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# Mimicking a 3.7v lithium battery with solar power Answered

I'm trying to replace a digital cameras' battery with a solar power source instead. The camera would only work in the sun with this power source, but that's exactly what i want.

The battery I'm trying to mimic is 3.7v at 760 mA. I was thinking of making a solar panel with an output of 5v at 1amp, and then using a voltage reg to bring the 5v down to a steady 3.7v.
Can I get a 3.7v regulator? Is there such a thing as an adjustable/customizable voltage regulator?

Would 4v be fine instead of 3.7v? is it close enough?

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## Comments

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I should have mentioned in my reply that if you are using solar as a trickle charger, that you only have to have capacity to replace what you use.

For example, if you took 100 pictures a day using the Canon Power Shot I described, you would use up 100/140 *  2300 = 1642 mAh. If you lived where you were getting 8 hours of useful sunlight, you would need solar cells capable of producing 1642/8 = 205 mAh average.

You also don't have to worry about power dropouts.

A fully charged Li battery has a voltage of 4.2V. When it is fully discharged the voltage is around 3V. The 3.7V rating that you see on Li batteries is approximately 1/2 charge. Often when you get a new Li battery it is charged to 3.7V, because a battery not in use will have longer shelf life if stored at this voltage.

Look again at the battery and check the 760 rating. This rating is mostly a charge rating not a current rating. The value's units, I suspect is 760 mAH not 760 mA. A 760 mAH rating means the battery can run either one hour at 760 mA, or 760 hours at 1 mA.

A one amp power supply for a camera is excessive. Especially if you are not planning to use flash.

The more normal way of setting up this type of project is to use a non-Li battery chemistry like Nickel Metal-Hydride, and keep the rechargeable NiMH battery in the camera and use solar power to trickle charge the battery. Li batteries do not trickle charge well.

The Canon PowerShot cameras that use 2 AA batteries, work well for this type of project.

You have to be very careful not to exceed the 4.2V full charge voltage with a Li battery battery, because most Li batteries have a protective over-voltage circuit built in that will open (and make the battery useless) if the voltage exceeds 4.275V.

What sort of amperage do you think would be appropriate for a digital camera? if I knew how long a charge on the battery lasted, and with the 760mAh rating on the battery, could I calculate how many mA the camera uses? I really want to circumnavigate the need for a battery entirely.

The amount of power being used by a camera depends on the camera and the functions that you are using.

Your idea of checking how long the charge lasted and using the 760 mAh would give you a good data point for an idle camera situation. The camera manufacturers usually spec the number of photos that a set of batteries can take when a picture is shot every 30 seconds while alternating shots at max and min zoom.

I was able to measure some currents on a Canon Power Shot A720IS (nicely functioned point and click, 8 mega-pixel camera). This camera extends the lens when you turn it on using a motor (high current). It has a LCD screen that use a lot of current when not taking a photo. When taking a photo the stabilization and focus systems come into play (a motor again - high current). Using the test listed above, the manufacturer says the camera will take 140 photos on a set of alkaline batteries (2300 mAh). This seems a little conservative based on my use of the camera, but not too far off.

Canon Power Shot:
Power Supply = 2 AA alkaline batteries (2300 mAh)
1.  Turning on camera (lens motor out) and LCD ~ peak of 720 mA
2.  Camera on, LCD monitor on, not taking photo = 460 mA
3.  Camera on, LCD monitor off, not taking photo = 105 mA
4.  Camera on, LCD monitor on, taking photo ~ peak of 750 mA
5.  Camera on, LCD monitor off, taking photo ~ peak of 550 mA
note: where the ~ signs are my meter can't sample fast enough

By contrast a simple key ring spy camera (eBay) which is similar to the camera in a cell phone, takes about 145 mA. It has a pin hole lens, no zoom, no focus adjust (i.e., no motors). This camera is powered by a Li polymer battery rated 3.7V, 200 mAh. An issue with this camera is that even when it is "off" it is running a real time clock, so the battery is slowly drained even when not being used.

My main concern for the success of your project is that if you do not retain some sort of storage element all your electronics is at the mercy of a cloud.

I hope this is of some help.

I'm pretty sure the 760 is mAh what your camera takes in mA will be something different.
How much continuous use can you get (hours) from a full charge?

L

Do you mean the lifespan of the freshly charged battery? about 1-1.5 hrs. And yes, the battery is 760mA, but the camera should only take the amps it needs, right?

Batteries are rarely quoted for amperage at that precision. If it's 760mAh, your camera is averaging 500-700 mA. I agree with ork' on the possibility of data-corruption, I wouldn't try it like this.

L

Will a 3.7v 1350mA battery charger charge a 3.7v 760mA battery (different mA)? The chargers seem to tell when a lithium battery is full when it reaches a charge of 4.2v. Is the mAH relevent in lithium battery charging?

I would expect the charger to work with either, they're somewhat "smart" like you say.

L

Yes, there are configurable voltage regulators.

I have no idea whether your camera would tolerate 4V. You could try asking the manufacturer.

Personally, I really wouldn't recommend this as you've sketched it. If the sun goes behind a cloud for a moment, the camera loses power. And the camera losing power in the middle of saving an image may not only garble that image, but leave your flash memory in an unrecoverable state. Much, much, MUCH safer to use the solar cell to recharge the camera when it is not in use.

Seconded the second half fully. is that a quarter?

If so, I appoint you quartermaster.

Quarterhorse or porterhouse?