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I am about to start construction of a new house, and unfortunately the property is not near where I live. I wanted to keep an eye on the construction site (No electricity) to make sure workers are showing up. To do so I want to put a cellular phone in a bird house, which will transmit a video from the camera over the cellular signal. (The Android/iPhone app is called "Alfred") My only problem is, how do I keep my cellular phone powered for 7 to 14 days without charging?

I would GREATLY appreciate any help anyone can give. I think the best option would be to connect a 6 or 12 volt motorcycle or car battery to the phone, but what do I need to consider other than converting the voltage from 6V or 12V to 3.7V? How do I convert from 12V to 3.7V? The screen on the phone will be off the whole time. All I will be running is the camera, and data transmition.

Does it make a difference if I connect the external battery directly to the phone so the phone thinks it is the internal battery versus connecting the external (6 or 12v) battery to the phone via its USB connector which the phone would think is a charger?

Solar won't work because of tree coverage. My other thought was breaking open laptop batteries and using the cells. I would like the least expensive and a simple way of doing it. Can I just connect a cigarette lighter USB plug to a car battery directly and plug the phone into the USB?

Thank you in advance for any help anyone can give.

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closed as off-topic by PeterJ, Daniel Grillo, Ricardo, Scott Seidman, Nick Alexeev Oct 2 '15 at 6:50

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – PeterJ, Daniel Grillo, Ricardo, Scott Seidman, Nick Alexeev
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Any chance you could edit your question to use paragraphs? It's a wall of text right now. \$\endgroup\$ – John U May 11 '15 at 14:20
  • \$\begingroup\$ As requested I put it into paragraghs. Thank you. \$\endgroup\$ – Travel May 12 '15 at 17:39
  • \$\begingroup\$ There are GSM enabled surveillance cameras that are intended to operate on battery power. Some can be activated by remote SMS or motion sensor. I expect some could be activated by a timer of you could add your own to the motion sensor input. There are also simple game cameras that will record all motion for long periods and save on memory card. \$\endgroup\$ – KalleMP Nov 1 '16 at 21:15
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Can I just connect a cigarette lighter USB plug to a car battery directly and plug the phone into the USB?

This is exactly what I thought of when I was reading your... text wall. Using an USB plug would be the cheapest way to power your phone, concerning both, costs and time.

The plug reduces the 12V from the battery to 5V for USB. There are still old plugs with a linear regulator out there, which waste more energy than the phone consumes. As they have to dissipate this as heat, they usually can not provide much power. So, pick one which can deliver 2-3 or more Amperes, as those plugs contain a more efficient switching regulator.

About calculation: My phone has a 3100mAh battery (3.7V) and lasts 5 days when idle. So, the battery contains an energy of 3.1Ah * 3.7V = 11.5Wh

A small 12V 4Ah scooter battery contains 48Wh , which is four times the energy of my phone. So, it in principle it will last 20 days.

Note that this is a rough estimate. Switching power supplies are also not 100% efficient, so may be it's only 70-80%.

In your case, check out how long the battery lasts when doing the desired job. Keep in mind that signal strength also plays a role.

Finally, it may not be legal to keep your workers under surveillance.


From the comments:

Anyone know if it is more efficient to connect an external battery to the phone via USB and charge the internal battery or connect the external battery directly to the battery connector inside the phone? Lastly there are 4V 20AH lead acid batt. Can I connect direct to phone?

It would be less efficient to charge your phone battery from the external battery, as charging always goes with some losses. However, if the phone battery is full at the beginning, you have the charge of the external battery plus the charge of the phone battery, and in total, the phone will last longer.

About the 4V: I doubt the phone will take the 4V from the external battery supplied via the USB port, as this is clearly below the USB standard. On the other side, a fully charged 3.7V LiIon battery has about 4.2V

This means it should be OK to remove the internal battery and connect to an external voltage of 4V. (It is not recommended to switch batteries, especially different types in parallel.)

On the other side, a lead-acid battery behaves similar: When it's full, it has up to 2.4V per cell, so 4.8V for your battery. This is only 0.6V above the maximum of the LiIon battery. I would guess, that it is OK, but I can't guarantee it. But if you measure the voltage of your battery, the actual voltage may not be that high, and then it's OK.

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    \$\begingroup\$ " lasts 5 days when idle." I was under the impression the phone would always be transmitting. Otherwise, how is it going to answer and then connect to the app? The "Alfred" app is really designed to be used with Wi-Fi, not a cellular connection. \$\endgroup\$ – tcrosley May 11 '15 at 17:14
  • \$\begingroup\$ I also don't expect many phones lasting 5 days when idle... This is just a number. @Travel should test his phone battery when the app is running. Also, it's not clear if the phone consumes the same power from USB as from its battery. \$\endgroup\$ – sweber May 11 '15 at 17:28
  • \$\begingroup\$ "Finally, it may not be legal to keep your workers under surveillance." It seems to be legal given the number of cameras in stores and restaurants etc. They're not just to catch customers. Think casinos. Cameras everywhere. And you know they're watching dealers too. But you may have to tell the employees they're being watched. Not a bad thing actually. \$\endgroup\$ – tcrosley May 11 '15 at 22:57
  • \$\begingroup\$ A bit of off topic, but it is completely legal (in the US) to video tape your employees unless the associated activity therein is otherwise illegal or breaks local/state regulations (every place has to be different) : workplacefairness.org/surveillance#1 \$\endgroup\$ – Kurt E. Clothier May 12 '15 at 5:32
  • \$\begingroup\$ In terms of the answer, the estimations are just that: estimations. My phone can go 6 days on one charge if I don't use it at all. My wife's identical model phone can't go 24 hours. It all depends on battery life cycle state, past usage, etc. The screen is the single biggest power drain, but being always connected to a network and transferring data while recording video (hi CPU usage) will be a big power draw as well. The only way to know for sure is to test the phone in question, then use the calculations presented with the measured data. \$\endgroup\$ – Kurt E. Clothier May 12 '15 at 5:36

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