How do I pick an inverter?

[KC10Chief]

Okay, I would like to be able to power my refrigerator, TV, lights and other small eletronic devices while I'm going down the road or parked. I'm getting a smaller, 5 window bus. I'll use it mostly for hunting and for dirtbike races and it may be parked for two days at a time or more. I have two Honda EU2000i generators if I need to charge. Anyways, how many watts do I need to run a small 4.0 cubic foot fridge, a 20 inch LCD, lights and a furnace? How many batteries will I need and what type? How about powering a roof A/C while going down the road? It gets miserably hot here in Oklahoma in the summer. On a travel trailer I recently sold, it would take about 2,000 watts to run my roof A/C. Is it possible to buy like a 3,000 watt inverter, run it off of four of those 8D batteries, and charge it with the bus alternator while going down the road? Would I need a bigger alternator? If I decided against the roof A/C, how many watts do I need to power the fridge and other stuff? I see a lot of inverters for sale on the internet for cheap. I always thought they were several hundred dollars for like a 600watt inverter. Thanks for your thoughts!

[lapeer20m]

you can get a 2000watt inverter for about $200

as for battery charging, once your batteries get run down i found it is more efficient to run appliances directly from the honda eu2k than it is to use the geni to charge your batteries. The ultimate charging system would be to use a 2nd alternator off of the engine that only charges your "house" batteries. A 200 amp leece neville alternator would be my choice. Chances are good your bus already has a leece neville alternator, they are very commmon among skoolies and big trucks. You're house batteries would charge and stay charged when you're driving. once you park, you could run your appliances off of the inverter for several hours or days depending on the size of yoru battery bank and the amount of load. Running an a/c from an inverter requires a huge battery bank.

for the expense and hassle involved, i would personally just stick with the honda eu2000 geni(s) and get a realatively cheap inverter and small battery bank. Use the inverter while driving, and the geni when parked.

[Les Lampman]

Originally Posted by KC10Chief

Okay, I would like to be able to power my refrigerator, TV, lights and other small eletronic devices while I'm going down the road or parked. I'm getting a smaller, 5 window bus. I'll use it mostly for hunting and for dirtbike races and it may be parked for two days at a time or more. I have two Honda EU2000i generators if I need to charge. Anyways, how many watts do I need to run a small 4.0 cubic foot fridge, a 20 inch LCD, lights and a furnace? How many batteries will I need and what type? How about powering a roof A/C while going down the road? It gets miserably hot here in Oklahoma in the summer. On a travel trailer I recently sold, it would take about 2,000 watts to run my roof A/C. Is it possible to buy like a 3,000 watt inverter, run it off of four of those 8D batteries, and charge it with the bus alternator while going down the road? Would I need a bigger alternator? If I decided against the roof A/C, how many watts do I need to power the fridge and other stuff? I see a lot of inverters for sale on the internet for cheap. I always thought they were several hundred dollars for like a 600watt inverter. Thanks for your thoughts!

Watts = amps times voltage. On your AC appliances look at the tag on it and it should give you the amperage required at a specified voltage (typically 115 volts). So, if it's 4 amps @ 115 that's 460 watts. It's the same on the DC side, which is why it's nice to work with wattage. 4 amps @ 12 volts is 48 watts. For the AC to DC conversion just use a factor of 10; it's easy math and usually makes up for inverter losses. So that 4 amp load on AC is a 40 amp load on the DC side. This isn't perfect but good enough for what you want to figure out.

A roof air unit pullling 10 amps on AC is consuming about 1150 watts and it'll pull about 100 amps on the DC side running through an inverter (not counting the starting load). Theoretically a 150 to 200 amp Leece Neville will keep up with that but there won't be much left to also charge the batteries if they're low or run other equipment that's turned on in the bus.

Unless you have a massive battery bank it's awfully tough to run the A/C on the inverter (when the engine isn't running). The load is so high (at 100+ amps) that the internal resistance of the battery will take the batteries down very quickly. For instance, if you have four 8D batteries at 225 Amp-hours each (900 AH total) you'll have about 80% of that available if you do a really deep depth of discharge, or about 720 amp hours. Now this means that the batteries can sustain a 30 amp load for 24 hours. The higher you go over the 30 amp figure the less life (on a diminishing basis) you get due to the batteries internal resistances (this is Peukert's Law). So, you wont get 7.2 hours of run time for your 100 amp load; it takes a chart (which I don't have available at the moment) or a complex calculation to figure out the run time exactly but it's probably in the 4.5 to 5 hour range. But then you have 720 amp-hours of energy to put back in the batteries. If these are wet cell batteries they shouldn't be charged at a rate over 25% of their total capacity; in this case 225 amps. Even the 200-amp Leece Neville can't do this; it actually can't put out 200-amps continuously as it's not hot-rated for that. Try it and you'll burn it up. You'll probably have to have an external regulator that will limit the charge current to perhaps 180-amps or so. You'll also have to allow for other loads on the alternator (bus lights, heater blowers, etc). That means you'll have to run for about 6 hours to recharge the batteries.

None of this means that it can't be done...I don't mean to imply that. Only that it's not an easy thing to achieve. It would actually be cheaper and easier to run the A/C off the Honda than to set up the DC system to handle the load when stopped.

More later...I have to go back to work!

[Les Lampman]

Ok..about the load questions. There aren't any specific answers as it really depends a lot on the specific item (brand and model sometimes). For example lets use the 4.0 cubic foot refrigerator. If it's an AC model it uses perhpas 1 amp or so; if you run that off an inverter you'll pull about 10 amps of DC. If it runs 1/2 the time that would be 120 amp hours that you'd take from the batteries. If you were treating your 8D batteries really nicely and only taking them down to a 50% depth of discharge it would take one 8D to run the refrigerator one day (assumming a 50% run time; that's probably a little high).

If you chose instead a Norcold DE-0041 (actually 3.6 cu ft), it runs on 1.6 to 2.8 amps on DC (a measly .2 amps on AC); less than a fifth of the AC refrigerator above. In other words, you could run the DE-0041 for almost 5 days on one 8D battery. It's an AC/DC model with a real compressor; it's not a gas absorption model.

The small 4.0 cu ft AC refrigerators seem to sell for around $150 or so (some less, some more). The DE-0041 sells for about $800. Ouch! But it's like having 5 times as many batteries aboard; it all comes down to what will work best in your situation. If you have lots of battery power and will be back where you can charge them every couple of days probably the AC refrig would work fine; if you're going to set up for a week or so and won't recharge the Norcold would be a better choice (if the price doesn't kill the deal).

As much of a PITA as it is you need to do an Energy Budget. It doesn't have to be fancy; just add up everything you're likely to run in a day. Just add up all the amps; if it's an AC unit that's going to run on the inverter take the amps on the plate on the appliance and multiply times 10 for DC amps. If it's rated in watts just divide by the voltage...for instance...a 5-watt light on DC draws .4 amps (5 divided by 12). An 800-watt AC coffee maker would be about 7 amps (800 divided by 115) on the AC side; running on an inverter it would be 70 amps from the batteries.

Use the internet to search for watts or amps on everything you want to run. Then figure out (even guessing for the most part!) how long each thing might be turned on and that becomes your amp-hours. So a 5-watt bulb on for 1.5 hours is 7.5 amp-hours. The 10-amp refrigerator running for a total of 8 hours out of 24 would be 80 amp-hours. Then add up the total amp-hours and that's your Energy Budget for the day.

Now...if you really want to be kind to your batteries multiply your total Energy Budget by 3 to get the total number of amp-hours you should have in batteries and recharge them at their 50% level (12.1 volts). Say you came up with 115 amp-hours per day; you should then go for about 345 amp-hours worth of batteries. And your alternator should be no smaller than 33% of this number (40% to 50% is better); or about 114-amps in this example.

Now, here is where another big trade-off comes...how important are your batteries. If you're not having to spend too much on them and you don't mind replacing them more often you can drag them down deeper (in terms of depth of discharge) and also charge them faster (if you have the amps available). This will kill the batteries much sooner than if you cycle them with shallower DOD (depth of discharge) and charge them properly but that takes money and time and depending on your use and the batteries it might not be worth either. If you buy really good (and expensive) batteries and want them to last as long as possible you'll have to treat them better and pay more attention to their DOD and charge rates. If you're boondocking a lot and/or trying to maintain a viable solar charge system this will be important; if you're weekending probably a lot less so.

Even if you're weekending and want to get the most you can out of your batteries (regardless of cost) get a good multi-stage (typically three...bulk, absorption, and float) charger that has settings for your battery type (usually wet cell, gel cell, and AGM) to plug in when you get home. This will charge them properly, top them off (your bus alternator (without an external regulator) never can) and keep them peaked up ready for the next trip.

I'm not trying to overwhelm you with technical stuff here; it just helps to know a little something about what you're after so you know whether or not you're going the right direction for what you want to accomplish.

If you've got questions...fire away!

[the_experience03]

I know everyone has opinions on this, including the manufacturers, but I'm going to throw this out there....

Most 8D starting batteries have a huge reserve capacity in the area of 450 minutes at 25 amps. This is, however, due to their shear size and not their design. I had the opportunity to cut into a used 8D packaged under our label at work. They are manufactured by East Penn (now owned by Douglas) so they are not an el-cheapo Exide. The plates are the same as any other starting battery. They are thin and closely spaced together to provide the approximately 1200 cold cranking amps these batteries are designed for. Basically, they have a huge reserve capacity, but are not a deep cycle battery and I think a DOD greater than about 20 is going to hurt a relatively expensive battery over time.

We do sell 8D AGM's that are meant to be deep cycled, but they are $$$. I still stand by golf cart batteries as the poor man's choice to portable power.

[KC10Chief]

Wow! Thanks for all of the very useful info! As far as the A/C goes, I was mainly concerned with running it while going down the road. When parked, I'd just use my generators. From everything I've read, it's not worth the trouble and I can't afford all that stuff anyways. I am going to put a rear rack on my bus. I figure if it gets too hot, I'll just run the generators back there to power the A/C while I'm going down the road! Anyways, where can I get some good batteries at? Thanks!

[the_experience03]

You have two Batteries Plus loacations in Oklahoma City. GC2's with ~220 AH should run right about $75 there with a possible price break if you buy several. Anything with the Werker in-house label is American made should that be of great importance to you.

[KC10Chief]

Since the GC2 is a 6V battery, do I just hook two of them upin series so I can get 12V? Could I then hook up two sets of batteries that are in series, and wire each set parallel so that I have 900ah of 12V power? Am I understanding this right?

[the_experience03]

Youve got it for hte most part. To get to 12 volts, hook positive from one battery to the negative of the other and use the remaining two terminals as the positive and negative to the load. That would be series. Then parallel groups of 12 volt banks by going positive to positive and negative to negative. The only thing is this...hooking in series will double the voltage, but reserve capacity will stay the same. Hooking in parallel will leave voltage the same but double the reserve capacity. Basically, that means that four 6-volt golf cart batteries will give you about 450 amp hours, not 900.

[Les Lampman]

Originally Posted by the_experience03

I know everyone has opinions on this, including the manufacturers, but I'm going to throw this out there....

Most 8D starting batteries have a huge reserve capacity in the area of 450 minutes at 25 amps. This is, however, due to their shear size and not their design. I had the opportunity to cut into a used 8D packaged under our label at work. They are manufactured by East Penn (now owned by Douglas) so they are not an el-cheapo Exide. The plates are the same as any other starting battery. They are thin and closely spaced together to provide the approximately 1200 cold cranking amps these batteries are designed for. Basically, they have a huge reserve capacity, but are not a deep cycle battery and I think a DOD greater than about 20 is going to hurt a relatively expensive battery over time.

We do sell 8D AGM's that are meant to be deep cycled, but they are $$$. I still stand by golf cart batteries as the poor man's choice to portable power.

Thanks for adding that! In my posts I was referring to Deep Cycle batteries and not Starting Batteries but I didn't say that...and it's important...my bad .

I agree too...the Deep Cycle wet-cell golf cart batteries are the biggest bang for the buck (the most amp-hours for the dollar spent). Other battery technologies (like AGM) only really start to have a better payback in specific systems. I have a whole program written only to figure out where than break point is; there are a ton of variables. Which is why I've planned for GPL4C AGM batteries (6-volt golf cart) but will most likely install GC2 wet cells to start with; it will be years before we're using the bus in a mode that would allow the GPL4C's to have the better payback. I can buy the Dyno 245 AH CG2B battery for $90 each; the GPL4C 220 AH AGM from the same supplier is $231 each. That's a big difference (times the number of batteries in the system)!

[pete c]

I am interested in doing the same thing as KC, but in my little toyota sunrader. We are making a trip out west this summer and will undoubtedly have need for AC while underway at times. It has been suggested that I need a genset to accomplish this, but, there ain't a whole lot of room for such a thing, other than strapping one to the rear bumper. But, I'm not crazy about this idea as that bumper ins't the strongest thing to begin with and I'll already have enough junk hanging off it.

I found a 160 amp alternator on ebay. I would think that this, along with a 2Kwatt inverter ought to do the trick. I don't need a real high dollar fancy inverter with super clean out put. Just something rugged and capable of putting out something resembling 120VAC.

There is already a house inverter. I would use this for other needs and use the hi amp one solely for the AC.

Any recommendations would be appreciated.

[the_experience03]

160 amps@14.4 vdc run through an inverter is basically going to give you 16 amps at 120 vac. However, this is the struck by lightning max output of the alternator. Chances are it's not going to put out near that and if it does, it's not going to be duty rated to do it for long. Don't think it isn't possible though. Heck, try it with that 160 if you can get it cheap. I may well be wrong if it ends up being a big Leece-Neville instead of a GM 12SI or similar. You could look into some of the alternators made for offroading (onboard welders, winches, air compressors, etc). Mean Green and Premier (Power Wleder) are both reputed to make a very nice unit that is duty rated at it's max output. Unfortunately you pay for them, but there's always eBay.