Designing effective usage time from 12v car battery


Need a little power design help:

I have a project I'm working on where I have measured that I will use approximately 14.4Ah over 24 hours. This is quite the load for a small timelapse setup where I need the camera clicking for 5 months. I need 24 hour power because once the camera turns off, it needs a human to press the power on button. I will have access to the site, but it won't be everyday access – most likely on the weekends.

I think I'm looking at needing to go with a deep cycle marine battery and a 25w solar panel to keep this system powered on and the battery charged.

As an example, if I have a 40Ah 12v car battery, and I know my draw is 14.4Ah a day, then this battery can give me 2 days of usage?

However my project requires only 5v, and my 14.4Ah is calculated from 5v.

This is where my math breaks up, but if I convert the car battery 12v to 5v output, and draw 14.4Ah a day from it, does this mean I'll get approximately 4 days of usage?

Ultimately I'd like to hook a 25w solar panel to the battery to try and keep it charged, too.

Thoughts? Thanks!

Edit: adding a little info about my project. It may help with the design.

I'm an electronics rookie, I know enough to get me in trouble, but not enough to help fix it 🙂

I'm looking to setup a long-term timelapse camera. The enclosure will consist of 1 camera and 1 mobile broadband device. I'd like to use solar power to try and keep the unit operational.

Some brief details on my setup:
The camera is a Canon ELPH 130 point and shoot which has firmware on it to take 1 photo every 10 minutes, between 6am and 6pm. When a photo is taken, it'll save it to an Eye-Fi SD Card. When the Eye-Fi receives a new photo, it'll connect to the MiFi and automatically upload it to the internet.

I've connected my Multimeter in-line with these devices to gauge their amp usage.

Here's my findings:

  • Camera: average current of 0.25a over 2 hours.
  • Mobile Broadband: average current of 0.35a over 2 hours.

Both devices showed various spikes – mainly when a photo was taken and uploaded. Then the current seemed to "settle down" back into the averages above.

The downside: I need these devices up 24 hours a day because if they lose power it requires human intervention to push a button to power them back on.

So here's my math:

  • Camera: 250mA x 24 hours a day = 6000mAh
  • Mobile Broadband: 350mA x 24 hours a day = 8400mAh
  • Totaling: 14400mAh battery required to keep this thing going for 24 hours.

One thought on the camera and current usage: The camera does not have a sleep function, so it's powered on all the time. It does have a power-reducing mode (LCD off), but that's about it. Perhaps I can connect some jumper wires to the power button to be able to control on/off without a human needing to press it?

Best Answer

To do these calculations, it's easiest to work in terms of power instead of current. For example, if your camera is averaging 250mA at 5V, then that is 5V x 0.25A = 1.25W. If you are running it for 24 hours, then that's 1.25W x 24h = 30 Watt-hours ("Wh") per day.

For comparison, your 40 Amp-hour ("Ah") battery, at 12V nominal, has 12V x 40Ah = 480 Wh of capacity.

With no other considerations, this would mean that the camera (by itself) would operate for 480Wh / (30Wh/day) = 16 days on the battery. Of course, there are many other considerations, including the inefficiency of converting your 12V to 5V...

Please look at this answer I made for a different application. It details the considerations and calculations for a battery powered system with possible solar input.

Good luck!