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trouble with inverter, car battery and car emergency set up

I had a recent power outage that lasted for 3 days. Not having a generator on hand, I decided to try using what I did have for short term power purposes. The items consisted of the car, 500 watt inverter, and I had an extra car battery available.

I hooked up the inverter directly to the car battery in the car. With the car running, I was able to run the 46" LCD TV, and a couple of CFL lights which was all I really needed to do. However, I ran into some problems. If the car wasn't running the entire time, the items connected to the inverter would start flickering on and off after a period of time. If off of the car battery inside the car, this might be within an hour, but with the separate battery, maybe 10 minutes unless it was coupled to the car battery in the car with the car running.

All in all, I was disappointed. I'm wondering why the items wouldn't operate correctly unless the car engine was running. There certainly wasn't a lot of wattage being drawn.... the TV uses like 100 watts with the lights maybe 10 watts each, and there were 2. I had hoped to just use the battery in the car to occasionally "jump" and charge the separate battery when it got too low, but never got to that point as the attached items would start flicking on and off.

I see portable power packs with internal battery and inverter at places like Walmart. The inverter is usually 400 watts or so, so I know this method should have worked. Any ideas as to why my set up didn't work well would be welcomed. If it had worked this time, I had planned on maybe a 1000 watt inverter at some point which I had hoped maybe if this happened in the future to be able to run the house refrigerator from the inverter/ car battery set up, but not if I have to have the car running the entire time.

Thanks for your help.

Pete

Reply to
pete
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So what was the DC over the battery in the 4 situations, battery idle, battery being charged, battery powering inverter only, battery and charger powering inverter ?

Reply to
N_Cook

Maybe you overdrew the battery?

A normal car battery has a power of maybe 5kW. It's normally run for a few seconds which means it normally supplies, let's say, 10kJ of energy. It's designed to release a few percents of the total energy stored within itself. Lets say 2%. So a fully charged battery contains about 500kJ of energy, however only a small percentage of that is repeatedly recoverable. If we say a car battery can be discharged to 10% repeatedly, that gives us 50kJ to work with. If we draw that energy at a rate of

200W, it will expire in 500 seconds, which is 8.33 minutes and close to your measurement of 10 minutes.

The critical question is at what internal charge level does the battery start dying? If the voltage at the terminals starts dying when the charge drops to 90%, that explains the performance of the extra battery. If the charge has to drop to 30% before the battery starts to fail, that explains the behavior of the in-the-car battery.

Obviously, my calculations assume undamaged batteries. If the batteries were heavily used and were overdiven previously, that would lower the total amount of energy present in them and ipso facto lower the amount of recoverable energy.

100% certain that the secret is in the battery.

For this scheme to work assuming lead-acid batteries, you want a battery that can be severely discharged, a so-called "deep cycle battery". These come in many varieties so shop around if you're interested. You can also make it work with NiCd batteries. Some have also suggested NiFe batteries that you can supposedly build yourself.

Reply to
Aleksandar Kuktin

A good inverter will shut off to save your battery so you can start the car later to recharge it. Thank the those that were thinking in the design.

Jamie

Reply to
M Philbrook

What did the wiring from the inverter to the car battery look like? If it involved either a cigarette lighter plug or alligator clips, improving that connection will help a little. Ideally you would have fairly thick cables (8 AWG or thicker), as short as possible, running from the inverter to the battery terminals. Remember, if the inverter is putting out the full 500 W, it will be drawing around 42 amps from the battery.

This was probably the low-voltage shutdown in the inverter trying to operate. Most of them will shut off the AC output when the DC input voltage gets too low, so as to avoid running the battery down too far. Usually this isn't adjustable, but on some inverters it might be... check the manual if you have one.

Have you measured it or looked at the nameplate? This is from 2009

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but 46" TVs drew from 107-310 W.

A 60 W equivalent CFL is usually 13 or 14 W. If you're playing games like this, a Kill-a-Watt or similar portable watt-hour meter is a good tool.

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Standard disclaimers apply: I don't get money or other consideration from any companies mentioned.

Matt Roberds

Reply to
mroberds

the OEM alternator, cost effective, works only on the highway where rpm pro duces power. Generalizing, a Ford 120amp alternator supplies lights and air at 50-60mph. Add a stereo amp, way over limits. The negative threshold com es slowly with yellow lights after 2-3 hours of asking a very good battery for more discharge than charge.

A 200amp alternator (Summit Racing) claims production of 120amps at idle bu t give it 15-20 mph. The OEM needs 65mph to give 120amps.

The battery your using may not be quality or fresh. Charging takes power an d voltage the system may not support but that the OEM alternator at 65mph d oes. The 200amp unit charges at 45 mph in abt 20 miles.

drivers 'charging' dead batteries at idle after jump starting are not getti ng their $$$ worth.

Reply to
avagadro7

roduces power. Generalizing, a Ford 120amp alternator supplies lights and a ir at 50-60mph. Add a stereo amp, way over limits. The negative threshold c omes slowly with yellow lights after 2-3 hours of asking a very good batter y for more discharge than charge.

but give it 15-20 mph. The OEM needs 65mph to give 120amps.

and voltage the system may not support but that the OEM alternator at 65mph does. The 200amp unit charges at 45 mph in abt 20 miles.

ting their $$$ worth.

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Google Images offers a menu of chart information or search your alternator

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Powerstream under 'technical resource' offers a wire size calculator

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Reply to
avagadro7

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