Is this a PRACTICAL use for an Inverter?

[BernardinTring]

My caravan is stored on a site which needs the grass cutting round the van edges at regular intervals.

I have a Strimmer for home use which plugs into the mains for the edges etc.

I also have a 'spare' battery which I use in the winter period in order to keep the Phanton alarm active whilst a re-charge takes place.

In order to cut the grass should I be looking to buy a petrol strimmer or would it be a more practical and cheaper solution to invest in an Inverter to plug in the domestic strimmer unit for this grass cutting task.

Any ideas how long in terms of hours minutes the charge in the battery would last for this task. Its a 105 amp Numax.

I have absolutely no idea or experience of this wonderful word Inverter as you can tell from the question I'm asking.

Many thanksto all in advance of some advice which I know will be informative and to the point.

Bernard

 

[WatsonJohnG]

I would think that it would be expensive to buy a big enough inverter for a strimmer power requirement

I have a B & D cordless battery strimmer and it will do a lot of strimming of edges on one charge

 

[Colin_374358138]

Hi Bernard,

All the inverter is doing is converting 12v DC battery to a 230v nominal AC sine wave.

(well actually its either sine, modified sine or square depending on the inverter - sine being most expensice and best)

It of course converts inefficently - 75% is the best you are realistically going to get over the whole range of conversion. Beware some may spec more but its at a specific set of input and output conditions. Not across the whole range. Designing and efficient power supply is one thing. Making it efficient over whole range is another.

Anyway lets say you have 1 1000w electric motor in your strimmer. It consumes 230v elec at about 4 ish amps when running. DOn't forget a big startup current in a motor and don't forget losses due to friction, heat and most importantly load on the motor when its being used. Prolly more like 6A running ann more on startup.

So it follows that your little ole inverter must supply this 1000w. It does this by sucking it out of your Numax. But your poor old Numax can only deliver it at the rate of 12v. Since volts * amps = watts this means that the numax must supply around 100A.

At 100A the 105Ah Numax will be totally empty in 1 hour in theory.

But don't forget MOST caravan battery chargers are the worst charger the caravan manufactuer can lay its hands on due to the price. What does the consumer care about the charger ? They are single stage 'dumb' chargers (you really need 4 stage smart charger) and only ever fill your battery up to 80% of its capacity.

So ow you've only got 80Ah to use.

But also don't forget that the Numax's chemistry will be ruined forever if you flatten it more than 60% empty. Let approximate and say that you shouldn't go lower than 40Ah left in it. In voltage terms that a battery voltage of around 12.2v (12.8v is full 11.8v is empty).

So now you've only got 40Ah to play with.

But lets not forget that your inverter doesn't convert all the leccy it gets from your battery into 230v mains. It looses some. About a quarter actually.

So now you've only got 30Ah to play with.

30Ah will drive your 100A strimmer for 30/100 * 60 minutes. About 20 mins. Not very long.

Also don't forget that although your Numax is a tough old bird its not designed to get 100A continuous for minutes at a time. The plates will warp inside and the battery will fail.

Also don't forget that you will need some hefty old wiring and fuse gear to pull 100A continuous from your battery for 20mins.

Of course if you buy a 500w strimmer then its all doubled.

BUT the motor of whatever strimmer you buy will have a big startup power much higher than the running power. You will need to get and inverter with a surge power rating of at least 1.5 times your motor (maybe 2 times). Then also don't forget you'll be pulling 200A from your weary ole Numax.

Also some electrical equipment does NOT like a modified sine or square wave inverter. Especially motors only like a sine wave inverter. These are best but most expensive.

All in all the battery strimmer mentionned before is best by far. Inverter option is costly I think.......

Cheers,

Col

 

[richard_5133120]

Hi

I bought a bosch rechargeable strimmer for this exact job last year... I managed to get it from argos out of season for 15.00... I t works a treat!!!

Richard