DC amps go up AC volts go down....

sheriffav8r · October 2009

I've got my truck battery/inverter system set up and it seems to run fine until I try and run my portable AC off the inverter. My setup is 4-255AH 6V GC batteries wired in series/parallel, 2 gauge wire and clamp on lugs, and a 2500W inverter. It's an AC Delco inverter I bought from BJ's but seems to work fine. I'm guessing it's a MSW and not a pure sine wave (duh, it's cheap). The inverter never hiccups or shuts down from the Air Conditioning power draw. There's an electronic DC battery isolator that separates the vehicle batteries from the GCs. Here's my setup, the bottom box is the inverter and the top one is the isolator:

Attachment not found.

Battery banks sit on opposite sides of the truck bed and the connecting cables are equal length at 6'. Wire run to the inverter is about 6', too. The cables never get warm and current draw is equal across all batteries and banks.

I power up the inverter and the air conditioner. The inverter pulls about 80 Amps DC and the AC pulls about 8-9 Amps AC while putting out 114VAC. The batteries start at about 12.5VDC for each pair and the pairs are equal. After a few minutes the DC amperage starts climbing and the AC voltage starts dropping. The DC amps peak at 150ish and the voltage drops to 105VAC and the AC compressor kicks off the line. While my DC battery voltage is steadily declining, it never falls below 11.6-11.8VDC and jumps back up to 12.4 when the AC kicks off. The AC unit will pull 8-9 amps AC all day long on shore power and the voltage is steady at 113-114VAC. Is this just a poor inverter? What's happening?

icarus · October 2009

Re: DC amps go up AC volts go down....

A couple of things,

First, the A/C compressor may not like the MSW inverter and it may be rebelling. Second, a ~150 amp draw on your battery bank (~510 ah capacity) is a HUGE draw to sustain very long. That's what? about 30% of capacity. One hour with that load would draw the battery down to ~70% soc, 2 hours to 40%. Even 80 running amps is a ~ 15%. Remember, starting amps for many motors are easily double, and often times even more than running amps. Add in the efficiency of the MSW inverter, and the trouble the motor might not like and I think you have a recipe for trouble.

8-9 amps @120vac shore power, might be considerably more through a MSW inverter.

Tony

Kamala · October 2009

Re: DC amps go up AC volts go down....

The MSW is the likely suspect. IMHO.

Tony's point about the draw is precisely why I ordered my camper without the A/C unit.

K

sheriffav8r · October 2009

Re: DC amps go up AC volts go down....

The 150 Amps is only what the load gets up to before the compressor kicks off. It starts out at 80 and systematically rises to 150 then unloads. As the amperage is increasing the AC voltage is decreasing. I'm happy with an 80 amp load for 2-3 hours to bring my 450AH bank down to 50%, I just can't figure out why the amps climb over about 5-6 minutes or so while the inverter can't keep 115VAC. The inverter keeps chugging like there's no problem. I just want to be sure it's the inverter and not some wiring snafu or other problem before I go spend more money on this stupid thing....

dwh · October 2009

Re: DC amps go up AC volts go down....

Hrmm. I'm not sure why, but for some reason that diagram makes me think that maybe the inverter is pulling more load from one side or the other - i.e., not equally.

As to why the DC amps draw goes up...seems to me that it might be because the DC voltage is going down.

niel · October 2009

Re: DC amps go up AC volts go down....

in any case you are drawing a lot of power for those batteries to handle. an msw inverter will draw around 28% more power and this is being dissipated in the form of heat in the a/c unit and is shortening the lifespan of the a/c unit. the initial power is about 1000w before its upward slide and add to that the 28% extra power due to msw and you're at 1280w minimally. there is also a huge surge upon turn on.
my recommendation is ditch the msw inverter and get a sine wave inverter and/or try to use another a/c unit that is more efficient that will draw less to help with the battery bank shortcomings. 8-9a is big and even many a/c units in the 7500btu range will draw around 6a, so downsizing the btus can be done too. if you can't or won't reduce the ac power draw then more batteries may be needed. see if the a/c unit does that on a generator of sufficient power or on utility power to rule out a problem with the a/c unit.

Brock · October 2009

Re: DC amps go up AC volts go down....

When you measured the battery voltage under load was it at the batteries themselves or at the inverter terminals? I would measure it at both places and I am guessing your loosing .2 to .4 or more volts in the wire, which may not seem like much but the inverter likely kicks out at 11.0v to protect the batteries.

I think you have a couple things going on here at once.

The AC motor doesn't like MSW as much as sine wave and it is pulling more power to compensate.

That loads the inverter more and the higher load on the batteries causes the voltage to start to sag which causes the inverter to pull even more amperage to compensate for the reduced voltage. At some point the inverter can't keep AC side the voltage up and the motor start to draw more amperage as well.

And I am guessing at those amperages the line loss you do have starts playing in to the cascade of more power demand. Even with larger wires I am guessing this would still happen because the batteries would still start to sag under load and start the runaway situation. I am guessing thicker wire would just lengthen the time before that happens.

For kicks how do you charge these batteries, from the engine alternator? Can you try charging them at the same time you running the AC to see if that helps keep the voltage up on the battery bank? It isn't a long term fix, but it would let you know if it is just the motor on the AC that doesn't like the MSW or if it is from the voltage sag on the batteries.

sheriffav8r · October 2009

Re: DC amps go up AC volts go down....

After further review, here are some findings and more questions:

I ran the A/C off house power: 120VAC, 7.6-7.8Amps with no wavering whatsoever.

I ran the A/C off my Yamaha YG2600 generator: 124VAC, 7.7Amps same as above.

I ran a 1500W ceramic heater/fan directly off the inverter: 124VAC, set at 9.5A AC, saw about 95A DC. Ran for about 15-20 minutes with no wavering, steady battery draw. Started battery bank at 12.5 and went to 12.2VDC during the test and pretty much stayed there.

I ran the A/C directly off the inverter: 124VAC, 7Amps AC, 75Amps DC, same battery draw as above. I ran it for about 10 minutes and the voltage started to go down, but no lower than 120VAC. DC amp draw very slowly climbed to 95A and the AC draw climbed to about 10.5 amps. I started the truck and the voltage held steady at 12.4VDC with an amp draw of 95 amps. The amps continued to climb and the compressor kicked off. While I didn't see 150ADC this time the amps unmistakeably kept climbing. Again, the inverter never even hiccuped and showed green lights the whole time.

Conclusions (for me at least):

1. My A/C motor does not like MSW inverters. Question is, what does my Yamaha generator have and why does it work fine with the A/C?

2. My battery bank should be able to hold an 80A draw for a couple of hours. While some may disagree, my heater experiment showed me that the batteries held up just fine. I'll do a longer test in the near future to see how long they last to 50% at 80A draw.

3. I need to find a pure sine wave inverter. Anyone have a Sunforce 2500W? They have them at Sams, Costco, NorthernTool, Camping World....

icarus · October 2009

Re: DC amps go up AC volts go down....

Your Yamaha genny produces pure sine wave power at 60hz. Clearly your conclusion about the msw is correct. If the A/C is drawing say 1000 watts, if it starts out at 120 vac it would draw ~8.3 amps. Now if you drop the voltage to say 105 vac you would now draw ~9.5 amps.

Take the same example, the same 1000 watts at 12.5 vdc would be ~80 amps. Drop the voltage to 11.5 would raise the amperage to ~87 amps. Now if the MSW were eating up an additional ~25-30% because inefficiencies the situation only gets worse.

I humbly suggest that you read some of the following: http://www.batteryfaq.org/ http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

While your battery bank "can" handle the load that you are subjecting it to, I think you have to consider if it is a good thing to do. A common misconception about "deep cycle" batteries is that they are boxes in which you can pour out electricity, and then pour it back in. While this is true in some sense, the reality is that repeated deep discharges (below 50% soc) lead to much shorter battery life. Some folks (including me) suggest that batteries never routinely get discharged below ~80% soc (20% dod). I am not a battery expert by any stretch, but it is my understanding that "deep cycle" batteries are designed to deliver (comparatively) low current draws over longer period of times, as opposed to the large current draws over very short times that starting batteries are designed to provide. It seems intuitively to me that you are having too big a short term current drain on your batteries. Of course if you are prepared to replace your batteries very much earlier than you might expect to.

I have been able to run T105 golf cart batteries on a small off grid application for 12 years by being quite conservative with the loading and the charging.

Good luck,

Tony

sheriffav8r · October 2009

Re: DC amps go up AC volts go down....

I wish my Yamaha was pure sine wave, but that is the new YG2800i. Mine is just a standard, run of the mill YG2600 industrial unit that is over 11years old. FWIW, it starts on the first pull, has never had it's oil, spark plug, or air cleaner replaced and runs like a champ.... This thing will run for about 18 hours on 5 gallons of gas and is priceless for reliability.... I wish there was a way to separate these generator/inverter circuits so you could run the inverter off of batteries and then start the genny to charge up the bank every so often....

PhilS · October 2009

Re: DC amps go up AC volts go down....

My guess is that the MSW is causing the A/C motor to heat up, and the more it heats up the more it draws. Then the more it draws, the more it heats up. "Death spiral" kind of thing IMO.

Phil

icarus · October 2009

Re: DC amps go up AC volts go down....

Your Yamaha IS most likely a pure sine wave. It is just not an inverter generator. An inverter generator alters it's power electronically to provide pure voltage/cycling (60 hz) regardless of engine speed.

A conventional generator maintains it proper 60 hz phase (what is the proper term to describe this?) by maintaining proper engine and therefor generator speed. It is no coincidence that nearly all conventional generators run at speeds that are multiples of 60. Most small generators run at 3600 RPM (60X60=3600) Larger ones will run at 1800 and some diesels as slow as 900 RPM.

Conversely, in a conventional generator, voltage is controlled by the exciter voltage, and in most cases this is not adjustable. If you adjust your generator so that the idle Hz is ~62 (with a kill-a-watt or scope) the voltage will probably be ~130 vac. Load that genny to 80% and the phase should be ~60 hz and the voltage ~120 vac. The mistake that folks make is slowing the genny down when the voltage seems too high. The basic spec for a 120vac system is +- 10%. So anywhere from ~108-132 vac is considered "normal".

For decades, my lister diesel ran perfectly, but would not run a motor load. The idle voltage was 220vac. I kept upping the speed, with little effect on the voltage. Once I got a kill-a-watt I realized that it was running too fast, and the phasing was too high, (65hz) I realized that this old fashioned genset had an adjustable resister controlling the excitation voltage. Simply moving the pick up point of the resister raised the idle voltage to ~242 vac. Fire of the bench grinder and the voltage would drop ~5 vts, and quickly recover, where before it would drop 30 volts and the grinder would never get up to speed and the genny voltage would never recover.

Tony

niel · October 2009

Re: DC amps go up AC volts go down....

generators are considered sine wave, but when we talk of a sine wave inverter or a modsine inverter it is the % off of true sine wave that determines its classification. our utilities are made to keep within 5% total harmonic distortion (thd) and as such our sine wave inverters must be 5% or less as well. the worst is about 28.3% and you could have a good modsine inverter at 6 or 7%, but few attain that and might just as well go the little extra to attain sine wave statis and extra $ it would bring.

sheriffav8r · October 2009

Re: DC amps go up AC volts go down....

Yep, got confused with the pure sine wave lingo on non-moving electronic inverters... Anywho, going to look into a day cab air conditioner when this one dies. I guess they're new to the scene for semis that can't idle forever like they used to....

icarus · October 2009

Re: DC amps go up AC volts go down....

Semi-tractor Aux. power units (APUs) are a great advance. They allow the cab/sleeper to stay warm or cool without needing to run the tractors engine, reducing fuel consumption dramatically as well as emissions. It might be an interesting topic to throw out there for conversion ideas for off grid, or other applications.

Tony

sheriffav8r · October 2009

Re: DC amps go up AC volts go down....

Looks like Dometic is coming up with some nice 7K and 10K BTU units, but can't find any actually for sale. I'd like to just get the compressor/blower unit, but looks like they have the inverter (MSW by the way) and everything else in the package. Are there any Danfoss compressor A/C units out there? They might be the way to go for power consumption....

dwh · October 2009

Re: DC amps go up AC volts go down....

I've looked all over for a/c for my camper. I even checked into the Danhard stuff (I dunno anything about Danfoss):

http://www.danhard.com/

The a/c unit that I'm almost certainly going to put in my camper is this one:

http://www.frigidaire.com/products/air-conditioners-dehumidifiers/window-air-conditioners/FAA055P7A

I emailed Frigidaire and one of their techs sent me the "No BS" specs on the unit:

> Low Voltage Start-Up: Systems are designed to start at 10% below their
name
> > plate rated voltage under normal operating conditions.
> > This is the voltage requirements: Read the manual for generator to
> > understand about start up voltages and requirements.
> > Electrical Information
> > Voltage 115
> > Amps - Cooling 4.5
> > Watts - Cooling 475
> > Starting Amps 11.5
> > Starting Watts 1,322.50 Note: Generator must be capable of
> > handling this amount of wattage for start up plus whatever else is on the
> > generator.
> > Fuse/Breaker (Amps) 15
> > Power Cord Amps Min 13

>> However to answer your question the fan comes on first and about 3 min
later the compressor comes on to keep the starting wattage down.

sheriffav8r · November 2009

Re: DC amps go up AC volts go down....

The Frigidaire unit looks nice and compact. Should work well for a truck cab. Danfoss is a type of compressor that uses less amps to run than a regular AC compressor. They use them in refrigerators that run only on 12VDC. I think my next step is just to get a pure sine wave inverter. What's a thousand bucks among friends?....

DC amps go up AC volts go down....

Comments

Categories