Power supply to garden workshop

[janliber]

I am developing a prototype garden workshop design that has an off-grid power supply. The current setup is 300Wp PV, MPPT charge controller with 20A rated discharge current, 266Ah (C100) deep cycle battery and a 3000W (6000W peak) modified sine wave DC 12V to AC 220V inverter. This setup happily provides power for all my tools including a 2.4kW chainsaw. However, it fails 2 crucial tests - the boil-a-kettle test and the mow-the-lawn test. The kettle is 3kW and when that is switched on the controller starts screaming and cuts the power off. The mower is a 1.8kW Bosch mower and that only works when the sun is shining brightly, and only if I keep it running. When I switch it off to empty the grass box, it tries to restart (it revs slowly but can't get up to full speed) and I have to leave it for about 30 minutes before it will go again (if the sun is still shining brightly). In overcast weather, it won't go at all. So I think the reason it works when the sun is out is because the panels are supplying the battery at 12V and constant voltage and thus keeping the the whole system voltage fairly steady at 12V. I don't think that the charge controller is switching the circuit off, rather that the inverter is cutting out because of the low voltage.

How do I fix this? A second battery to provide 24V to an uprated inverter? An uprated inverter on its own? What else….?

 

[TedM]

Buy a low power kettle, you can get them 800-1000W for £25.

You cannot expect your battery to last long with those sort of loads.

 

[saggers]

Couldn't you get another battery and parrellel them up.

 

[janliber]

Buy a low power kettle, you can get them 800-1000W for £25.

You cannot expect your battery to last long with those sort of loads.

I take your point about the kettle, but this is a prototype system and the point about using the 3 KW kettle is to stress the prototype to find its failure points, and 3 kW is the rating of the inverter. What I can't understand is how the system easily copes with the 2.4 kW chainsaw but not the 1.8 kW mower. On your point about the battery, after about an hour's usage of the chainsaw on a cloudy day with the PV switched off, the battery level went down to about 97% of capacity as indicated by a metre measuring its voltage and confirmed by a hydrometer reading. The battery seems to be coping well with that load.

 

[janliber]

Couldn't you get another battery and parrellel them up.

Yes I was thinking that's probably the way to go. Is there an issue with the two batteries having different capacities?

 

[Sunshine777]

What I can't understand is how the system easily copes with the 2.4 kW chainsaw but not the 1.8 kW mower.

Is it because the saw doesn't draw the full 2.4 kW at start-up (the motor speeds up as the trigger is pressed) whereas the mower always wants to run at full speed?

 

[Darkwood]

The KW in a motor rating is not an electrical term' it the mechanical output or the turning force that can be delivered by the shaft so its wrong to look at it as an electrical evaluation.
A 2.4kw chainsaw then can deliver upto 2.4Kw of mechanical power but crucially when you set it off it will generally be off load until you approach the job in hand, an electric mower on the other hand is probably already sat in its work ready to cut so fires up on load... the inrush on the different motors may also play a part.

1hp which equals 746watts is the output mechanical force but after allowing for losses a ball park figure would be 1kw electrical energy per HP or 764w mechanical energy.

 

[HandySparks]

The KW in a motor rating is not an electrical term' it the mechanical output or the turning force that can be delivered by the shaft so its wrong to look at it as an electrical evaluation.
A 2.4kw chainsaw then can deliver upto 2.4Kw of mechanical power but crucially when you set it off it will generally be off load until you approach the job in hand, an electric mower on the other hand is probably already sat in its work ready to cut so fires up on load... the inrush on the different motors may also play a part.

1hp which equals 746watts is the output mechanical force but after allowing for losses a ball park figure would be 1kw electrical energy per HP or 764w mechanical energy.

The power shown on the rating plate of an appliance (rather than a bare motor) is usually the maximum electrical power input, unless otherwise stated.

 

[Darkwood]

The power shown on the rating plate of an appliance (rather than a bare motor) is usually the maximum electrical power input, unless otherwise stated.

Yes you have a good point ... been working with motors too long can't remember last time I looked at the rating plate of a domestic appliance. :wink5: but the rest of the post is relevent though.

 

[Richard Burns]

It sounds a bit like your battery is not fully connected to supply the loads, logically the fact that the sun is shining should be irrelevant to the load as this should be just topping up the battery and 300W compared to 3000W is minimal.
Is the battery connected directly to the load or is it connected via the charge controller?
With a 3000W kettle running the system will need to supply 250 or so amps at 12 V (ignoring any losses in the system) and the battery should be able to manage this for the duration of boiling a kettle (just). However the battery voltage will drop rapidly and this may be why the charge controller struggles as it is trying to load a draining battery and the voltage will drop so low that the inverter will cut out.
It is also possible that the mower has some electronics in it that does not like the modified sine wave from the inverter and so does not run well.
To only lose 3% of capacity after drawing nearly that batteries limit for an hour? the battery should have been almost totally fried.