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Getting my toes wet. All help welcome!
Greetings! This will be my first forum post here.

I, as many newbies, have big plans to eventually build a nice pack for my long-in-progress-dream-camper-conversion, but as a starter, I figured i'd take on a smaller project:

My father's birthday is coming up soon, he recently bought land and started building stuff about but he does not have the tools necessary, I've spent some money on a few second-hand cordless 18v powertools which work fine but their batteries (NiCad) are lacking. I've tried all the "Revival" techniques in the book and managed to make 1 use-able battery out of the 4 bad ones I've collected but I figured, why not try to make a battery pack myself? would be great practice for my future project.

I'm now appealing to you, seasoned 18650 wizards to look over my plan and call out any mistakes, I'm going to spill it all out, what I think can go wrong and what's my approach, hoping that this will be a great learning experience for me and my father will have two functional batteries to work with. 

My first "oopsie" came from the choice of powertools, I went with 18v because I figured, more voltage = more power right? ([1]Correct me if I'm wrong, I figure at least half of my statements in this post will be incorrect) I've done the math [2] and figured I would need a 5sXp configuration, but upon looking for a BMS for 5s [3] I've noticed my options are scarce, I then thought, could it be that the 5s configuration is not stable?[4] perhaps something to do with the voltage not being exactly optimal for use?[4] I remember I've read somewhere that achieving 12v with 18650's is awkward[5] because you'd need 3.5sXp, but didn't know anything about 18v. 

My second "oopsie" happened when I realized that power tools are, well, power hungry[6] and they can discharge their NiCad/NiMH batteries at 4C [7] (Side note, I have no clue what 4C means) I'm really confused about max draws, but a glimmer of hope came when I remembered high school physics class which taught me that wiring stuff in parallel spreads [8] the load so making the pack 5s3p [9] (Pure guesstimation on the 3p part) could/maybe/should allow the power tools to work in their max draw case? [10].

My plan of action is currently as follows:

Step 1: Aquire 16 cheap 18650 batteries with similar capacities already tested by people smarter than me from the german Ebay Kleinanzeigen website.[11]

Step 2: Look for a BMS on Amazon[12].

Step 2.5: Wonder why BMS boards have this strange "A" rating and what does it mean. (Image 1)

Step 2.78: Do a fruitless search about this ampere rating of BMS boards, guess that it has something to do with how many you put in parallel, pick the BMS with the most understandable wiring diagram and a bigger "A" rating, because perhaps it's important. [13]
step 2.99: order this: (Not yet ordered, awaiting input from you guys) 

Step 3: Solder [14] 3 cells together, + to + to + and - to - to - [15], using some thick-ish copper wire [16], repeat 4 more times. 
Step 4: Solder [14] the copper wires from the first +++ set to the first pin of the BMS, from the second + + + set to the 2nd pin of the bms, from the thirds + + + set (Does it have to always be on the + side?[17]) ... until the last + + + copper wire, which I think may be called a bus bar? [18]
Step 5: Solder [14] the bus bars [18] + to - to + to - to + to - to + to -, solder the first - and the last + to the BMS, also solder the last + to the + of the battery housing, solder the - of the battery housing down to the BMS accordingly.
Step 6: Apply generous amounts of Solder [14] to the spot pointed by the red arrows below, hope that's how you short circuit it and it actually helps with the initial power surge [19] (Image 2)

Step 7: Wonder if it was such a good idea to Solder[14] everything without even a single attempt at self-electrocution by making a DIY spot welder from one of the two broken microwaves you have.[20]
Step 7.2: Wonder if the charger from the NiCad batteries is suitable for your use case[21], given that it charges at 18V 2A when my DIY battery should technically reach 21V. [22] (Image 3)

Step 7.47: Wonder if the battery should ever reach 21V [23] since the NiCad batteries provided seem to top off at 18V (Image 4)
Step 7.86: Wonder why there are no power rating stickers on any of the power tools, consider opening one up to learn more about the circuitry but figure that you probably don't have the knowledge to understand how to calculate[23](Would love to know how it's done) if 21V will fry it or even the required knowledge to put it back together.
Step 7.92: Figure that the original charger won't put the cells over 18v, the tools won't break and you'll get excellent [24] life cycles from these cells since you only charge them to 50% all the time.
Step 7.99: Keep wondering if 2,0 A of charge will be enough to ever charge this battery. (I guess yes, in like 15 hours?) [25]
Step 8: Put everything together, drain the battery pack to 18V and after putting it in a power tool try activating it with a comically long stick to see if it works [26]
Step 9: Success?!

Thanks for reading this far! Feel free to reply a list pointing at the references I've made to let me know what I'm wrong or right about, I'm open to suggestions for other BMS boards as long as they're from amazon and would ship within the week. (I'm on a schedule, dad's birthday won't come around again for another year). I hope this was the right sub-forum to post in.

After thought: I realize that before connecting any battery to any other I might want to make sure they give a similar voltage reading, not sure if it's 100% important since they'll be connected to the BMS, but do correct me if I'm wrong.

I have a e-cig that uses 18650 batteries, I'm wondering if I can creatively use it alongside a multimeter to bring all the cells to a similar charge state before starting the soldering work. preferably 3.6V since 3.6*5 is 18
Have you considered just replacing the old cells with new NiMH cells? Will definitely be easier.

"1C" means that the cell is capable of safely discharging completely in 1 hour.
"4C" means " " " in 1/4 hour (15min).

So a 3000mAh cell capable of max 4C discharge can power a tool that requires 12Amps or less.

You mentioned Germany, so:
Modular PowerShelf using 3D printed packs.  60kWh and growing.
(07-15-2020, 07:03 AM)ajw22 Wrote: Have you considered just replacing the old cells with new NiMH cells? Will definitely be easier.

"1C" means that the cell is capable of safely discharging completely in 1 hour.
"4C" means " " " in 1/4 hour (15min).

So a 3000mAh cell capable of max 4C discharge can power a tool that requires 12Amps or less.

You mentioned Germany, so:

Hi, thanks for your reply! Indeed I initially considered replacing the cells and call it a job well done but a part of me wants to "get my toes wet" in this field, and if that also comes with making a better battery pack for my dad then I'm all for it! Thanks for taking the time to find me these links. I think the price is similar to taking the 18650 route, I'm curious if my battle plan is all-in-all correct, also thanks for explaining what the "C" rate means, of course it has something to do with fractions, that's why I couldn't wrap my head around it. So basically I would need to do 5s4p to be safe about the max discharge? Also, I'm not entirely sure that these NiCad cells even have a 4C discharge rate, I can't find a reliable source for them in particular, I'm just going with the highest value I could find, thinking that anything below that is safe.
ajw22 gave you some good advice and links.
But you better get some Hochwasser Stiefel (Waders)
As with that project you will get more than your pinky toes wet


For Info Google Drive

Great ABB shunt breaker seller River City Industrial
Great prices and superb service.
 [Image: em2566%20(1)__19172.1521541365.jpg?c=2]

Not your average Wolf       
Never converted power tool batteries myself, these are just my educated guesses.

[1] Math works out
Original pack has NiCd cells in 15S.
NiCd has about 1.4V ( x15 =>21V ) at full charge, ca 1.0V ( x15 => 15V ) when empty.
LiIon 5S works out perfectly at 4.2V ( => 21V ) full and 3.0V ( => 15V ) when empty.
*note charging issue [24]*

[12] The (A)mpere rating of a BMS is the maximum current it can safely handle. If the tool tries to use more, the BMS _should_ detect an overload and cut the electricity.

[21,22] Generally, only "nominal" (sort of "average") voltages are specified on labels, not the min/max voltage. AC/DC adapters usually specify the max voltage, but I guess this is technically a charger, and a mismatched voltage might confusion...? In any case, 100% sure it supplies upto ca 22V to the battery.

[23] I guess that's because there are no typical work cases for power tools. The power output of the tool will differ greatly depending on the voltage (battery charge state), and how hard the drill is working. Eg. It will be extremely high when the battery is full and driving a big screw into hard wood, and very low when driving a thin screw into soft wood.

[24] NiCd/NiMH chargers have fairly complicated circuitry to detect a full battery, which is not compatible with LiIon charging. You'll either have to replace the circuitry, or add an extra charge port to the battery.

Step 1. is figure out the voltages - done

Step 2. is to figure out how much current (in Amperes) the motor needs under _really_heavy_ load, because every decision after this will depend on it. Probably easiest to google. Measuring it is possible, but you'll need some equipment to do so.
The "4C" rating you mentioned just says that the battery pack was empty after 15min.... Unfortunately a useless information without knowing the capacity of the battery pack.

Step 3. Look for a 5S BMS that can _comfortably_ handle that current
3.1 Figure out charging

Step 4. Look for LiIon cells that can _comfortably_ handle that current. You'll probably want high performance cells, not high capacity laptop cells. Yes, you can put cells in parallel to split the current between multiple cells, which also works to extends the runtime. Of course, you'll want everything to still fit inside the original battery case...
Modular PowerShelf using 3D printed packs.  60kWh and growing.
Thanks everyone for your replies! I'm genuinely learning so much so quickly! This community is great.

I'm currently stuck at finding a suitable power supply for charging which also does not break the bank. I have a 65W 20V Lenovo AC/DC adapter available, but I figure I will need some more tech on the other end to be able to safely charge the battery without burning the house down. Sadly I have no clue what this tech may be, is there a way to get some step up/step down magical circuitry that would convert this PSU to be able to power the battery? I found a source of e-bike cells which I guess will fit the bill, since they're made to provide powah and provide it now, unlike laptop batteries which are made with capacity in mind.
I do realize this PSU is a bit too beefy for my application. from my own research I've learned I should be pumping 21V at 1/4 the A of the battery pack, how does one regulate the output?

My cell options:
(07-16-2020, 08:37 AM)Calin Wrote: I have a 65W 20V Lenovo AC/DC adapter available

The label should say that it can supply a max of around 3.25A  ( 65W / 20V )
Unfortunately, that AC/DC adapter is not useful for this project. Those adapters try to supply 20V whatever come, and then shut off when the current exceeds 3.25A.
The battery on the other hand tries to suck in as much current as it can, given high enough voltage.
This results in the adapter just shutting down as soon as a near empty battery is connected, as that will for sure suck more than 3.25Amps.

What you need is a so called CC/CV charger like this: (the 21V / 1A version) 
It looks like a basic AC/DC adapter, but with the difference that it does not shut down when the battery tries to draw more than 1A.  Instead, the adapter simply lowers the voltage until the battery draws only 1A.  Then it keeps adjusting the voltage up until it hits 21V.  It's still considered a crude charger, but it will charge the battery.

You can get basically the same functionality by combining an AC/DC adapter with a "buck converter with current limiter", such as this one: (note the 2 adjustment potentiometers, one for voltage, and another for max current)
But these can only step down the voltage, hence your 20V adapter would not work.  It'll probably need at least 24V to charge your battery to 21V.

These methods I mentioned do not look at individual cells, so it is up to the BMS to ensure that the charge level of all cells are balanced, and especially that no cell is getting overcharged.  Putting these to work with the old charger should be easy.  Just rip out everything inside the charging station, and solder on the new charger to the connection tabs.

Another approach is to use a balanced charger, such as this one:
It needs connections to each cell (so 6 leads in your case, much like the BMS), and the charger ensures that each cell get charged to exactly the same level.  This would not work with the old charging station, due to the extra leads.  You'll have to drill a hole in the battery to somehow put in a charging port or connector.
Modular PowerShelf using 3D printed packs.  60kWh and growing.

Went with this PSU, I believe it to be CC/CV, I wish I knew about the balance charger earlier, would've saved me some hassle but I think this option is better because it will be easier, just plug & play for my dad.
(07-16-2020, 11:32 PM)Calin Wrote:

Went with this PSU, I believe it to be CC/CV, I wish I knew about the balance charger earlier, would've saved me some hassle but I think this option is better because it will be easier, just plug & play for my dad.

That'll work.  You just have to rip out everything inside the old charging dock and connect the leads of the new charger to the contact pieces of the dock.
1.8A charging is too fast for most 18650 cells to handle safely, so you'll probably need to put 2 or more cells in parallel to share the charge current.  So you'd be looking at 5s2p, or 5s3p, ...

Make sure not to leave anything charging unattended, particularly DIY packs with the cheapest BMS and Chinese no-name chargers.  Or charge/store within range of flammable materials, where it could get wet, etcetc.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
I took the extra precaution to order the right temperature sensor for the BMS. I'm still thinking rather than remaking the battery pack and working in a crammed space, I will make a 5s6p pack which will more than happily take 2A charging and take out two 10mm² copper wires out of the battery that he can plug into the tools, he can keep the battery pack on his belt/backpack, I still haven't figured how heavy this will be, I'm expecting under 1.5kg, but it feels like a better option than having it attached to the tool, being DIY I feel it's safer to have a bigger capacity pack that is serviceable rather than a crammed 5s3p that might break on the first drop.

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