Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

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?

share|improve this question
    
power it for how long? –  Brian Drummond Jun 11 at 20:23
    
Indefinitely. Later we will be adding a solar charging system, but for now I just need to get it powered through a day. So at max capacity I'd say it should be able to run for 16 hours with no more external charging. –  user45440 Jun 11 at 20:26
    
The way you've described it, I'm assuming that if I plug the cloud sensor into a source of 15VDC rated to supply at least 1 ampere the cloud sensor will be happy, but I'm confused about the computer's power supply. Can you provide some more details? –  EM Fields Jun 11 at 20:57
    
If you want something off the shelf, Goal Zero makes a 1200 W-hr model with lead-acid batteries that costs ~$1400 and weighs about 100 pounds. goalzero.com/p/140/goal-zero-yeti-1250-solar-generator –  Matt B. Jun 12 at 0:37
    
65W sounds fairly high for laptop power draw. What's written on the wall adapter would be more of a worst case power draw e.g. charging laptop battery, CPU full on, ports powering accessories etc. In practice it may be half that or less so your big battery pack may give more runtime than it appears at first. For long enough specified runtime it may be cheaper to get a lower power laptop (or Raspberry Pi etc) than a larger external battery. –  Matt B. Jun 12 at 3:47

2 Answers 2

up vote 5 down vote accepted

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.

share|improve this answer
    
It doesn't need to be terribly portable, just fit in a weatherproof case. There should be a decent amount of unused space in the case as well. Cost does come into play as we'd like to make multiple of these and we are limited to a budget. It doesn't have to last too long as it is a prototype and test bed for a later project, but a long term solution would need to be found later. –  user45440 Jun 12 at 5:27
    
@user45440 Deep cycle lead-acid batteries are probably your cheapest option at this juncture. They'll be big, bulky and heavy, but that doesn't appear to be a problem for your current application. –  horta Jun 12 at 14:03
    
@user45440 Rereading your question, it's not clear how much power you'll actually need. If your laptop isn't crunching numbers all the time and is idle most of the time, you'll need hardly any battery compared to what I've quoted. You should get a kill-a-watt and use that to measure how much power your application will take when it's up and running in the field. Then use that to determine how much battery you need. –  horta Jun 12 at 19:12

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.

share|improve this answer
    
Deep cycling a car battery will kill it. –  Aron Jun 12 at 8:25

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.