Lance Allen

The internal resistance of any power source is a representation of that batteries ability to provide power to a load. This can be thought of as a battery capable of delivering infinite current at no voltage drop in series with a resistor, that resistor being the internal resistance. So a small 12volt battery will have a higher internal resistance than a big 12volt battery and a damaged battery will have a higher internal resistance than an undamaged battery.

You can determine internal resistance by loading the battery and watching the voltage drop across its terminals. If a 12 volt battery drops 1 volt under a load of 1 amp then its internal resistance is 1 ohm. Internal resistance will vary with the state of charge so compare batteries at a common condition, such as full charge.

Manufacturers data is usually comprehensive on a batteries ability to deliver power at different rates, temperatures etc.

A lead acid battery is fully charged at a terminal voltage of 12.6 volts, 25% discharged at 12 volts and flat at 10.8 volts. DO NOT discharge a 12 volt Lead Acid Battery below 10.8 volts or it will be damaged over time.

So (from manufacturers data) a Hitachi 12 volt 7 Amp Hr Lead Acid battery has a fully charged internal resistance of 25 milliohms and can supply a maximum current of 40 Amps for 5 seconds. The maximum charging current is 2.1 Amps.

Alan King on using caps in low power applications

...[I found some] black filter caps with gold lettering said "gold cap" on them. Sure enough, those numbers really said 2.3v 10F, big F, no pesky little u in there! Charged it up to 1.93v and put a superbright red led and 1K resistor on it, and it's still glowing well four hours later. I can't wait to see how long I can get them to run a PIC doing some low power data sampling. Definitely an alternative to batteries for some low power apps, about 1/2" shorter and 1/8" less diameter than an AA cell. A bit pricey, $7.50ea from DigiKey in singles, but not as bad as I was expecting, and drops to around $3 each at 1K, probably less from some other distributors. And the current version is rated 2.5v, so you only need two to start out with 10F at 5v. I'm only 30, but I remember how they used to say it'd take a railroad car size to have a 1F cap not so very long ago, much less 10F. Neat to have a small train's worth of cap powering your PIC..

See also:

• Hawker Energy 100% deep cycle and cranking power
• http://www.xs4all.nl/~jwasys/diy4.html
• Linear Technology make what they call Micropower switching regulators (LT1073)
• TELCOM TC55 series. Available from Digikey. I've got two on a board, with a total quiescent current of <3 uA out the ground pin with no load. <100mV drop out at low loads, and CHEAP.
• LM2936 regulator for low current consumption supplies a maximum current of about 50ma but has a quiescent current of around 10 microamp actual Same TO92 package as a 78L05

• Power Supplies
  • Astrodyne - Switchers and DC to DC converters
  • Autec - Switchers including power supply used in DRC190
  • Ault - Extensive line of international wall warts. Available from Scott Partlow at Powerspec at +1 408 748 9900 x229
  • CET Technology - phone +1 603 894 6100 - Wallmount, desktop, linear and switcher
  • Elpac - Universal input AC adaptors and more
  • Friwo - AC/DC wall adaptors, including international. Pages list part numbers for DC output wall warts for US, Euro and UK use. The MultiPower Plug 10 has interchangeable input plugs for US, Japan, Europe, UK, and Australia.
  • Golden Pacific - External switchers
  • Jerome Industries - Various, including wall wart
  • Newport Components - DC to DC converters
  • Operating Technical Electronics - Universal input switchers, including external
  • Phihong USA - Universal input switching wall warts, and more
  • Pillar Industries - AC/DC adaptors, including international
  • Rodale - Wall warts, transformers, power supplies
  • Shogyo - Transformers and wall warts
  • Total Power International - Wall warts and table top
  • Xentek
• Semiconductors
  • Power Integrations - Switching power chips
  • STMicro - SGS
    • Viper100 - 100W SMPS chip