You can never tell about these Mobilepro batteries. I mean they really last a long, long time. But when they die, they are pricey little devils to replace. Even if you get one on sale, there is a chance it will be no good because they have been sitting in a warehouse for years doing nothing. My favorite shopping place, eBay, often has them, and I've picked up good working batteries for as little as a buck, and have seen others sell for as much as $75 or more.

Like, like laptop batteries, they are easily rebuilt if you feel up to the task. There is really nothing to it, and it can save you big bucks.

The following is an unofficial file that I have created that demonstrates how to rebuild a NEC Mobilepro 770/780/790 series handheld battery. The same principles apply to the 900 series battery. There may be better instructions, but I haven't found them out there on the internet. Disclaimer: You do this at your own risk. Optimally you would perform this using proper tools such as an antistatic mat, grounding straps...you know, all the electronic safety tools that electronic technicians use. Since I don't have access to all of that, I will just use an X-acto knife, some scotch tape, and of course, a soldering gun while I sit at my desk.

The first step is to open the battery...carefully!

Now, using a new sharp blade in an X-acto type knife, score along the seam of the battery until you have cut completely through the case. This picture shows the seam (I highlighted it with a white line). The scoring and separating of the case is the most difficult part of rebuilding a battery. It simply takes patience, and about 100 scores on each side and the ends.

When you are done, simple remove the top half of the case to expose the "innards" It may be held in place a little bit by the adhesive NEC uses, but it will pull apart fairly easily.

The existing batteries and the circuit board can now be removed from the bottom half of the case. This part is simple, just untape the wires, remove the metal leads from the existing batteries and solder the replacement batteries in place. Be sure and note the direction (polarity) of the existing batteries. The positive terminals point towards the left as you see it sitting in the case in the next picture.

The batteries themselves are really cheap and easy to find. I have a couple of different batteries here in the same picture that all work fine. One is a Sony Energytec 18650S, another is a Panasonic CGR18650.

Where you buy them doesn't matter, shop around for the best price. I got mine online from a surplus parts warehouse. In my case, the batteries were part of some type of cellular phone battery pack, but when you took the pack apart, you discovered two brand new cells. Not bad for only $5. By the way, this battery is the most commonly used battery in laptops too.

If money isn't a problem, shop around for the highest capacity battery you can find. 2600mAh rated batteries will give you the longest time on a charge.

By the way, the next picture shows the power control circuit. This circuit limits the charging of the battery pack to the optimal level and prevents overcharging the lithium ion cells. Handle it gently. Ruin it and you are out of luck.

The final step in the process is simply putting the halves back together again. After all the work you did, you might be hesitant to re-glue it. I never did, but if you do decide to do so I would suggest that you simply use a black colored silicon adhesive, like you would see in bathtub caulk (epoxy works well too, but is pretty permanent). Rubber band the halves together and then fill the seam with the adhesive and let it dry. It will be very easy to take apart again should you ever need to do it.

Or you can do as I did here. I simply used some cellophane tape and carefully ran a strip lengthwise on both sides (you don't need to tape the ends) and you will find that this holds pretty darn good, and doesn't look bad either. When I was all done, I plugged it in my Mobilepro and let it charge up. I found myself with a battery that lasted hours like brand new...which it essentially is.