Wanted: inverter that will drain a battery all the way?

August 12, 2007 12:53 AM

My brother is looking for an inverter to use with a deep cycle battery while camping.

He says:

I use a 12v DC -> 120v AC power inverter hooked up to a deep cycle
battery while camping. Unfortunately the inverter I use emits a high
pitch whine when the output from the battery falls too low, presumably
as a warning. Eventually the inverter shuts off and the camp goes
dark. When I recharge the battery my meter indicates that it's still
got about a 60% charge remaining. The point of the deep cycle battery
(as opposed to a regular car battery) is that it can be fully
discharged without damaging it. Does anyone know of an inverter which
will let me use more than 40% of the battery's capacity and which
won't make an annoying noise when it gets low?

posted by croutonsupafreak to technology (8 comments total)

What's he measuring with that meter? If it's voltage, and the voltage is 60% of nominal under charge, that doesn't mean the battery is at 60% charge - it means it's pretty much dead flat.

Instead of switching to an inverter that lets him extract 99.9% of what the battery is capable of instead of merely 99%, he may well be better served by buying a second battery.
posted by flabdablet at 2:43 AM on August 12, 2007

"... The point of the deep cycle battery
(as opposed to a regular car battery) is that it can be fully
discharged without damaging it. ..."

Not really. Some deep cycle batteries (Optima & Lifeline) can be discharged to near 100% of rated capacity repeatedly without permanent loss of capacity, and even stored for up to a month uncharged, but other lead-acid batteries of deep cycle design (like flooded plate batteries) that are fully discharged for any length of time are going to be damaged. So the first thing is to be absolutely sure that the specific battery that he is using is going to be OK with discharge cycles that take it below 50% charge. Even batteries that are designed for this are going to be less efficient in supplying the last 50% of charge, than they will be in giving up the first 50%, simply due to increasing internal resistance developing as battery charge is lost. Inverters will generally drop at least a little bit in efficiency, too, as discharge progresses. So, even in the best set ups, the "last" 50% won't be, truly, 50%.

The next thing is to be sure he's charging his deep cycle batteries with a charger made for them. A deep cycle charger will typically have 3 charging rates and volts, the last one being about 15 volts, to fully charge the battery in the minimum time. But these type chargers aren't designed for use with normal batteries, and most automotive charging systems won't fully charge deep cycle batteries.

All that said, the choice of a specific inverter is going to depend on total load being driven, peak load, whether the load needs sine wave corrected power, and the amount he is willing to spend for superior inverter hardware. Of these considerations, the most important is to find out whether any of the loads he is powering need sine wave corrected power. If not, the extra expense and lower efficiency of a sine wave corrected inverter is pointless. He'll up to 20% longer operation with a non-sine wave inverter, by far. Things like toasters, blenders, and fans won't generally care much about non-sine wave power, but radios, televisions, and some kinds of lighting may.

Next, he needs to characterize the current demands of his loads, to match the choice of inverter characteristics and features. As an example, if he is driving a highly variable load, like a big stereo or color TV, that kind of load can really benefit by having a big capacitor connected across the battery in the fully charged state, since that kind of load can present very high current demands that change many times a second. That kind of highly variable, high current demand load is terrible for battery efficiency, and the addition of a big capacitor that smooths it out can pickup a lot of performance. But a 20000uf computer grade electrolytic capacitor, in a ruggedized temperature compensated package, is $20 component by itself, to say nothing of the current limiter circuit usually used with such added elements, in more capable inverters.

Finally, he should match the current capability of his battery to his inverter, and his inverter size to his loads. If needs 1000 watt of AC power for 4 hours, with a peak of 2000 watts, he's going to have buy batteries and inverters to power the 2000 watt peak load, but these will be more expensive by far than his average load would otherwise require, and the larger inverter will not be as efficient, generally, in powering the 1000 watt average load, as a 1000 watt inverter could be.

For a one size fits all recommendation, I've had pretty good luck with the xPower 3000 Plus RV inverter. It has decent efficiency (+ 90%) across it's operating range, some capability for surge supply (up to 5000 watts for 5 minutes), and is a modified sine wave (not a pure sine) output. But I've never used it with Optima or Lifeline type batteries, instead using it with Exide marine batteries, down to about 65% charge levels, which is exactly the situation your friend is complaining about. This inverter will go to low voltage warning at 10.5 volt input voltage, so if your input battery can keep above that until it is exhausted, the inverter wouldn't have a problem. But I doubt that even a Lifeline battery can do that.
posted by paulsc at 2:48 AM on August 12, 2007

By the way: the right way to check whether he's getting full use of his existing battery capacity is to insert an ammeter into the circuit, right between the positive terminal of the battery and the inverter; measure the current drawn; then time how long he can draw that current before the inverter's alarm goes off.

Multiplying the number of amps drawn by the number of hours of runtime will give him a figure in amp-hours that he can compare to the battery's nameplate rating.

To be kind to his batteries, he should be sizing them such that they can run his camp for about ten hours before going flat. If they discharge in less time than that, he won't be getting the full capacity they're capable of. Battery amp-hour ratings are quoted for the 10 hour discharge rate. If you discharge a 50Ah battery at 5 amps, you'll get your full ten hours; discharge it at 50 amps, and you might get half an hour if you're lucky.

Deep cycle batteries actually lose capacity in response to rapid discharge to a greater extent than car batteries do (car batteries are designed to provide brief bursts of hundreds of amps to starter motors).
posted by flabdablet at 2:55 AM on August 12, 2007

Another thing: I hope his lighting isn't incandescent. If he's not using fluorescents, he's doing it the expensive way.
posted by flabdablet at 3:05 AM on August 12, 2007

He responds:

Thanks, that's all useful information! So I guess getting the battery below "60%" may not be realistic after all.

Here are some more specifics on my current gear and how I'm using it. I'm willing to upgrade, but likely can't afford the top of the line options:

Battery: "EverStart deep cycle trolling, part: 27DC-C". Amp Hours: 115; Reserve Capacity: 160.

Inverter: Tripp-Lite 500 Watt "PowerVerter"

Lights: 10 sets of X-Mas lights, each 0.34 amps running at 120v.

I use this setup at camping festivals and am currently gearing up for this years Burning Man. I'd like to be able to keep the lights on at my camp over night without having to run the generator or having the inverter emit that awful high pitched whine. The lighting is just standard strings of x-mas lights, about 30' long each. I am interested in switching to LED's but haven't fully researched viable
options yet.

I have also tried running a stereo off the inverter and while the equipment would turn on and behave like it was working, it wouldn't play any sound. I had assumed it was because the power output from the battery just wasn't enough, but I guess that it may have been a matter of the sine wave. At any rate, while being able to run the
stereo off the battery/inverter would be a nice bonus, it's a lower priority.

I have a battery charger, the "Schumacher Speed Charge", which can be switched between car and deep cycle battery mode and which supports charging at 2, 12, and 25 amps. It also reports the batteries current amount of charge, which is how I got the "60%" figure.

During the day, as needed, I charge the battery using my generator. The generator has a built in inverter that outputs 120 volt AC (incidently, the stereo runs just fine off it) and also has a 12v DC outlet and came with a set of cables that connect directly between the battery and the 12v outlet. So, I use the 12v outlet on the generator
to charge the battery rather than the "Speed Charge", because I figure it's probably more efficient than stepping up to 120v to run the charger and then back down to 12v to feed the battery.

Typically I'll switch the camp from generator to battery power and then I'll go wander and check out other camps for a few hours. By the time I get back the inverter is generally squealing. I need to do some tests to determine exactly how long the battery actually does last before that point, but my general feeling is "not long enough".

I imagine adding a second battery to the mix would basically double my run time. I don't have any experience with running car batteries in parallel, but am open to the idea.
posted by croutonsupafreak at 12:57 PM on August 12, 2007

Ten sets of 120V xmas lights at 0.34 amps each is 10 * 0.34A * 120V = 408W (watts). Divide that by an assumed inverter efficiency of 90% and we get 450W. Supplying 450W at 12V requires a battery discharge current of 450W / 12V = 38A (amps). Your battery capacity is 115 Ah (amp-hours), and 115 Ah / 38A = 3 hours. This is significantly less than ten hours, which means you won't actually get the rated 115Ah capacity, which means you should actually expect a bit less than three hours. So, it sounds like your setup is functioning exactly as designed, and you don't need a better inverter.

What you need is more efficiency, and using LED lighting strings running directly off the battery will definitely give you that. LEDs are about ten times as efficient as incandescent lamps; so even allowing for a bit of waste in the LED current limiting circuitry, you should easily be able to achieve 20+ hour runtimes of equivalently bright lights off your present battery.

Low voltage DC is much easier to design fancy control electronics for than 120V, too, so if you go for LEDS you could probably get some pretty spiffy moving light effects going without too much trouble.
posted by flabdablet at 4:19 PM on August 12, 2007

flabdablet nails it. And based on his calculations, a second battery is warranted, if you want 10 hour run time. You also need to examine what that charger is really doing, for deep cycle battery operation, on its various charge settings. I strongly doubt that a 25 amp charge rate for a deep cycle battery is a good idea. Even a 12 amp charge rate on a deep cycle battery isn't a great idea. But what really eats you is "So, I use the 12v outlet on the generator to charge the battery rather than the "Speed Charge", because I figure it's probably more efficient than stepping up to 120v to run the charger and then back down to 12v to feed the battery." It may be true that a direct DC charge system is "more efficient," but it is highly unlikely that a random 12 volt DC outlet is the optimal way of charging your deep cycle battery. For best charging of deep cycle batteries, you need a charger capable of understanding their charge profile, and changing the charge rate and voltage appropriately, as they come up to full charge. The final charging voltage on many deep cycle chargers is well above 12 volts, usually from 14.4 to 15 volts DC. Before I used the generator's 12 volt outlet directly to the deep cycle battery, I'd use the Speed Charge off the generator, if I was looking for maximum battery life.

And before your brother heads out to Burning Man, he should build and verify his rig, including a thorough check of his deep cycle battery. Sounds like that battery may have been through some tough times, and might need to be replaced.

For your camp lighting, ditch the inverter entirely, and use direct 12 volt DC LED light strings. The strings on this link can be wired "end to end" as most houselight strings can be. However, because of the low voltage, you may get more reliable operation if you don't do this, and Underwriters Labs only recommends series strings of a maximum of 3 strings, if you do choose to run "end to end." Each 12 volt DC string needs to come from a central distribution point close to the 12 volt battery. Think of a "star" pattern for your camp lighting, where the battery can be at the physical center of the star. If your battery can't be at the physical center of the star, any "distribution" wiring you do needs to be heavy gauge (8 or 10 gauge) copper wire, at a minimum, to minimize resistive losses. Low voltage = greater current for equivalent power = much greater wire guage than for 110 volt systems.

Your brother also needs to do some dark night comparison of LED light strings and 110 Volt incandenscent bulbs, to see if the light output from LED light strings is going to be sufficient to his purposes, as general camp lighting. LEDs are way more efficient than incandescent bulbs, but part of the secret in getting that efficiency is focusing of output light in certain directions, so that, as general illumination, they might not be what he hopes. Also, the 12 volt DC light strings I'm linking are 20 light links, whereas many 110 volt strings are 35 lights.

To run 5 12 volt LED light strings with minimal resistive losses and voltage drop, you'd need to make or buy a 5 outlet "octopus" with appropriate connection clamps for your battery, and 5 cigarette style outlets. Or, you could go the professional route, and eliminate cigarette style connectors altogether, in favor of power distribution blocks. Use relatively heavy (10 or 12 gauge) copper wire for your battery connection, and you've essentially got "lossless," silent lighting operation, down to whatever your battery will suffer (no low voltage cutoff).
posted by paulsc at 6:28 PM on August 12, 2007

As usual, paulsc nails it too.

Heavier gauge wire is indeed better, but you don't need to go completely over the top with that just for LED strings.

Power losses in a 12V LED-based setup due to slightly undersized distribution wiring are actually going to be comparable to the losses for a 120V incandescent-based setup, simply because the 10x efficiency gain from going to LEDs will compensate for the 10x voltage reduction from 120V to 12V. You will end up with about the same current flowing in your distribution wiring, and therefore about the same power loss.

Yes, that same absolute amount of power loss will now be a higher percentage of your total load; but if you use ultra-heavy-gauge cable to try to reduce that, all that will happen is that your LED strings will light up a little brighter and draw a little more current, and your running time will actually go down.

Distribution wiring for a camp site also needs to be convenient to handle and tough enough to avoid damage when people kick it around; you don't want to be shorting out a lead-acid battery through a heavy-gauge 12V rated cable by carelessly crushing it with a camp chair, unless you aim to be the Burning Man :-)

If you can't manage the optimal star topology that paulsc suggests for your entire site, I think your next best bet would be to do any extra distribution you need using lengths of heavy-duty 3-core outdoor extension cord (look for a 20-amp or better rating) and just live with a small amount of power loss.

Cut the 120V plugs and sockets off, wire the earth and neutral wires in parallel to cut the losses a little more, use decent automotive connectors (I'd favour 5/16" spade lugs), keep all the wiring runs as short as you can manage, and it should all work pretty well.
posted by flabdablet at 8:41 PM on August 12, 2007