Question:
What is the best battery solution for a portable dimming situation? We are using a 4 watt LED lamp and need it to run for 10 minutes and have 4 hours of standby time.
Thanks for your Help
Andrew
Answer:
We must calculate the battery requirement for the load (4W for 10 minutes), then add the standby current for the receiver and dimmer. An RC4Magic DMX2dim draws approximately 200mA.
First, we need to choose a working voltage. If you are using LED devices with built-in current limiting, they are probably designed to operate at 12V. In that case, 4W / 12V = 333mA. Thus, the running load is 333mA + 200mA for the receiver electronics = 533mA.
A very common small 12V sealed lead-acid (SLA) battery is rated at 1.2Amp-hours. This A/h rating is based on a 20 hour discharge rate. On other words, it can deliver 60mA per hour, for 20 hours. But battery chemistry gets less efficient when the load is higher. In fact, if you put a 1.2A load on that battery, it will work for only 30 minutes (or perhaps slightly less) before discharging to the lower limit recommended by battery manufacturers. The efficiency curve is non-linear.
1.2A / 533mA = 2.25. If the battery efficiency curve were linear, a 1.2A/h battery would drive the dimmer with 4W load for over 2 hours. Worst case, you will get better than 1 hour. That’s a whole lot more than the required 10 minutes.
1.2A / 200mA = 6. If the battery efficiency curve were linear, a 1.2A/h battery would drive the RC4Magic DMX2dim receiver, with the load turned off, for 6 hours. In fact, it will be less — approximately 4 – 5 hours. That meets the specified requirements.
Thus, I recommend using a 1.2A/h SLA battery. It’s small, readily available, and quite inexpensive.
However, these current requirements are low enough that NiMH batteries could also be considered. This is particularly viable if you are using an LED component, without built-in current limiting. Then you can choose a low operating voltage, and use a simple resistor for current limiting. In that case, I recommend using a pack of 4 AA cells. Each cell produces 1.25V, for a total of 5V. Although this is less than the 6V specified as the minimum voltage for the DMX2dim, it does work. Just remember that you don’t have much room for the battery voltage to drop further, before the dimmer electronics will cease to function.
At lower voltage, 4W demands higher current. 4W / 5V = 800mA. The standby current for the dimmer-receiver is unchanged at 200mA. So you need to deliver 1A for 10 minutes, 200mA for 4 hours.
Capacities of AA cells vary, but a common mid-range value is 2000mA/h, which is 2A/h. This means you will get longer running times than noted above for a 1.2A/h battery. Just be sure to calculate the current-limiting resistor value properly. You’ll find a useful calculator for this at http://ledcalc.com/.
I hope this helps!
Jim
RC4