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'salvaging' A Laptop Computer Battery

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So....I have a laptop battery that, inadvertantly, was allowed to drain to completely empty whilst the laptop wa sleft unattended. Although far from new it still held a good two hours running time of power when fully charged.

Now, connected back to the AC power adapter and chraged back up, my Windows7 battery icon at bottom right of the taskbar (near the clock) has an ominous red cross/warning sign overlaid on the battery symbol. The message, when hovering the mouse over it, is "there is a problem with your battery, so your computer may shutdown suddenly" and separately "Consider replacing your battery".

Question: Has discharging it really damaged it in some way so that its capacity is now seriously reduced? or is this a software/settings related issue and can somehow be 'reset' so that Windows will again 'see' it as a healthy battery?

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Have a look here :- http://batteryuniversity.com/learn/article/low_voltage_cut_off

Li-ion batteries contain a protection circuit that shields the battery against abuse. This important safeguard also turns the battery off and makes it unusable if over-discharged. Slipping into sleep mode can happen when storing a Li-ion pack in a discharged state for any length of time as self-discharge would gradually deplete the remaining charge. Depending on the manufacturer, the protection circuit of a Li-ion cuts off between 2.2 and 2.9V/cell. (See BU-802b: Elevated Self-discharge)
Some battery chargers and analyzers (including Cadex), feature a wake-up feature or “boost” to reactivate and recharge batteries that have fallen asleep. Without this provision, a charger renders these batteries unserviceable and the packs would be discarded. Boost applies a small charge current to activate the protection circuit and if a correct cell voltage can be reached, the charger starts a normal charge. Figure 1 illustrates the “boost” function graphically.

Remember.... it could blow up in your face!

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Completely discharging Li-Ion batteries is very bad for them, which is why most battery packs have a built in low voltage protection circuit which is meant to cut all power until such a time as they are recharged.

Some batteries feature an ultra-low voltage cutoff that permanently disconnects the pack if a cell dips below 1.5V. This precaution is done to prohibit recharge if a battery has dwelled in an illegal voltage state. A deep discharge causes copperplating, which can lead to a short circuit in the cell.

So if the protection circuit worked correctly, then regardless of what you did with your computer the battery should have protected itself.

If you left it for months on end then it is indeed possible that it can no longer be charged properly or the battery is permanently damaged.

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if you carefully open the battery case you will probably find it's just standard li-on 18650 cells inside. Basically you can try and charge and coax them back to l life individually,separate from the circuitry in the case, and you can often find that not all the cells are bad and just replace a couple.

However, it's worth bearing in mind, that li-on battery fires from shorting are bad and I've had quite a few even being careful.

Also I have a spot welder to put the nickel strips on to make the packs back up and soldering can easily wreck the cells but obviously not worth the investment for a one off. On top of that the good quality cells are expensive and you don't need many to replacing to defeat the object. It's essential to check on ebay the price of a replacement before doing anything I have wasted a couple of hours trying and failing to revive a laptop battery with new cells, for it to turn out it was the circuitry that had gone and a new one was only a tenner.

With a lot of modern electronic kit you really need to treat any repair attempt as a hobby interest rather than a guaranteed money saver and check the price they're going for first on ebay.

Edit to add: I should have just cut to the chase there really and said don't bother repairing laptop batteries.

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We had a pair of rather unused Acer Aspire Netbooks a few years back, and occasionally they'd end up totally flat.

What was interesting was that irrespective of charging from flat while the system was off, which didn't work, what I did find made a difference was the OSes (dual boot Ubuntu and the Windows).

This Must have been down to the battery management software or settings and some toggle switches on the batteries themselves- but Windows would allow the cells to charge and Linux wouldn't. To be fair to Linux I think it had decided the battery wasn't functioning, so had excluded it from the system.

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