Author Topic: Reviving dead laptop battery  (Read 802 times)

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Offline tooki

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Reviving dead laptop battery
« on: March 18, 2017, 01:09:21 pm »
My old MacBook was sitting on the shelf, and I didn't notice that its MagSafe cord had come unplugged, allowing the battery to deep discharge. The computer recognizes the battery (so it can read out the total capacity, charge cycles, serial number, etc), but will not charge it. Measuring the external battery terminals showed 0V (so presumably it must be triggered by a signal), opening the battery itself and measuring across the only two large solder joints on the board (labeled VG and VP) shows 3.8V. The whole battery is nominal 10.8V, and the six cells inside are labeled 3.7V. It's hard to be sure about the topology without destructive disassembly, but as best I can tell, it's three pairs of paralleled cells, in series, with two thermistors in between the stacks.

This picture indicates two solder joints for the cell junctions: https://forums.macrumors.com/attachments/a1181-battery-board-jpg.433649/
Measuring from VG to the 2/3 junction shows 3.8V (actually a few mV higher than between VG and VP), and VG to the 1/2 junction measures 2.35V. This makes me think that cell pair 1 is at 2.35V, cell pair 2 at 1.45V, and cell pair 3 at zero (it measures -43mV actually).

Here's someone else's pix of the insides of this battery model: https://www.ifixit.com/Teardown/MacBook+Core+2+Duo+Rechargeable+Battery+Teardown/5173

Can I wake this battery up by applying a charge voltage (10.8V?) directly to the cells for a few mins with a bench PSU, just long enough to get them above whatever the controller's threshold is? Or is that just asking to set my house on fire?
« Last Edit: March 18, 2017, 01:11:36 pm by tooki »
 

Offline Jeroen3

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Re: Reviving dead laptop battery
« Reply #1 on: March 18, 2017, 01:30:14 pm »
Quote
Measuring from VG to the 2/3 junction shows 3.8V (actually a few mV higher than between VG and VP), and VG to the 1/2 junction measures 2.35V. This makes me think that cell pair 1 is at 2.35V, cell pair 2 at 1.45V, and cell pair 3 at zero (it measures -43mV actually).
Cell 2 and 3 are certainly not in good shape if this is true. There is a reason the battery controller disabled itself, and is not enabling when you apply voltage. You might need to change cells and reset the controller.

I wouldn't mess with these pyrotechnics you put on your lap when in use.
« Last Edit: March 18, 2017, 01:32:40 pm by Jeroen3 »
 

Offline tooki

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Re: Reviving dead laptop battery
« Reply #2 on: March 18, 2017, 02:39:27 pm »
You might need to change cells and reset the controller.
In which case it's not worth it to DIY; it looks like the cells cost $12-17 each on eBay for no-name ones, so six of them is really close to the cost of a new good-brand replacement battery ($55-90). Quality cells from Conrad are $45 each.  :-DD
 


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