This one is for the physics teachers....

[Guest tak1]

Okay, so I rigged my 315 to run on a 9 volt battery. The question is: How much longer or shorter will a 9V run as compared to two AA's putting out 3 volts? My best guess tells me that it will go 3 times longer. Any input?

 

LK

[Guest peter]

quote:

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Originally posted by tak1:

Okay, so I rigged my 315 to run on a 9 volt battery.

LK

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First question would be "Why?" AA cells are readily available almost anywhere in the world and in a variety of different chemistries. Why switch?

The second and third questions are what type of 9 V battery and how did you modify the 315? The runtime of your GPSR will depend on the energy capability of the battery, i.e. its voltage under load multiplied by the current to give power, and then multiplied by time to give energy. So the energy available from a pair of 1600 mA-hr NiMH cells is given by:

2 x 1.2 V x 1.6 A-hr = 3.8 Watt-hr.

The 315 uses about .4 Watts, so it'll run for about 10 hours on the NiMH AA cells.

So if you give us the mA-hr capacity of your 9V battery and the power lost in any voltage regulator you added, then we can easily calculate the runtime of the GPSR with that battery.

[Guest logscaler]

Peter; Along the same line, I am using a mag315 rigged to run from two (2) " D " cells into the external power system, same as the cig. lighter input. 3.8 volts in, the same as the "AA"s. My thinking might be flawed but bigger battries should last longer. Any input accepted. (Or did I just waste time and money.)

[Guest Nemesis]

quote:

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Originally posted by peter:

The 315 uses about .4 Watts, so it'll run for about 10 hours on the NiMH AA cells.

So if you give us the mA-hr capacity of your 9V battery and the power lost in any voltage regulator you added, then we can easily calculate the runtime of the GPSR with that battery.

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I agree, if the voltage regulator has to cut the voltage down to 3 V for the 315, at least two-thirds of the power from the 9 V battery will be lost as heat (assuming that you haven't used a switch-mode regulator, this would be more efficient).

 

A typical high quality Alkaline Manganese AA (1.5 V) battery (Energizer or Duracell) has a capacity of around 2500 mAhr, the same quality of PP3 (9 V) battery would have a capacity of around 550 mAhr. So you can see that your 9 V battery will last around one-fifth as long as two AAs (and will probably cost you twice as much too!).

 

Cheers,

Donovan Govan.

[Guest peter]

scaler:

Peter; Along the same line, I am using a mag315 rigged to run from two (2) " D " cells into the external power system, same as the cig. lighter input. 3.8 volts in, the same as the "AA"s. My thinking might be flawed but bigger battries should last longer. Any input accepted. (Or did I just waste time and money.)

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This is much more reasonable than using a 9 V battery (esp. if it's one of the little ones used in small radios and smoke detectors). The D cells will give much longer life at the expense of size and weight, but I would have wired them directly to the AA battery terminals which are intended for a nominal 3.0 V input rather than the external power input that is designed for 3.8 V and probably has an extra series diode voltage drop.

Unless you have an application where you can't change batteries for a long time, the approach I prefer is to use two sets of rechargeable NiMH AA cells and swap as needed between the GPSR and the charger. It's much cheaper in the long run than even the cheapest non-rechargeable cells.