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I am currently using an old lithium-ion laptop battery pack for my portable sound system. The battery seems to work pretty well for its age. I simply hooked up a cable to the positive and negative pins (which I needed to guess) which directly leads into the amplifier. For charging I simply put the battery back into the laptop.

However, is there anything I have to care about? What would happen if the battery pack is empty? Is there an integrated underdischarge protection? And does the voltage output fall over time or is it regulated internally (I don't own a multimeter to test that)?

The pack is rated at 14.8V and the capacity is about 5000mAh.

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That method will work fine. In fact many hobbyists similarly repurpose laptop batteries as universal power supplies, e.g. see here. Beware that some laptop batteries also require that another pin be grounded to enable output, e.g. the "System Present" pin on Dell batteries - see the linked page.

There is no need to worry about overdischarge since all modern laptop batteries incorporate a battery management system that includes overdischarge protection. It will disable output when any cell reaches too low voltage (typically between 2.8V - 3.2V).

Laptop batteries don't typically include any voltage regulation, so the output at the terminals will indeed decrease as the cells drain, typically from around 12.6V to 9V for a pack with 3 cells in series, or from 16.8V to 12V for a pack with 4 cells in series. More recent packs may use cells that charge to 4.35V so then the fully charged packs will be 13.05V and 17.4V respectively.

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  • \$\begingroup\$ Thanks for this info. I would vote up if I had enough reputation to do this. \$\endgroup\$ – Kim-Yannick Jürs May 8 '16 at 15:06
  • \$\begingroup\$ The laptop is a very old one with a Pentium 4. Are even that old laptops protected against underdischarge? \$\endgroup\$ – Kim-Yannick Jürs May 8 '16 at 15:15
  • \$\begingroup\$ @Kim-YannickJürs Almost certainly so but you should check to be be sure. It is easy enough to test. For example, drain it till your laptop shuts down, then see if it sill works in your other device. If so, check that the discharge protection kicks in fairly quickly. If not, give it a very slight charge in the laptop, then do the protection test in the other device. There may be a slight difference where it shuts off in each device due to varying loads from the devices (which causes varying voltage drops due to internal resistance in the cells). \$\endgroup\$ – Bill Dubuque May 8 '16 at 15:23
  • \$\begingroup\$ The battery has more than just 2 pins, doesn't that already mean it has some sort of battery management system in there? \$\endgroup\$ – Kim-Yannick Jürs May 8 '16 at 15:33
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    \$\begingroup\$ @Kim-YannickJürs Almost surely so, but one can never be too safe when dealing with Li-ion cells. \$\endgroup\$ – Bill Dubuque May 8 '16 at 15:41

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