Finding out how much charge is in my battery?

boycee · 26 Jan 2009 at 09:55

Ok so...

Thinking of getting on on those cigarette lighter adapters so I can use 240V, mind you I only need 90W for a laptop. However...

I'd like to know somehow when my car battery is running low, as to not completely drain my battery. Otherwise it will go flat and I won't be able to start the car. Something like, woah, you only have 30% charge left in your battery, best stop draining it.

Does such a thing exist? I'd like to wire it to the battery and have it so that I can read the output from inside the car too.

Possible?

BullBoyShoes · 26 Jan 2009 at 10:14

Simplest thing you could do would be to monitor the voltage and turn off your laptop or whatever when it reaches 10v ish

Shadez · 26 Jan 2009 at 10:22

boycee said:
Ok so...

Thinking of getting on on those cigarette lighter adapters so I can use 240V, mind you I only need 90W for a laptop. However...

Most inverters have a low power cut off that will shut the inverter down if the battery starts getting to low.

Hodders · 26 Jan 2009 at 11:29

Mains invertors are still not the efficient. Laptops need DC why not by a car charger specifically for your laptop, should be a lot more efficient.

Also you could always fit a 'leisure' battery like on a yacht or caravan. That way you can drain it and still drive home. You will need to can some wiring work done to 'plumb' it in though.

Dogbreath · 26 Jan 2009 at 12:30

BullBoyShoes said:
Simplest thing you could do would be to monitor the voltage and turn off your laptop or whatever when it reaches 10v ish

Run a car battery down to 10v and it's not going to be starting any engines!

It's almost impossible to accurately tell the state of charge of a lead acid battery purely by voltage, since it changes with the load applied, the temperature of the battery and the general condition of the battery.

Either limit the time you have the laptop plugged in for, or use a secondary battery as suggested.

paradigm · 26 Jan 2009 at 13:13

Alternatively, only run the inverter when the engine is running. Then it's not running off the battery

Clarkey · 26 Jan 2009 at 13:19

paradigm said:
Alternatively, only run the inverter when the engine is running. Then it's not running off the battery

If we're going to be pedantic about this then it is still running from the battery

paradigm · 26 Jan 2009 at 13:20

Clarkey said:
If we're going to be pedantic about this then it is still running from the battery

Is it?

boycee · 26 Jan 2009 at 13:42

Hodders said:
Mains invertors are still not the efficient. Laptops need DC why not by a car charger specifically for your laptop, should be a lot more efficient.

Also you could always fit a 'leisure' battery like on a yacht or caravan. That way you can drain it and still drive home. You will need to can some wiring work done to 'plumb' it in though.

Hmm the cut off would probably be ok, as it's not going to be used very regularly though. I guess the more I spend on the inverter the more efficient it will be?

Any recommendations?

Dogbreath · 26 Jan 2009 at 17:20

Clarkey said:
If we're going to be pedantic about this then it is still running from the battery

If the engine is running, and provided the alternator output is sufficient, then there will be a net current flow into the battery, so wouldn't be running from the battery.

Tony Williams · 26 Jan 2009 at 18:48

What about just taking a spare battery, keep it in the boot -- just incase?

Al-Ectrician · 26 Jan 2009 at 20:35

Dogbreath said:
It's almost impossible to accurately tell the state of charge of a lead acid battery purely by voltage, since it changes with the load applied, the temperature of the battery and the general condition of the battery.

Either limit the time you have the laptop plugged in for, or use a secondary battery as suggested.

Thats the best answer I would say, you need to be able to read the current available aswell as voltage.

What you need is a ammeter and a multimeter.

Al

paradigm · 26 Jan 2009 at 20:49

Al-Ectrician said:
What you need is a ammeter and a multimeter.

You mean an ammeter and a voltmeter? Given that a multimeter can read amperage

Al-Ectrician · 26 Jan 2009 at 20:54

paradigm said:
You mean an ammeter and a voltmeter? Given that a multimeter can read amperage

I mean a voltage indication device and a proper ammeter to accurately read the current being drawn.