I'm working on a high altitude balloon APRS tracker and data logger, which seems to be working great, but only when powered by my Mac's USB port.
The setup is an Arduino Uno with a Seed SD shield, an Argent radio shield, and a GT-320FW GPS module. I have the GPS powered by the Arduino's 5V pin.
On batteries the radio shield will sometimes not respond, and the GPS will sometimes not find a position, or everything will work for a while, then just stop responding. I've read that this GPS module will act like it's working on a fix, but never does if it's not getting quite enough power, which seems to happen sometimes.
For batteries I've tried both 4 and 8 normal AAs or Enloop nimh AAs. I don't think I understand enough about how to determine how much current a set of batteries is capable of producing, or how to measure the current my setup is trying to pull.
There's a possiblity I damaged the voltage regulator, as 8 normal AAs puts out too much voltage, which made the regulator got very hot. I don't have another Uno handy to try it on though.
I'm thinking about trying one of those rechargable USB cell phone batteries into the USB port next.
Does anyone have any thoughts on what might be the best way to power this?
The FIRST thing you need to do is measure how much current your set-up demands. If you don't have a test meter then the revised FIRST thing to do is go out and buy a test meter.
Once you know the current demand you can determine what AH of battery you require (based on how long you want the system to operate once released)
Increasing voltage does not generally increase operating time; it simply means your regulator wastes more energy pulling the voltage down to the desired level. If you need more AH than your selected battery can supply you could fit extra ones in parallel (not recommended). It is better to get ones large enough to meet the demand since batteries in parallel can give added problems.
As a related issue, keep in mind that with high altitude balloons that battery failure or very short duration can result from the very cold temperatures such balloons can reach. I've even read of some that keep the battery in a heated container to maintain power capacity. However as heating itself requires energy from the battery I would think careful calculations are required to make sure the whole system works as designed for the mission duration and maximum altitude reached.
I do have a meter, though I'm not sure if I'm using it correctly. I just hooked up a set of 4AAs, which seems to run the Arduino and GPS, but the radio shield doesn't respond to the serial commands, like it does on USB. I setup my meter to read DC amps, and connected it between the battery's positive line and the Arduino. It reads between .09 and .10 amps. That seems a bit low to me, so I'm wondering if that's the proper place for me to connect the meter.
As for the temperature, people seem to use the common Energizer lithium batteries, but we'll also have a chemical heat pack inside the enclosure. Hopefully that, combined with the heat of a the radio itself, will work. However, I have friends who have flown payloads using off the shelf APRS trackers on plain old alkaline AAs and haven't had any trouble. I think being inside an insulated cooler helps too!
I just tried it again using an 8 AA pack, which puts out around 12V. The meter read 1.2A, so I think I now know that my issue is just lack of power.
The reason I don't like the 8AA pack is that the voltage regulator gets too hot to touch. I thought 12V was in the acceptable range, but maybe my board is just a bit off.
bensinc:
I just tried it again using an 8 AA pack, which puts out around 12V. The meter read 1.2A, so I think I now know that my issue is just lack of power.
The reason I don't like the 8AA pack is that the voltage regulator gets too hot to touch. I thought 12V was in the acceptable range, but maybe my board is just a bit off.
Well keep in mind that it's not the specific voltage value feeding an arduino board that determines how much heat the on-board +5vdc regulator will generate. But rather it's the product (input voltage - 5.0) / current. So the more current you are drawing from the arduino 5V shield pin (if you indeed are) to power external components and modules the hotter the on-board regulator will run.
As your system is being battery powered and battery duration is important you might want to consider using a external buck switching mode voltage regulator to power all the things using +5vdc power. This will help greatly in the overall efficiency of your total system. Here is a typical 5vdc switching regulator:
Thanks for the help! I'll take a look at using an external regulator.
If I'm understanding how batteries in series work, with the 8 AA pack I only have somewhere around 2000ma/h at 12V, so if I need 1A I'll only have 2 hours of power. According to the predictions, my flight should be around 2.5 hours, so I'll need to get more power somewhere.
I think a rechargable battery pack of some kind might be good. It could be lighter weight too!
bensinc:
Thanks for the help! I'll take a look at using an external regulator.
If I'm understanding how batteries in series work, with the 8 AA pack I only have somewhere around 2000ma/h at 12V, so if I need 1A I'll only have 2 hours of power. According to the predictions, my flight should be around 2.5 hours, so I'll need to get more power somewhere.
I think a rechargable battery pack of some kind might be good. It could be lighter weight too!
Two hours only in a mathematical sense. Most battery manufacture state their mAH capacity rating based on a pretty small continuos current draw, say 100ma. As you draw a higher continuous current the effective mAH rating is actually a lot lower, perhaps even as low as the 50% rating. Study datasheet from battery manufactures if you want to try and see what the 'real' mAH rating will be at a specific current draw. A better way is to just test and measure your system while on the ground to see what duration it can really deliver by measuring the battery voltage as the system discharges the battery over time.
If you are feeding your 4xAA into the regulator it's unlikely to be working correctly (since it generally needs a couple of volts greater than the output (7 volts). If all your shields are rated for 5 volts you'd be much better using an external 5 volt switching regulator rated for say 3 amps and power this by a decent capacity RC battery pack. This regulator output could then feed the arduino directly. However, if some of your peripherals are not stackable shields you should feed these directly from the regulator rather than off the arduino VCC line.
As to battery temperature, a really good insulated container with all the electronics contained therein should keep the batteries warm enough. The waste heat from the gizmos should keep things warm. If you have a spare data channel you might want to monitor this temperature, and the battery voltage. There's nothing worse than your data failing for some "unknown" reason