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I want to make a very simple external battery pack for a laptop, connecting to its 19V DC power jack. I plan to use a series of four lithium batteries that should provide between 11V and 16.8V of power.

The internal battery pack consists of 3 Li-Polymer cells, which I assume provides the laptop with approximately 8.1V to 12.6V of power.

When I connect the 4-cell external battery pack to the 19V DC input, what are the effects? Does it make sense to assume that the laptop is fine with voltages below 19V? Will it take whatever voltage it needs to run, which should be related to what the internal battery provides, i.e ~8.1+V, and direct only excess voltage to recharge the internal batteries? Are there any terrible inefficiencies going on? Does this even make sense at all?

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The answer is simple: We don't know because the laptop is only documented to run at 19 volts. It may work fine with less, or it might not work at all. Although remote, it is possible that even if it "works" fine with less voltage, running it for an extended period could damage something due to parts of it's power supply getting too hot.

We just don't know what is inside of your laptop. We also don't know the internal workings of every laptop in the world. Just because it might work with one laptop, it doesn't automatically mean that it'll work with any laptop.

If you want to be safe, take the output of your battery pack and bump it up to 19v. Otherwise, all bets are off.

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  • \$\begingroup\$ I see. I'm guessing I can't easily figure out if it is safe or not either. Thanks for the heads up. \$\endgroup\$ – baggage_lump Feb 29 '12 at 0:29
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I agree with everything David said. In addition, note that many laptops don't just have 2 wire connections from the charger. Look carefully, and there is likely a third line. This is almost certain if it's a Dell.

The third line is so the laptop can talk to the charger and act funny if it's not one specifically authorized by the manufacturer. Dells are known to operate with just 19V but won't charge. There is of course no electrical reason for this since 19V is 19V, just the manufacturer trying to keep third parties from making chargers. Yes, I think this is obnoxious too. Some day when I have enough time I'll look at the communication line and see if I can reverse engineer the protocol.

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  • \$\begingroup\$ This is why I won't buy a Dell laptop. So far, laptops from other manufacturers use two wire power supplies (usually ones that are even branded differently, for example, my Acer laptop uses Lite-On power supply). \$\endgroup\$ – Pentium100 Feb 29 '12 at 3:55

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