El cheapo inverter

[Matty F]

I've Googled for this but it's hard to know what websites can be trusted. In here, if someone makes wrong statements there's an army waiting to pounce!

I'd like an inverter to run off my car battery. I can't seem to find its ampere-hour rating but it's 350 mm long and is the biggest battery in any shop around here. There's also a V8 engine to charge it if need be. I want to run a bunch of 230 volt tools and appliances off the inverter - one at a time.

An angle grinder (what else?!) rated at 500 watts. Maybe a circular saw at 700 watts. Maybe a water blaster at 700 watts. When one of those is not being used I'd want to run a fridge/freezer (probably under 100 watts), maybe together with a desktop computer with CRT monitor and a Philips LCD TV.

There's a cheap inverter that says it's rated at 1500 watts continuous (3000watt max). It will be something like a square wave output. So really, will the above appliances run on that without damage?

[Tabby]

These 3 will be motors with near zero load at 0rpm, so all they need to run is their VA rating, which will be about 1.2x the wattage figure.

These need a current peak at switch on, but 1.2x700w should give enough VA for that. I gather they tend to run a bit hot on square waves, so compressor life may be reduced some.

These don't care about startup current or waveform shape, and power demand is small compared to the tools.

Yup. Drawing 70A off the car battery it wont last long though, in either sense. A 35Ah battery should manage nearly half an hour, or a big 70Ah battery near an hour. Don't run the battery down too much though, or its life expectancy falls badly.

NT

[Matty F]

The batteries are marked 100AH 760A. I have two of them. I'll run the car often when using the tools. Othewise if I have a power cut at home I'd like to run the fridge and my connection with the outside world. I already have the house wired up with 12V lighting.

[Thomas Prufer]

The 70 Ah rating is at I10, i.e. the life when drawing 7 amps, and it'll last a

*lot* less drawing 70 amps...

If you are going to use the tools for a bit here and there, ok, though they may have trouble starting and may perform poorly compared to mains. Try it and see, and if the tools trip an overcurrent protection, run them of a long extension cord. Or wire a lightbulb in series with the tool, and bridge that with a switch once the tool is running.

TV and PC should be fine, though I wouldn't run the car with them attached to the inverter, in case of overvoltage, nasty spikes etc etc.

Still, seems to me you have to requirements, which are (at least partially) mutually exclusive: a small, continuous load like TV, PC, fridge, that require a quiet power source. And then soemthing like an angle grinder, where it doesn't matter if the power source is noisy, the angle ginder will be noisier:-) Maybe a smaller inverter, and a cheapo genset?

Thomas Prufer

[Terry Casey]

In message on Fri, 25 Feb 2011

11:18:12 + 0100 Thomas Prufer wrote:

It's important to use the correct type of battery. Car batteries are designed for just that purpose: supplying very high current for very short periods with very little demand otherwise. They are not designed for deep discharge and can be damaged by it.

For continuous demand, you should use a deep-cycle battery, often refrred to as a 'leisure' battery. It is designed to supply a continuous current demand for long periods and other applications which require frequent disharge cycles but is not capable of the high instantaneous current demanded of a car battery.

[Matty F]

I've bought the 1500 watt inverter now. It may run my bigger tools out in the wilderness. I need to grind some welding off railway lines and waterblast the points. If any one puts a wheel clamp on somebody's car the angle grinder might be useful :) And I'm sick of having nothing to do when there's a power cut.

[Andrew Gabriel]

A generator would be better for all the appliances you list. Battery load is going to be very large, higher than a car alternator can normally supply, so even running the car continuously won't keep up with the battery drain you are talking about.

[Matty F]

OK, maybe not the waterblaster then. The grinder and saw would be used for very short periods.

[Dave Liquorice]

700W @ 12v requires 58A from the battery. Hefty bits of cable from battery to inverter and probably not much more than 30 mins of life. And such a heavy and rapid discharge of a car battery is likely to kill it. Bear in mind if this battery is also required to start the engine don't be surprised if it can't manage that after only a few minutes...

The best way is to keep the vehicle battery for the vehicle and have a separate deep discharge (leisure) battery on a split charge system for the inverter. Just like caravans have.

If you can start it...

A generator has been mentioned but dismissed. A small light 850W two stroke would probably do what you want, though a 4 stroke 1.5 - 2Kw would be a better bet but they are bigger and heavier. Or a 1 to 2kW inverter generator these are quieter and lighter than a normal set of similar rating but have a rather larger price tag.

Most small gensets have a 12v "battery charging" outlet of somewhere between 5 and 10A. Hang a lead acid battery across that to stabilise it a little (I suspect it might vary wildly as the main load on the genset changes) and you have your nominal 12v for lighting.

[newshound]

I was going to say that too. I got mine from an eBay dealer (can't recall who), to run lights at a stables with no mains power, and it is excellent.

[Tabby]

train robbery? :)

NT

[Jules Richardson]

Wasn't it someone on this ng a few years ago who was planning on driving a large generator via a pair of rollers? When they needed power, the plan was to park the vehicle on the rollers, chock the non-driven wheels, and throw it into gear...

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Tabby saying something like:

Hah. Tell that to my old TV which turned its PSU toes up - twice, after running from a mod square inverter. Otoh, I was happily running my old PC and CRT from the same source, but via a UPS, which was running with a slight buzz.

[Matty F]

Actually track maintenance! There's some spot welding that needs to be ground off. So, not more than half a minute with an angle grinder, then drive the car to the next point, leaving the engine running. There is a road next to the track but no power supply.

I've now tested the inverter. The 700 watt circular saw cuts 2" timber with no problems. My 1380 watt Skilsaw runs OK but the battery leads warm up, as I would expect with 115 amps or more. One day I'll get a generator as well but this inverter will do in the meantime. Some of you may have heard that the power has been offf for the last week in parts of Christchurch NZ after the earthquake. Up until now I've had emergency lighting and gas cooking but nothing else..