Quote:
Originally Posted by darrellx
Hi All
Steady progress is being made on finishing the observatory. But I have one niggling issue I am having trouble getting my head around. I am hoping someone here might be able to help me with electrical theory.
My Laptop and the current draw on a 240 volt system compared to a 12 volt system.
I have a MSI laptop that I use exclusively for astro work.
On the transformer, the Output is stated as 19.5v 7.7a
The Input is listed as 240v 2.7a
When I use my Amp meter, plugged into 240 volts, during boot it peaks at 0.31 amps.
During normal use, it sits around 0.22 amps. In terms of watts, it moves around 42 to 48.
The laptop is fully charged.
Now, when I plug this onto a 12volt battery with an Inverter, the figures essentially stay the same.
All good so far.
What I don’t understand is the impact on the battery.
My thoughts on this.
- Forget the Output on the transformer. The listed 7.7amps is a maximum.
- The important figure for the laptop is the third of an amp it seems to draw at maximum.
- The other important figure is the 2.7a listed on the transformer.
So, ignoring any loss and inefficiencies, the transformer requires 240v at 2.7a. It is then delivering the 0.31 amps that the laptop requires.
Does that mean that the loss from the battery is still only the 0.31 amps or 2.7? I don't think so.
240v 0.3amps = 72 watts
72watts at 12 volts = 6amps
So, to run the laptop for say 6 hours - 36amps 
I suppose if I wanted to get picky I would say 240v 0.22amps = 53watts
53watts at 12 volts = 4.4 amps
6 hours of runtime would then be 26.4 amps
Does that seem correct?
Thanks
Darrell |
Darrell
I note that no one actually taken up your request for help with your electrical theory. Well here it goes:
Your analysis of the issue is largely correct. The figures for the transformer (power supply?): input 2.7A is nonsense, probably refers to fuse rating. Output 7.7A max. at 19.5VDC is consistent with your measured max input current of 0.31A. (0.31A input translates to 0.31x(240/19.5) = 12.3A, but only at 100% efficiency).
For the battery and inverter case, again the efficiency of conversion has a significant impact.
If your inverter supplies an accurate 240VAC output the draw from the battery will be 0.31x(240/12)= 6.2A but more likely about 7A due to inefficient conversion. Your calculation based on power is correct at 6A.
Now, about battery capacity. Your calculation is correct but what you got was 36 Amp hours not amps. You need a minimum battery size of 36 Ampare Hours, AH for short. This is basically a good car battery. Nasty things car batteries, better to use SLA (Solid Lead Acid) batteries. At 36AH they are not cheap if availably. The largest one at JAYCAR is 17AH but they may have larger. Need to check.
Andras