells very soon so the usable capacity is very small. Not really worth it.
 
You could take a look at the main IC that controls everything Even tho it's wired up to 6s, it might be easily adapted to 5s. On the BQ769x0 chips I'm working with, you can change the number of cells by connecting to balanced legs together. This tells the IC to bypass that reading and adjust accordingly on the overall calculations. Like for the 5s IC (BQ76920), you connect balance lead 3 and 4 together and it tells the IC that this is a 4s setup, regardless of the rest of the electronics that surrounds the IC.
 
Korishan said:
You could take a look at the main IC that controls everything Even tho it's wired up to 6s, it might be easily adapted to 5s. On the BQ769x0 chips I'm working with, you can change the number of cells by connecting to balanced legs together. This tells the IC to bypass that reading and adjust accordingly on the overall calculations. Like for the 5s IC (BQ76920), you connect balance lead 3 and 4 together and it tells the IC that this is a 4s setup, regardless of the rest of the electronics that surrounds the I
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So I tried both doubling up one of the balance connectors and connecting the pins I believe corresponded on the IC, neither of which worked. Both still indicated the battery was undercharged. If anyone cares to look, the IC was labelled 95F012k. I wasn't able to find a data sheet with those exact numbers but another forum suggested it was MB95F564k, which does have an available datasheet.

I also found a couple of different 5s 15A bms for ~$8 so I guess when you're looking at a tool battery, $18 still isn't that bad.

I'm almost positive there is a reason why but if the cells are properly balanced within the battery with a bms, is there a reason why the original nicd charger wouldn't work? $18 To fix a battery seems decent enough but if you have to spend upwards of $50 on another charger it doesn't seem worthwhile unless you have a lot of batteries. I bought s charger because i think I'll have enough use for it. But for someone who just has a drill with one or two dead batteries, it almost seems like more trouble than it's worth.
 
So I'm sure you guys get this all the time but this is killing me. I bought two BMS/Protection circuits from Aliexpress a month or so ago (they were available on amazon just 5x the cost).


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https://www.aliexpress.com/item/5S-...-21V-Cell-Protection-Circuit/32815500275.html


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https://www.aliexpress.com/item/5S-...ack-Protection-Circuit-Board/32870120376.html


They both say "battery charging/charger protection" so I thought one of them would be able to charge the board safely, I was leaning towards the one that has the barrel plug because it seemed logical, but now that I have both, it seems neither of them are capable of charging appropriately. They seem to be pretty much the same with the exception that one has a temp probe and the other balances. They may both balance, I'm not sure, I've not done any testing yet.


So I guess my question(s) are:

1) Is there such a thing as a 5s protection + charging board (when trying to search for 5s charging, mostly iPhone stuff comes up)?

2) If not, is there a dedicated 5s charging board that I can use to make a cheap dedicated charger?

3) I have a barrel plug 20.5 V charger from another drill (at the moment I'm not sure what the current is, I can add that later), I know this may not be able to fully charge, but assuming the current is not too high, could I use this safely?

4) I know that Lithium batteries are supposed to be charged Constant Current/Constant voltage, but I believe I've read somewhere (possibly here) that you could use constant voltage the entire time, assuming the current is under the acceptable range. I have some variable boost converters that are up to 1A. If I input 5v 2A and boosted to 21v it would be <.5A, the cells are 1500 mAh, 15A rated. Could I use this to charge? I know it would take a long time, but if its safe and effective and would cost me nothing, it would be ideal for my current situation.
 
You posted the picture/link to a 5s balancer. How does it not work with the 5 cells? I'm confused on that one. Before you were trying to use a 6s
 
Korishan said:
You posted the picture/link to a 5s balancer. How does it not work with the 5 cells? I'm confused on that one. Before you were trying to use a 6s
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It's not that either of them do not work for the 5 cells, its that I was hoping to have a set up which did not require a hobby charger. I have one, which is fine and these would be great if it was for me. I'm trying to convert dead nicd batteries for my father, who doesn't have any dedicated lithium charger. So I'm trying to figure out the cheapest/easiest way that I can return them to him without telling him he needs to buy a $40 charger, or wait until I visit to recharge them.

Similarly, I also bought a handful of button protection boards for single 18650 and was disappointing to find they are not adequate for charging either. I have several TP4056 charging/protection boards and thought the button BMS served the same function and would make it easier to integrate into small projects, but everywhere I've read says even if it's protected, it still needs the special charger.

So basically I'm having a hard time understanding why/how protection boards prevent overcharging, but you still need a special charger.
 
Ohhhh, ok. gotcha. just get a buck converter that'll match the input voltage needed for the bms charger. The input voltage would be what the NiCD charger outputs
 
1) + 2) How do you define "charging board"? What should it do? Are you looking for a charger fitted to a PCB that in turn can be fitted to a battery so that you supply AC from the mains or DC from a PSU to it and it then does a CCCV charge of the battery?

3) Probably no.

4) No, not unless the converter also has CCCV.
 
DarkRaven said:
1) + 2) How do you define "charging board"?
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Something similar to the TP4056 modules for a single cell, but for a 5S cell. So I could hook up (within reason) a voltage source abovethe 21v cutoffand it would provide the CC/CV, but also protect the cells from overcharge/discharge, etc.

Do you agree with Korishan about the buck converter working with the original Nicd charger? Would that allow for the CC/CV? Would a buck converter end up providing too much current?

This would be much easier to figure out if I actually had the charger, but my father lives a few hours away so I won't have access to it for another week or so and I was hoping to have the packs assembled by then. But I suppose it'd be better to wait and make sure everything is as it should be than to assemble it and then cause problems.
 
I think something like a charging board in this sense doesn't exist. At least I'm not aware of something like this. But maybe you could put alle cells in parallel and use a boost converter to power the tool. Boost converters exist in small form factors and single cell charging boards are widely available, like the TP4056, so maybe that's a solution. But then you might want to add an on/off switch as well.

I don't agree with Korishan on that one but I don't necessarily disagree either. I don't know how the NiCd charger works internally. If it's just a PSU, then yes, it would work. But if there is some intelligence added to it and it's more a charger than just a PSU, then no, it probably won't work. Generally a buck converter with CCCV function would work. There are ones without and they wouldn't work.
 
So I talked to my father yesterday and his charger has an output of 2.8A, no voltage listed(This is for a Dewalt 18v battery btw). So I'm not sure if trying to use that input is the best option or if I was going to use a buck converter anyways, I could just use an old laptop charger or something. I found this video, which seemsto be the most helpful thing I've found, as I've never used one before and this basically gives the exact instructions I'm looking for, but as any youtube video, there are people saying it is wrong and not the correct way etc. So I want to double check before I start buying things.


I am unsure about his method for setting amperage, as I understand it would be different under load, and my multimeter wouldn't even read a current the way he set his up.

If all of this is good, my last question/confirmation would be the settings. I have a 5s battery rated for 1500 mAh, and 15A.

So I know it's recommended 0.5C to 1C, so 750 mA- 1500mA, but I've also read the higher C rated cells can be charged at a higher rate "with little stress" as battery university states. But it doesn't specify the C rating they can handle. And the BMS has a Voltage rating of "19V/21V", but I don't know if that means it can charge with 19-21V or it will cut off somewhere between 19-21V.

My plans are tentatively 20V 1000 mA
 
So update to those following along. I got the battery pack assembled, I used the 20A bms with the barrel plug, I ordered a buck converter for the charger but before I finalized the charger I wanted to make sure the battery worked. If I try to use any of the tools full power it doesnt work, if I start slow it works up to a point. Obviously the BMS overcurrent works, but based on what I've read, power tools seem to be between 10-20A, the cells were continuous rated to 15A and the BMS 20A. Either the dewalt set used more current or the bms cuts out much sooner. My multimeter is rated to 10A so I'm not sure how to go about testing it. Suggestions? I've seen other 5S boards rated for up to 100A but I'll need new cells if that is the problem. I'm just wondering if the BMS is cutting it at a lower amperage.
 
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