Power supply question
Moderators: [nope] cartel, team nopesport
19 posts
• Page 1 of 2 • 1, 2
Power supply question
I have been asked to look into using inverters and leisure batteries for running laptops and other electrical equipment at events where a generator might be too noisy or intrusive. Can somebody advise me as to how to decide what inverter and what battery to buy?
- Mr Chips
- orange
- Posts: 110
- Joined: Sun Jun 06, 2004 6:14 pm
- Location: London
Guess that's me then.
There's a few real world differences that change the answer but you can get close enough by using schoolboy physics:
Watts = Volts x Amps
If your laptop draws 0.5A at 240V (see bottom of the powerpack) then 0.5 x 240 = 120Watts. At 12V this will be 120/12 = 10A. There will be some losses in the inverter but as I said, the schoolboy stuff will get you close. To run more than one laptop/other device then simply sum the number of Watts.
Most car batteries will be rated in two ways: Ampere hours and cranking power (Watts). Ignore the cranking power, but total the number of amps you need and multiply by the number of hours you need them for. You can vary it a bit e.g. 1 amp for 3 hours and an additional amp for the middle hour = 4Ah. As long as the car batteries life exceeds that (and it's fully charged!) You'll be ok. If you need more than a battery can last you can connect two or more in parallel and you will have the sum of the Ampere hours to use.
Hope that wasn't too confusing.
There's a few real world differences that change the answer but you can get close enough by using schoolboy physics:
Watts = Volts x Amps
If your laptop draws 0.5A at 240V (see bottom of the powerpack) then 0.5 x 240 = 120Watts. At 12V this will be 120/12 = 10A. There will be some losses in the inverter but as I said, the schoolboy stuff will get you close. To run more than one laptop/other device then simply sum the number of Watts.
Most car batteries will be rated in two ways: Ampere hours and cranking power (Watts). Ignore the cranking power, but total the number of amps you need and multiply by the number of hours you need them for. You can vary it a bit e.g. 1 amp for 3 hours and an additional amp for the middle hour = 4Ah. As long as the car batteries life exceeds that (and it's fully charged!) You'll be ok. If you need more than a battery can last you can connect two or more in parallel and you will have the sum of the Ampere hours to use.
Hope that wasn't too confusing.
-
FatBoy - addict
- Posts: 1042
- Joined: Thu Apr 15, 2004 1:46 pm
Many thanks. My schoolboy Physics petered out somewhere around 1965. I recognise the formula but wanted confirmation that it really is as simple as that. Does the laptop's power adaptor draw its stated input current (e.g. 1.5A) regardless of what current the laptop is drawing from it?
- Mr Chips
- orange
- Posts: 110
- Joined: Sun Jun 06, 2004 6:14 pm
- Location: London
No, it'll probably be a peak current. If the laptop is fully charged and not doing much then it'll be much lower. If it's charging and e.g. playing a DVD it'll be around 1.5A. There's no easy way of measuring the average draw except by trying it, because "average" depends on what you're using it for therefore the manufacturers can't really say what average is. If you base your calcs on 1.5A you'll certainly have spare in the tank at the end!
-
FatBoy - addict
- Posts: 1042
- Joined: Thu Apr 15, 2004 1:46 pm
I tried to use a laptop with a flat battery from a 300W inverter last week. There was a loud beeping noise and the inverter cut out. Fortunately it was a second laptop I was trying to startup and the event went ahead smoothly (once I realised why the split printer wasn't working any longer).
There is a moral there somewhere.
There is a moral there somewhere.
- SIman
- brown
- Posts: 515
- Joined: Wed Dec 31, 2003 5:09 pm
having used all sorts of power supplies (big genys, invertors etc etc) over the past few years for small events you cannot beat a generator that screw fix do for flexibility.
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=43168&ts=76698
its quiet, uses hardly any fuel and is so cheap its daft
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=43168&ts=76698
its quiet, uses hardly any fuel and is so cheap its daft
-
stodge - blue
- Posts: 411
- Joined: Wed May 05, 2004 12:02 pm
- Location: Milford
£99 is an amazing price...
As I said in the other thread... it's possible to run laptops, thermal printers and even a laser printer from batteries... (pref: Leisure/Deep Discharge/Trolling/Marine types)...
For small/medium events a battery works... for bigger events a generator makes more sense.
As far as calculations are concerned...
P=VI
Inverters are typically 80% efficient.
Don't forget failure... 2 generators... 2 batteries/inverters.
Have Fun,
12vGeek
PS
I prefer batteries... total quiet... (apart from the SI read unit bleep and a whir of thermal printer..) heaven in a quiet forest night event...
As I said in the other thread... it's possible to run laptops, thermal printers and even a laser printer from batteries... (pref: Leisure/Deep Discharge/Trolling/Marine types)...
For small/medium events a battery works... for bigger events a generator makes more sense.
As far as calculations are concerned...
P=VI
Inverters are typically 80% efficient.
Don't forget failure... 2 generators... 2 batteries/inverters.
Have Fun,
12vGeek
PS
I prefer batteries... total quiet... (apart from the SI read unit bleep and a whir of thermal printer..) heaven in a quiet forest night event...
- 12vGeek
hmmm...800 watts - Fatboy is the electrician
what's the power draw of the laser? wouldn't like to run our Ricoh off it
we have had 2xlaptops, 4x60watt light bulbs, 2xsplits printers and an inkjet all off the generator at night
I always have a 200 watt cigarette lighter inverter on standby
what's the power draw of the laser? wouldn't like to run our Ricoh off it
we have had 2xlaptops, 4x60watt light bulbs, 2xsplits printers and an inkjet all off the generator at night
Don't forget failure... 2 generators... 2 batteries/inverters
I always have a 200 watt cigarette lighter inverter on standby
-
stodge - blue
- Posts: 411
- Joined: Wed May 05, 2004 12:02 pm
- Location: Milford
Please correct me if the following argument is naive or ill-informed.
(1)A laser requires high power (e.g. 1000W) only in short bursts (half a second?)
(2)An inverter only takes power from the battery in proportion to the power that it is supplying (about 8% more, I have read).
If one uses a big enough inverter (say 1200W) to provide for the laser's occasional power peaks one will draw no more from the batteries between peaks than one would using a smaller inverter (say 600W). Drawing 1000W (say ~4 Amps at 240V) for half a second amounts to about 0.0006 Amp hours.
Or is this complete rubbish?
(1)A laser requires high power (e.g. 1000W) only in short bursts (half a second?)
(2)An inverter only takes power from the battery in proportion to the power that it is supplying (about 8% more, I have read).
If one uses a big enough inverter (say 1200W) to provide for the laser's occasional power peaks one will draw no more from the batteries between peaks than one would using a smaller inverter (say 600W). Drawing 1000W (say ~4 Amps at 240V) for half a second amounts to about 0.0006 Amp hours.
Or is this complete rubbish?
- Mr Chips
- orange
- Posts: 110
- Joined: Sun Jun 06, 2004 6:14 pm
- Location: London
My laptop power supply seems to require 192W input (0.8A at 240V AC) and to provide 48W output (4A at 16V DC). That sounds very inefficient to me. As an alternative to inverters has anyone on this forum considered using DC-DC converters such as those found at http://www.eurobatteries.com/sitepages/power-adaptors-laptop.asp ?
- Mr Chips
- orange
- Posts: 110
- Joined: Sun Jun 06, 2004 6:14 pm
- Location: London
Mr Chips wrote:Please correct me if the following argument is naive or ill-informed.
(1)A laser requires high power (e.g. 1000W) only in short bursts (half a second?)
(2)An inverter only takes power from the battery in proportion to the power that it is supplying (about 8% more, I have read).
If one uses a big enough inverter (say 1200W) to provide for the laser's occasional power peaks one will draw no more from the batteries between peaks than one would using a smaller inverter (say 600W). Drawing 1000W (say ~4 Amps at 240V) for half a second amounts to about 0.0006 Amp hours.
Or is this complete rubbish?
Sorry been skiing
Not rubbish - all correct on paper. However in the real world batteries drain in different ways not perfectly linearly, so you may find a sudden 4A draw does more "damage" to the charge level than simply 0.0006Ah. Repeated similar draws may significantly reduce the battery life.
The only difference in the situations you describe between a big inverter and the small one is cost. To build a big current inverter you big cables which cost more.
-
FatBoy - addict
- Posts: 1042
- Joined: Thu Apr 15, 2004 1:46 pm
19 posts
• Page 1 of 2 • 1, 2
Who is online
Users browsing this forum: No registered users and 190 guests