An electrical question

[Morella]

I have a bit of a problem with what is otherwise a really good backup power system. I use a 2500w Cobra inverter attached to the spare batteries with four 4-gauge cables. There are two 70 amp-hour batteries in the belly, which, ideally, should give me 1680 watt-hours of energy. If I use DC power, I can have all I want, but as soon as the alternator is switched off, the battery voltage goes down to 11.9, then 11.8, and when it hits about 11.7, the inverter squeals "low voltage" and shuts down, even when the load is less than 100 watts (or is it kVA?). I don't fully understand the AC sine wave, much less the squared-off PDC wave that the inverter puts out, but anyway...

Does anyone know if an inverter's low voltage cutoff can be adjusted, or a way to raise the battery voltage? Lead-acid batteries will drop steadily in voltage as they are drained, which is fine for DC circuits, but if the inverter has to have more than 12v to function, then it would take three or four alternators to keep it going continuously. So why the fuck do they make a 2500w inverter? I'm only able to run my Christmas lights on the battery and inverter for an hour or two before it shuts down, which really sucks ass. It also doesn't make much sense, because I ran a window air conditioner on the batteries once. Maybe driving it around town isn't charging the batteries enough...

I know. How bout if I plug a battery charger into the inverter and use it to charge the batteries? Then it should go on producing power forever, like a perpetual motion machine. I think I am losing my mind, or it just isn't responding to massage therapy today.

Seriously...does anyone have a better idea for backup AC power, aside from throwing a generator in?

-denise

[Highbars]

Seriously...does anyone have a better idea for backup AC power, aside from throwing a generator in?

No.

[BIGEVIL]

The inverters I have used start to squeel at 10.5 volts.

Yours should NOT squeel at 11.7v. The only time I had a problem is when a large a/c draw happened suddenly (ie an electric motor under load or in my case the air conditioning compressor). The fix was 0 gauge cable and stiffening capacitors (16v dc) close to the inverter. I had a total of 6 farads.

[Morella]

Can you give me a diagram for hooking up the capacitors, and maybe part numbers?

When I was young, I remember someone saying that a 1 farad capacitor had never been built. Have we progressed beyond that?

This is really good information, and I would gladly spring for the caps, if it will solve the problem.

thanks

-denise

[Venom]

car stereo instillations regularly use multi farad stiffener caps the size of a tallboy beer can.
google stiffening cap or ebay search, there are many to choose from.
Stinger and Monster brands make good stuff

Expect to pay about $150 or so for a 1.5F Monster
stingers and metras are cheaper, I think they have a 5F for about $200

as for wiring, here are a couple of diagrams, just substitute your inverter for the amplifier.

http://www.caraudiobook.com/car_audio_power_capacitors/car_audio_power_capacitor_installation.htm

Also using deep cycle batteries would help immensely as they hold a steady voltage longer. They dont have the long gradual drop off that a normal starting battery has.
Their discharge curve is much steadier and they are designed to be charged and then run dead, unlike a starting battery.

[Morella]

This is very helpful, thanks. I do have deep cycle marine batteries.

Do the stiffening caps get hot? I'm wondering if I can install them under the mound, in the compartment that is cooled by gasper fans. I got a remote thermometer to measure the temperature down there.

-denise

[BIGEVIL]

No they don't get hot.

But as you remember, they are for short discharge bursts. So for sudden a/c loads on your inverter they would be perfect. For long consistant draining situations they won't do much help.

I still don't think your inverter should be unhappy at 11.7v though. Most shut off at 10.5v Maybe the inverter is getting too hot? Maybe a weak cell in one of the batteries?

[Morella]

I'll have to put the battery charger on, make sure that they're fully charged, and then see what the voltage actually is when it cuts off. Appliances like heaters and air conditioners pull a big surge when they start up, so the cap is probably a good idea.

-denise