Among DIYers there is this thing of building your own battery, the cost of batteries has been high for some time and it hasn’t been so bad if you buy from our shop. but you have a choice of what was £3,500 or some 18650 used cells which has been very popular on line.
Building, recycling 18650 battery cells is dangerous, and requires expert knowledge in pack building and loading. its not far off making a bomb or defusing one. I totally DO NOT recommend doing it.
We are proud to say we test all options and usual ways of doing anything before having a say, this give us first hand knowledge and real life rather than some Muppet who repeats what someone else is selling.
25 days of testing cells together with days of dismantling, a few pant liners, and reactions to the toxic batteries.. I forget how long this took to make a pack from used cells.
The lesion I learnt was that if you buy new cells, you could save a massive amount of time. Not only this but the risks are massive when trying to get apart some of the packs.
If you have time on your hands, then this may still appeal, however mixed packs means mixed battery cells. This then becomes a nightmare when you have to read all the specs. That is so you can limit your pack specs to the lowest performance cells. Or you have to change about to library the cells into groups, and then see what you have before you can begin thinking about what you can build.
The Cake Size.
Oddly most systems are nominal, so you have 12v, 24v, 48v, and 36v, I happen to also have a 192 volt system, but that’s another story… within these nominals, are a voltage range, typically based on lead acid battery voltages.
So your common 12V will be 11.5 to 15v.
You will start to see the immediate problem when looking at lithium cells and there 2.7 – 4.2 voltage range. 3 cells are often used for 12v lithium batteries.
so your voltage range for 12C lithium is 8.1V -12.6
You are likely to find that your inverters lower voltage limit is 12V. and your battery will want to sit around 10.8V.
A 24V system, well that’s different you would use 7 cells and not 6, and with 48v you use 13.
the Normal voltage range of a 24v inverter is 20-30V
The 7S battery voltage range is 18.9-29.4v Therefor well within the specs.
We want to be under the top voltage and under the bottom voltage but use as much as we can of the cell range. for many the conservation of the cell, will range the cell from 3v to 4.1v giving you a range of 21-28.7.
Capacity.
Having capacity is key and therefore the battery should be in parallel, and there are two main considerations;
if they can, and can do it safely.
I don’t think I have heard any DIY battery build mention the limitations of the strips; or the need for a collection bus bar. Lets take a look at this as its IMPORTANT!!
Current carrying capacity of pure nickel strips based on their width and thickness.
| size | Optimal | Acceptable |
| 0.1 mm x 7 mm | < 3.0 A | ~4.5 A |
| 0.15 mm x 7 mm | < 4.7 A | ~ 7.0 A |
| 0.2 mm x 7 mm | < 6.4 A | ~ 9.6 A |
| 0.3 mm x 7 mm | < 10.0 A | ~ 15.0 A |
When considering the current flow pattern in a 7S 18P battery, we need to understand how the current moves within the battery during discharge. Let’s break it down:
- Series Connection (7S):
- The battery cells are connected in series, forming a chain. In a 7S configuration, seven cells are connected end-to-end.
- The positive terminal of one cell is connected to the negative terminal of the next cell.
- The total voltage of the battery is the sum of the individual cell voltages.
- Parallel Connection (18P):
- Within each series group, there are 18 cells connected in parallel.
- All the positive terminals of these cells are connected together, and similarly, all the negative terminals are connected together.
- This parallel arrangement increases the overall capacity (mAh) of the battery.
- Current Flow:
- During discharge (when the battery is providing power), the current flows from the positive terminal of the entire battery pack (the combined voltage of all 7 series cells) to the negative terminal.
- Inside each cell, the current flows from the positive electrode (cathode) to the negative electrode (anode) through the pack.
- The current then passes through the nickel strips connecting the cells in parallel.
- These nickel strips act as conductors, allowing the current to flow between the cells.
- The overall current capacity of the battery pack depends on the total number of cells in parallel (18P) and their individual capacities.
- Flow Structure:
- To handle a 100-amp current, consider using a pure nickel strip with a width of approximately 100mm and a thickness of 0.15mm
- Think of the current path as in series, this is shared by the parallel. For example :
If we look at 7S1P this could be 7 amps. Each P would add to the current.
The final terminals (the positive and negative) would need to pass the full current. (18 X 7 =126Amps).
To be fair, with used cells due to the mixed range you will want to limit the current, which is Nominal 2 amps and Max of around 4 Amps, This is 36 and 72 Amps giving you 932 Watts and 1,864 Watts respectively.
This relates to the discharge currents of the cell based on the manufactures spec.
There is nothing stopping you from making parallel battery packs to boost the out put performance, that what I did..
Another risk is the failure of any parallel cell, this can be a significant problem.
in a 7S1P ( that’s 7 single battery cells) if one cell has a issue, it can be seen easily, but if you have 20 cells for it to hide in, it will not be easy to find the problem and its hiding with 20 others. You may not see this issue show up in the voltage within the packs. but you could have a drain on the cell voltages ( out of balance) which is good, a short on the other hand will dump the packs power into that faulty cell.
Fault checking:
While testing is one thing, weeding out faulty, risk, damaged, other otherwise imperfect cells is a task all on its own.
To identify faults in 18650 cells, follow these steps:
- Salvaging: Begin by removing the battery pack from its original casing. Use pliers or wire cutters to carefully disassemble the pack and extract the cells. Visually inspect the cells for any signs of damage.
- Multimeter Test 1: Use a multimeter to measure voltage, cells below 2.5 volts should be recycled, Do not use cell that are under 2v..
- Visual Inspection: After removing the cells, visually inspect them to ensure they are not physically damaged and show no signs of leaks. Discard any cells with cracks, dents, or leaks.
- Cleaning: Clean the cells to remove any glue, paper, or tape residue. This reduces contact resistance and prevents contamination. Use a solvent or specialized cleaning solution, along with a metal scrub brush or sanding wheel. Take care not to make a short!
- Charging: Charge the cleaned cells fully while monitoring their temperatures. Abnormally warm cells or those that don’t hold a charge should be discarded.
- Capacity testing: A full charge and discharge with a capacity tester, using the same one will give you regularised results. Recharge after testing.
- Self-Discharge Test: Leave the charged cells sitting for a week. A significant voltage drop indicates a faulty cell.
- Load Test: Perform a load test to simulate real-world conditions. Check for voltage drop under a given load. If unsure, compare several cells.
- Multimeter Test 2: Use a multimeter to measure voltage. If the voltage dropped below 3.7 volts, the cell is dead and needs replacement. Between 3.7 and 4.2 volts, the cell is healthy12.
If you are using new cells, then you do not have all the steps above. Check out the battery section of the shop for Batteries and Testers, you can simply put the pack together, and just take note of the strip and bus bar requirements.
One response
so just so it is clear. Making a battery like this is time consuming and dangerous. even when it is made, you run the risk of a short or fault cell which would result in a fire. if you use multi packs to make one big one, you have to limit the power ( current) that you use to the weakest cells, if you do not they will over heat and then you have a fire.
if on the other hand you were using NEW cells the risk is slightly reduced and you can have smoother power in and out of the battery, however with large cells, for example 25Ah LFP cells. giving you 4, 8, 15 or 16 cell packs that’s a lot less to deal with than the few hundred of smaller cells.
check out the battery section of the site for options…