Lithium-ion laptop battery overhaul

Introduction

I've read about rebuilding laptop batteries for a while now, and finally got around to trying it for myself. I own a four year old Dell 600m and suffice to say the battery does lot last like it used to. When new I recall having runtimes above 3 hours. Recently, the battery has been pretty unusable, lasting less than an hour. The capacity was so low (around 14500 mWh) that the laptop would constantly flash the orange LED four times quickly followed by a short green pulse, indicating a battery problem.

First, let's get familiar with how these battery packs operate. They are actually very simple. The cells are arranged in such a way that their voltages add up to the total voltage of the battery pack. In my case, two cells are connected in parallel to form a pair. This gives an output of 3.7 V and twice the capacity of a single cell. Each pair is then connected in series, yielding the sum of the voltages of each pair (3.7 + 3.7 + 3.7 = 11.1 V).

The cathode and anode (+ and -) of this series of pairs is then connected to a small PCB circuit. This "smart" circuit is the brain of the battery pack. It has a thermistor in case the battery overheats (by breaking the circuit) to help avoid fire or explosion (lithium-ion batteries are known to overheat and explode from time to time). There is also circuitry that monitors charge/discharge rates to give your computer some accounting (through ACPI) of how much power is remaining, last full capacity, etc. Some of this data is even stored on a small EEPROM so the accounting is more accurate and follows the slow decline in capacity that affects lithium-ion batteries over time. This is why you may fully charge an old battery pack only to have "100%" mean that it will last for 25 minutes.

We intend to replace the old cells with new ones, and hope that the circuitry will adjust to appropriately reflect the new capacity after several full charge/discharge cycles. It is a straightforward procedure, slightly more complex than changing the batteries in a flashlight.

Required tools and items

Before embarking on this project, you need some items at your disposal.

• New lithium-ion cells matching the old cells in both size and capacity
• A soldering iron and solder
• Screwdriver or knife (or other means of opening the battery pack)
• Pliers (for pulling off the metal contact strip)

My laptop battery is the standard 6 cell lithium-ion pack that comes with the Dell 600m laptop. I gather there are many manufacturers, and mine happens to be Sanyo. Upon opening the battery pack, I discovered the cells are pretty standard 18650, 3.7 V and around 2500 mAh.

Depending on your laptop, finding new cells can be a challenge. In my case, I found a reasonable set of 6 new cells from DealExtreme costing $9 USD per pair. What an eXtreme deal! There are cheaper ones, but I picked these because I figured a higher priced pair is more likely to have a higher capacity. I also bought a charger for the 18650's so I could attempt to recycle the old cells into bicycle lights and other projects.

Procedure

OK, here we go. The first step is to get the battery pack opened. For this you may need something like a knife or a screwdriver. If there are screws you are lucky, normally these seem to be glued shut and there is risk of damaging the plastic enclosure. Be careful. Take care not to injure the PCB as you go.

Once the battery pack is opened, remove the cells and PCB (it's usually all one piece, soldered together). We're going to remove the cells one by one, being careful to leave the metal contact strip intact since we will use this with the new cells.

(Above) The original battery pack removed from the casing. You can see I've started removing the contact strip and two cells have been removed.

Keep going, removing one cell at a time, and carefully peeling back the contact strip so as not to short circuit the remaining cells.

(Above) The two more cells removed, leaving the last two.

The next step is adding some solder to the new cells so we can easily re-attach the metal contact strip. Try to apply the solder as quickly as possible so as not to overheat the cells.

Each cell should have a dab of solder applied to each end. Make sure the solder is well applied, otherwise the contact will not be strong resulting in a faulty circuit.

Next, we will begin soldering the cells to the metal contact strip.

Note: Before starting to solder the cells together, be aware of how much space you have available in the plastic battery pack casing. In my case, there was extremely little room for error, and in the end I could not fit the rebuilt pack inside without cutting some pieces of plastic out to give it more room. Even then, the battery pack did not fit together well, and I had to squeeze it into my laptop to make it fit.

(Above) Replacement of the cells. Note: Pay attention to the orientation of the cells. They must be put back together in the same way! If you get one backwards you might expect a small explosion!

Four cells replaced, two more to go.

The last cells are installed and the contact strip now connects all cells together. Once the cells are soldered together, test the voltage with a voltmeter to make sure your soldering was good. Now the fun task is trying to fit it back into the enclosure. As I noted above, I had to cut some plastic out of the case to get it to fit.

Results

While not achieving the original runtime, an average runtime on the new battery appears to be nearly two and a half hours. This is far better than the less-than-an-hour on the old cells.

The figure below shows the battery voltage versus time for the laptop running on battery power. I had read somewhere that the cells drop voltage quickly to within their operating range (see 0-5 minutes), maintain voltage within this range for the majority of their discharge cycle (see 5-120 minutes), and then drop out of their operating range (see 120-145 minutes) until there is insufficient voltage remaining to power the laptop.

One of my fears was that the circuitry in the battery pack would not recognise the higher capacity, and would limit the charging/discharging to values stored on the EEPROM. However, everything went fine, and after 7 full charge/discharges, the last full capacity increased to reach the design capacity. After each complete discharge, where I would just let the computer turn off due to insufficient battery, the last full capacity would be updated to reflect the longer discharge. See the figure below.

I generated a graph for the charge cycle because I thought it was interesting. Charging lithium-ion batteries requires a high voltage and current for the majority of the charge cycle, until the voltage reaches a certain point. Then, the charging circuit sends short pulses of low-current to top up the battery until it reaches it's capacity, a process that can take as long or longer than the first half of the charge cycle. This is evident from the figure below. Don't ask me what those deep troughs in the plot are, hopefully just some irregularity...

References and further reading

• How to rebuild a Li-Ion battery pack
• DIY Laptop Battery Rebuild
• Charging lithium-ion battery
• Wikipedia article on lithium-ion batteries
• Smart Battery System Manager Specification [PDF]

Donn Morrison, 9th April 2008

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