electrical help please

I would just but a 12 volt fan from super cheap around 20 bucks.

Cheers.

Current is an instantaneous value so it will draw the same at all times. If you're trying to work out how long a battery will last you can divide the amp/hour rating of the battery by the current of the fan. i.e a 7 amp/hour battery can supply 7 amps for 1 hour or 1 amp for 7 hours.

Inverters are a really inefficient way to supply power and you would almost certainly be losing as heat, as much as you got to actually use. A dedicated 12v appliance is always a better option if possible.

Lyndon,

If you run the fan for 1 hour you will use 1.16 Ampere Hours.

Regards,
Bob

If you use it on 240V it will draw 1.16Amps.

When connected to a 12V battery it will use about 300Watts or ..........wait for it 25 Amps.

In other words a large N70 cranking battery will last about 2 - 3 hours before it is fully discharged.

Hi Lyndon,

280 W /240 = 1.167 Amp (at 240 volts)

If the inverter is 90% efficient, 280w/.9 = 311 Watts

311 W / 12v = 26 Amps, so the inverter will be drawing 26 Amps from your 12v battery.

So if you started with a fully charged 80 AH battery you would only get about 2 hours before the battery voltage was below 12v

The Ampere-Hour rating of a battery is normally done at 20 hour rate but if you discharge a battery down to the voltage they use you will drastically reduce the life of the battery.
Cheers,
Pete.

Lyndon,

here are some figures for you:

fan: presuming the fan blades are driven by an induction motor (brushless, the ones with a cylindrically shaped capacitor), the active power of 280W has to be divided by the power factor of around 0.75 to get to the apparent power of 373VA.

Divide this by 12V and you get 31A.

Divide this again by the efficiency of the (has to be pure sine for the induction motor to work properly) inverter of 0.9 and the battery current is almost 35A.

Divide this by 3 and the current per battery is 11.5A.

Looking up the discharge tables for 50Ah spiral wound AGMs, this will result in 4 hours of run time which coincides nicely with your expectations.

Two important things: have a low voltage cutout device, set to around 10.8~11.0V, to prevent the batteries from getting over-discharged (this might be an in-built feature of the sine wave inverter).

Charging: using your generator, the batteries will be about 90% charged after 2.5 hours or so.
If you expect the batteries to give you the specced cycle life (around 300 at 100% DOD), you have to push at least 100% of charge back into them before discharging again.
This may take another 10 hours of slow absorption/float charging.
Solar might be an option for this.
If that's not on the table, the batteries will only be good for a sustained 3.5 hours run time (at some loss of cycle life).
Occasional topups and equalisation charging is definitely recommended for optimum battery health in your application.

cheers, Peter

Just trying to think outside the box here. If you have a generator or some other 240v power to charge the batteries ("even while the fan is running" ) why not just run the fan from that supply???