NinjaKitten
100 µW
Hey all, new to the forum and had some quetions.
I'm planning on making a 12V battery from used cells so I can power some LED light strips. Getting ready for hurricane season. But during my research for everything I had some questions and confusion that popped up and I haven't been able to find a reliable answer for some stuff. So I figured I'd ask about it all here.
Balancing Salvaged Cells Before Building The Pack, and Selecting Cells To Use
Thickness Need For Nickel Strip Series & Parallel Connections
From what I researched, a 0.15mm strip can handle about 7a. So if my math is right….
Say I have a 3S5P 18650 battery. Each battery is rated for 15Amp Discharge, 5Amp Charging, and 2500mah. Each series connection (5) x 15 Amps, means this battery can supply a max of 75 Amps Continuous Load. Or 75a x 12v = 900w maximum load.
However the actual battery capacity will be 2.5ah x 5 = 12.5ah or 150wh (12v x12.5ah)
Anyway, I would need to use 2 of those 0.15mm strips on each series & parallel connection in order to handle 15A from each battery, so the full 75 Amps is distributed evenly across the entire pack. (14a is close enough to 15a for this example
)
Of course, I wouldn’t plan on pulling that many amps from such a small battery pack. But I just want to make sure I’m doing the math right more than anything.
For the charging, I would be able to use a 12.6v MAX 5Amp (since each cell is rated for a max of 5A) charger, I think.
Using the correct AWG wire for the main Positive/Negative Leads & Connectors, & Selecting a BMS
After doing the math..
The main leads from the battery for positive and negative would need to be at least a 4AWG wire (which can handle 85Amps) and I would need a BSM rated for more than 75 Amps.
Now my next question about this part. Would attaching the main leads ONLY to the ends of the positive and negative part of the battery be enough for that full 75amp load? I’ve seen people have the wire itself soldered all the way down the entire length of the main positive and negative ends on top of the batteries in parallel.
I’m thinking that since that’s where the full load will leave the battery, the nickel strips on those two parts will need that extra wire to handle the current and load, since the connections themselves can only handle 15amps max continuous? So running the leads down that path will help with the flow of current? That and bigger batteries with higher loads should be setup that way, otherwise more of the cells towards the top will be under most of that current load….
Correct me if I’m wrong though! But this is what I mean by having the wires go down the path –
But in essence, I should be able to use a BMS and wires for whatever AMPS that match (or ideally is rated for a bit more) the power needed for the demand, so long as it doesn’t exceed 75 amps in this example.
I wouldn’t necessarily NEED to have a 80 amp BMS or 4 awg wire unless I need to pull the max power I can from this battery.
Securing & Wrapping The Battery Pack
My plan for my 12v battery in terms of wrapping it and finishing it up:
Use plastic 18650 spacers for each cell
Use the ring terminal insulators on the positive ends
After spot welding nickel strips on and soldering my main leads for the positive and negative ends… wrap the whole thing in some Kapton tape
Wrap it in some heat resistant foam on the outside as well for some added protection and then heatsink it all together.
If anyone has opinions or suggestions to properly insulate and wrap a battery though, I’m all ears!
After watching so many YouTube videos, and reading posts on reddit. I think I finally did enough research to get started. I don’t plan on making an ebike battery anytime soon. And if I do, I’ll use new cells instead of salvaged ones. I only plan on making a decent sized 12v battery to power some lights for hours since we got the hurricane storm coming up. But I couldn’t help but ask these questions since I couldn’t find any in depth answers.
Preciate any help or opinions!
I'm planning on making a 12V battery from used cells so I can power some LED light strips. Getting ready for hurricane season. But during my research for everything I had some questions and confusion that popped up and I haven't been able to find a reliable answer for some stuff. So I figured I'd ask about it all here.

Balancing Salvaged Cells Before Building The Pack, and Selecting Cells To Use
- Balancing Cells Correctly – Do I just hook them up to a charger, make sure they don’t get too hot, and match the ones that all charge up to 4.2 volts?
- Checking Capacity – Do I just use a cell charger to charge, discharge, then charge them again to get the reading of the mah capacity? How CLOSE would I match usable cells together? Would 2000mah and 2050mah (50 mah difference) be acceptable?
- Cycle Count – How would you know that you can use a Cell without knowing its cycle count? Is that where the rated vs actual capacity comes into play? Would you recommend only using cells that are at least 85% of their rated capacity only? OR is there a better general idea I should go by.
- Mixing Different Sourced Batteries – Generally I know you should only match batteries with the same brand/model in a battery pack. But what if the specs of the 2 different batteries are same, just different brands? Would you recommend using used laptop batteries mixed in with old power tool batteries? Or keep it strictly the same brand/model no matter what?
- Self Discharging – After the cells are charged, how long do you let them sit before checking voltage again to make sure they aren’t self discharging? And is this necessary for low drain applications (battery for 12v light strip – lower amps) instead of high drain applications (ebike – higher amps)?
- Internal Resistance – How would you check this accurately? I know there are tools you can by to do so, but I’ve read that Internal resistance can change depending on the state of charge, full, half way, empty. Also can be different from the make, type of cell, etc. Is it generally speaking good enough to charge the battery to full 4.2v then check it? And when checking it, what’s a good general number for used cells? Anything under 50? Of course a good number to look for would depend on the application I’m assuming. Higher drain cells would be lower from my understanding, and low drain cells would be generally a bit higher.
- To summarize, so I can have a general idea of what to go by, does this seem like the necessary matching criteria to pair cells with?
- Voltage must be at 4.2v when full, should not get too hot when checking capacity
- Voltage should stay the same with very little movement if any after sitting for a few days to a week.
- Capacity mah rating needs to be within a 50mah difference between cells
- Internal resistance of each cell, (depending on application) should be no more than ~50 +/- mOhm
- Batteries should be match with the same brand/model to avoid major differences
- Amp Specs need to match (Max Amp Discharge/Recharge)
Thickness Need For Nickel Strip Series & Parallel Connections
From what I researched, a 0.15mm strip can handle about 7a. So if my math is right….
Say I have a 3S5P 18650 battery. Each battery is rated for 15Amp Discharge, 5Amp Charging, and 2500mah. Each series connection (5) x 15 Amps, means this battery can supply a max of 75 Amps Continuous Load. Or 75a x 12v = 900w maximum load.
However the actual battery capacity will be 2.5ah x 5 = 12.5ah or 150wh (12v x12.5ah)
Anyway, I would need to use 2 of those 0.15mm strips on each series & parallel connection in order to handle 15A from each battery, so the full 75 Amps is distributed evenly across the entire pack. (14a is close enough to 15a for this example

Of course, I wouldn’t plan on pulling that many amps from such a small battery pack. But I just want to make sure I’m doing the math right more than anything.
For the charging, I would be able to use a 12.6v MAX 5Amp (since each cell is rated for a max of 5A) charger, I think.
Using the correct AWG wire for the main Positive/Negative Leads & Connectors, & Selecting a BMS
After doing the math..
The main leads from the battery for positive and negative would need to be at least a 4AWG wire (which can handle 85Amps) and I would need a BSM rated for more than 75 Amps.
Now my next question about this part. Would attaching the main leads ONLY to the ends of the positive and negative part of the battery be enough for that full 75amp load? I’ve seen people have the wire itself soldered all the way down the entire length of the main positive and negative ends on top of the batteries in parallel.
I’m thinking that since that’s where the full load will leave the battery, the nickel strips on those two parts will need that extra wire to handle the current and load, since the connections themselves can only handle 15amps max continuous? So running the leads down that path will help with the flow of current? That and bigger batteries with higher loads should be setup that way, otherwise more of the cells towards the top will be under most of that current load….
Correct me if I’m wrong though! But this is what I mean by having the wires go down the path –
But in essence, I should be able to use a BMS and wires for whatever AMPS that match (or ideally is rated for a bit more) the power needed for the demand, so long as it doesn’t exceed 75 amps in this example.
I wouldn’t necessarily NEED to have a 80 amp BMS or 4 awg wire unless I need to pull the max power I can from this battery.
Securing & Wrapping The Battery Pack
My plan for my 12v battery in terms of wrapping it and finishing it up:
Use plastic 18650 spacers for each cell
Use the ring terminal insulators on the positive ends
After spot welding nickel strips on and soldering my main leads for the positive and negative ends… wrap the whole thing in some Kapton tape
Wrap it in some heat resistant foam on the outside as well for some added protection and then heatsink it all together.
If anyone has opinions or suggestions to properly insulate and wrap a battery though, I’m all ears!
After watching so many YouTube videos, and reading posts on reddit. I think I finally did enough research to get started. I don’t plan on making an ebike battery anytime soon. And if I do, I’ll use new cells instead of salvaged ones. I only plan on making a decent sized 12v battery to power some lights for hours since we got the hurricane storm coming up. But I couldn’t help but ask these questions since I couldn’t find any in depth answers.
Preciate any help or opinions!