Battery Backgrounder

Batteries are becoming an increasingly important part of astronomy - not just for the traditional users of batteries, like the astro-imagers and people that use Goto scopes, but increasingly even visual and dobsonian observers are using batteries to run laptops for starcharts.

The question comes up, “what kind of battery do I need?”

Unfortunately, this is not a simple question, because it depends a lot on what you are going to do with the battery, and for how long. Someone looking to power a simple set of digital setting circles for an evening can make due with a 9volt battery. An imager on a 4 day observing trip might need 3 car-battery sized deep cycle batteries though.

Here is the important question to ask:

• How much power am I using?

Batteries are rated in amp/hours. A 10 amp hour (ah) battery in theory can run a device that draws 1 amp for 10 hours, or 2 amps for 5 hours. So the important factor for judging battery need is how many total amps do my devices draw.

If you are lucky, your devices will list how many amps they draw. Unfortunately, my experience is that this is rarely the case.

Those little power blocks that electronics use specify the maximum amperage they can draw - which is far more than what the device hooked up to them typically draws.

Other devices are measured in watts. Fortunately, you can get from watts to amps easily - simply divide the number of watts listed by the voltage. So a 1200 watt hair dryer running on 120 volts actually draws 10 amps.

There are a couple of ways. Some multimeters (sometimes referred to as volt meters) have an amperage gauge. You hook the meter up “in series” with your 12 volt devices (batter→multimeter→device→battery) and the multimeter measures the amount of power being drawn. These work very well.

For 120volt devices, at least one inverter I own (a Vector unit from Target) also has a digital watt meter that displays how many watts are being drawn. I love this solution.

If you can’t measure though, you should either:

• read the instruction manual for your device

• contact the manufacturer
• as a last resort go by the rating of the power block (per above)

Once you know how many amps you are drawing, it is pretty easy.

Laptop batteries are usually marked with how many milliamp/hours they have in them. One amp/hour is 1000 milliamp/hour’s. Take the battery out and look at its case.

Batteries sold for astronomy are almost always rated in amp hours.

Deep cycle car batteries are always rated in amp hours.

Take the number of amps you are drawing, and multiply it by the number of hours you need to keep running for. This will give you the number of amp/hours of power you will need.

Keep in mind that you will not want to discharge your battery more than 50 percent. Doing so will greatly shorten its lifespan. Deep cycle batteries usually are rated for 50 deep cycles (full discharge). In practice, the number is much lower than that. Fully discharging a typical battery just a few times will destroy it.

AH is usually measured at a 20 hour discharge rate, so a 55AH rating would be at 2.75 amps. at higher current outputs, you’ll get less total AH out of the battery

lead acid batteries should be stored charged whenever possible, they aren’t like NiCad or NiMH batteries where there is some memory effect such that if you always discharge only 20% then recharge they will no longer give you a full life if you later need it. (btw, I’ve read numerous reports that this is less true with newer ni*** bats, also that it was only ever really a problem with slow discharge rates, that if your application involves higher current output, memory was a myth).

deep cycle lead acid batteries generally have a relatively fixed -lifetime- AH they’ll output, figure this at about 100 or 200 times their AH rating.... so if you discharge them to their rated AH then fully charge, you’ll get about 100 (or maybe 200) charge cycles, if you discharge them to 50% each time, you’ll get about 200 (or maybe 400), etc. this relationship isn’t linear, its just an approximation. Overly discharging a lead acid -is- definately bad for them and will rapidly shorten their working life.

When a battery is fresh off a charger, it has a ‘float’ or ‘surface’ charge which will cause its voltage reading to be unnaturally high. Either wait a few hours or put a mild load (say, 1 amp, a 12W 12V lightbulb or similar) across the battery for a few minutes to bleed off this surface charge, then measure the voltage.

As far as determining charge state from battery voltage... The standard numbers for a flooded lead-acid 12V battery at 75F are..

  100%     12.65V
   75%      12.45V
   50%      12.24V
   25%      12.06V
    0%       11.89V or less

Probably the most accurate thing you could do would be to characterize your battery under your load via some testing...

To characterize the voltage / load relationship, measure the voltage under your maximum working load, disconnect the load. wait 20 mins, and measure the voltage again. Record these readings, and repeat this at a few discharge states. Use the above table to determine the charge state from the open voltage, and mark your data points on a graph where the X axis is charge state, and the Y axis is voltage, and draw two lines through the open and load voltages... it should be quite linear in the 90% to 10% discharge range.

Also, if you’re still using that old automotive battery charger you’ve had in the garage since the 80s... toss it out, and buy a new solid state automatic charger, they are -MUCH- smarter about how they charge, temperature compensated, and do a sophisticated 3-state charge cycle. To properly fully charge a lead acid, you first use a constant current until a specified voltage is reached, then use a constant voltage for about 5 hours(!) to fully saturate the charge (this is called the absorption phase), then reduce this voltage to a ‘float charge’ level where the battery can be maintained. If the absorption phase isn’t properly done, the batteries service life will be greatly shortened.

temperature DOES affect the output current capacity of the battery, but it has a very small effect on the float voltage. a charger must be temperature compensated to avoid overcharging, however. batteries will charge best when they are warm (but not hot), but are best stored cool.

Inverters versus dc-dc converters. Inverters (that convert 12 volts to 120 volts) used to be big, ugly, noisy, and inefficient. Recently this has changed, with some having quite good efficiency. My favorite is a little 400 watt inverter I picked up at Target that even turns off the fan for quiet operation when not needed. They work well.

Dc-Dc converters seemingly skip a step. Your laptop runs on 9-20 volts depending on brand. Your battery is running at 12 volts. If you use an inverter, the power is converted from 12 volts dc, to 120 volts ac in the inverter, then back to 9-20 dc volts in the laptops power block. A dc-dc converter goes directly from 12 volts dc to the 9-20 volts dc. Hidden in this process is the fact that it still goes through AC power, but not necessarily at 120 volts. These can be quite efficient.

Both solutions work though. In general, if I were just powering a laptop and all my other equipment didn’t require power or ran on 12 volts, I’d probably go with a dc-dc converter. If I had other 120 volt equipment as well I’d probably go with the inverter.

Batteries are substantially less efficient in the cold. They lose voltage quicker and don’t last as long. Take this into account when judging your power consumption. I assume a %50 loss during the winter.

Be careful how you charge. Sticking a regular car battery charger on a gel cell battery will ruin it for example. Ask the person selling the battery what the best charging method is, and stick with it.

Lead acid versus gel cell versus Optima (AGM style) batteries

All the above are lead-acid batteries. Gel cell’s have their acid in a gel form. Optima style batteries have the acid held in place in some kind of fiber mesh. They all work great.

Gel cels are finicky when charging. Inappropriate charging will turn them into paper weights.

Lead acid batteries (car batteries) come in two types. Starting and deep cycle. You want deep cycle. They are cheap and powerful, but if you drop them (or if they tip over) they will lead acid. This can ruin your car and make a mess that will not go away.

Optima style batteries are better at being dropped or tipped over. They are more expensive though.

Don’t forget to make sure you have set the power properties of your laptop to an efficient level. Turn down the screen. Make sure the hard drive is powering down. These make a huge difference.

Also, if you are running your laptop off a bigger external battery, you might want to take its internal battery out. Just be careful with the big hole in the back of your laptop now. If you leave the battery in, it will suck juice actually keeping itself charged. The only advantage of leaving the battery in is that it gives you some protection if your big battery runs out.

A quick writeup on Optima batteries...

The blue top is a marine battery. It is also designed to be a “starter”, and deep discharge.

The yellow top is designed to be deep discharge - e.g. its used in motorhomes as auxiliary batteries (and for thumper car stereos).

The “starting” battery is a “red top”. (Yes, you don’t want this.) (We got one in the 67 mustang and you can practically drive the car w/ just the starter.)

I buy yellow tops at four wheel parts in San Jose. They stock them. (You don’t want to pay the shipping on these things.)

Fwiw, I picked up a microprocessor controlled charger (Schumacher) at the auto store for about $50 – http://store.schumachermart.com/ssc-1000a.html This charger kicks butt on these batteries. Works much better than the 2 amp $20 old-school charger I was using.

There is at least one (?) other sealed battery brand out there- excide? I believe kragen carries these.

The main reason for going w/ the optima, imo, is a) long life, and b) NO SPILLS whatsoever. Not cheap, but I’d rather pay a bit to avoid doing the battery-acid-carpet-hole thing. (Fwiw, it seems that any trivial amount of battery acid will just eat cloth with a vengeance. I have first hand experience.)