DEAD High Quality Metabo DRILL BATTERY - Can it be REPAIRED? 

7 years running a battery rebuild company 10k batteries rebuilt. Mixing cell capacities technically works but you risk the longevity of the cells they will under/over charge eventually. I would always replace all cells at the same time. Some chargers have very good balancing Bosch for example very easy to rebuild the batteries. You can buy rolls of nickel strip it's thinner and spot welds easier. You'll not spot weld the original metal without a commercial battery welder. Also take your ring and watch off there's a risk of a dangerous arc burning you!

As someone who rebuilds medical batteries, I can tell you a few things. Putting in mismatched cells that have a difference and internal resistance or capacity can cause lower capacity cells to go into a reverse charge during normal discharge. If the protection board is correctly monitoring all of the cells, it might prevent this from happening. Getting those thicker tabs connected, I would suggest spot-welding down thinner nickel strips with an extended tab coming off the end to solder onto the thicker tabs by the manufacturer.

Just wanted to thank you for sharing your interest and experience in fixing things with faults. I have the same hobby and while I’ve been sharing my iPhone repairs to RU-vid, I’m now doing the same with random broken items I find which I challenge myself to fix. Your videos are so encouraging in that manner and it’s a reason why I enjoy your content. Thanks Vince

The thing you have gotten mixed up with, the BMS system in those older drills are unlikely to do Cell Balanced Charging, but will simply only monitor the change and when any single cell gets to a cutoff voltage it will cancel charging. It takes a lot more electronics on a BMS board to do balanced charging even in this day. The other issue you will likely run into - as it has been touched on in comments here, and you kind of touched on it yourself. As you discharge the battery pack, the lower capacity (and worn out cells will make this worse) will drain to a lower voltage than the other batteries. the pack Should technically signal that it is flat and stop the drill. However once you try to charge the batteries, the inconsistencies in charge state and battery worn level will result in the charge cycle taking a different length of time. As it is likely the green cells will show as fully charged quicker, this will then mean you have a "full" battery that actually consists of 2 well charged batteries and 8 partially charged ones. As you then use this again, the situation gets worse and worse. It may then be possible that you also get into the possibility of over discharging once pair of batteries whilst using the drill, which will then take the battery again requiring you to once again repair it. The general rule of thumb is to try to keep batteries with the same wear pattern together, as well as the same charge cycle and capacity. which would lean towards replacing all batteries in the drill. On the standpoint of changing them all to higher capacity batteries - the pack, and charger will technically be able to handle this, but the charge time will increase. As for the drill itself, if it is a brushed motor, it will work without any issue - a brushless motor may however be able to use more current from the battery pack, so long as the pack is capable of delivering it. Technically this shouldn't cause anything to break, unless it ends up over-torquing the motor or linkages. However, i suspect at some point the original manufacturer would have come out with an increase capacity pack anyway and the risks i have mentioned wont be valid.

Hi Vince, when adding new cells to a BMS, you should have the cell voltages matched to within 0.1V. Otherwise you are placing a strain on the cells as they try to balance, and high current will flow between differently matched cells. As to which cells you can use, its not good to mix various technologies and ratings. In the video you mentioned using new cells of 5.6Ah. This is possible, as the voltages will remain the same, but the increased Ah rating means the battery will last longer during use. 😊

Metabo are still going, UK office in Southampton. Returned a drill driver to them for repair last week. They replaced the motor for under £40 inc VAT and postage, sent on a Saturday, returned the following Friday. Superb products and service, highly recommended.

I have that same spot welder but I hooked it up to a lead acid car battery. It is able to penetrate deeper through the thicker nickel. Also pressing down hard on the probes helps too

Vince as you well know. The people with the least knowledge, give the most advice.

I can recommend the kweld for spot welding. It's a DIY kit and I printed the case for it. The capacitor board from Kweld is highly recommended. I think one of the best spot welders in the hobby area.

This was a super interesting video Vince. It would be great to revisit this one soon

Looking forward to the follow up with all the answers to the questions raised

Hey Vince, i purchased a spot welder off Amazon, Seesii Farad I believe the model, around £120 I paid and welds up to 0.3mm, over 100 different settings.. have never found a battery strip it won't weld through. Not sure of the reputation/quality but I've used it many many times and has never let me down.

I’m sure you had one Vince, but would be good to show the viewers you have a BigClive-style Explosion Containment Pie Dish on hand. I keep an old saucepan and lid ready so I can get it out the back door if necessary. Stay safe. PS - if you look at the £ of a 5ah battery (and given the work you are doing to get into the pack) it makes sense to replace the old cells .. Maybe use the displaced but still working cells in a power bank or other project.

That spot welder is good, Vince. Just use a car battery. I use the same for thicker plates, too.

What great timing this is i was just thinking i wonder if Vince uploaded

I know nothing about this but I sure enjoyed this video and actually learned something thank Mate. From here in California 😊

Nice Job Vince 👍 I bought 10 faulty Milwaukee batteries over a year ago, repaired 8 of them and they all still seem fine. Was thinking if you were worried about the cells capacity you could have removed all of the cells, and just fitted 5 x 2.5ah cells which would have given slightly more capacity than the original pack with half the weight 🙂 Keep up the great work 👌👏

Another option: Get rid of all the old batteries and only take five of your green ones in series - > roughly the same capacity.

More than the charging, I would worry about discharging.. the greens might encourage the Reds to over discharge.. not sure if by a lot, but might be a good idea bit to run the drill to 0

What your missing is the C rating on the old batteries, which tell you how many amps they can push out. C rating of 10 means they can run trough the battery at 1/10 of an hour. 1C takes an hour. To get the max amps multiply the C rating with the capacity. So 3000mah battery at 10C can run at 30A for 6 minutes. Serial extends the time, Parallel extends the amps. When intoducing new cells to a pack you can leave them to self balance between themselves if it's not too big of a difference.. otherwise you risk pushing short circut amps from a full cell to an empty one, which is what the spot welder does basically. With charging you might have issues as you dont have any BMS... otherwise there would be wires on single cells. I wouldn't let them charge fully and leave them for a bit to self balance before using it. And lastly most chargers without a BMS simply mesure resistance, not voltage.. when it starts spiking, it's full. Hope that helps somewhat.

I have a Sunkko 797DH spot welder. It really works quite well.

I reckon anyone with a bit of know-how and an adventurous spirit can totally make something useful out of a spot welder like yours. There's probably no need to spend the big dollar! Let me explain... I bought one of those crappy aliexpress £15 spot welders about 2 years ago before I knew anything about electronics. It was absolutely rubbish. Even though I was using decent high-current LiPo packs, on its max setting the welds were still barely visible. So I kept upping the battery voltage until the damn thing fried. A few months later, I dug it out of the parts drawer and analysed the circuitry and made a mental model of the schematic. Firstly, I realised it had no gate driver for the 5x FDBL9403 mosfets (awesome chips btw) which meant that they experienced excessive amounts of punishment every time they dumped current. Two of them were fried, so I ordered some replacements. The next thing I noticed was the crappy 8-bit microcontroller was obviously also fried. So I deleted the IC, and soldered on some wires for ground, VCC, mosfet gates, and continuity detection (which is how it knows you've got the electrodes pressed to a conductive surface). The plan was to write a bit of Arduino code to do the job right. So I made a simple breadboard circuit: - a mosfet gate driver chip (MCP1407 in this case, but there are MANY chips that do the same job) - a pair of optocouplers (PC817) from the parts bin - literally 2 resistors (for current limiting the PC817 inputs), and a 1uF capacitor to feed the gate driver One optocoupler for passing the continuity detection signal to the microcontroller, and the other for triggering the gate driver. I wanted total galvanic isolation because when the spot welder fried itself, it literally exploded the copper ground plane right off the circuit board. You don't need them kinda transient spikes anywhere near something that's gonna be plugged into your laptop USB port. With that I was now able to have full control over everything with just a few pages of code. I can customise precisely the shape and duration of the current pulse -- nickel strips thick and thin, it doesn't matter, I can set the pulse duration to anything from a flea's fart, right up to a busy night for the North London fire brigade. I also control exactly how it's triggered, for maximum comfort in the timing between the probes making contact, and the pulse. And I got rid of the annoying beeps: instead there's a nice 8x8 LED matrix with appropriate animations and flashing for precise feedback. I just need to get around to learning how to use Altium or something so I can get a custom PCB made instead of balancing a mini-breadboard on top of a battery pack. Seriously, it was such an easy project. Although I am a veteran software engineer, whereas I think most people find the coding to be the trickiest part of such projects. No worries, anyone can have my code -- or I'll write some for whoever does something similar to the above but needs a bit of code to make it work. (For the sake of clarity I should add that increasing pulse duration is absolutely no substitute for a weak power source. By my careful measurements with a 1.5 mΩ shunt resistor and an oscilloscope, a healthy pulse is easily 1000-1500 Amps. When using a 3S pack (11V - 12.6V) that translates to a sub-10mΩ internal resistance.)

For cordless power tools I use 20A discharge rated 18650 cells, like the samsung 25R. To be able to use your spot welder you need to buy nickel strips 0.15mm thick, and replace the original nickel strips that came with the battery. All cells must have the same voltage when assembling the new battery pack.

I think it will be OK - obviously better spot welding lol! But I think you are right about when it's drained, the green cells will have a higher voltage than the red, and then when charging it will reach the turn off point sooner perhaps. But I think over lots of cycles it may kinda balance out?!?

Just watched this video and I had a look on eBay for those metabo battery packs and there are a few reasonably priced ones on there...but I think you'll have looked yourself... anyway keep on making your very watchable videos they're very watchable.... I've been inspired by you to try fixing things myself...so far I've fixed the kitchen TV with gravelly sound ( Not the speakers!) A Sony turntable, an electric razor and the air con in my wife's Fiesta.....so thanks for making me feel that I could possibly do it! Keep up the good work...

Second from youtube, I love your channel vince, great work 👍

This is a common fault on many other battery packs . one of the groups dies because that is the group that the voltage measurement device uses and it keeps drawing a few microamps , and worse when the rest of the battery is flat that those two batteries are drawn down until they die. Reminder to charge the batteries every few months

You should probably invest in a silicone ring Vince, I use mine for electrical tasks.

I design lithium ion battery packs for consumer electronics and don't recommend anyone modify or repair a failed lithium ion packs due to a variety of possible hazards. If your spot welds are not perfect there is a possibility of local overheating during discharge which could result in excessive cell temperatures and a cell related thermal event. The original cells were closely matched for internal impedance and cell voltage, mismatched cells will result in poorer performance and life than well matched cells. During charging the lower capacity cells will finish charging soonerr and your 2.5Ah cells will not long term be fully charged, this might or might not be an issue depending on cell balancing and external current leakage paths in your BMS. I would keep any modified pack in a fireproof battery box. Otherwise I enjoy your videos!

Prob could have used low melt solder in that instance as doubtful the cell cavity would have gotten so hot it would melt low-melt stuff. As for me I am a Parkside fan all the way, the £20 Lidl's cordless drill I bought is still going strong 5-6 years later, the rotary cordless kit nearly as long and I dunno what 18650's they used but I can leave any of them for months and not lose any charge. Parkside is also a German company and why I liked my drill, rotary, impact etc is they are all made in Germany, think the only Asian parts are the cells themselves. I bought the Parkside drill in frustration as the major brand expensive one I had bought two years prior was shagged out after a year of not very heavy duty. My Parkside impact driver which has the 20v battery is such a game changer for me, just eats wheel nuts off the thread, buzz off and buzz on again and it will punch screws through the hardest timbers and surfaces, great for setting concrete anchors effortlessly. The rotary tool again bought when my very expensive Dremel burnt out, £20 and 5 years of quite hard duty and still running perfectly. BTW do check out Lidl's at mo, they got a really decent bench vice for £9.99 which has an attachment to clamp a dremel or soldering iron into plus soft jaw shells and quite a decent jaw gap, I bought one today and already worked it hard, just putting my rotary tool into it so I can do some watch case polishing, so much easier with a static tool :D

Search for '5000W Handheld Spot Welder, Docreate Portable Spot Welder Machine DIY Kit for 18650 Battery Pack Welding Tools, Adjustable Battery Welding Soldering Machine with Nickel Strip and Spot Welding Pen'... I've been building e-bike batteries for years and this spot welder is the only one i'll use, it has it's own internal power supply and a foot pedal, the leads are 0 awg, can spot weld 1/16th of an inch thick nickle if needed.

I don't think your spot welder is the only problem here. Since you are still using the existing nickel plated strips, you will never have a flat surface in contacting with the battery, after you dissembled the battery pack, in order to meet the spot welding surface requirement, plus, your nickel plated strip is thicker and no matter how hard you press onto the battery, it just won't make a good contact to form a reliable spot weld. if you use a brand new set of batteries with new nickel strips, then the spot welding will be good, otherwise, I would just try a soldering method. I have powerful spot welder, but it just doesn't work for me when I am repairing a battery pack by replacing dead battery (with the exact spec).

Yeah I found the same issue with my spot welder not being able to spot weld such thick nickel strip. I think only a commercial spot welder would have the power to spot weld such thick strip. In the past I have double up on nickel strip by spot welding one strip on top of another as the strips are thinner the spot welder can weld the the strip without issue but I am unsure if doubling up on the strip doubles the current carrying capacity of the strip.

My experience with spot welding is that you should not press down too hard. You need the higher resistance from light pressure to actually weld. When pressing hard resistance becomes low and current flow without doing anything.

Typically, the charger will stop when any cell in a series pack is fully charged. Likewise when discharging the device will shut off when any cell is empty. The biggest problem with an imbalanced pack is that you won't get the full runtime. (Just like with the suitcase). In this case it really doesn't matter as the new cells will never fully charge or fully discharge as the weaker older cells will finish first both ways. You can test that behavior yourself by measuring the cells as they charge and when they cut-off.

I have a similar spot welder to you, but always run it off a proper car battery - might be your boost pack which is letting you down.

I think this will be fine. The green cells will have the easiest time ..not really empty when the drill stops working and not really full when the charger stops charging .. this is the best case scenario for Li-ION cells. The only worry would be the discharge rate of the new cells. The other cells having a relatively low capacity suggest they have heavy duty discharge capability. In most Li-ION cells it is a tradeoff between capacity and discharge rating. But I would not worry about it..

My grandfather had a Bosch (Might have been Black and Decker though) battery powered drill in the 80s. Probably early 1980s. It was pretty weak and always running out of charge, at the time I just used to wonder why he bothered. They definitely got a lot better. Seems their first was in 1978 and I can't find anymore details than that. I remember it being orange and silver.

Seeing you handle that battery pack with exposed terminals whilst wearing a ring made me shiver

Batteries used in powertools usually have a greater C-rating than the ones used in regular electronics. Using higher capacity cells usually means lower C rating, meaning lower sustained current a cell can provide. Neither voltage, nor capacity rating mean a lot, so when replacing cells you should try to search for a cell datasheet, and choose the ones with 20+ C rating for powertools. It appears that this battery pack has NO cell ballancing, which isn't uncommon, but contributes to early demise of these battery packs.

Recently found the channel and enjoying the repairs. I've bought some random returns from auctions in the past and had generally good luck in getting stuff that wasn't really broken or easily repaired, not even close to the depth you go to, so that Icould use them. I have recently bought a tool and battery that is probably likely to have the same problem as you show here I think. Should be 18V but only shows about 12V after 'charging'. Shame since the battery pack (5Ah) looks totally unused so likely an issue from new, hence the return. I'm chancing my arm with the warranty but if i can't get a replacement I doubt it's going to be worth the stress to repair myself since I don't have the spot welder etc to hand but good to have an idea what the problem is.

i love the math, awesome breakdown

Awesome fix, Vince will it have an update to see how it performs? And charging?

I don't mind working with live voltage and power supplies but there's something about batteries going bang that fills me with dread. It's a shame the manufacturers don't provide adapters so you can use newer batteries and chargers on old drills. I have a Bosch SDS and it's brilliant.

I've had a few issues with batteries or chargers that's for sure. The weirdest were: A Black and Decker hammer drill where the battery charges fine in a compatible DeWalt charger but doesn't in the original Black and Decker. I tested at the connections to the battery and it didn't look right so opened the charger and found that the adapter was supposed to be supplying something like 24v AC to the board and it had failed. I couldn't find an AC adapter compatible so I looked at the options of wiring a DC adapter into the circuit instead. In theory it could have worked but I gave up on the project partly because I lost the items I bought to fix it.. Doh! An Atlas Copco 12v hammer drill which had some issues in use which I resolved and then had to get a charger. I got one and after cleaning it I noticed that the charger just wouldn't work (I hadn't got it wet) and got a replacement. That worked for probably 2 weeks before it also failed and not clear why. I can charge the battery with another independent charger but it isn't ideal when I'm trying to sell the thing as a working drill. It should work but it doesn't. Going to be fun trying to figure out why that has failed.

If it's 2.2Ah and there's 2 in series that means the stock cells are 1100mah. Which is pretty horrendous but it is very old. What you should do, for safety and quality is swap out all 10 cells with 3.5mah cells, you'll have a 7Ah battery. It will last over 3 times as long as well.

Great video

Trying to fix a Bosch Athlet hoover. I had the same problem trying to weld 18650’s with the same welder. Bought the 99 Gears type spot welder and it wouldn’t weld 18650’s either. Ended up soldering very quickly with flux. That worked and didn’t blow up. However l shorted out the disconnected control board and let the smoke out. The battery bank was disconnected, not sure what shorted.

There is no happy end when doing partial repairs with old battery packs - it may work for some times, but new issues will develop sooner when later. Get cheap chinese semi brand cells (e.g. Liitokala VTC5A) for 2€/cells - and replace all of them. The pack will last at least 10 years and the capacity as well as peak power will be higher. Also: When replacing power tool batteries, you have to look up the spec sheet that they are high current. You may cause thermal runaway, when drawing too much current. This has noting to do with capacity, but is an independent parameter, often giving as "xC" x = multiple of its capacity rating in mAh or directly as Amps. Power tool batteries should have at least 20-30 Amps as 1P and ~15 Amps in 2P configurations. Another good option, is to convert all tools "20V" tools to one battery standard (e.g. Makita or rioby) by replacing the battery receptable. Then you only need one type of charger and a 2-3 good packs to run everything, even the older tools.

Matthias Wendell did a nice investigation on the large dewalt batteries that seem to fall over. Seems if they’re used in a high current drawing tool when partially charged the middle cells in the pack go bad.

Thanks for the informative video and the accompanying discussion. Wider spread understanding of battery repairs are going to be critical if EVs and home solar are to affordable long term also to make this technology as green as possible. Looking forward to Vince buying a knackered Tesla now.

An important thing to consider is the C rating of the cells because that dictates the safe charging current. For example, a 2,500mAh cell with a 1C rating can be charged at 2.5 Amps and will take an hour to charge, once at Low Voltage Cutoff (LVC). A 2,500mAh cell with a 2C rating can be safely charged at 5Amps (2 x 2,500mA) and will take half an hour to charge from LVC. By mixing different C rated cells and cells with different mAh values, you are likely to run into balancing issues. I use very powerful LiPo packs in my large remote control monster trucks and use a hobby grade balance charger, always charge at 1C and always put the cells in a flame proof charging pouch. Be careful!

On the weak cell, you can bump charge it up to the point where it likely will work fine. Bump charging take advantage of the fact that AC has voltage on a sine wave so you will have voltage almost all the time. this places a voltage to the load at power on then the circuit takes over and removes the voltage. turning the charging circuit on and off (power strips work well) you can force power into the cell. after about 20 or so, just leave the 18650 single cell charger attached to the battery that was allowed to over discharge. It cannot hurt to try this.

In my experience the li-ion cells in a pack go short when other batteries try to force reverse voltage in the completly discharged cell/pair. So the one cell that lost some capacity will be destroyed by the healty cells. And the cell that is not shorted can be revived no problem by slowly charging it.

The middle prong is for the temperature probe.

Vince, I think the answer is that it will work and is not a safety issue. However the battery will become more imbalanced and perhaps even cause the adjacent battery to fail because they are parallel and it will carry most of the load. If the charger has a proper balancing circuit you could get away with replacing both parallel batteries together. But no charge balancer in the world will compensate for am unmatched pair in parallel.

Yes they still exist. The bought Hilti in the USA. Great brand

Let's get a vid of you fixing that squeaky door😂😂

Thank you!

Hi Vince love to hear which spot welder you end up purchasing.

Metabo is a massive brand, of course they still exist lol

Use cotton wool to fill the screwdriver socket , it is so easy to put in and take out. Love the videos , please keep up all your excellent , entertaining work!

Vince you was spot welding wrong, You was "live and earthing" the same bit of metal you need to earth the battery terminal and live probe the strip joiner with the probe so the electric travels from probe through the connector, through the battery terminal to the probe. And I bet when you read this and watch back the video you will see it. Great Vids as always x

Hello Vince, changing 1 pair only is very bad idea. Change it all to get higher capacity of pack.

if you make the pack 5aH the charger wont mind... it'll just take longer to charge than the 2.2aH packs. probably about twice as long as the 2.2packs take. BUT the drill will run twice as long as it did with 2.2aH packs. the few tool packs i have repaired by only changing one cell went bad again very shortly after the new cell was installed. those 25R cells are great cells. a pack full of them will breathe new life into your old drill.

power tool batteries tend to be of a low amp hour, but they are usually of a high amp output. i have an aldi powertool battery made of lg 18650s. they are only 2ah but 20ah output.

Very interesting Vince but I would replace all of them and take anything metal off saves shorting you ring out and losing your finger

You should always make sure you are matching or exceeding Amp Draw ratings. When I rebuilt my Dyson Robot battery, I used cells rated for 10A draw. Sanyo NCR18650GA, 3500mAh 10A Cells. This ensures the cells can provide the power when needed. If you use anything below the required Rating, you can experience tool cutting out or excessive wear and tear.

Since you had all the cells out, you should have charged them all up, to have the same voltage. I like to replace them all at the same time with higher capacity. That way that drill will be more powerful and last longer. I did this with an Elektrolux stick vacuum cleaner.

I use kWeld from keenlab. It spot welds nickel up to 0.3mm. Works for me.

I use a super capacitor spot welder and its good enough for up to .3mm strips. Whats your thickness?

I've not used cordless tools for years as I found the batteries always failed. I might have a look at my digital camera though to see if that battery pack could be repaired.

I would be interested to see the voltages it charges each of the pairs of cells up to ''after'' you've drained the battery pack. I suspect it will charge the old cells quicker than the new cells and you might only ever get a half charge into the 2 new cells.

27:57 Perhaps a future video could be 'Trying to fix... a squeaky door' 🤪

Green has bigger capacity so that's why it charges slower - last to reach max voltage. Also BMS inside can drain highest cell with around 0.2A, that's why it takes ages to balance them fully

there is no balancer / BMS in the Battery,so you can´t mixed with different Cells.

Many other comments regarding mixing cell capacities and older cells may be nearing their charge lifetimes. I didn't see any balancing mechanism but there may be one. The sunko 737G welders are ok but ensure spare copper pins are included. Practice makes perfect (easy to blow holes). I used my drill now because I was messing around with that omega watch (1:21). Are you and Chris the same person? WD40 required on door or wife or kids.

Currently using the MinderRC DH30 which can be brought on ali... not let me down so far

Good video vince tbh I'd change all the cells does it matter if you wind up with 5.0ah it just basically means the drill will drill for longer before going flat my ryobi one plus as a range of 1.5ah all the way to 9.0ah

A good balancer should discharge the green cells when they hit 4.2v if the other cells are lagging behind, that’s the whole point of a balance Charger it constantly monitoring the cells in the charge cycle, it stays constant current until each cell hits 4.2v then switches to constant voltage and the current falls off, I don’t know why you didn’t just take one of the cells from the parallel side and just made do with reduced capacity and you would of had some spare cells if it goes bad again

Metabo still exists, still a very good brand 👍

Just use 5 cells active cell balancer module,easy to use very small you can fit in or you can put just one cell instead of 2 in order to make them same capacity,but internal resistance is also important i recommend use active cell balancer for both batteries because the other one is also unblaced,because full charge battery pack should reach 21 volt not 18 v

You could have tried cutting off most of the existing strips at the top of the battery pack and soldered or spot welded thinner strips to the remaining tabs and then spot weld to the batteries.

why not just put a single cell in there? replace your two i.iah batteries in parallel (2.2ah) with a single 2.5ah? putting 2 in seems a waste to me.

Molicell make 26a quick charge ,discharge cells , I got some for my vape ,and they're still going strong today ,from close ,towards two years

can sometimes repair the old cells by trickle charging (at a very slow rate) up to their nominal voltage

Mod your Spot welder that is a Good one or your time was to short to welder it ? Was the input voltage and ampere correct? For such things you should use car battery! Thank you for your informative video's!

Will have issues with the higher amp batteries. They will discharge at a different rate and the drill will stop working even if the older batteries have enough power (I would think). I would say the drill can be used with higher amps or lower amps but all batteries need to be the same.

I'd replace them all with 18650s so they have the same capacity, likely at that voltage and configuration you'd not notice any power draw difference the issue with what's in there now is like you said they're different capacities and will cause issues of equalization the chargers and BMS of that era normally only measure via voltage, so as long as it gets fully charged to the nominal voltage the BMS will say "yep you're charged" but now it can't do that quite right since they're different capacities

The batery charger will go off a voltage charging curve, it wont really have anything to do with capacity. Once they reach full charge the voltage will drop off slightly at that point the charger will know its fully charged. Or somthing along thise lines. The problem you have with different capacity cells is the the smaller ones will deplete alot quicker. So you will run the risk of over discharging the lower capacity cells. Thw bms will charge each cell individually to balance them so charging isnt so much of an issue.

As to the balancing problem, would it not be a good idea to use the drill until it stops working, then charge it. It may well balance the pack better than just charging with two cell holding a full charge.

Hi Vince, where did you get the super duper extra long screw drivers from

Ordered my spot weld from Aliexpress with a built-in high current low voltage transformer, input 220v

Make a spot welder from a MOT with secondary removed and wind 1 or 2 turns of heavy gauge wire for output

Metabo i think i remember being sold at Focus DIY years ago before they went under...

Not your style Vince, but I've seem around some projects of spot welders made of old microwave transformers. Very low voltage, lots of amperage.

What you need is a big vise, a massive hammer and some grit and determination. A lot less fiddling about with plastics and such.

Maybe you could use some cells from old laptops for example with the same capacity to replace the faulty ones with.

It's ok with the batteries if the (5s) BMS is doing the low voltage cut off (2.5V). ;) I am soldered my battery packs for ages now, nothing blew up and no battery degradation. It's pretty much an urban legend that bad things happen if you solder it. That said, a spot welder is much easier (and faster) to use if you choose the correct settings. If you choose the wrong settings you can damage the battery. Strange, nobody warns you about that ...

I see this a lot in tool batteries. Moreso where it's one cell that has failed. Could you link to your spotwelder please?