Thursday, May 7, 2009

Wherefore Art Thou, Camping?

Exactly how many doohickeys can you take with you camping and still be considered to be 'roughing it'? I know many people for who camping means 'in a camper', i.e. they need their flat panel TV and fridge full of beer or they're not going. I will sleep in a tent, so I consider myself somewhat superior to these people. But being an electrical engineer I just LOVE gadgets and I can barely stop myself from bringing them along. I know not to even THINK about bringing a laptop, but I'm still bringing our battery-powered fan. Of course, instead of batteries I'm using an emergency jumpstart car battery thingamajig with a cigarette lighter power adapter. If you want to power AC devices you'll need an inverter - just make it has sufficient maximum power (I've experienced the letdown of not having enough power firsthand when my friends and I figured out we couldn't run a smoke machine off of an inverter plugged into a car. Memories!) I haven't graduated to thermoelectric coolers yet but soon I will.

The question is how to you power all of your gadgets in the wild? A car battery is a good start but for a perfectionist like myself it just won't do. Car batteries are heavy. If you have to carry anything any distance you'll with you hadn't brought it. You can find better batteries than the lead-acid car batteries. Nickel-Cadmium, Lithium-Ion and Lithium Polymer are all better options (i.e. greater power density per kilogram). Technology is only one thing to look for when choosing a battery. There are four main things to look for in a battery: Voltage, Capacity, Current Capacity and Technology/Battery Type (Ni-Cad or Li-on or whatever).

Voltage is easy for the most part. Car batteries are 12V nominally and for the most part that is the magic voltage. Since your car runs on 12V everything that plugs into your car runs on 12V so there's lots of devices out there that will happily run on this voltage. Other popular voltages are 9V and 5V, but if you have a 12V source you're in luck because you can use dead-simple linear DC-DC conversion to get down to those voltages. Look up the LM7805, or in fact, everything in the LM78xx line (the last two digits are the nominal output voltage). They're really simple to use - for an electrical engineer anyhow. Fun fact: a battery is not a single unit - it's made up of cells. Depending on what the battery is made of, each cell has a different voltage. It's intrinsic - entirely dependent on the technology used in the battery. You get other voltages by stacking these cells up in series. Lead-acid is 1.5V per cell I believe, so a 12V battery has 8 cells in series. Another fun fact: it's not called a battery unless there is more than one cell used in it.

Capacity of the battery is also straightforward. It's measured in Amp-Hours. If my battery is rated at 7A-Hs then it can in theory deliver 1A for 7 hours before it's empty. Of course this is simplistic. Voltage degrades as the battery empties, so if you need 1A at 12V for seven hours you may not be in luck. After hour three the voltage may go down to 11.5V, then down to 11V next hour, etc. If you're using linear step-down conversion then you're out of luck for a couple of reasons:

  1. You need a certain minimum voltage difference between the input to the voltage converter. If your battery voltage goes too low then you won't meet this requirement and you won't get any power at all. This can be alleviated with a switched-mode voltage converter

  2. Assuming the power output is constant, lower volts means higher amps. As your voltage goes down your converter will draw more amps just to have the same amount of power and this will cause your battery to drain faster. Switched-mode voltage converters are not immune to this

The degradation in voltage is a function of the battery technology, so your mileage may vary.

Current capacity is distinct from the other capacity. Current capacity is the ability of the battery to source large amounts of current. Not all batteries are equal in this respect. As with any non-ideal voltage source a battery has a certain amount of internal resistance. This means as current flows out of the battery, power is dissipated in the internal resistance. This creates heat. Heat causes explosions, especially when it's applied to exotic chemicals. Remember when Dell's batteries were exploding? They got too hot. They got too hot because despite the fact that Dell spec'd the batteries with a peak current of say 5A for one minute, the manufacturer ignored that and went cheap on the batteries. This caused them to heat up when the laptop pulled the amount of power from them it thought safe. They got so hot that the chemicals got angry, and *POOF*! Exploding laptop. In general batteries are high-performers for current sourcing - that's the reason your emergency jumpstart kit works better than the alternator on your Yugo - current capacity.

Technology is a good one. I'm not into chemicals and ions and things but it's somewhat exciting to see the effect that all of that sciencey stuff has on actual performance and use. As I said before, technology affects all sorts of things on a battery: nominal battery voltage, voltage degradation over time, capacity to weight ratio, discharge capability, charging method, discharge behavior, etc. In general your newer battery technology has better capacity to weight ratio, less voltage degradation, higher capacity and greater current capacity. What it doesn't have going for it is simplicity and reliability. Simplicity in that they're more difficult to charge. Lead-acid is dead simple: apply 13.8V to a 12V battery and let it go for a while. It will charge completely. Lead-acid is surprisingly hard to damage as long as you don't exceed its recommended voltage. Newer technology is harder - it's rather finicky about how much current you push into it and what voltage and you must stop trying to charge it when it's done or else you'll break it. As for reliability consider that you can discharge a lead-acid battery almost down to 0V and it will still live - just charge it up again (deep cycle maritime batteries are the best for this). Most other battery types can't handle this. If you let them get too low then they're dead.

My fingers are tired so I'll leave until a different time the discussion of the solar-powered power unit I'm making. For my camping gadgets of course!

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