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I 'm planning to use a zinc–carbon 9 V battery to power up two circuits (the one that battery belongs and one more). I 'm thinking of using a breadboard in order to achieve this.

The second circuit is BeagleBoard-xM (5 V, 300 mA), that will be powered up, via buck converter LM2596S-ADJ.

My question is:

Is this possible or there will be voltage drop? I think that I can do this, but this way battery will last less. If I am not correct, please let me know.

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  • \$\begingroup\$ I wouldn't use a zinc-carbon battery for anything these days. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 16 '14 at 14:34
  • \$\begingroup\$ It would really depend on how much power the original circuit takes. There will be a voltage drop, the question is how much? Check these graphs out for an idea of what kind of voltage/current curve you will get: powerstream.com/9V-Alkaline-tests.htm \$\endgroup\$ – horta Apr 16 '14 at 14:52
  • \$\begingroup\$ @IgnacioVazquez-Abrams The carbon rods can be removed and used for welding or making an arc light. The "heavy duty" Z-C batteries are "okay" for light-duty (eg. 9V multimeters). Not for hundreds of mA or A. \$\endgroup\$ – Spehro Pefhany Apr 16 '14 at 15:20
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If you mean 6F22 9V zinc-carbon battery - this is VERY BAD idea.

1. You need much bigger battery (or a bit bigger modern LiIon or NiMH rechargeable battery)

2. Zinc-carbon is ancient type of battery and you can't draw such big current from it.

That kind of battery will be overloaded, may leak horribly. Stuff that leaks from that kind of battery is acidic may damage your PCB, hands, pants etc.

Converter with such design parameters (9V -> 5V/300mA) will draw about 150mA current from 9V, but there will be about 5-7V instead of 9V due to battery internal resistance/overload.

Maybe new battery will not drop that much, but after few minutes you may observe, that inverter draws even more (200-300mA), battery voltage dropped and overload is getting worst, because inverter tries to maintain output voltage.

For that load (5V * 0,3A = 1,5W) you need battery with 15Wh capacity.

Wh capacity = voltage * Ah capacity

10 hours discharge is safe for almost all batteries on the market.

To get close to 15Wh capacity you need

  • 5 cells AA size NiMh 2600mAh 1,2V or
  • 2 cells 18650 LiIon 3.6V 2000mAh
  • 1 cell D size 13500mAh 1.2V (and boost converter)

There are also batteries designed for high current draw (RC model batteries, some modern NiMh), you can discharge their full capacity much faster than 10 hours, but they wear out faster. However I would not recommend them for you, just stick to 10 hour safe discharge.

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  • \$\begingroup\$ Thank's for the advice. I 'll use 9V Z-C only for the circuit that is designed for and NiMH batteries for BB-xM. Is it ok to use 4 cells AA 1.2 V 2600mAh or 5 cells are better? I edited my question with BB-xM's electrical specification. \$\endgroup\$ – dempap Apr 16 '14 at 21:07
  • \$\begingroup\$ 4x AA 2600mAh is much better. However notice, that fully charged NiMH have 1.32-1.35V, and discharged will drop to 1.0V or a bit below. That means 4x1V = 4V at end of battery cycle and you may need diffrent kind of converter it to get 5V. \$\endgroup\$ – Kamil Apr 17 '14 at 3:50
  • \$\begingroup\$ Is there any way to calculate time for batteries to discharge in this case (300mAh)? \$\endgroup\$ – dempap Apr 17 '14 at 10:55
  • \$\begingroup\$ @dempap "Typical battery" load should be no more than 0.1C (0.1 of capacity). 0.1C charge/discharge for 300mAh is 30mA. By typical battery I mean standard Li-Ion, Ni-Mh, Ni-Cd, lead-acid and alkaline batteries. \$\endgroup\$ – Kamil Apr 17 '14 at 11:14
  • \$\begingroup\$ @dempap And if you are asking about discharge time of 300mAh battery with huge current overload - maybe it's possible to calculate this (read time from discharge curve from battery documentation), however 300mA load for 300mAh ZnC battery may be not covered in battery documentation because that current it's probably outside chart :) My guess is leak and battery death after at 15-20 min. You may try it, just be careful with that corrosive leaking stuff. \$\endgroup\$ – Kamil Apr 17 '14 at 11:18
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Electrically, there is no problem running both devices off of the same battery.

However Zinc-carbon 9v batteries only have a capacity of 400 mAh, and Alkalines are not much better at 565 mAh, so neither is going to power your BeagleBoard much more than an hour or so (you can't run the battery all the way down). And that is not taking into account the original circuit you want to use the battery for, with an unstated current consumption.

Instead you should switch to a higher capacity solution, such as using six high-capacity AA Alkaline batteries with a capacity of up to 2600 mA, over six times the capacity of the individual 9v Zinc-carbon battery.

A better option, if the two devices do not need to be portable, is to use a 9v wall-wart (be sure it's regulated), or use the one designed for the BeagleBoard and run your original device off the 9v battery.

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  • \$\begingroup\$ I think Z-C battery will overheat and leak horribly after 10-15 minutes overloaded by buck converter (it will work like "constant power" converter and draw even more current when voltage drops). \$\endgroup\$ – Kamil Apr 16 '14 at 19:05
  • \$\begingroup\$ @Kamil I didn't mean to imply he should use a Z-C battery, just that a 9v Alkaline wasn't really suitable either. I've edited my answer to indicate he should only use AA batteries or higher capacity. \$\endgroup\$ – tcrosley Apr 16 '14 at 19:18
  • \$\begingroup\$ I know, I'm just sharing my opinion/experiences :) \$\endgroup\$ – Kamil Apr 16 '14 at 19:27
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Yes, you can, yes there will be some voltage drop according to Ohm's law. How much depends on your battery's internal impedance, wire gauges and trace resistance. Why use a carbon-zinc battery?

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