Replacement battery purchase for Compaq / HP H3800 H3900 series

I made it my own way 😂. Since those batteries are nowhere to be found in the entire world - nothing to mention my homeland Ukraine - and even if you find it in another country - delivery would cost almost as much as the battery and most countries would not be allowed to send batteries in here - I decided to just find a similar battery and make a new one 🙌🏻.

The original battery is around 85 mm x 58 mm x 4 mm - and you should not go any bigger or especially thicker because it will put pressure onto the screen from behind - trust me, I tried 😝. And the original battery is 3.7V 1600 mAh.

I managed to find a new battery that has a little smaller footprint and has a capacity of 2100 mAh - definitely better than my 20+ years old battery anyway. Here they are next to my iPaq H3835 - the first one in the lineup that didn't even have bluetooth 😎:

So I just carefully desoldered an old battery from a BMS board and then soldered a new one instead.

Here it is connected to the device:

And the device successfully booted with a new battery

But there are a few more important things:

1) if your new battery is 4mm - try to put a battery management system PCB next to the battery and not on top of it to make the whole construction as thin as possible

2) battery terminals are way to flimsy and easy to break, but can be soldered no problem with a good flux

3) probably the most important: at first my new battery didnt want to charge and was not really recognized by the device:

So I had to open the device and connect it directly to the bench power supply set up at 5V 2A for like 5-10 minutes. After that I plugged it back into the device and it got immediately recognized:

Hope that helps many other retro gadget enthusiasts out there 🙌🏻