Inverters

Cathnrodg · Newbie

Inverters

Are We Lost · Guru

Running the inverter isn't the problem. It's what you plug into it that matters. A single battery is not designed to run high amperage output. Running a high wattage device briefly should be OK, but not for long.

So for example a typical 1800 watt kettle or jug would draw 150 amps at 12 volts. It should be OK to boil that once, but a cuurent draw that large would very quickly bring down the battery voltage enough to shut down the inverter.

That kettle, if it runs for 1 minute would take about as much power from the battery as running a laptop for an hour (back of postage stamp calculation). And the higher the current, the greater the discharge rate. i.e. 150 amps for 1 minute would consume more than 15 amps for 10 minutes.

Also, a 3kw inverter would have more standby current usage than a smaller one, so it would be important to only switch it on when needed.

Whenarewethere · Guru

Many batteries in parallel, & big batteries if you are running loads a long time.

Look at the data sheet of your battery. Calculate how many amps your inverter needs when you run at maximum load. Add another 10% to the figures to cover losses.

Look at the maximum load of the battery can handle. You may need 2, 3 or 4 batteries in parallel.

At maximum load the battery has a far smaller capacity (see data sheet for my batteries, 4 in parallel). So if running large loads for a longer period you will need plenty of capacity.

To reduce stress on the batteries, for example use a 750 watt Birko kettle instead of a 2000 watt kettle. Ok it does take 3 times longer to boil but it is a lot better on your batteries.

If running air conditioning install a home 2.5kW split system, they are far more efficient than the inefficient units normally installed, & you can use them as a dehumidifier to reduce wattage even further.

Calculate wire resistance for the total length of + & - if wiring is not heavy enough an additional 10% of amps will be going up in heat!

Do you actually really need a 3.0kVA 2700watt inverter.

I have one this size in a UPS (uninterruptible power supply) to run a bank computers. Can on last 20 minutes at 2700 watts. You need to look at what you are actually going to run.

If wifey wants to use a hair dryer, Firstly break the contacts so it doesn't work on high, & then turn off other device while it is being used.

-- Edited by Whenarewethere on Sunday 29th of August 2021 02:58:13 PM

PeterD · Guru

Roger, there are a lot of members running 3 and 4 100 Ah batteries in their van to drive their large inverters. A common size of a battery bank is 100 Ah per 1,000 W of inverter output minimum. The large power items are only used for short periods as explained above.

The current drawn by a 1,800 W inverter given by Are We Lost is not achievable, it does not take account of the power losses within the inverter. To work out the current drawn by a load you divide the input power of the load by 10, not 12 as he did. The figure of 10 is a good rule of thumb as it allows for the inverter and cable losses. That means an inverter delivering 1,800 Watts of power requires at least 180 A of current to drive it. Some of the less efficient models could require more current

I have seen many salesmen at caravan and 4WD shows suck people into purchasing large inverters without telling them the complete story. A serious size inverter needs a serious size of battery power to drive them and that will then only be available for short periods, not all morning or all afternoon.

Dick0 · Guru

Hi Cathnrodg and welcome.

I assume you want to run 240V appliances for a short time from one battery, possibly an AGM?

Difficult to answer not knowing all your battery/charging circumstances.

So, I will outline my system and how I do not cook the battery.

I have 2x135AH LiFePO4 batteries solar charged from 4x200W fixed/1x200W portable panels through an 80A MPPT controller. The inverter is 2000W rated/4000W peak.

I use the inverter for breakfast usage of Coffee-pod, microwave and toaster appliances, run separately for about 4-5mins each average.

I may also use the same appliances and a sandwich press for lunch/dinner.

During daylight hours my solar charger supplements much of the inverter wattage usage. Outside daylight hours the batteries fully deliver the inverter wattage usage.

Still, the inverter usage is kept to a minimum but sufficient in the convenience of meals preparation. Of course, I could draw upon more power but do not have a need to at the present time.

With my inverter usage requirements, I don't see any possibility of cooking a battery.

I would not, however, run the aircon through the inverter. I could, but I wouldn't.

In addition to the above, I also run a 60L fridge freezer, Laptop, TV, Radio, Modem, Chargers and other various 12V devices of long term usage.

Hope this helps.

Peter_n_Margaret · Guru

Cathnrodg wrote:
Can anyone tell me how people manage to run a 3Kw inverter off a 12v battery without cooking the battery

Nothing you do will "cook" the battery.

If the load is too great for the battery to deliver, it simply won't deliver it and the voltage will drop to a point where the inverter will switch off.

3kW is around 250A at 12V. Your starter motor probably draws more for a short time.

Cheers,

Peter

Whenarewethere · Guru

238amps for my starter motor on 2.0L petrol engine for about 1 second.