Laptop Battery Rebuild – Why it failed

The problem: Battery still appears as “empty”, even if  I charged it all day long.

For the last six months, I been carrying a 3 meter extension cord on my university backpack. Why? Because a replacement battery for my DELL 6400 laptop costs $325.000 pesos. To get an idea of how much it hurts, think about earning $325 dollars (just for informative purposes: 325000 COP = 170 USD)

However, the net is plenty of stories about ¿easy? and ¿successful? laptop battery rebuilds, so I gave them a try.

Edit: As you already guess, this procedure failed. But I posted it anyway because it teaches how to open sealed plastic cases, among other li-ion battery stuff

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Open it

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Almost all people who tried and documented this hack ripped the battery case open. There is a more “glamorous” way to do it.

First step is heating an utility knife. If the grip ctaches fire, you are doing it wrong

Cut as shown, following the existing separation between both covers. Watch the depth!

From time to time, use a flat screwdriver to separate the covers. Go easy! If you find resistance, switch back to the knife.

Keep going, and going, and going

It’s obvious, don’t cut over the battery contacts!

One last cut, and Ta dah!

Hell yeah

A nice old feature: LED bar display gauge

Here’s a view of the whole device, notice the circuit board.  The two black wires are a thermocouple. Three cells are wired in series, then the two resulting groups are wired in parallel.

Now it’s time to buy new cells, and we are ready to go!

Or perhaps not.
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Damn smart Batteries

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On old battery packs replacing the cells fixed the problem. But in my case the cells are fine, there is an evil intelligence inside the battery that labels them as inoperative. Its called Smart Battery

Yeap, the battery gauge. The chip keeps track of the the manufacture date, Cycles Count, Full Charge Capacity and Permanent Failure Flag. I’ll explain each one of this items

Full Charge Capacity: How much current the battery can provide. I’ve explained that in layman terms on this post. The capacity drops over time, as you are about to see.

The manufacture date: For every year under storage, the battery losses a percentage of capacity

Cycles Count: Every time the battery is fully charged and fully depleted, 1 cycle occurs. More cycles mean less full charge capacity. After certain number of cycles (200-500) cells will fail.

Permanent Failure Flag: Li-Ion Batteries are very emotional. To keep them under control, the “brain” permanently disconnects the whole pack if one of the following conditions occur:

• The cells over-discharged
• One of the cells gets shorted and consume its neighbors.
• Too much cycles have been reached.

This is actually a good feature: For example, recharging a fully depleted Li-Ion cell without the Smart Battery system may lead to explosions and other unfriendly results. But the bad part is, good cells can be labelled as bad.
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What can I do?

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If you don’t have fancy equipment like an SMT soldering station and a EPROM programmer, nothing. I’ve checked the datasheet of the BQ20857 and the BQ29311PQ chips, and they are very intelligent (or I am very dumb). This is a job for a professional battery rebuilder, so I gave up.

Remember:

• Don’t buy old batteries, even if they are cheap as dirt.
• Don’t leave them fully charged. 40% is good for storage.
• Li-ion batteries don’t develop memory effect. Constantly using and recharging a lithium ion battery does not shorten the battery life more than normal usage would.
• Don’t allow the battery to discharge completely.
• Don’t expose them to high temperatures. All right, the computer will overheat them anyway, but don’t make it worst leaving the computer on a car under the sun.

Here are some interesting links

http://www.buchmann.ca/article21-page1.asp – A more serious explanation of the SM Bus

http://en.wikipedia.org/wiki/Smart_battery_system

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