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skyl4rk · 12-20-10, 10:18 AM

I did some testing on Cree 1W Warm White Star LED's to see if they would work within the range of a 12V lead acid battery system, with 4 Star LEDs in series using no voltage regulator.

12V lead acid battery voltage range:

Min: 50% discharge at 67F: 12.61V
12.61V / 4 LEDs = 3.15 V across each LED

Max: Maximum recommended charging rate: 14.4V
14.4V / 4 LEDs = 3.6V across each LED

Note that there may be an inrush current when the LEDs are switched on, this does not take inrush current into account.

Voltage range for each LED: 3.15V - 3.60V

I did some testing using 2 LEDs in series, setting the Voltage using a V regulator, and measuring A. The single LED V is one half of the measured V. Note that amps should be the same for a single LED.

Voltage - single LED V - measured A
7.35V - 3.68V - 0.43A
7.23V - 3.61V - 0.40A
7.21V - 3.60V - 0.38A
7.10V - 3.55V - 0.35A
7.03V - 3.52V - 0.34A
6.84V - 3.42V - 0.30A light still bright
6.71V - 3.36V - 0.26A
6.48V - 3.24V - 0.21A
6.32V - 3.16V - 0.19A
6.24V - 3.12V - 0.17A light still relatively bright
6.04V - 3.02V - 0.12A light still usable though not as bright
5.72V - 2.86V - 0.06A
5.41V - 2.70V - 0.03A
5.18V - 2.59V - 0.01A
4.94V - 2.47V - 0.00A light appears, very dim

At the battery system max, 3.6V across one LED, the measured current is 0.38A, which is higher than specified at 0.35A. Brightness is very bright.

At the battery system min, 3.15V across one LED, the measured current is 0.19A, and brightness is satisfactory.

Ideally, at battery system max (3.6V per LED) current should be the rated current (0.35A), which would indicate a resistance of 3.6V / 0.35A = 10.3 ohms.

A four LED in series system shows 0.38A at 3.6V, or a resistance of 3.6V / 0.38A = 9.5 ohms.

Adding a 1 ohm resistor in series would reduce that amperage at battery system max to just under the rated 0.35A. 3.6V / 10.5 ohms = 0.34A

It might provide some protection against inrush current when switched on.

It would diminish the amperage at the battery system min as follows:
3.15V / 10.5 ohms = 0.30A

At 0.30A the light was still bright.

Each LED needs 1 ohm. With 4 LEDs, 4 ohms are needed.

So with a 12V lead acid battery system, I would recommend using 4 Cree 1W Warm White Star LEDs in series with a 4 ohm resistor, 1W or better, in series. NO REGULATOR IS NEEDED.

12-20-10, 10:18 AM	#10
skyl4rk Helper EcoRenovator Join Date: Sep 2010 Posts: 66 Thanks: 3 Thanked 6 Times in 4 Posts	I did some testing on Cree 1W Warm White Star LED's to see if they would work within the range of a 12V lead acid battery system, with 4 Star LEDs in series using no voltage regulator. 12V lead acid battery voltage range: Min: 50% discharge at 67F: 12.61V 12.61V / 4 LEDs = 3.15 V across each LED Max: Maximum recommended charging rate: 14.4V 14.4V / 4 LEDs = 3.6V across each LED Note that there may be an inrush current when the LEDs are switched on, this does not take inrush current into account. Voltage range for each LED: 3.15V - 3.60V I did some testing using 2 LEDs in series, setting the Voltage using a V regulator, and measuring A. The single LED V is one half of the measured V. Note that amps should be the same for a single LED. Voltage - single LED V - measured A 7.35V - 3.68V - 0.43A 7.23V - 3.61V - 0.40A 7.21V - 3.60V - 0.38A 7.10V - 3.55V - 0.35A 7.03V - 3.52V - 0.34A 6.84V - 3.42V - 0.30A light still bright 6.71V - 3.36V - 0.26A 6.48V - 3.24V - 0.21A 6.32V - 3.16V - 0.19A 6.24V - 3.12V - 0.17A light still relatively bright 6.04V - 3.02V - 0.12A light still usable though not as bright 5.72V - 2.86V - 0.06A 5.41V - 2.70V - 0.03A 5.18V - 2.59V - 0.01A 4.94V - 2.47V - 0.00A light appears, very dim At the battery system max, 3.6V across one LED, the measured current is 0.38A, which is higher than specified at 0.35A. Brightness is very bright. At the battery system min, 3.15V across one LED, the measured current is 0.19A, and brightness is satisfactory. Ideally, at battery system max (3.6V per LED) current should be the rated current (0.35A), which would indicate a resistance of 3.6V / 0.35A = 10.3 ohms. A four LED in series system shows 0.38A at 3.6V, or a resistance of 3.6V / 0.38A = 9.5 ohms. Adding a 1 ohm resistor in series would reduce that amperage at battery system max to just under the rated 0.35A. 3.6V / 10.5 ohms = 0.34A It might provide some protection against inrush current when switched on. It would diminish the amperage at the battery system min as follows: 3.15V / 10.5 ohms = 0.30A At 0.30A the light was still bright. Each LED needs 1 ohm. With 4 LEDs, 4 ohms are needed. So with a 12V lead acid battery system, I would recommend using 4 Cree 1W Warm White Star LEDs in series with a 4 ohm resistor, 1W or better, in series. NO REGULATOR IS NEEDED. Last edited by skyl4rk; 01-16-11 at 05:07 PM..