Repairing 12v 9Ah Lead Acid Battery and Adding More Capacity

In this instructable, I’ll make a powerful 12V 14000mAh of capacity Lithium-ion (Li-ion) Battery Pack by recycling dead Sealed Lead Acid battery with reclaimed 18650 lithium-ion cells from bad Dewalt 36V cordless tools batteries. I do not only increase the capacity but also reduce the weight.

How I did it - you can check by looking DIY video or you can follow up instructions below.

For this project you will need:

Materials:

Faulty 12V Lead-acid battery 7ah, 8Ah, 9ah, or any other you want to recycle and repair

18650 Lithium-ion cells ( I used reclaimed cells from faulty Dewalt 36V cordless power tools batteries)

18650 cells holder

3S 40A BMS module:

1 unit - https://bit.ly/3aKGP87

3 units - https://bit.ly/2Z2QcOj

0.15 of thickness 8mm width pure nickel strips https://bit.ly/3AP9BiF

10awq silicon wires ~10cm length (a pair)

5min epoxy

How to recycle used lithium-ion cells: https://youtu.be/6GpvOvw5oEc

Tools:

Hack saw

Multimeter

Clamps

Tin snips

Chisel

80 grit sandpaper

Spot welder

Soldering iron & solder

And some random bits and bobs from around the workshop.

Step 1: Faulty Sealed Lead Acid Battery

This is probably the most popular Sealed Lead Acid battery type in the world after the car battery of course. They are widely used in electronic devices which need a 12V constant power supply. Sooner or later they end their life and should be replaced. The same situation happened with this particular battery. It is dead flat with a bit more than a one-volt of charge. 

Instead of buying a new lead-acid battery I’ll recycle and improve by converting it to a lithium-ion battery.

Step 2: Take a Look What Is Inside

To disassemble, the easiest way will be to cut along this line with a hacksaw. Before taking any disassemble action make sure your battery is totally flat or has not more than 1 or 2volts of charge. In case you have a bigger voltage - drain it, with a 12V car bulb for example.

Step 3: Removing All Internals

Here is how it looks inside. Those better-looking battery banks were removed very easily in one piece, which could not be told about those crusty brown ones. They oxidized and expanded, which trapped those banks inside their compartment. Scratching and picking piece by piece released the tension and finally, the battery body was empty.

Next, I need to remove this internal divider. I started with tin snips to remove the major part and what’s left was finished with a chisel.

Step 4: New Power Source

The case is ready, let's talk about the batteries. For this project, I’ll use this pile of INR18650-20R batteries made by Samsung. They are rated for 2000mAh of capacity and could give up to 22amps of maximum discharge current.

As you can see they were spot-welded already. And the reason is - I recycled them from a pair of Dewalt 36V faulty batteries which I cheaply bought online. If I remember correctly those cells have 80 euro cents per cell on an average cost, which is a steal. 

Those cells were fully tested - and important information was written on each cell, like the voltage which cell had when it was recycled, and the measured capacity. I leave a link in the video description to a full explanation video of how to recycle, test, and separate good usable cells from the trash. 

Step 5: Arranging All Cells in 3 Groups

I will make a 3s7p battery configuration which means 3 groups of cells connected in series while in each series group will have 7 cells connected in parallel. I want each group in series to have as close as possible capacity, so I rearranged some cells. 

Now each group of 7 cells will have almost identical measured capacity.

Step 6: Battery Holder

To keep cells in place I 3D printed a simple battery holder. A similar holder could be bought online too, but the 3D printed option was a time saver for me instead of waiting for the order.

Step 7: Connecting Cells to a Battery Pack

Before connecting cells, I checked if all were within close voltage. They do, so I took 0,15mm of thickness pure nickel strips and spot welded all parallel cells first. After that connected those 3 groups in series.

And if everything was done right - I should get around 12V. 

Additionally, I tripled the nickel strips on the main positive and negative battery pack terminals. This was done to avoid any heating and increased resistance issues when I will pull maximum amps from the battery pack.

Step 8: Connecting BMS Module

To keep those groups of cells nicely balanced, ensure proper charger and discharge - I’ll use this 3S 40A BMS module. It is designed for 3S battery configuration and could handle continuous 40A load, with short peaks of 60A. In the market could be found two very similar looking 3S 40A BMS modules. Take your attention and choose the proper one - with a balancing function, while another one has protection from overcharge and over-discharge only. The best visual difference is this part. On BMS with a balancing feature, you will see 3 big resistors. I will leave a link in the video description for the right product.

Pre-soldered all terminals and with insulated sticky spacer BMS was glued on the battery pack. One by one soldered battery pack terminals to the BMS board by starting from the most negative one. And lastly soldered a pair of silicon wires on the main BMS terminals.

Step 9: Placing Battery Pack Inside Lead Acid Battery Housing

Before placing the battery pack inside the old battery case I sanded down both surfaces to a perfect joint.

To keep the battery pack inside sturdy I glued small plastic parts around the battery perimeter.

Step 10: Last Step - Closing the Lid

Pre-soldered old terminals and soldered negative and positive battery wires to them. All exposed cells were covered with insulating sticky paper.

Mixed 5 minutes epoxy and glued battery case back to one piece.

Step 11: The Result

And this is it - recycled 9ah lead acid battery to 14ah lithium-ion battery. It not only has 50% increased capacity but also weighs more than twice less. This battery could be charged with a smart charger like iMax B6 or any constant voltage power supply with 12.6V. 

At the moment this battery could give up to 40A of continuous and 60A of peak current for any 12V device because that’s the BMS limitation. In case you need more - there is an option to bypass the BMS module and connect directly to the battery terminals, through the fuse for safety reasons. In that case, the maximum discharge current of this battery pack could be pushed up to a stunning 154A.