Thursday, November 13, 2008

Amps, Volts, Watts... Power Inverters & Batteries

I don't have a 2nd battery hooked up yet in my van. All I've been using is a Xantrex power inverter with my cigarette lighter. I am thinking about upgrading, so a good portion of my free time lately has been dedicated to learning all that I can about battery basics and related topics.

I've really ended up learning about all sorts of interesting things that are connected to eachother like a big spiderweb ~ I'm so excited that I finally understand the difference between volts, amps, and watts! I've also learned a lot about batteries in general, wire thickness, and power inverters. It is pretty simple stuff once you understand it, and since it is fresh in my mind, I'm going to type it out in this post. Then I can refer back to this page the next time I need to remind myself how to figure out watts or amps again (cheat sheet). I understand all this now, but I am guessing that I will forget it all if I don't keep refreshing myself on it. Here is what I have recently learned:

VOLTS - voltage (like the pressure of water in a pipe)
AMPS - amperes (speed/quantity of flow per unit of time/measure of current)
WATTS - power

When you have got two of those, you can figure out the third. Here are the formulas:

WATTS = volts x amps

AMPS = watts / volts

VOLTS = watts / amps

(Example: A light bulb that uses 120 volts and has 2 amps running through it is a 240 watt bulb, because Watts=Volts x Amps. 240=120 x 2)

Also: If the watts aren't known, you can figure out the ohms.
Ohms = the value of resistance/force/restriction to flow

OHMS = volts / amps

AMPS = volts / ohms

VOLTS = amps x ohms

Amps are how many electrons flow past a certain point per second. Volts is a measure of how much force that each electron is under. Think of water in a hose. A gallon a minute (think amps) just dribbles out if it us under low pressure (think voltage). But if you restrict the end of the hose (ohms), letting the pressure build up (higher voltage), the water can have more power (like watts), even though it is still only trickling at a gallon a minute. The power can grow enormous as the pressure builds, to the point that a water "knife" can cut a sheet of glass. In the same manner, as the voltage is increased, a small amount of current can turn into a lot of watts.

Anyway, once you make a list of all the things you want to power in your vehicle, you can then figure out all your amps, volts, and watts. One you've got that info, you can decide whether a simple power inverter will work to run everything out of your 12 volt cigarette lighter plug, or whether you should hook up a 2nd battery in order to power your gadgets. You can use your total watts to determine how big of a power inverter you need, or your total amps to determine how powerful of a battery you need, based on battery AH (amp hours).

I used the above formulas to figure out the amps & watts of several of my appliances. That gives me a rough idea of how much power I may possibly need from a 2nd battery (or if I've got a strong enough inverter to hold me over). My laptop adaptor says 19.5 volts and 4.7 amps, so lets see.. that means my laptop uses 92 watts (19.5 x 4.7 = 92). I looked at my old high-power blender, and it said 4.5 amps. So if I assume that it is meant to be plugged into a 120 volt AC outlet, I figure it uses (4.5 x 120) about 540 watts. Another thing to keep in mind is that, even though my blender runs at about 500 watts, I know it takes more than that to start it. I think I've heard to double that amount, so I will assume that my blender would take 1,000 amps to start it, which is how many cranking amps I would also need in a battery.

POWER INVERTERS:

To figure out how big of a power inverter you need in your vehicle, determine the total watts of everything you want to operate. Remember, amps x volts = watts. If you only know the amps, multiply your amps by 120. Your amps x 120 = watts. Once you've got your total watts, pick out an inverter that supplies at least that wattage constantly. I've also heard that it is important to add 15% to your total watts, just to make sure you'll have enough power to meet your requirements. Inverters have both a peak rating and a constant rating. This means that a 400 watt inverter will supply constant supply of 400 watts, but will deliver a peak of 800 watts. The peak voltage doesn't last very long, but it allows extra starting power.

I found a neat list of various devices and the approx. number of watts that they each use.
Here are some of the common wattages for several appliances:

full size microwave (1400-1750 watts), 42" fan (1235 watts), chest freezer (1200 watts), mini microwave (1000 watts), coffee maker (600 watts), portable vacuum (525 watts), computer & monitor (450 watts), blender (450 watts), refrigerator (360 watts), home stereo (350 watts), 3/8" drill (320 watts), tv/vcr combo (300 watts), quartz halogen spotlight (250 watts), 3-speed fan (130 watts), computer printer/fax (150 watts), laptop computer (90 watts)

BATTERIES:

I started looking around at different batteries, trying to pick out what I would like, and I discovered several things. First, a Marine Deep-Cycle Battery is definitely the type of battery that should be used as a backup power source in a vehicle. Deep Cycle batteries provide continuous power for long periods of time (a trolling motor for a small boat, auxiliary RV power, traction power for a golf cart, etc.). They can also be used to store energy from a small wind turbine! (neat fact) They are designed to have a long, continuous discharge period, followed by a complete recharge. They've got fewer, thicker plates (in order to have a greater capacity) and a low cranking ability, but they have lots of reserve capacity. They endure a lot more discharge/recharge cycles than starting batteries. I found this site, titled "Deep Cycle Battery FAQ," which has tons of good info. I also read that AGM (absorbed glass mat) batteries are one of the best types of deep cycle batteries out there.

When I drove around, looking at deep cycle batteries, I discovered that they have AH (amp hours), Reserve Capacity, and Cranking Amps. One battery I looked at had 44 A.H., 78 min, Reserve Capacity, and 1,200 Cranking Amps. Another one had 52 A.H., 124 min. Reserve Capacity, and 1,100 Cranking Amps. That gave me more researching to do...

AH (Amp Hours) = amount of amps flowing (load in amps) x number of hours used.

In other words, amp hour capacity of a battery is the certain amount of amps that a battery may deliver for a period of time, depending on the battery's size.

AH / load in amps = estimated life of that battery (amp hours divided by the load in amps = how long the battery will last)

Example: a 72 A.H. battery, with a 10 amp load should last 7.2 hours (72 / 10 = 7.2)
Example: a 100 A.H. battery can deliver 5 amps for 20 hours (100 / 5 = 20)

If you determine that all the appliances you want to run equal, say, 20 amps, then you could run everthing simulaneously for either 2.2 hours on a 44 A.H. battery (44 / 20), 3.95 hours on a 79 A.H. battery (79 / 20), or 4.6 hours on a 92 A.H. battery (92/20). You probably won't be running everything together, however, for that entire time. That was just as an example to test my math skills :)

Now onto Reserve Capacity. Reserve Capacity is the time in minutes that a new, fully charged battery will deliver 25 amps at 80 degrees and maintain a voltage of at least 1.75 volts per cell, or 10.5 volts for a 12 volt battery (I had to look up that official definition). It represents how long the battery will continue to operate if the alternator or vehicle generator fails. Basically it is the battery's ability to sustain a minimum vehicle electrical load, in the event of charging system failure.

Cranking Amps are also listed on a battery. There are regular cranking amps (CA), and then there are cold cranking amps (CCA). In a round-about description, regular cranking amps determine how much power you have to start your car in most climates, and cold cranking amps determine how much power you have to start your car on cold winter mornings. It takes more power to initially start something up (ex: starting your car), which takes a high discharge rate in amps for a short period of time. To start something, the battery must "crank" (rotate the crankshaft) while it maintains a sufficient voltage. The higher the rating, the greater the starting power.

Misc. Basic Informational Tidbits:

Household electrical current is 110/120 volt AC (alternating current - an electrical current thats direction reverses cyclically/the direction of energy flow periodically reverses)

Automobile/battery current is 12 volt DC (direct current - an electrical current that flows in a constant direction)

Extention Cords:

While reading about power and amps, I also came across a neat chart regarding extension cords. I do need to get a new extension cord for my van, so this chart is pretty handy... (I use an extension cord to plug the outside of my van in to any regular 110/120 AC outlet, and it powers the wall outlet that I've got near the floor inside my van). Here is the table that shows the wire thickness (gauge) required in an extension cord, depending on the cord's current draw in amps and the length of the cord: (The larger the AWG number, the thinner the wire, and the less current it can bear. So if you want to power 14 amps and have a 50' cord, the chart recommends that it should have a 14 AWG thickness. If you don't have a 14 AWG cord, then you can use a 12 (but not a 16), - you can go to a smaller number, which is a larger size):

Well, most men out there probably already knew all that stuff since it is really basic info.
But I like having it all typed out anyway :)
My next step is to find a battery that I want and then learn how to hook it up in my van with wires, an on/off switch/solenoid? etc. OH - and I need to figure out where I'll put it, so I can measure and figure out what size battery to get. The batteries I've looked at were all sorts of different sizes.

When I figure out exactly how to hook up a 2nd battery, I will let you know. I really enjoy teaching myself stuff. I think it is a positive way to spend extra time.

**Also, thanks for the tip, via the comment!! Definitely check out How To Have Electricity at cheaprvliving.com! Bob has some wonderful info there, and it is extremely helpful.

amps25'50'75'100'125'150'175'200'
0-10.018 AWG18 AWG16 AWG16 AWG14 AWG14 AWG12 AWG12 AWG
10.1-13.016 AWG16 AWG14 AWG14 AWG14 AWG12 AWG12 AWG12 AWG
13.1-1514 AWG14 AWG12 AWG12 AWG12 AWG12 AWG12 AWG
15-1814 AWG12 AWG12 AWG12 AWG12 AWG12 AWG

7 comments:

LifeArtist said...

I have found a lot of interesting info on a site dedicated to "cheap rv living", including an article on electricity. http://cheaprvliving.com/howtohaveelectricity.html

I have yet to try out any of the tips, since I still live in a regular house. It's fun to explore, though! Thanks for sharing your experiences.

Anonymous said...

I had a friend who owned a 79 vw westy that had a dual batt setup that, if i recall correctly, was factory. There was a batt on each side of engine compartment in the rear. In general these types of systems will have an isolator system so that one of your batts will not be used for powering your devices so that you will always have some juice to fire your engine. you may be able to find the isolater at a regular auto parts store and souldnt be a problem at a rv supply place. If there are pick and pull junk yards there you may be able to get one there for cheap

Jesus said...

Really amazing website, pretty bus, and good info on electricky stuff..

A few extra tips that you don't often hear about:

Don't power anything from an inverter unless there's no other way of powering it. Most devices can run from 12 Volts directly, there's very few that do not. Most commonly, the ones that don't include non-solid-state components, such as electric motors and vacuum tubes. Even for those, there's usually some device out there doing the same job with a 12 Volt motor or 12 Volt vacuum tubes. It's worth investing in these, as an inverter is a tremendously inefficient way of doing things. In any conversion of power, you have losses. Inverters usually convert the DC to AC through an Oscillator and then use a transformer to bring the volts up. The transformer converts electric power to magnetic power, and back into electric power.

So, in fewer words with less techie terms, if you want to power a 100 Watt device, you need an inverter rated at least at 100 Watts constant power, but this is going to use 120 Watts of your battery power, because of the losses. The bigger the inverter, the more your losses. Also, the oscillator and transformer do their thing even when there is no power required on the outlet. So if you leave an inverter on with nothing plugged in, it will draw quite a bit of current from your battery. Some manufacturers include this in the specs as "idle current". The bigger the inverter (bigger Watt rating) the more the idle current it draws.

So for a device that would draw 100 Watts directly connected to your battery, if you would get 120 Watts drawn through an inverter, you would have a 20% higher consumption.

This is easily avoidable for things like cell-phone chargers, walkmans, lights (unless you need a 5000 Watt work spotlight), etc..

Finally, beware of most things sold as "leisure batteries"... If it has a cranking current rating, it's probably not a true deep-cycle battery. True deep-cycle batteries should never be used for cranking an engine, and therefore they are never tested for a CA or CCA rating. The dual-purpose ones are also best avoided. They're not good at cranking and they're not good at deep-cycling.. So they're not really good at any of the two purposes. Best way to do it is to have the existing starting battery solely to crank the engine and provide the spark to the sparkplugs while it's running, and have a second, deep-cycle battery solely to power the appliances when the engine is off.

Good luck! Awesome blog! Feel free to contact me through the website for any technical or other info on van dwelling.

Jesus said...

Really amazing website, pretty bus, and good info on electricky stuff..

A few extra tips that you don't often hear about:

Don't power anything from an inverter unless there's no other way of powering it. Most devices can run from 12 Volts directly, there's very few that do not. Most commonly, the ones that don't include non-solid-state components, such as electric motors and vacuum tubes. Even for those, there's usually some device out there doing the same job with a 12 Volt motor or 12 Volt vacuum tubes. It's worth investing in these, as an inverter is a tremendously inefficient way of doing things. In any conversion of power, you have losses. Inverters usually convert the DC to AC through an Oscillator and then use a transformer to bring the volts up. The transformer converts electric power to magnetic power, and back into electric power.

So, in fewer words with less techie terms, if you want to power a 100 Watt device, you need an inverter rated at least at 100 Watts constant power, but this is going to use 120 Watts of your battery power, because of the losses. The bigger the inverter, the more your losses. Also, the oscillator and transformer do their thing even when there is no power required on the outlet. So if you leave an inverter on with nothing plugged in, it will draw quite a bit of current from your battery. Some manufacturers include this in the specs as "idle current". The bigger the inverter (bigger Watt rating) the more the idle current it draws.

So for a device that would draw 100 Watts directly connected to your battery, if you would get 120 Watts drawn through an inverter, you would have a 20% higher consumption.

This is easily avoidable for things like cell-phone chargers, walkmans, lights (unless you need a 5000 Watt work spotlight), etc..

Finally, beware of most things sold as "leisure batteries"... If it has a cranking current rating, it's probably not a true deep-cycle battery. True deep-cycle batteries should never be used for cranking an engine, and therefore they are never tested for a CA or CCA rating. The dual-purpose ones are also best avoided. They're not good at cranking and they're not good at deep-cycling.. So they're not really good at any of the two purposes. Best way to do it is to have the existing starting battery solely to crank the engine and provide the spark to the sparkplugs while it's running, and have a second, deep-cycle battery solely to power the appliances when the engine is off.

Mark said...

Be careful with your assumption that a 75ah battery with a 10A load will last 7.5 hours that would mean a totaly flat battery and even deep cycle batteries dont like to be discharged more than 60-70% or the lifespan of the battery is considerably shortened I too am trying to get my head around the same situation as you and will save you page in case I ever need a refresher at the moment in trying to get my head around voltage sensitive relays sounds like a simple way to set up a second battery but I'm struggling to work out what sort of amps a discharged battery will draw so I can work out cable size ....good luck ..mark

Mark said...

Ha ha I just noticed the dates 2010 you've probably finished your project by now

Anonymous said...

Nice share!