# Toying with Voltage Ratings

#### xibalban

Joined Apr 4, 2013
11
I was wondering if the following makes any difference, technically.

Say, some load is rated at 100 W at 230 Volts (resistive load assumed)
By using Power formula:

I=P/V [Power factor=1 assumed]
So, I=100/230 = 435 mA

Let's connect a 12 V source [battery] across this same load:

I=P/V
So, I=100/12 = 8.3 A

Hence, if a 40 Ah, 12 V battery is connected across this load, it would last for approximately 5 hours:

Total hours, h = Ah/A
h = 40 Ah/8.3 A
So, h = 4.8 hours

I know that this setup would require thicker connecting wires due to higher amperage, but I have the following doubts:

• Is it practical?
• Can a 230 V rated load be connected to a 12 V source?
• What happens if the load were to be inductive/capacitive?

#### #12

Joined Nov 30, 2010
18,167
You forgot to convert the resistive load to how many ohms it has. When you connect a 12 volt battery to something that can withstand 230 volts, a lot less current will flow.

#### MrChips

Joined Oct 2, 2009
19,734
#12 is correct.

Since power = V^2/R

The new power at 12V is 100*(12/230)^2 = 0.272W

#### xibalban

Joined Apr 4, 2013
11
Thanks, I realise that too now.

Why aren't loads rated in impedence/resistance instead of common voltage and wattage ratings?

#### #12

Joined Nov 30, 2010
18,167
Thanks, I realize that too now.

Why aren't loads rated in impedance/resistance instead of common voltage and wattage ratings?
Because 99% of people don't know about Ohm's Law and Watt's Law, and sometimes, even the ones that do know make mistakes. Better to keep it as simple as 12V, 2A or 120VAC, 1200 watts, and sometimes, that still isn't simple enough for people that aren't into electronics at all.

#### xibalban

Joined Apr 4, 2013
11
Would my assumption about the Ah to h conversion have been valid, if we introduced a voltage converter after the battery i.e. 12 V to 230 V converter, and feed the load with rated voltage?

#### #12

Joined Nov 30, 2010
18,167
Yes, except for the inefficiency losses of the converter.

#### gerty

Joined Aug 30, 2007
1,267
That would change the dynamics of the whole load, and introduce the losses of the inverter.

Ya beat me!!

#### #12

Joined Nov 30, 2010
18,167
"Some days the magic works and some days, it doesn't." (Chief Dan George in the movie, Little Big Man)

#### gerty

Joined Aug 30, 2007
1,267
I know it didn't take me 5 minutes to type that sentence #### xibalban

Joined Apr 4, 2013
11
I see, so just in order to confirm:

"A 40 Ah, 12 V battery would light a 230V, 100 W bulb for 4.8 hours provided the output voltage is converted from 12 V t0 230 V with 100% conversion efficiency"

Right?

#### #12

Joined Nov 30, 2010
18,167