Looking for batteries to power a project I'm working on

Question

Sorry if this is really basic, but I don't really know much about power consumption and voltage. I'm building a sensor system with a small computer with a 19V, 65W power supply, as well as a cloud sensor with a 15V DC - 1A power supply. A few other things will be added on such as a low power consumption anemometer, but those should be run directly from the computer.

If I'm looking for rechargeable batteries to power this, what should I be looking at? I'm not sure if I should be looking at one large battery or a bunch of smaller batteries together. Are there any other things I should look out for?

Answer 1

80 Watts. 100 Watts to be conservative. That times 16 hours is 1.6 kW*Hrs.

In no particular order here are some of your options:

1. Lithium ion: if you don't care if the batteries degrade within 3-5 years, lithium ion batteries would work. They'll be light and small.
2. Lead acid would also have degradation issues in the 3-5 years range. They're heavy but cheap.
3. Nickel-iron batteries have a very long life time and by weight have more energy than lead-acid. By volume they have less.
4. Lithium iron phosphate have long life times and and have about 1/2 the energy density of lithium ion and about as pricey.
5. Nickel Metal Hydride (NiMH): Next most energy dense technology behind lithium batteries. Cheaper than lithium type batteries. Limited lifetime to 3-5 years varying largely depending upon use/abuse.
6. Nickel Cadmium (NiCd): requires a smart charger to prevent memory effects. Older technology, less energy dense than NiMH, cheaper than lithium ion.

Your choice really depends on what you're after. How portable does it need to be? How much of a factor is cost? Which should be weighted more, energy density or specific energy? How important is lifetime of battery to you?

All of these battery types have relatively high usage in some field, so they all have some advantage or allow for some balance between options.

If it was me, for a battery that size, I'd likely go towards something pre-manufactured to be near my specs. If you're interested and knowledgeable in battery balancing, charging and discharging, you could work with individual cells to come up with a nice solution and possibly save a bit of money. Otherwise, you're best off buying something off the shelf with your specs: 20V, 1.6kWHr.

Answer 2

You're asking for quite a lot of battery. Let's say you want to use car batteries, since they are cheap and easily available. From horta's answer, you want a total of 1600 watt-hours. 1600 watt-hours / 12 volts = 134 amp-hours. Furthermore, you do not want to discharge a car battery to less than 50% of capacity, so your actual capacity should be 268 amp-hours.

So, how much capacity does a car battery have? Well, the appropriate number is RC, reserve capacity minutes at 25 amps. If a battery has an RC of 60 minutes, it is a 25 amp-hr unit. And this is about what you get at the low end of battery sizes, so I'll use it as an example. 268 / 25 = 10.72, so for this size battery you'll need 11 batteries. As you can see, you'll need quite an installation. And, as I'll discuss later, you need at least a day's extra capacity, so you'll actually need about 22 batteries. If you use bigger batteries (and I've seen ratings as high as about 150, you'll need fewer, but they'll cost more per battery.

And then there's the solar cells you expect to use. In the long run, battery charging is about 60% efficient, so you'll need a total daily solar panel output of 1600 / 0.6, or 2700 watt-hours. This will take place over a period of 8 hours, but most of the time you're getting way less than peak power, so let's size the array for 4 hours. 2700 watt-hours / 4 hours is 675 watts. And that assumes that every day is sunny. At a minimum, you need to assume at least one day's worth of reserve, so your minimum solar size is 1300 watts. Just as an example, Harbor Freight is selling a 45 watt panel kit, with a size of 3 ft x 3 ft. So just as a rough number, you'll need 2700 / 45, or 60 panels, with a total area of about 540 square feet.