# Advice on battery powered project

Discussion in 'The Projects Forum' started by jboavida, Mar 17, 2009.

1. ### jboavida Thread Starter Member

Jul 10, 2008
23
0
Hi,

I have one circuit ( 12V battery operated) that drives about 60 LED's.
The LED's have about 2V voltage drop, so I use 5 in series and limit the current to 20ma. That gives me a total current of 240mA.
The battery is charged by a solar panel. I must reduce the power needs, so the solar panel be as small as possible (because they are expensive). currently we use 20W panels, but I must find a solution to use 10W or less.

The Leds are on 500ms and off 500mS.

The battery and solar panel must be 12V.

Can someone give me an ideia / orientation. This 1st solution that came in mind is using a step up regulator and power the LEDs with 24V and using 10 in series (Total current will be half).

Another is using a capacitor to store energy, cut the power, discharg to the Leds and connect the power again to another cycle. (a little like a flash). Is that possible ?

JB

2. ### SgtWookie Expert

Jul 17, 2007
22,199
1,801
Why don't you post your entire circuit so far?

How are you regulating the battery charging current?

Are you trying to run the LEDs all day/all night?

If you double the voltage to cut the current in half, you won't have changed the power used, but the boost circuit will have (at best) around 90% efficiency. You'll wind up with a net loss.

Your power consumption in your LEDs is roughly 3 Watts. Somewhere you're losing efficiency.

3. ### jboavida Thread Starter Member

Jul 10, 2008
23
0
Here is a brief description of the circuit:

Led's are controlled and must flash 24/7.

The solar panel as is own regulator and charge controller. I believe they are quite efficient.

My circuit as a micro controller that reads a light sensor, and pwm's the LEDs
so we can have automatic brightness control. The 60 Leds light at once and are switched on/off by a mosfet (connected to the micro controller). The 12 volts from the battery is connected directly to the anodes. The mosfet gives ground to the cathodes via a current limiting resistor. Power supply for the micro controller is taken from the 12V using an standard 78L05. The circuit itself takes around 10mA.

I have up to 4 sets off 60 LED's. In the worst case they light up all at once. This gets 12W from the battery. A 10W solar panel is not enough.

I must have a 5 day autonomy without sunshine. The goal is reducing the size of the panel and the batteries.

Thanks
JB

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
8
You need 12 W * 24 hours * 5 = 1440 Watt-hours. At 12V, this is 120 AmpHours. I recommend 140 or 150 AmpHours worth of batteries to have a safety margin.

To gauge minimum panel size, one needs to know the peak sun hours where your gizmo will be used. http://www.oynot.com/solar-insolation-map.html

If this must function in winter, it would be better to use the winter numbers for peak sun hours.

I suggest a minimum panel size, in Watts, of 1.2*(rated output)*(peak sun hours)/24