I am making a handheld radio telescope which needs 13-18v to power and currently runs off 2 9V batteries. However it eats power like mad, drains them so hard that it actually needs to be recalibrated every 10 seconds. Now I am looking for a way to give it a stable power supply and more importantly to make it rechargable.

Now I was wondering if it would be possible (and smart) to take appart one of these chargers and wire it in with a swich. one position would make the device run off the batteries and the other side would charge the batteries and run the divice on power (which comes from the plugged in cord)


Alternatively, if that is a really bad idea for some reason, what about making it run on a laptop power supply? (19V but should also work) I got some BNC connectors, so I could swich the head and make it work off of that. It would not be rechargable, but atleast I could use a long cable.

  • \$\begingroup\$ Knowing current drain would be useful | Charger shown very likely has batteries with one side common when charging (eg 2 x -ve joined). You'd have to do some internal cutting to add a switch to change them from semi parallel to series. | Those batteries are not nice. Low capacity and often not a real 9V (measure to check). | Laptop supply is viable. Old laptops are about free. IF you can find one with a > 13V battery pack then you can run scope from battery and charge while using or when not in use. If LiIon pack (all newer ones) V is 7.2 / 10.8 / 14.4 nominal for 2 /3 /4 cells in ... \$\endgroup\$ – Russell McMahon Jan 3 '14 at 11:27
  • \$\begingroup\$ ... series and minimum of about 6 / 9 / 12V . The 14.4V/12V one could be run down till at 13V if scope needs >= 13V. That's 3.25 cell which is not empty but has used much of capacity. | Any regulated mains power supply 13-18V of adequate current will do. | More specific answers available if this sounds useful. \$\endgroup\$ – Russell McMahon Jan 3 '14 at 11:30
  • \$\begingroup\$ It does :P So you are suggesting a laptop battery + charger cirquit? Can I smash/cut/fiddle that out of a laptop and keep it working? \$\endgroup\$ – Coolcrab Jan 3 '14 at 11:40
  • \$\begingroup\$ Easiest is to find an old laptop, gut out definitely unwanted parts, wire to internal contacts to pick up battery voltage and use that way. Modern laptops will usually have the charger proper inside the laptop. It MAY be a board all by itself and it MAY be able to be excised easily so you only need battery + charger plus battery contacts and charging input jack. | Some laptops have the mains-AC to 19VDC supply inside the laptop (I've seen Compaqs that do this and there are sure to be others). You then have the whole unit in one piece. - mains in, battery inside, battery leads out to scope... \$\endgroup\$ – Russell McMahon Jan 3 '14 at 12:04
  • \$\begingroup\$ ... Even if charger is too hard to excise you can remove screen, keyboard, hard drive, floppy & DVD/CD if present, maybe motherboard - so getting quite light. | But, you can get power supply modules on eg ebay or sparkfun or ... for modest cost that boost any sensible battery voltage to voltage in your range at a few amps. Some examples here and here and here. YMMV / Caveat Emptor / NIH / IANAL / Whatever ... :-) \$\endgroup\$ – Russell McMahon Jan 3 '14 at 12:09

You've not said anything about current requirements or available space/weight constraints, but I'd suggest looking at 18650 size cells. They're now readily available on the Internet, have standard chargers available, decent capacity, and a nominal voltage of 3.7V.

Stack four of them in series and you have a nominal 14.8V pack. You can get preassembled packs and chargers designed for use with R/C models, where this configuration will be described as "4S".

  • \$\begingroup\$ How exactly can I charge these guys up then? Is it doable within the cirquit? \$\endgroup\$ – Coolcrab Jan 3 '14 at 12:37
  • \$\begingroup\$ @Coolcrab: you may want to search for "battery management IC". \$\endgroup\$ – RedGrittyBrick Jan 3 '14 at 13:59

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