UKC

On a tangent from SARS thread about sleeping in his SUV, when sleeping in the car I'm always paranoid about depleting the car battery and was wondering if any mechanically minded folk could give me an idea of how much it takes to drain a car battery.

I know it's a bit of a "length of string" type question probably dependent on how big your battery is, how cold it is, how new it is and obviously presuming it's fully charged but for example, how long could you have your stereo on before the battery was too weak to start the car?

I've toyed with the idea of getting a converter for the cig lighter to charge a laptop etc. Would it be a case of only using it while the car was being driven or would the battery be able to cope with it while the car was off.

Also, I've heard conflicting theories about how the alternator charges the battery. Is there a difference between how much it is charged while the car is stationary and while it is being driven?

Cheers

Ben

In reply to Sharp: not an expert but having a top spec battery would help

In reply to Sharp:

A car stereo or a mobile phone charger don't use much power at all compared to a car battery - for proof compare the amp hour rating of a big lead acid battery with that of your mobile phone so the answer to your question is quite a long time. As for charging, at tick over it will still charge but not as much as when you are running at normal driving revs.

In reply to Sharp:

I connected my i pod and listened to it as I was taking down the tent and packing away the camping gear last year.

probably an hour or so ( it's a big tent )

battery was as flat as a pancake.

In reply to Sharp:

For around £100 you can get a portable battery charger which you can use to both jump start your vehicle and also to charge items such as ipods etc. Problem solved!

In reply to coinneach:

Then you must have had a short circuit somewhere, how many amp-hours do you think a tiny ipod battery is? And running just on that it normally lasts for a lot more than an hour.

An ipod battery is less than 1 amp hour, whereas a car battery is a minimum of 40 amp hours.

In reply to Sharp: Alternator output is related to rotational speed so the charging output at idle speed is very much lower than at higher rpms hence batteries will take much longer to charge at idle.
You probably don't have space to hide another battery in a car but an extra 'house' battery with a split charge relay would be the way to go if you intend using your car as a camper regularly. The relay senses when the alternator is charging and allows the alternator to charge the 'house' battery when the engine is running. When the engine is off the relay disconnects the 'house' battery from the starting battery preventing it being run flat

In reply to woolsack: A "leisure battery" would be ideal but for the amount I actually use it would probably be overkill. Weekends away aren't a huge problem as I can do without it but if I've got a week in the car it would be nice to be able to recharge the lappy and listen to a few tunes.

Thanks for the info on how the alternator works. I'm not sure if it's possible to answer but would an alternator running at idle be giving more charge than would be being depleted by charging a laptop?

In reply to Sharp:


Yes it would normally with an alternator, this is one of their advantages over the old dynamos. To check see if your lights dim when you gradually slow down the engine to tick over - if your lights stay bright then your alternator is providing more power than the lights are using. If the head light dim then this is not the case... so you may have a problem depending on how much power you want to draw from the battery.

Camping cars and such like use two batteries and reserve one for starting. You could always buy a small battery for radio use and charge it up with a mains charger before every trip if you didn't want to rig up a proper charging system as suggested by woolsack.

In reply to Sharp:

You could get a simple plug in voltmeter to tell you the state of the battery charge. Here are some random examples from Amazon.
http://www.amazon.co.uk/Alarm-Temperature-Thermometer-Voltage-Monitor/dp/B0...
http://www.amazon.co.uk/Cigarette-Lighter-Voltage-Voltmeter-BuyinCoins/dp/B...

A 100W plug in inverter at full load will probably run for 5-10 hours off a fully charged car battery (depending on size of battery). Much more powerful than that and you'll probably blow the fuse for the cigar lighter. An Ipod or phone charger will run for days.

In reply to Bruce Hooker: Cheers for your reply, I've noticed my headlights dim when I'm at low revs, i.e. starting to move forward from standstill. Is this a sign that the battery is weak and needs changing?

In reply to Sharp:

No, it just means that at those revs your alternator isn't providing enough power to run all your lights, especially the headlights. If you add up all the bulbs that makes perhaps 200 watts, so with a 12 volt system 200/12=17 amps approximately - which is quite a lot. As you speed up the alternator produces more power and can cope with the lights and charge the battery.

When I used a car for camping I had just a single small lamp on a long lead - maybe 12 watts (can't remember but it simplifies the maths) so 12/12=1 amp. A 60 amp hour car battery can supply 1 amp for 60 hours to light up your dinner that's fine. As for music and charging an ipod you can look up the power consumption of each and do similar calculations to see if you are going to be alright or not.

Just in case keep a couple of jump-leads in the boot to get you out of trouble if you do flatten the battery

In reply to Bruce Hooker:

It is true that in principle a 60Ah battery can provide 1A for 60 hours, but in practice car batteries are not designed to be discharged. They are designed to provide a very large current to start the car, and then to be recharged back to full. They are not designed to provide low-current power for extended periods of time. In theory using more than a very small fraction of the total battery capacity can permanently damage the battery since they are not designed to be significantly discharged.

By comparison a 'leisure' lead-acid battery has thicker lead plates with a smaller surface area cannot provide a large starting current, but can be more completely discharged (typically to about 50% of rated capacity) without being damaged. They are also called 'deep cycle' batteries.

So generally speaking running anything significant off a car battery for any length of time is probably a bad idea.

In reply to Bruce Hooker:

Are you sure you are not assuming a 100% efficient transfer of charge?
I did this experiment last year, charging a Samsung Galaxy S2 from my car's "cigarette lighter" socket with the engine off. Didn't time it or look at how much charge was on the phone battery before and after, but it was plugged in for about 2-3 hours. It flattened the battery. There was no short circuit or other electrical issues with that car (we jump-started it after the experiment).

In reply to Sharp:

I think I'd just buy a standalone portable digital radio and a torch.

In reply to Blue Straggler:

Did you check what the voltage had fallen to when it wouldn't start? Modern engine management systems won't activate below a certain voltage whereas an old carburettored engined car will still crank over and fire at a lower voltage

In reply to Sharp: I recently purchased a new battery as the old one was on its way out.
There were a number of options, where a lot of people will just go for the cheapest or middle of the road.
I went for the most expensive purchased through Tanya batteries (the battery was £40 cheaper than most places quoted suh as kwik fit and automotive centres).
I went for the varta silver dynamic which had a capacity of 77ah and a cold cranking amps of 780A, compare this to the cheapest I cold get which is heir black dynamic capacity 56ah and cold cranking 480A.
Now I have a volvo so the lights are on as soon as the ignition is turned and wanted something that was going to be reliable for the Mrs.

Like others have said, those that are having the battery die after charging a phone need to have their battery checked as it may be on the way out or the previous owner could of put on a cheap battery during their ownership.

In reply to Blue Straggler:

I wonder whether some of these people reporting flat battery from 2 hours of iPod etc have cigarette lighter sockets that are only on when ignition is on? If so it is more than likely draw from other things (fan, fuel pump, ecu etc) that's flattened the battery not whatever you plugged into lighter socket.

A fully charged car battery in good condition should easily have enough juice to charge a phone / ipod for several days and still crank over at the end of it.

Think of it in terms of energy, 12V 40Ah to discharge that in even 4 hours you'd be drawing 10 amps = dissipating 120 watts, I think your phone and charger would have burned through the seats in short order.

In reply to Blue Straggler:


Well if this is the case and there was no chance of an electrical problem then I stand corrected. Maybe a charger uses a lot more power than a lamp? I've only ever charged phones with the engine running as my cigarette lighters only seemed to work when the engine was running. I've used a lamp on a wire, but probably only for a few hours while cooking for example but perhaps the power used was much lower.

Caravans seem to manage well enough, and on a boat we used the lights without the engine running but I suppose only for a few hours each evening... The only thing to do is experiment, and have someone nearby with another car just in case!

Thanks for the replies to the report of my little test. TBH I did not examine all factors, it was literally just a "will charging the phone kill the battery?" test. The car ran happily for several months after than before I sold it. I'll ask the buyer if he has had battery problems