240 Or 12v Usage Calculations ~ My Brain Hurts!!!!

[P20]

Help!!!
We’re trying to get our heads around the proposed battery system for our conversion.
Our converter has recommended the LPS [URL]https://www.claytonpoweruk.com/lps/[/url] stating that we could run an induction hob off it if we wanted. This idea appeals. However trying to get our heads around battery life is more than I’m capable of. I’m fine working out the 12v side, but how long can the system run for on 240v? My basic (very basic) understanding is the dometic fridge for example will be significantly more efficient on 240v, but I don’t understand how long it would then last for? Surely you lose something???
Example:
200w appliance.
On 12v draws 16.6a. So would last 6 hours on 100amh battery
On 240v draws 0.87a so would last 114hours on 100amh battery.
Is that right? I know I haven’t taken efficiency of inverter in to try and make my life simple.
In my mind we’re getting something for nothing here. I feel you should get significantly less because you’ve upscaled the voltage.

the end calculation would be if we can run a double induction hob without killing the battery

 

[Neilson]

You haven't included the difference in voltage. The battery will be supplying 12 volts. I think you need to calculate the wattage;

12v x 16.6a = 199.2w

240v x 0.87a = 208.8w

Presume you'll supply the 240v from an inverter? which will be more inefficient.

 

[Dellmassive]

As above use watts for a better conversion.

If using SLA batteries you can only use the top 50% of DOD....

So you need to half the final time....

If a 17A @ 12v dual hob on a 100Ah battery... will be 5.8hrs..(theoretical)

Half it to 2.9hrs.....(50% dod)

Then take off losses etc you might end up with 1.5-2hrs.

Which would leave you with a totally flat battery that will take 20hours to recharge with a EHU charger....

So you need to be realistic with your expectations...



.

Advice would be get a decent pure sine inverter and as large a battery bank as possible.

Add some solar and EHU for extra help.

 

[andys]

Help!!!
We’re trying to get our heads around the proposed battery system for our conversion.
Our converter has recommended the LPS Lithium Power Supply - Clayton stating that we could run an induction hob off it if we wanted. This idea appeals. However trying to get our heads around battery life is more than I’m capable of. I’m fine working out the 12v side, but how long can the system run for on 240v? My basic (very basic) understanding is the dometic fridge for example will be significantly more efficient on 240v, but I don’t understand how long it would then last for? Surely you lose something???
Example:
200w appliance.
On 12v draws 16.6a. So would last 6 hours on 100amh battery
On 240v draws 0.87a so would last 114hours on 100amh battery.
Is that right? I know I haven’t taken efficiency of inverter in to try and make my life simple.
In my mind we’re getting something for nothing here. I feel you should get significantly less because you’ve upscaled the voltage.

the end calculation would be if we can run a double induction hob without killing the battery

Yeah your sums are suspect. 200W pulls 0.83A at 240v so multiply by 20 to get the current that the inverter will pull from your battery, giving the 16.7A, but let’s make that up to 20A to cover inefficiencies in the inverter and wirng. As @Dellmassive said, your 100Ah battery will actually give you 50%, ie 50Ah so your 200W appliance would last max 2.5hrs. That’s if it was using 200W constantly though, and a fridge will be dipping in and out as it cycles the compressor. If you were using a 1KW induction hob, it would kill your battery in less than 30 mins, they’re really only viable with lithium or a big battery bank with solar.

 

[Dellmassive]

On from @andys above...

A 1kw 240v hob..... will need a 1500w inverter....

That will draw 100A per 1000W per hour...

Which would be 1hr theoretical and 30mins minus losses possible..... so maybe 29mins runtime, leaving you with a totally flat battery....

Then a 20hr recharge on EHU !!!

Or 20hr driving via a dc-dc to recharge...



So if your going to run an induction job off grid...... get a big battery bank and a decent dc-dc + ehu for the recharge..

 

[P20]

Thank you all! The brain hurts a little less now

 

[Phil_G]

In simple terms, to drive a 240v device, your inverter is multiplying the battery voltage by 20.
In so doing it needs to draw 20 times as much current from your battery.

When you plug a 240 watt mains bulb into the mains, will draw 1 amp at 240 volts from the grid.
To drive this same bulb, your inverter will draw 20 amps from your 12v battery.

As an example, the Jura E6 is a typical mains-powered bean-to-cup coffee machine. Its 1450 watts.
To power it, your inverter will draw 120 amps from your battery which could drain a typical leisure battery in 15 minutes

Take any mains appliance, read the wattage label, divide this by 12 and thats how much current your inverter needs to draw in order to power this appliance.

This assumes an ideal, 100% efficient inverter - in practice the current draw could be 10-20% higher.

 

[OllieGBR]

I'm looking at an induction hob for my conversion. I have 400w of solar and have yet to decide on the type and size of battery.
Regardless, I made the decision to only use the hob when on shore power - too power hungry. It will only be used to heat water/ soup that kind of thing and therefore all other cooking will be outside.
If I'm not on power then jetboil and BBQ!