Building a battery pack


Jul 2, 2005
Hi everybody,

I want to make a battery pack for a laptop (not really a laptop, more a suitcase-pc I am assembling).
I know some basic electronics, but I am a complete noob at battery technology.
Here is a brief description of the project:

I have:
4 Toshiba PA2487U Laptop Li-Ion batteries, 10,8V, 4Ah each
1 Toshiba PA2488U Charger, seems to be broken (connected to mains, the status LED is green, if put a battery in, it starts blinking red after a few seconds, stops only if I disconnect & reconnect main power)

I want:
A battery pack with a included charging circuit. I can provide 12 V (either car outlet or AC-DC PSU) for charging.
It must be possible to run the pc from the pack (it runs with the 10,8V, but 12V output would be better, because I also want to connect some other stuff) or charge the battery while running the PC or running the pc with the external power when the batterys are full. This should be done automatically, without any manual switching (just like the internal battery of a laptop)

I started researching and thinking about the project and ran into the following questions:
-The PA2487 has 10 connectors on the front an 2 on the top. The 2 on the top seem to be security connectors. I have to brige them to get current on two front connectors. What for are the remaining 8 connectors? I think the might be for temperature sensors or so.
-I found out that Li-Ion cells output 3.7V, so my guess is that there are 3 Cells in each battery. The charger has an output of 12.6V, that would be 4.2V charging voltage per cell. Is that right? I read that the charging voltage has to be very exact for Li-Ions, but you don't have to disconnect power when they are full. Is that correct?

So my naive approach would be connecting all the batteries in parallel, also parallel to a removable stabilized supply of 12.6V and the output to the PC. This would be without any temperature sensors or any other security. Is that an option or just way to kill some good batteries? Besides, how can I generate exactly 12.6V out of "more-or-less" 12V i get from my car or the psu?

I am thankfull for any tips, links, circuits, ideas, etc.

Thank you for your help and greetings from Munich,


Sep 15, 2002
I think you need an engineering forum instead of a general computer help forum. I dont quite have that much electrical knowledge :)

<A HREF="" target="_new">My precious...</A>


May 21, 2005
Hi mate,

I wouldn't bother trying to do this yourself as there are many considerations to make when charging Li-ION batteries (i.e. precise charging cutoff to avoid overcharging, precise current regulation, etc.)... otherwise you battery life is going to be zilch (I'm talking % capacity through longterm usage - the archiles heal of all rechargeable batteries).

The hassle of successfully wiring up a Laptop battery as well is going to be major!! Those fiddly propriety connectors are not designed to be used outside of the laptop they are intended for!!

Li-ION cells are about 3V each as you suggest.

I got a couple of 120Watt external batteries for my laptop and they do 16V or 19V and weren't too expensive ($300.00 I think total from memory).

I guess you'll be at till the cows come home and then end up blowing up your batteries and subjecting your laptop to a reverse polarity input/explosion (thats what happens when I try and do this sort of thing :)... But hey whatever floats your boat :)

Just my $0.02!!