Generic battery packs...

I have a toshiba genio pocket pc 2002 and am becoming more and more dependant on it every day. The only problem I have is that now I am starting to put more cards into and use it more often so that the battery live is becoming shorter and shorter, down to about 4 hours with constant use of a wireless card and mp3 software. I have seen battery packs advertised on e-bay. They hold 4 AA batterys and supposedly will give you up to 2 charges of your original battery...this made me think, which is really, really dangerous. What if I had a battery pack like this that accepted 4 9000 mamp rechargeable "D" batteries? the voltage on rechargeables are, I think, 1.3 volts, so that 4 would be 5.2 volts. My pda takes 5.0 volts.... any ideas? has anyone tried anything like this? I know it is heavy but I often travel in China by train or bus to very remote areas...it would be very useful if I could run my pda for at least 20 hours with all the bells and whistles going ...

T

Nickel metal hydride cells have a typical float of 1.2V. They may read 1.3v when freshly charged, but that drops quickly to float value. It should be perfectly safe to use, but the slightly lower voltage might not properly recharge your onboard lithium ion pack. But it shouldn't drain it either. Any halfway decent PDA would have built in polarity inversion protection, but just to be on the safe side you should implement either a diode block or a bridge rectifier into the interface. That will not only prevent blowing out the PDA if the external batteries are inserted in wrong, but also prevent backflow. (which is unlikely anyway). I use a 5-cell setup myself, with a float a 6V through a bridge and then a 7805 voltage regulator and a current clamp zener. Crude but effective. Those 9amp D cells are great. Just the thing if you go walkabout. Several days worth of use out in the woods and perfect for mounting in a trail bike with a dynamo.

mine can read even 1.45 of course only for some minutes....

Thanks! Does anyone know, by chance of a source that has plans to do these things? I live in Tianjin,China. I think I know where I can get a battery case but am not sure about the electronic end of it. Is this something simple to rig up?

Somebody in the forums has made two MP 790 batteries into a dual MP 790 battery. search for the thread

If your device uses a regular barrel plug, then it is dead simple. Just wire it like a regular battery pack. You can use generic battery holders or make your own. If using NiMH cells, then just count 1.2v per cell till you reach your device's required power input or a little higher. You will have to charge the 'booster pack' separately though. Or you can do what I did when going on extended walkabout on a mountain bike. Use a small sealed lead acid deep cycle battery (used on cellphone relay towers, traffic lights and marine bouys) and charge it with a dynamo in the drive train. Depending on the load on the battery (lights, radio, mobilepro...) and the rating of the dynamo, you actually top off instead of drain the battery during the trip, and arrive with a fully charged SLA battery good for 2 days worth of constant use.

PengYou, its a fairly easy setup. If you are going for the 4XAA cell setup with regulator, it goes something like this:

http://www.national.com/ds/LM/LM341.pdf

Input power source is maxed at 15v for the 7805 chip from national. Other company's chip may vary.

http://www.national.com/pf/LM/LM78M05.html

Edited by Snappy! 2005-07-09 11:32 PM






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7805.JPG (11KB - 8 downloads)

Thanks for your help! I guess....if it is 1.2v per cell - I am hoping to go with "D" cells at 9000 mamps each - I will either have to settle for 4.8, less than I need, or 6.0, more than I need. I am guessing that more is the way I have to go. Will the resister shown in your drawing tolerate that voltage?

T

That's not a resistor, it's a voltage regulator. It can take from 3.5v to 15v as input, and it puts out a constant 5v. They are very cheap, simple to use and fairly tough. But not very efficient when driving large loads. Since this is to power a PDA, it should be more than adequate. The circuit diagram Snappy posted is just about all you need in addition to the batteries. I have a similar setup as well using just that 7805 with a 5-cell 6v source and a regulated 5v output for powering discrete electronics. You can use a 6-D-cell battery enclosure, modify it to hold just 5 batteries, and the entire circuit will easily fit in the space left over from the missing 6th battery.

Note that it is usually a good idea to put buffer capacitors before and after the voltage regulator, and an anti-reverse-polarity diode on the output.



Edited by Zapper 2005-07-10 6:11 AM

A picture is worth a thousand words... Note the components are not in scale.

From left to right:

battery bank
power pack switch
input buffer capacitor
voltage regulator
output surge capacitor
polarity inversion protection diode
overload fuse

As you can see, it takes very little soldering. You won't even need a circuit board.

Note that if you use a 7805 made by National Semiconductor, the voltage regulator needs an input voltage of between 7.5v and 15v for best regulation, and with a maximum input of 35v. Other manufacturers have slightly different specs. The included schematic should be plenty 'good enough' for keeping a PDA topped off.

Edited by Zapper 2005-07-10 7:24 AM






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PS6-5vr.gif (20KB - 8 downloads)

hey, that's a really good idea to use one of the cell space for the circuit! I have a four-cell case and am wondering how to fit in the circuit! ... good tip!

oh, on that, I have a USB female connector so that any USB charging/sync cable can use the battery pack. Combining with your idea of using the last cell space, the connector can fit snugly inside the case too!

EDIT: On your diagram, did I say that its awesome? No, well, its awesome!

Edited by Snappy! 2005-07-10 8:41 AM

Thank you, kind sir.

Excellent idea with the USB connector. That definitely seems to be a trend that's catching on. USB bus powered devices that might not even use the port for anything *but* power. Neat trick. I should consider putting that in on the next one I build.

An alternative might be doing the same thing that Wally did in another forum. Wire a mini-hub into the enclosure. Then you have power as well as data.

With regard to the circuit, I use the exact same thing with a 12v sealed lead acid battery for my mountain bike. The only difference is the use of a 7812 to keep from burning out the onboard electronics during a hill descent with the dynamo on charge. The onboard 12v lights and radio... And a 7809 for the mobilepro...

... that's really neat, having the 12v thingie ... yeah, with the various 78xx chips, its fairly easy to mod the battery pack for different devices.

Good thing about using the AAs is that they are aplenty these days and rechargeables are so easily found, likewise alkalines ...

Cool! I know that most of the ppc's use 5 volts. what about things like the Jornada, the mobilpro, the aero, etc? is it more like the 9-13 volts of most notebooks? or are we still talking about low voltage? If I were willing to lug around 8 of these 9000 mamp "d" cells can I still connect the cells in series or should I make two sets, each as shown in the diagram and then connect them in parallel? I am guessing that the latter would reduce the heat build up. Hmmmm are there any little flash light thingies that work on 5 volts? When I travel I often like to read in bed after the lights have been turned out...What would be really cool is if I could create a little power strip on the case of batteries so that i could plug a couple or few 5 volt devices in, maybe even my mobile phone to recharge it! Wow...this is really freeing...you are all a great inspiration...and yes, as a matter of fact, I am a little crazy...but that's what makes life interesting

...and...if I put the whole shooting works in a case would it be possible to build in a recharging system also? Keep in mind my desire target is 9000 mamp d cells...preferrably 8 of them...


Edited by pengyou 2005-07-11 1:20 PM