# Power a laptop LED backlight with NiMH batteries

I'm powering a laptop LED backlight with an external 24,8V power supply, because I have problems with the internal WLED driver. The display used to work at 27V, but with the 24,8V of the power supply it's just fine. Now, I want to use three 8,4V NiMH batteries connected in series to power the LED's while working without a plug. With fresh charged batteries, I get around 30V. Will it damaged the LED circuit? I know that the voltage will drop quicky, but I would like to use the batteries as soon as I remove them from the charger. If that's dangerous, can someone suggest an circuit to achieve a constante 27V with the three 8,4V batteries?

Thanks, best regards.

We don't know what's in the LED backlight circuit, so your question can't be answered. For example, if there is a switching power supply in there that regulates LED current, then a higher input voltage would probably be OK. However, if it's just LED strings with a resistor in series with each, then the higher voltage will cause excessive LED current, which would significantly shorten their lives.

One way to tell is to vary the voltage from 24 to 27 V and see whether the display brightness changes. If it does, assume the LED and resistor topology and don't give it 30 V. If the brightness stays reasonably constant, then something is regulating the LED current, and up to 30 V is probably OK.

Another thing to look at is the current drawn as you perform the test above. If the current goes up with voltage, then assume the LED and resistor case. If the current actually goes down with increasing voltage, then you have a switching power supply in there. If the current stays relatively constant, then there is a linear regulator in there. You can probably get away with 30 V in the second and third cases, although be careful about the linear regulator overheating.

One problem is that 30 V is a common maximum rating for transistors. In a high volume product intended to work with a bit less than 30 V, quite possibly some of the components are only rated for 30 V. No matter how the tests above work out, you should be careful to not exceed 30 V.

The safest thing to do is to put the batteries in parallel instead of series, then make a boost converter to produce a regulated output voltage in the range that you know the backlight is intended to handle.

• Thanks Olin. When I change the voltage, the brightness changes with it. I think the current regulator was in the circuit that had been damaged, in the computer motherboard. So, I had by-passed it with the power supply. Commented Nov 21, 2014 at 17:29
• Now I would like to by-pass it with the batteries. I was thinking in connect a resistor in series with the batteries to dissipate the current of the extra 3V... is it right? I measured the current when using the 24,8V powersupply and it was around 60mA. So, can I use a 100 ohm resistor in series with the batteries, to reduce the voltage in the LED backlight? Commented Nov 21, 2014 at 17:36
• @tiag: You can use a resistor, but the light will keep getting dimmer as the battery voltage goes down. Commented Nov 21, 2014 at 18:01
• Olin, I just try it a couple of minutes ago, and measure with the current with different resistors. I realised that it was happening, after some minutes the light was dimmer. And now, I turned on the computer and read your message. :) Commented Nov 21, 2014 at 21:07
• I thought I could use a pot, so I can control the value of the resistance. When the voltage of the batteries goes down, I can turn the pot to 0 ohm. And also I can change the intensity of the backlight. But, I would like to ask if you know a good circuit to boost the 8,4V to 27V... thanks again Commented Nov 21, 2014 at 21:11