Parts of an electrical system - what would I need?

[MegA]

Here's the current situation: My van used to have a wheelchair lift in the back. I sold it and the buyer, when he removed it, also removed the battery that was back there. But the battery compartment still has the wires that were connected to the battery. When I bought the van the seller told me the lift wouldn't work unless the engine was running, so I'm guessing the battery was dead but the connection between the engine and battery was still there. The lift did work then and when I operated it for the pictures for the ad.

I'm hoping to be able to use those wires for a new battery, and with any luck it will charge from the engine (alternator?). But I'm not sure; I don't know how to test it. Honestly, electricity makes me really nervous.

I suppose even there is no connection with the engine I could just buy a battery and put it back there. But what else do I need? I'm having a lot of trouble figuring this out.

- Inverter - to be able to use AC power? How does it connect to the battery?
- Inverters have AC outlets on them, don't they? What would I do if I didn't have enough outlets? Can you use those multiple outlet thingies? I have a few of those; they're a standard for apartment dwellers. And can you use extension cords? (When I say "can" I mean "is it safe to")
- DC power - How do I get that from the battery? Is there some kind of device that needs to be connected to it?
- Something to monitor how much charge the battery has left? I think you're not supposed to let it drain past - 50%?

I'm asking not just for information, but because my budget is shrinking drastically and until I find out what I'd need I don't know what I can afford. If it was just a battery it would be fairly simple, but I suspect it will involve more than that.

Here's what I'm thinking I'd use electric for:
- Computer - Normally - when I'm not laying carpet in a van - I spend a lot of time on the computer - probably 8-10 hours a day. I do some work online and am hoping to increase that in the future, to increase my income.
- Lighting - for the computer area, which is also where I do my crafts (I make jewelry). So one light there and another somewhere else - maybe movable depending on whether I'm cooking, reading on the futon/bed, etc.
- Cooking - sigh... This is hard. I know I can buy a little camp stove that runs on propane or some other gas but I'm not good with gas cooking. Honestly - I haven't used either the (gas) stove or the oven in this apartment in months. I do everything with the toaster oven and the microwave, and I have a bread machine that I use once a week or so for whole wheat bread. I know those are all major power draws. I have noticed there are also DC appliances you can buy, even microwaves, and I've read that inverters use power so DC appliances are more efficient. But even DC appliances that heat will still use a lot, I think. And the bread machine - I'm guessing I'll have to give that up. Reluctantly.
- Heat - not sure what to do about this. I have a radiator-type (oil filled) space heater - another apartment necessity - but once again, it's a major power draw. I know there are alternatives that run on gas. I just haven't looked into them because I don't have propane tanks and I'll bet they use more than a little camp stove would.

I really can't think of anything else I'd use that would be electrical. I'm planning on using an ice chest for food I need to keep cold. I don't think I'd need a freezer; I'm basically vegetarian so I don't need to keep meat cold, and if I buy anything frozen I was thinking I could just cook it as soon as I got home; then it would just need refrigeration.

I read somewhere that I could figure how much electricity I'd need by dividing watts by volts (to get amps?) and multiplying that by how long I'd be using something. But I'm having trouble finding wattage on things and I'm not sure exactly how to do that anyway; do I use 12v or 120v? Also, I think this is supposed to help you decide what batteries to buy, but I have a feeling that decision will have to be made based on budget rather than amperage (is that the right term?)

I hope that helps explain my situation. I've tried to give as much info as possible so you know where I'm coming from, but you can probably tell this is pretty confusing to me. If I need to clarify something please let me know. And thanks as always for your help.

Meg

[markopolo]

the battery compartment still has the wires that were connected to the battery --- my guess is that they would still be connected to the vehicle charging system

I suppose even there is no connection with the engine I could just buy a battery and put it back there --- batteries need to be recharged and actually kept charged or you will have to replace them often. My guess here would be that a professional installed the lift and may have included an isolator. If so, that is exactly what you need. Pop the hood and visually trace the battery cables. The red or positive wire from the battery might lead to an isolator which could red or blue or silver, some have cooling fins are about the size of a foil wrapped pound of butter from the grocery store. ...... couldn't think of anything else That positive battery cable might lead to the alternator and a wire from the alternator might lead to the isolator. I keep mentioning isolator because it is important. It separates the two batteries so that you can't drain the van engine battery and not be able to start the van.

Inverter - to be able to use AC power? How does it connect to the battery? --- positive cable to positive battery post, negative cable to negative battery post, ground wire to vehicle frame

Inverters have AC outlets on them, don't they? What would I do if I didn't have enough outlets? Can you use those multiple outlet thingies? --- yes, but don't exceed the cord amp rating or the inverter wattage rating

DC power - How do I get that from the battery? Is there some kind of device that needs to be connected to it? --- 12v DC power is what you get from a battery, usually, in an RV, it will connect to a distribution panel with fuses and then your various 12vdc loads connect to the panel.

Something to monitor how much charge the battery has left? I think you're not supposed to let it drain past - 50%? --- correct, you will buy and learn to use a ac/dc multimeter if you tackle this part of your project - actually, lots of inverters now have a digital display that shows you the battery voltage. Don't let it go below 12 volts. (12.2 volts is approx 50% discharged) When charging you might see the voltage as high as 14.4 volts. A fully charged newer battery at rest will show around 12.7 volts.

Computer ----- laptop could use 100 watts AC, basic desktop could use 300 watts AC
Lighting ---- 12v lighting with LED or Florescent bulb are most efficient
toaster oven ---- very high current draw 80 to 150 amps DC if used through an inverter - the battery would not last long
microwave ----- very high current draw 80 to 100 amps DC if used through an inverter - short duration of use makes it doable when done right
bread machine --- don't know anything about them
Heat ------ maybe the oil filled heater is your least expensive option - that, and a nice sleeping bag
ice chest --------- doable with ice and freezer packs if you are near someone who can freeze the freezer packs for you

I read somewhere that I could figure how much electricity I'd need by dividing watts by volts (to get amps?) and multiplying that by how long I'd be using something. ------- correct, divide by 120v to get AC amps and divide by 12v to get DC amps. For inverter use I would divide the AC watts by 10 (instead of 12 DC) to factor in inverter inefficiencies.

It is a complicated subject and the installation would be best done by someone who does this type of work. It will be your home so you have to be safe. I'm just trying to give you as much general info as I can.

Are the battery wires in the rear of the van taped up or exposed?
Will you almost always be where you can plug in and get electricity to the van?

[markopolo]

Take a look at The 12 volt Side of Life Website
http://www.marxrv.com/12volt/12volt.htm

From the 12v side of life website:

Really good info there.

The diagram I posted shows you the typical RV electrical setup.

[MegA]

Thank you for your answers, markopolo. I wrote a really long post and I appreciate your taking the time to go through it.

...My guess here would be that a professional installed the lift and may have included an isolator. If so, that is exactly what you need. Pop the hood...
I didn't see this post until after it got dark so I'll check this out tomorrow and let you know what I find.

...don't exceed the cord amp rating or the inverter wattage rating
I checked all the extension cords I have and none has an amp rating, even the grounded one. But I do understand that inverters have wattage ratings so I suppose I could use that.

12v DC power is what you get from a battery, usually, in an RV, it will connect to a distribution panel with fuses and then your various 12vdc loads connect to the panel.
OK - this doesn't sound like something I can handle. (Actually, none of this does.)

Computer ----- laptop could use 100 watts AC, basic desktop could use 300 watts AC
I have a laptop but I'm using it with a standard size monitor. I also have a 3-in-1 printer (also a copier and scanner) but don't use it a lot; I turn it on when I need it.

Lighting ---- 12v lighting with LED or Florescent bulb are most efficient
I guess I'd need that distribution panel then.

toaster oven ---- very high current draw 80 to 150 amps DC if used through an inverter - the battery would not last long...

microwave ----- very high current draw 80 to 100 amps DC if used through an inverter - short duration of use makes it doable when done right
The toaster oven is 1400 watts, the microwave - no idea; I couldn't find that info on it. I guess it doesn't matter; this micro is way too big for the space I'll have in the van and I can't afford another one. So maybe I'll just have to force myself to figure out how to use a camp stove.

bread machine --- don't know anything about them
630 watts *but* it takes 4 hours to make a loaf of bread. I guess I'm not keeping it.

A question here about finding wattage on some of these things: Where do I look? On the computer stuff, would I use what's on the power supply box, or is it somewhere else?

Heat ------ maybe the oil filled heater is your least expensive option - that, and a nice sleeping bag
It's interesting you said that; I read something similar on another site. It surprises me because the heater I have is 600/900/1500 watts, depending on how you set it, but I think it would need to run for long periods - several hours at a time. It sounds like it would be a huge power draw.

...It is a complicated subject and the installation would be best done by someone who does this type of work. It will be your home so you have to be safe. I'm just trying to give you as much general info as I can.
Thank you for confirming that it's complicated; I was beginning to think I was just being stupid. I do appreciate the information. But who would I talk to about actually doing this? An electrician? An RV tech? I don't know anyone who does electrical work.

Are the battery wires in the rear of the van taped up or exposed?
The people who bought the lift tried to cover them with whatever they had; I think they used bags rather than tape. I admit I haven't looked; I've tried to stay as far away from them as possible. I don't trust them, even with the engine off. (The battery compartment is below floor level so that makes it a bit easier to work around whatever's in it.)

Will you almost always be where you can plug in and get electricity to the van?[/quote]
Probably not; I can't afford it. I may have to budget periodic stays, like once a week or every couple of weeks, but even that's a stretch right now. I was hoping to find a work camp job come spring; that would help with the electrical (and water) stuff. But I'll probably have to find other work before then. Or win the lottery. (Maybe I should buy a ticket...)

Meg

[MegA]

A bit of an update: I think I'm going to forego electric cooking. I've been doing some research about baking on a stovetop (camp stove) and it seems there are ways to do this; there are even devices made just for that. So unless I'm missing something - which is always possible - that leaves the computer/monitor/printer and lights.

I think I found the wattage for the laptop and the printer - on the power supply boxes - but the monitor doesn't seem to have that anywhere that I can see. I did take pictures in case anyone wants to verify what I'm reading or maybe find what I'm missing. Here's the laptop:

Here's the printer:

And here's the monitor:


It looks to me like the computer is 90 watts and the printer is 40, but dividing those by 12 doesn't give the same aH as what's on the power boxes so maybe the "A" there means something else. I've never tried to read these before.

So, am I on the right track for reading these figures? The 90w on the laptop goes with what markopolo guessed (100). I do wish I could figure out what the monitor is using.

Meg

[markopolo]

I think you have an understanding of this.
voltage x amps = watts so it would be 60 watts for the monitor ----------- 5 or 6 DC amps when drawn through an inverter and a 12 volt battery.

My comment about the oil filled heater being a good option assumed you would regularly have access to grid power.

Propane is the best practical fuel for heat when camping without hookups but I don't know that it can be implemented safely on a low budget. Burning propane consumes oxygen and introduces carbon monoxide. That can be deadly. In a factory B, the propane furnace is vented and exhausts to the outside so it is safe. Some RV'ers use a catalytic propane heater but my understanding is that they should be vented to the outside also. So a vented catalytic heater is what I would look for.

Recharging batteries is the biggest problem to overcome at this stage of planning. If you will be driving daily or every other day for an hour or so then that might be enough. That will be easy enough to calculate. Actually, you may not have any other choice but to drive daily to recharge the batteries because of up front costs. Driving would heat the van......

My guesstimate for solar would be that you'd need 400 to 500 watts of panels on your roof to be able to run a computer & stuff for 8 hours per day every day. Cost could be $1,200 used plus wiring & mounting etc. so maybe $1,500 DIY - just a guess. Maybe 300 watts of panels combined with some regular driving would work ? ? ?

You might be able to get a small really quiet portable generator (1,600 to 2,000 watts) for $800. Will you be parked where you could run it all day? Where would you store it and the gas for it?

I'll post this much for now but will also post some ideas re: a simple electrical circuit and inverter etc.

[markopolo]

Long topic here: http://www.classbforum.com/phpBB2/viewtopic.php?t=362 about mounting a generator on the rear hitch. If you could afford it a Honda 2000EU mounted in the Hayes vented box would solve your power problems (provided you would be where you could run it all day. I'd go with a proven manufacturer like Honda or Yamaha because you will be depending on it. The Honda 2000 can be run in that Hayes box.

Let me know if the generator idea is doable.

A question for everyone reading this: Would the van engine be harmed by extended periods running at idle? Like 2 or even 3 hours per day?

Edit: considering that it is for housing, heat, work, etc. what is the carbon footprint compared to someone who consumes more and has a house, commutes to an office etc? Is it a wash? Also, how prevalent are anti idling laws? Just thinking of the practicalities here.

[Mike]

Originally Posted by markopolo

Long topic here: http://www.classbforum.com/phpBB2/viewtopic.php?t=362 about mounting a generator on the rear hitch. If you could afford it a Honda 2000EU mounted in the Hayes vented box would solve your power problems (provided you would be where you could run it all day. I'd go with a proven manufacturer like Honda or Yamaha because you will be depending on it. The Honda 2000 can be run in that Hayes box.

Let me know if the generator idea is doable.

A question for everyone reading this: Would the van engine be harmed by extended periods running at idle? Like 2 or even 3 hours per day?

Edit: considering that it is for housing, heat, work, etc. what is the carbon footprint compared to someone who consumes more and has a house, commutes to an office etc? Is it a wash? Also, how prevalent are anti idling laws? Just thinking of the practicalities here.

Another thought about generators. If there will be situations where you just don't want to get up, and go to the outside
(rear end?) of your van to start the generator, there are retrofit remote starter kits for some Japanese made generators,
or some of them come with remote starters built in. They're more expensive, but may be worth it in the long run.
Then your challenge will will be finding a secure way to mount the generator, and figure out a safe way to feed it with
gasoline or whatever it's fuel might be. Just a thought. Call me lazy, but if I were building a camper from scratch, I'd try to make as much of the functional stuff as easy to use as possible, including remote controls for as much stuff as possible. As marko said, if you could find a really, really, quiet model, it might be enough to power your oil filled heater.

Another thought for heat - what do ice fishing huts use? Is there an easily vented propane or gas hut heater that might work in an RV application? Maybe see if there's an RV retailer/service center near you that might be able to suggest a solution to your winter heating needs. Maybe there's something here that would work safely?
http://www.mrheater.com/
Some safety info
http://www.ehow.com/facts_5660266_ventl ... afety.html

As far as idling, I have no research to back it up, but over the years I've observed cabs, buses, mothers picking up kids
after school in minivans, police cars, and many other vehicles similar to RVs idling for extended periods of time, seemingly without major issues. It's all about maintaining the engine. I would think you might shorten your oil change schedule even if you aren't actually in motion, as running the engine on idle will dirty the oil over time. 2 hours at 30mph? Add 60 miles a day to the idling engine's odometer for oil change purposes?

[MegA]

Thank you both for the information. First, about whether or not a battery could be recharged through the engine/alternator - I did pop the hood but couldn't see anything that looked like what you were talking about, markopolo. It could just be me, though, so I took a couple of pictures:



Maybe they'll help. I did see the red wire coming off the battery and I think that's it with the yellow arrow in the 2nd picture, but I'm not sure. The 2nd picture is the area to the left of the 1st. (In case it's not clear, the battery is on the driver's side.)

About idling laws - we do have them here in Denver but I've never seen them enforced - probably not enough officers to do it. I think it varies with the area. I don't like the idea of doing that, because of the gas wastage as well as the air pollution. And while I really appreciate the generator and solar ideas, there's no need - neither is an option at this point, for the usual reason which I won't bore you with. So I guess the first thing to determine would be if a house battery could be recharged through the vehicle. If not, a couple of other possibilities:
- Setting up somewhere where I can plug the computer in. I know the number of places with free WIFI is growing, but using their electricity probably depends on the place. I used to do chair massage at a coffee shop; people were always plugging in their laptops (so they could stay all day there ) and the manager didn't care. The only place I've ever used mine outside of home is the library (when I couldn't get internet access here) and I didn't ask because I didn't need it.
- Solar charger for the laptop. Last night it occurred to me to check and there are chargers available. The price varies; the cheapest were on eBay but I'd rather use a site that has reviews. Apparently the quality varies too.

Also - At this point I'm rethinking how much time I'll spend on the computer. Mostly I'm doing writing, but that's fairly flexible. I was going to try to sell jewelry online, which would definitely need consistent internet access. But then I'd have to find room in the van for mailing supplies and be near a post office. I may just try to sell locally at flea markets. It's a bit of a lifestyle change but I'm starting to realize I'll have to make those in order to do this. I suppose if I can relearn how to cook I can change my computer habits.

About propane heaters - I had heard of these and knew they had to be vented; there are lots of sad stories about people who didn't. I'm just not sure how much propane they'd use, meaning - how much I'd need to find room for. For a little camp stove I think I can get small bottles, but a heater would probably use a lot more. Alternative to that: staying someplace warm until the seasons change. (With so many restrictions - mainly due to budget - I'm trying to be creative about alternatives.)

Meg

[markopolo]

Battery math.

One 12v deep cycle RV type typically has a capacity rating of 110Ah.
If you don't discharge it more than 50% that gives you a usable capacity of 55Ah.
Your 90 watt Laptop by itself could consume power at a rate of approx 7Ah.
One RV battery could be discharged in less than 8 hours of laptop use.
The RV battery would probably not last 8 hours because as the battery gets weaker the current draw becomes more punitive.

So, you are most likely going to need two batteries for the living area.

Replacing that 55Ah by running the van could take 2 hours (my guess).

[markopolo]

Yes, you have a Battery Isolator!



This is good news. The rear battery can be used without you having to worry about discharging the engine battery and becoming stranded. Both the house battery and the engine battery will get charged when the van motor is running.

Is the battery compartment in the rear under the floor?
Is it vented to the outside?
How big is the compartment?
Can it hold two batteries?

A group 27 RV battery rating is usually 110Ah.
Two group 27's would give you 220Ah

[Mike]

Originally Posted by markopolo

Battery math.

One 12v deep cycle RV type typically has a capacity rating of 110Ah.
If you don't discharge it more than 50% that gives you a usable capacity of 55Ah.
Your 90 watt Laptop by itself could consume power at a rate of approx 7Ah.
One RV battery could be discharged in less than 8 hours of laptop use.
The RV battery would probably not last 8 hours because as the battery gets weaker the current draw becomes more punitive.

So, you are most likely going to need two batteries for the living area.

Replacing that 55Ah by running the van could take 2 hours (my guess).

My RT came with 2 GR27 batteries and the RT owner's manual suggests 2 hours to replenish both batteries.

"Accordingly, if the battery is not being used, it should be recharged monthly by connecting to an outside
electrical power source, by operating the generator (if equipped) for at least 12 hours, or by running your
vehicle’s engine for a minimum of 2 hours."

I wouldn't bet the farm on the accuracy of that claim, though. Wouldn't it depend on the alternator output
rating in amps, minus some draw to the engine battery?

[markopolo]

Yes Mike and also the gauge of wire running from the isolator to the rear battery. The wire gauge and the isolator will both contribute to a voltage drop seen at the end of that wire run. The battery will absorb the charge quickly at first and then it tapers off. My guess was an 80 to 100 amp alternator with maybe a 40Ah starting charge. Maybe there is a high output alternator in the van. That would be a bonus.

[MegA]

The battery compartment *is* below the floor. Here are a couple of pics that hopefully show the situation - and also my imperfect carpeting job . (Actually I wasn't sure what to do with that area so it seemed a good idea to leave it unfinished.)



In the second picture I tried to show the openings on the rear of the compartment because they seem to go to the outside; in fact if I tilt my head at a weird angle I can see the street through one of them. I don't know if that's enough venting but that's what's there. (I couldn't see behind the front holes to know what they open to.) There was never a cover over the compartment; it was always open.

The compartment measures 9" x 12" and 12" deep which I think will hold only one battery based on some research I just did. I'm not even sure a Group 27 would fit in it. I guess since it was designed to hold a wheelchair lift battery there wasn't a need for anything bigger.

Meg

[markopolo]

Put the biggest 12v Deep Cycle battery that fits and you can afford in that battery box. Maybe buy it from a shop that will install it for you. An AGM (Absorbed glass mat) battery would be ideal as it is maintenance free an very unlikely to ever outgas when being charged. You could use a wet cell battery but if you do make sure the battery compartment is sealed off from your living space by putting a top on your battery box. I'd probably duct tape the box top down and cover it with carpet. Your box is vented to the outside (that is good) but you don't want engine exhaust fumes to enter the van through the box.

Probably the simplest way to get some AC power in your van will be to install an inverter. The wires can come up through the battery box lid. A 1200 watt inverter would let you run a 700 watt microwave or a 700 watt toaster for example. I would run the van motor when using a large current draw appliance like that. That way, they would almost be powered entirely by your van's alternator with very little impact on your house battery.

I'd choose an inverter with a 12v accessory outlet and a USB power port if possible. I have one with those features and it has been handy. The USB port can be used to charge a cell phone. Another a nice feature available with inverters is a digital display that shows your battery voltage. Most inverters have 2 AC outlets and you could plug in a power bar to have more outlets.

[MegA]

Thank you, markopolo. I was thinking I could use some of the leftover wood from the floor to make a cover for that compartment; I just need a hole for the inverter wires, yes? I have enough pad to put on top of that but no carpet left, but a piece that size shouldn't be hard to find. I almost rescued a whole bunch from a dumpster the other day... There's a lot of renovating going on in my neighborhood.

Just a couple of other questions:
- What is involved in the installation of the battery? I was thinking it would just be putting the battery in the compartment and connecting the wires there, basically a reverse of what the buyer of the lift did. I've already called a couple of places to ask about putting the battery in the van since there's no way I can carry one of those; they all seemed OK with that, they probably get asked about it a lot. But if there's more involved I'll have to ask around. If I got the inverter and the battery at the same time they could be connected at the same time.
- Inverters - I've read that there are 3 types - pure sine wave, modified sine wave, and square wave - with pure sine wave being the best (and most expensive) choice. But I've also found fairly cheap inverters just in looking on Wal-Mart's site; it doesn't say what type they are so how could I find out, or should I just avoid them? Do I need a special type for what I'm going to be doing? Also, does it matter what kind of wattage I buy? Does it depend on the appliances I'm going to be connecting to it or does the battery amperage get considered too?

I'm basing some of my questions on the info I found on these pages:
http://www.ehow.com/list_5910725_inverter-types.html
http://www.invertersrus.com/inverterfaq.html

If you disagree with any of that please let me know.

I didn't know inverters came with other outlets, though - that's great. I've seen other kinds of "adapters" that do but that would be really helpful.

Meg

[MegA]

One more thing, just to be sure I'm on the right track here - The figure that comes from dividing watts by volts, amp hours, is how many amps I would use if I ran that device *for an hour*, yes? So as an example, if I'm using a 700-watt microwave for 5 minutes, that's
700/10= 70
70/60=1.17
1.17 * 5 = 5.85 aH used

There may be some rounding errors there; if I do
700/10/60*5
I get 5.83 aH

Which would mean if the battery is 100aH there would be about 94 aH left to completely empty it or 44 aH left to take it down as low as it *should* go before recharging.

I think this is basically what you did when figuring the laptop usage; I just want to be sure I'm doing it the right way.

I used 10% instead of 12% to allow for the inverter's usage.

Meg

EDIT: And if this is right, I could make a spreadsheet for each appliance so I'd know (or least closely estimate) what it would use for any period of time I want. So I could almost predict when the battery would run down, or what I could/couldn't use depending on the voltage left?

[markopolo]

The battery installation is just as you thought.

Two types of inverters to consider really; pure sine wave and modified sine wave. Pure sine wave is better. It costs more but is the best choice. I've used both and my next one will probably be modified sine wave unless I can find a good deal on a pure sine wave inverter. Popping popcorn in a microwave oven using a modified sine wave inverter might take 4 minutes instead of 3 minutes with a pure sine wave inverter or when on grid power.

I'd suggest at least a 1,000 watt inverter. 1,200 watts should do the job perfectly. I've based that on a 700 watt microwave oven. It might actually use 1,000 watts.

Yes, your calculations look right to me. Inverters are often only 90% efficient hence dividing by 10 instead of 12 volts. It makes the math quicker.

Track what is left in the battery by voltage after it has been at rest.
12.7 volts = fully charged
12.25 volts = 50% charged
12.0 volts = 20% charged

A spreadsheet of your specific appliances is a great idea.

A larger current draw has a greater effect on a battery.
Peukert's law - As the rate increases, the battery's available capacity decreases.
So the large impact of the 70 Ah microwave oven running for 5 minutes current draw will be more punitive than the 5.85 Ah in your example. You'd probably have to add 1Ah.

AGM batteries are less affected by this than flooded wet cell batteries.

[MegA]

OK - Peukert's Law is mostly over my head (I did try to read the Wikipedia entry) but I think what you're saying is, for example - a 700-watt microwave running for 5 minutes will use more of the battery's capacity than a 90-watt laptop running for the same amount of time, yes? I'm sorry to keep repeating this back to you but it helps me understand.

When you're talking about 1000-1200 watts for the inverter do you mean the continuous usage? On most inverter specs I'm seeing 2 figures, a "continuous" and a "peak." The peak is usually twice the other figure.

I did set up the spreadsheet last night; here's a screenshot:


I had to crop it for space here but it actually figures all the way out to 12 hours (and could go further if needed). The aH is the watts divided by 10 and the aM is my own invention; it stands for ampMinutes and it's just aH divided by 60. I did it that way to figure usage of less than an hour, it seemed easier than dividing aH by many different decimal points. The other columns just multiply the time by either the aM (if they're less than 60 minutes) or the aH (if they're more than 60 minutes).

This is not intended to be exact, just a close estimate. But if it's right I'd be happy to share the formulas here if it will help anyone else; it wasn't that hard to set up. And anyway, it seems like a good idea to inventory all the electrical devices because it's more realistic. For example, I was thinking I just had the computer, monitor, and printer. Then I remembered the speakers, the charger for the camera battery, and the cell phone charger, and who knows what else I'll find? It's so easy to take this stuff for granted - until you have to find portable power for all of it.

Looking at the spreadsheet figures for the chargers reminded me - I'm not clear on how you came up with the monitor wattage based on the picture I posted. Did you add the input and output A (which I assume stands for amps? Is that the same as ampHours?) and then multiply by 12, or is there some other figure on that label that shows it? I've run into the same issue with the chargers and the speakers - no wattage on the label - so I want to make sure I'm basing my calculations on the right figures.

Meg

[markopolo]

Re: Peukert's Law - maybe I don't really understand because I really have difficulty explaining what I think means.
Lets say a person only has enough energy to walk 20 miles at a steady pace. Total of 20 miles distance covered.
If that person decided to run at full speed for the first 2 miles then they would only have enough energy left to walk 15 miles. Total of only 17 miles covered. They used up their available energy at a much quicker rate so there is a shortfall at the end of the day of 3 miles.

Really high amp draws have the same effect on a battery.

Spreading the load over multiple batteries reduces that loss.
AGM batteries are not effected by this as much as flooded wet cell batteries.

Re: your monitor - I multiplied the output volts by the output amps to calculate the output watts.

And all this reminds me to mention that appliances don't always consume as much as they are rated for. My laptop power brick is rated at 75 watts. I measured the AC amp hr rate yesterday when I plugged the laptop in because the battery was down to 28% remaining. The AC amp hr draw was .5 at 120 volts so my laptop energy output was 60 watts. When the laptop battery is fully charged the energy output can be as low as 26 watts consuming only .22 Ah AC at 120 volts.

Here is a post with my ammeter test and it really illustrates inverter loss. http://www.classbforum.com/phpBB2/vi...hp?f=29&t=1038