[Power] Fwd: Charging XO-1 batteries having extreme cell imbalance

[Adam Holt]

From: <xoapart at xoshare.org>
Date: Tue, Mar 26, 2019, 3:52 PM
Subject: [support-gang] Charging XO-1 batteries having extreme cell
imbalance
To: <support-gang at lists.laptop.org>

Charging XO-1 batteries having extreme cell imbalance.

Here is our procedure for charging batteries with very short run times
(minutes) followed with very short charge times (minutes). Tested on series
4 and series 8 batteries. On information, some series 1 batteries may not
have cell balancing circuits. Comments welcome.

Some laptops received for repair have batteries that will not power the
laptop for more than a few minutes or, in some cases, not at all. These
fall into three groups: infinite trickle charge, green battery LED but will
not power XO-1 or charge, or battery run times of minutes followed by short
charging time. In the past, all required drilling pin holes in the plastic
case and testing the individual cells and, where cells were not open or
shorted, charging from external sources. Using “bat-recover” in the newer
firmware, the batteries in the last group would start to charge and when
the voltage reached 7.500 volts, the charging current would go to 0.0 mA.
After, several minutes, the charging would start again and the charge /
nocharge cycle repeated continuously with very little accumulation of
charge. Hence, the batteries were perceived as not restorable unless an
invasive procedure was used.

Recently, it was discovered that if the charging power was interrupt 5
seconds every 60 seconds, the charge would accumulate much faster. After
the accumulation of 200 - 500 mAh, charging was continuous, as normal. One
battery falling into this group would not power the XO-1, but did show some
charging time. When the charging power was interrupted while running
bat-recover, the XO-1 would shut down. The cycle of power XO-1, start
bat-recover, and interrupt charging power was repeated until about 50 mAh
had accumulated, where the XO-1 remained powered when the charging power
was interrupted.

The working procedure is to accumulate, using bat-recover, sufficient
battery charge to keep the XO-1 running when the charging power in
interrupted. Then, interrupt the charging power for 5 to 10 seconds after
the charging current drops to 0.0 . When the charging current ceases the
periodic dropping, continue bat-recover until the battery is fully charged.
Interruptions of more about 10 seconds does not seem to increase
accumulation rate.

To minimize residual imbalance, use bat-recover (no AC power) to charge the
battery (AC power) to about 15% (500 mAh). Power off, power (no AC power)
on again and let the XO-1 run normally until it shuts down with the red
battery LED. Use bat-recover to fully charge the battery again.  Expect the
full accumulated charge to range between 2700 - 2900 mAh. New battery spec
is 2800 mAh.  Values near 3000 may be observed if the battery is completely
discharged with bat-recover, but only the 2800 will be available in OLPC
use.

A developer key is needed to access the firmware / bat-recover on the XO-1.
Key is not needed with the XO-1.5 . It appears that the linearity in the
current - voltage curve in partially discharged cells and the non-linearity
near full charge makes this procedure possible. Furthermore, it is
postulated that slower charging rates allow the cell balancing circuit in
the battery to accumulate greater balancing charge.

N.R.

Michigan XO Laptop Community Repair Center

xoapart at xoshare.org

http://xoshare.org
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