1
\$\begingroup\$

I am the proud owner of a new very old laptop, which comes with two very old NiCad 9.6V 2400mAh battery packs. Toshiba PA8708U, if you care.

The power supply appears to be failing (it won't turn on), but it is charging the batteries --- so I let it do that for an hour or so. One of the batteries, from stone dead, has charged to about 12V. The other is at about 10V. The voltage being produced by the charging terminals is 15V.

I know that NiCads tend to charge at fairly high voltage and are current-limited, but I wouldn't expect the battery voltage itself (while disconnected from the charger!) to go above the rated 9.6V.

So I am deeply suspicious. Is this normal behaviour? Does it indicate that the PSU is overvolting the batteries? Will they catch fire?

(The batteries are now safely disconnected, BTW.)

\$\endgroup\$
  • 1
    \$\begingroup\$ NiCd batteries don't catch fire. NiCd cells are quite abuse-resistant, at least when compared to Lithium batteries. (Further reading: brief on NiCd cells.) \$\endgroup\$ – Nick Alexeev Feb 9 '17 at 20:17
  • 1
    \$\begingroup\$ Though their nominal voltage is 1.2V/cell they can reach 1.4V/cell when charged. That voltage will decline fast under load though. \$\endgroup\$ – Brian Drummond Feb 9 '17 at 20:34
  • \$\begingroup\$ I'm expecting that these cells are years old and full of huge sulphur crystals, resulting in really low capacity. This would explain why they charged to such a high voltage so quickly, yes? \$\endgroup\$ – David Given Feb 9 '17 at 21:36
  • \$\begingroup\$ Rapid "charge" to 12V is indeed a cause of concern. What is the resulting voltage under a modest load, say, 50 Ohms? 2V for 10 seconds? Did I guess right? \$\endgroup\$ – Ale..chenski Feb 9 '17 at 23:08
  • \$\begingroup\$ The question appears to be moot now --- the PSU board appears to be fried, so while it'll (potentially incorrectly) charge the batteries, having the charged batteries does me no good. (The computer is now running, more or less, off an ordinary ATX power supply.) \$\endgroup\$ – David Given Feb 11 '17 at 11:00
2
\$\begingroup\$

The 9.6V is the nominal voltage. 1.2V NiCad cells will charge to 1.45V when fully charged. This represents the 90~100% of charge. Under load, the cell will settle down to its 20~90% voltage, which for NiCad is the nominal 1.2V. So 8 series cells will charge past the 9.6V nominal of their combined cells. 12 Volts is about the highest voltage that you will see for that pack.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ cough NiCad, not NiMH cough \$\endgroup\$ – David Given Feb 9 '17 at 21:33
  • \$\begingroup\$ NiCd cells are very similar in voltages. \$\endgroup\$ – Turbo J Feb 9 '17 at 21:46
  • 1
    \$\begingroup\$ @david corrected. That's not an important distinction here though, as NiCad and NiMH cells have pretty similar charging properties. \$\endgroup\$ – Passerby Feb 10 '17 at 0:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.