batteries, inverters, and power math for the masochistic

[meeotch]

As much as I hate creating Yet Another Power Thread - I've been wading through online info for hours now, and still haven't figured it out. Hopefully, this thread will at least serve as a fully-worked-out example solution for future waders. So we're trying to come up with a power plan for 2011. Some background:

Total power draw: a stereo (60W?), a bunch of strings of lights, maybe a blender. I'm guessing < 200W total. Probably more like 100W. (At 120VAC.)
Generator: we have an RV (*not* my idea) that has a genny, but - $3/hour fee to run it. Also, if it consumes 1 gal/hr (a guess), then there's not enough gas in the RV tank to run it all week. Plus, that's total overkill, power-wise (and gas-wise).
Constraint: we're coming all the way from the other side of the country, and therefore can't go with anything that we're not able to buy, set up, and test in Nevada in two days. (And everyone in our group is poor.)

So my next thought was deep-cycle batteries, charge off the RV genny, then run off batteries all day. I checked out those "emergency powerpacks", which seemed like a good no-brainer solution: [url]http://www.amazon.com/Duracell-DPP-600HD-Powerpack-Starter-Emergency/dp/B000TKHMWK
[/url] But from the reviews, you're just as likely to get a dud (due to sulfation) as one that works. Also, they're only rated at 28 Amp-hours, and take 36 hours to charge via AC. Again, we don't have much time to test this rig once we get to NV.

At say 100W, we'd be pulling 8.3A from a 12V battery. Figure 85% efficiency from the inverter, and it's more like 10A. If the batteries we use are 80Amp-hours, and only happy to be discharged halfway, then that's only 4 hours runtime off each battery. I have no idea how long deep-cycle batteries take to charge, but my understanding is that it's not quick. So figure we'd be running the RV genny 50% of the time, to charge the batteries... And it sounds like the cost of two (or more?) batteries, plus a decent inverter isn't going to be worth it vs. the gas & genny fee we save. (Remember that this is a one-shot deal - it's not worth shipping the batteries back to the East Coast after the burn.)

How is my math? Is our least-bad option to bite the bullet, load up on gas cans full of dinosaur juice, and poison the sky with our generator all week? That seems unnaturally painful, just to run a small stereo and some lights.

[FIGJAM]

Find out if the rv has a battery bank.

If it does, those batteries can be charged from the genny and the motor of the rv.

Check the internet, but I think the rv motor will charge at a higher amp rate than the genny, like 5x faster.

[meeotch]

Thanks for the tip. I'd read elsewhere that many RV's have a cab battery (for drivin'), plus a couple others that run the interior lights, etc. when parked... But I'd assumed this wasn't a solution, as the source seemed to be saying you didn't want to run heavy appliances like the microwave off of them. (I'm not the organizer of this particular RV - or even a huge fan of RV's in the abstract - so my knowledge on that end is somewhat limited.)

Is that what you're referring to - or something additional? If the RV essentially has the entire solution pre-built into it, and all we have to do is run the genny/motor periodically, that would be pretty nice.

[FIGJAM]

Microwaves use a lot of power for a short amount of time.

Just start the genny long enough to work the microwave.

You have to find out what the rv set up is, but running the motor will keep you from rental time on the genny.

[some seeing eye]

Get a battery you can move across the playa. A wagon is ideal and can be towed by bike. Charge your battery at Snow Koan solar. Figure out a way to charge it up to start in your start city. While on playa, you might enjoy talking with the AEZ village, energy experts, and sometimes they have solar baked chocolate chip cookies on their tours!

[AntiM]

Get a battery you can move across the playa. A wagon is ideal and can be towed by bike. Charge your battery at Snow Koan solar. Figure out a way to charge it up to start in your start city. While on playa, you might enjoy talking with the AEZ village, energy experts, and sometimes they have solar baked chocolate chip cookies on their tours!

We have a garden cart we always take to the playa. One year Hushville was quite near the Snow Koan; our neighbor would load up his deep cells which ran his CPAP on our cart, and walk them down the street for a recharge. Very workable solution in this instance.

[DustHand]

The blender is a high current draw item. Run the genset for that. For your stereo and light, use the inverter and get one of these to keep it topped off during the day.

http://www.google.com/m/products?oe=UTF-8&client=safari&q=solar+battery+charger&hl=en&bl=/m/search?q%3Dsolar%2BbTtery%2Bcharger%26ie%3DUTF-8%26oe%3DUTF-8%26hl%3Den%26client%3Dsafari&source=search&channel=products&ei=rv4qTuCSGcS_8AaQp5cg&ved=0CBcQzQM

Lead-acid batteries take a lot linger to charge than lithium-ion and having one of the solar chargers attached all day should keep it charged. Look for a watt rating that is high as possible. It will charge faster.

[FIGJAM]

Just finish going over a lot of fishing forums.
Turns out the guys useing electric trowling motors are rechargeing their deep cycle batteries with jumper cables from their cars for an hour in the evening after useing those batteries on the lake all day.

No genny for me this year!

[meeotch]

Thanks for the continued stream of info. Yah - my feeling was that we'd pay for genny time when anyone wanted daiquiris, and try to run lights & tunes off the batteries.

What I'm still shaky on is the math of discharge / recharge... If my figures above are in the ballpark, just lights/fans/stereo will burn through a couple of 80Ah batteries a day (50% discharge level). Would solar charging really be able to replenish them at that rate (or even half that rate)? If my math is right, we're discharging at 10A. If we're running the batteries for half of the sunlight hours, we'd need to charge at 5A to keep pace - or 60W, which seems a lot more than a solar trickle charger could muster.

For charging from the RV genny: Googling indicates that the max safe charging rate is 10-20% of the capacity - so 8-16A for our hypothetical battery. Unless I'm missing something (and again assuming 10A discharge rate), that means running the genny 40-60% of the time, just to keep the battery charged for the other 60-40%. Maybe this is where multiple batteries come in? Presumably, you could charge four batteries in parallel in the same time as one, then run for 4x as long when they're charged.

I know people do this stuff all the time, so it must work. But the numbers sure make it sound like it's not economical for a group with our particular constraints. (i.e. not being able to amortize the cost over more than one burn.)

[percussivepaul]

OK, I just did a lot of power planning for my camp, so I can help a little.
First read this page: http://www.templetons.com/brad/burn/burn-power.html "Desert power"

Something else I learned is that big batteries take a long time to recharge, and this is limited by the output current of your charger. Cheap chargers put out like 6 amps which means it will a full day of generator use to refill an 80Ah battery. My camp just purchased this 40 amp charger so we can refill a big battery in an hour or two. Ask me next year how well it worked out on the playa. http://www.amazon.com/Black-Decker-VEC1 ... sim_auto_5 (it was half price when we bought it, you can probably find it or something similar for $80 somewhere). Or as Figjam says you can try recharging off the motor. I have no experience with this.

You can rent a small generator from a place like hertzequip.com for about $150 a week. Fuel consumption on these was something like 0.8 gallons per four hours. You might save money with such a scheme compared to your RV genny if you are running it a lot, and it will probably be quieter.

Strings of solar LEDs can be found for $25 and are pretty good. Not super bright, but a few of them will light up a small camp well enough that you won't trip. My camp will be using battery-AC LED strands for a few hours each evening, and then switching them off and relying on a small number of solar strands from midnight to sunrise, or so.

To do this proper you need to plan your loads in terms of battery amp-hours. One hour of 60W stereo use is 5 Ah at 12V, and 40W of lighting is say 4Ah. So if you want stereo for four hours a day and lighting for 4 hours an evening, you need 36 Ah of battery per day. So an 80 Ah battery would work, if you recharged it fully every day.

To charge via solar, if you have a 30W panel (about $250) and get ten hours of sunlight a day, that's 300Wh, which at 12V makes 25Ah per day. It's in the ballpark of being able to maintain the charge for the above scheme if you were to cut your power consumption a bit.

[Rice]

Try to stay away from using an inverter. You will lose between 10% and 30% of your battery life just running the darn think. There are lots of 12V products that will meet your needs for a modest price. You should be able to go a couple days on one battery if you plan things out right.

Get some solar powered xmas lights, they are self-contained & have their own charger.

Hand powered blender? I have seen lawnmowers and chainsaws converted to run a blender as well (A gallon of gas should do you there).

I suspect the biggest drain on your battery will be your sound system.. Do you really need 10000W?? Are you doing a theme camp and expect to have people around all the time? Or are you going to be at your camp to sleep and maybe hang around during the hottest part of the day?? -- if so, then a simple 100Watt car stereo probably will do fine, I have seen camps get by with a solar charging panel and a couple car batteries. Guess it depends on you needs.

Taking untested products to the burn is a gamble. The times I have done that have resulted in complete and utter failure of some critical item. That totally sucked, since I then had to rely on my neighbours. Not exactly radically self-reliant... {sigh} Anyhow, test what you can now, see how long things run. Then you will have an idea what you are up against.

Good Luck!

See ya on the playa,

Rice.

[Trishntek]

I'm betting anything your RV has an inverter. If it has a tv/dvd player, I'm almost 100% sure it will have an inverter. That inverter runs off the camper battery which is recharged by either the genny or the truck's engine and does not affect the battery that runs the truck itself.

Some rental companies will cut you a deal on the genny hours if you tell them you will be staying in one location for several days. I know last year, the outfit I rented allowed 3 hours per day before charging $$ for its use. 21 hours of genny time will charge a lot of battery. And they only burn about 1/2 gallon per hour. Take an extra 5 gal can if you are concerned.

[unjonharley]

\
Few years back I found a 12v 10ah Battery pack for cheap. I use it for lights and music.

It has two cig lighter ports.. It can be charged off a car/truck lighter or a 110v transformer.. There is a charging diode in the 110v line..

Another burner saw my pack and built one for him self.. Mine is in a fancy plastic case.. He got a brief case from good will for his case.. Then he up graded his to two 10ah 12v batteries.. Then added a $3 volt/amp meter.

For lights I have two Northwest Territory 20LED camp lamps (from BigK).. These lamps will burn 24/2 on three D cell each.. They should be able to run 24/7 easy with the battery pack..

[meeotch]

Thanks paul & others - awesome info. (And numbers! I love numbers!) Sounds like my math was pretty much in the ballpark. If we were able to spread the cost across multiple burns, a fast charger & 2-3 batteries sound like the way to go.

Info from our RV company: the outlets aren't connected to the storage battery (the one that runs the lights, fridge, etc.). So we either have to run the genny ($3/hour, no free hours, no bulk discount possible), or get a cigarette lighter style inverter - which means draining the cab battery, which I'm leery of. The RV people said the genny eats 1 gal / hour... Though most of the models on their website seem to have Onan Microlight 4Kw generators, and some googling indicates maybe as little as 0.5 gal / hour. Maybe they're just being conservative.

I'm sort of surprised that a rental genny would be quieter than the one in the RV. You'd think RV genny's would be selected for quietness. But again, I'm not an RV aficionado. Anyway, at $3/hr * 10hr/day * 6 days, our fee is about $180. After calling around, I've been unable to beat that on a rental. Nor would that figure buy us the equipment we'd need for a battery solution, unfortunately. So poison the sky, it is...

[DustHand]

Keep in mind fuel consumption is directly related to the load on the generator. So, if you are runing your genset to power a few lights, it will sip fuel. If you are running all the lights, the microwave, cranking out daquiris and have 2 AC units blasting cold air, expect a greater consumption rate.

These are specs for a random Onan 2.8 KV genny...most likely NOT what you have:
Average fuel consumption:
Gasoline No load Half load Full load
60 Hz 2.8 KV 0.2 Gal/h (0.8 L/h) 0.3 Gal/h (1.1 L/h) 0.4 Gal/h (1.6 L/h)

[meeotch]

Yeah - that's why I had the feeling the CruiseAmerica lady was just feeding me a line. When she said 1 gal / hour, I asked if that included when it was just idling - I don't think our dinky home stereo receiver and xmas lights are going to load a 4kw generator much. Her response was that it's the same, 1 gal/hour.

I'm going with 0.8 gal/hour for my calculations, and will try to convince folks to spring for 2x 5-gal cans of gas. Then we can go nuts wasting fossil fuel on blender drinks, and if the RV runs dry, we've still got enough gas to get out of there on monday.

[DustHand]

Yeah - that's why I had the feeling the CruiseAmerica lady was just feeding me a line. When she said 1 gal / hour, I asked if that included when it was just idling - I don't think our dinky home stereo receiver and xmas lights are going to load a 4kw generator much. Her response was that it's the same, 1 gal/hour.

I'm going with 0.8 gal/hour for my calculations, and will try to convince folks to spring for 2x 5-gal cans of gas. Then we can go nuts wasting fossil fuel on blender drinks, and if the RV runs dry, we've still got enough gas to get out of there on monday.

I like your plan. We brought 4x5gal cans last year and filled up in Gerlach on the way in. Never needed the cans, but felt secure running the generator when we needed to.

Also keep in mind, most rv gensets feed from the main fuel tank at the 1/4 mark, so when the generator "runs out" of fuel, you still have a quarter tank with which to drive.