I need a simple circuit that will take the 16V DC output from a laptop power supply and step it up to ~20V and limit the current to 440mA to turn it into a slow C/10 constant current charger for a 16.8V, 14-cell 4400mAh nicd battery pack. A shutoff timer that can turn off charging after 14-16 hours would be useful as well.

I'm working on a project that uses an old laptop. The internal battery pack is dead, so I'm powering it with either the external 14-cell battery pack, or from AC. I want to be able to use the 16V power supply both for powering the laptop directly, and for charging the external pack (i.e. I don't want to use a separate charger for the battery pack).

Any suggestions would be appreciated.

 

Hi,Is there anyway you can get to the terminals of the dead battery inside the laptop and bring them out to use for charging your new external battery pack? This way you don't need to build a charger... just use the one inside the laptop.If the external battery is a higher capacity version than the internal one the charger should still work, but just take longer to charge.Regards,Ifixit

 

Thanks for replying, ifixit. I'm not sure if that'll work. The laptop's internal battery is a lower voltage (10.5V, I think), compared to the 16V input from the power jack.

I had initially considered running leads from the laptop's internal battery connector to a *10.5V* external battery, but I think there are a few problems with this. The internal battery connector has something like 10 prongs or contacts, some of which provide status monitoring, etc. and it doesn't supply power unless these are connected. So, it was a lot easier to just have a 16V external battery and run it through the laptop's power jack.