# REACH: Power from a car battery

Hello everyone,

I’m looking to deploy EMLID REACH antennas at static stations on a forthcoming Antarctic project. Basically, the ice on which I’m installing the antenna will move over time, and I’m looking to have the REACH record positions for a period of at least 3 weeks.

Because of this, I need a high capacity power source and something like a car battery springs to mind. I have a regulator - http://uk.rs-online.com/web/p/switching-regulators/6664379/ - to step the voltage down, but I have concerns about the current that will come from the battery. Does anyone have any experience of a similar thing that they could share?

Cheers,

Electrical spepcifications for the Reach module are:

Input voltage on USB and DF13 connectors: 4.75 - 5.5 V
Average current consumption @5V: 200mA

Input Voltage: 6.5 - 36V DC
Output Voltage: 5V
Output Current: 1A
Efficiency: 93 - 84%

As I’m sure you are aware, your selection is more than adequate to power a Reach module. In fact it could power 5 Reach modules (5 * 200mA = 1A)

You should not be worried about current flow from the battery. Think of it this way:

• A car battery produces 12 Volts and is capable of delivering up to 500 Amps (approximately)
• Your switching regulator is capable of handling 12 Volts at the input and it will draw up to 99mA from the car battery
• 5 Volts * 200 milliAmps = 1 Watt
• 1 Watt / 84% efficiency = 1.19 Watts
• 1.19 Watts / 12 Volts = 99 milliAmps
• Your switching regulator produces 5 Volts and is capable of delivering up to 1 Amp
• Reach is capable of handling 5 Volts at the input and will draw 200mA from the switching regulator.

I’m not familiar with what kind of battery is normally used in Antarctic conditions, but after looking at the freeze point of a typical lead-acid battery in cold weather, it doesn’t look good. For example, at -40C you can discharge a flooded lead-acid battery from 100% down to 65% percent. Any further discharge will raise the freeze point of the electrolyte up to -40C and the battery could be permanently damaged by freezing1. Because of that, I would assume that some type of dry-cell battery would be more appropriate, unless you have a heated enclosure.

But let’s not let that prevent us from doing a little more figuring! So you want to run for 3 weeks? Let’s see how much power is needed:

• 3 weeks * 7 days * 24 hours * 60 minutes = 30240 minutes
• 99 milliAmps * 30240 minutes = 2994 Amp-minutes
• typical group 24 size 12V deep cycle battery is rated for 920 minutes at 5 Amp draw
• the above rating at 5 Amp draw was the lowest published figure available (the lower the draw, the more efficient it is)
• 5 Amps * 920 minutes = 4600 Amp-minutes
• 35% usable battery capacity at -40C (from 100% down to 65%)
• 4600 Amp-minutes * 35% = 1610 Amp-minutes of available power per battery
• 2994 Amp-minutes requred power / 1610 Amp-minutes per battery = 1.86 batteries required

So, in conclusion, two (2) typical BCI “group 24” RV/marine deep cycle batteries would be able to power one Reach module for three weeks at -40C.

As a side note, the same pair of batteries would last three times as long (9 weeks) if the temp was above zero C.

I hope that is as helpful as it was fun to figure out.