A generator from the early '60s might only deliver ~10 amps at idle.
Figure the headlights, marker & tail lights also used 10 amps, plus the ignition system drew ~3 amps.
So extended idling at night was marginal even without the radio.
Iâve always been nervous about playing the radio for more then ten minutes or so. Iâm probably overly conservative there, Iâd imagine.
You could get something like the Duracell Powerpack 300 to run the laptop, taking some load away from your car battery. That would also give you the ability to top off the air in your tires if needed.
Interestingly, Iâm updating the navigation system in my Acura today. It takes two hours with two CDs and they warned to keep the car running the full two hours so that the battery doesnât discharge. I donât know what they do in places like Minneapolis where they have a law against idling cars. Maybe a battery charger.
I have run my car radio for over an hour many times with no perceivable drain on the battery. But I donât have it cranked either, and I tend to have mostly had big cars with the biggest batteries that will fit in the allocated space.
@Whitey: A pretty much bog standard modern laptop has at least a 65W power adapter to operate it and recharge the battery. Certainly the spinny things like the hard drive (if itâs not solid-state) and the CD/DVD ROM (if it even has one) use some juice. But these items are nothing just compared to the current draw for the motherboard, processor, and screen backlight. If an old school CD Walkman could run off a couple of AA batteries for hours, thatâs about as much power as the laptop CD/DVD drive takes when playing a CD or movie. A mosquito feeding on a buffalo.
More worrisome IMHO is if you are sitting in the car using the radio or whatever, are the umpteen electronic modules/computers/carâs own systems that arenât likely going to sleep like they would when you lock your car and walk away. Itâs very likely that these are as much or more of a factor in draining the car battery than the radio or anything youâre plugging into the cigarette lighter socket.
@oblivion, youâre forgetting the laptop computerâs cooling fan, if it has one, and Iâm betting this particular laptop computer does. Youâre also not counting the power wasted by using an inverter to turn DC into AC, before the computer supply turns it back into DC so it can power the laptop computer. Inverters are inherently inefficient, because they have to turn electricity that flows as a sine wave into electricity that flows at a constant rate equal to the crest of the sine wave. Iâm probably butchering the terminology here because this was covered in a Physics class I took about 24 years ago, but I think you get the idea. Lastly, some inverters come encased in heat sinks, but those that donât come with their own fans, so include that one too as another moving part that is running on the carâs battery.
This is how to pencil it out⊠A typical car battery has about 30 amp-hours capacity. The radioâs current draw depends on how loud you play the speakers. A home stereo is around 50 watts/channel say at max volume. In a car, what the typical speaker volume youâd use? Itâs less than youâd use in your living room probably. And you wouldnât use max level in your home either. So say the audio volume inside the car corresponds to 6 watts per channel, or 12 watts total. Since Power = voltage * current, the current corresponding to 12 watts would be 12 watts/12 volts or 1 amp. Since you have 30 amp-hours total battery capacity, you could listen to the radio for 30 hours until the battery was fully drained. If you wanted to reserve at least half the battery capacity to start the car, you could listen for 15 hours. Whether 1/2 battery capacity will start the car depends on the temperature and newness of the battery. And then thereâs the unknown of how much extra current is used with the key in the accessory setting. To be safe, Iâd test it first. Try listening 4 or 5 hours at a reasonable volume in your driveway and see if the car will start. If it does, youâve done the experiment. If it doesnât, bring out the battery charger.
@Whitey: Youâre not wrong. The fan is another mosquito (even sounds like one on some cheap laptops), but the inverter is a power waster for sure, if you use one. Speaking of fans, the stereo/nav system in my car actually has a cooling fan tooâyou can hear it running when you turn off the car after using the unit at a decent volume for a while, until you open the door and the power shuts off.
Getting back to the laptop, if we assume a 65-watt laptop adapter (which may even be low, as some of the mobile workstations we have use 180-watt adapters), not even counting inverter losses, youâre drawing about 5 1/2 amps continually if the adapter is running at capacity or close to it. With a healthy car battery in a good sized car, you can probably get away with doing that for a few hoursâabout the same as leaving your headlights on.
I suppose you should also factor in that someone drives a short trip to the store, leaves their kids in the car to listen to the radio/watch videos/play video games/whatever, then half an hour later comes back, starts the car, drives a mile or two back home, repeat. The battery would never be fully recharged in this case and will soon be low and lose capacity over time due to the beating itâs taking.
âWith a healthy car battery in a good sized car, you can probably get away with doing that for a few hoursâŠâ
That brings up another consideration. The battery in my Civic is about twice the size of a motorcycle battery, indicating the type of car the OP intends to use a relevant factor. 30-40 years ago, when we were going to drive-in theaters, car batteries in even the smallest cars were larger than many of the car batteries we use now.
The 2007 Corolla the OP intends to use probably has a relatively small battery. I think his best bet is to charge the laptop computer before and after he uses it to play music, but not while the car is off. It has its own battery, so why not use it?
The laptop adapter is sized to operate the computer and charge the battery simultaneously.
The backlit LCD display also uses substantial power.
Out of curiosity I connected a laptop (HP EliteBook 8470p) to a watt meter.
The adapter is rated 65W output.
Adapter not connected to laptop: <1W
Laptop off, charging ~78% full battery: 60W
Laptop on, full bright screen, no apps running, batt charging: 76W (occasional jumps to ~82W)
As above, battery removed: 10W (jumps to ~25W)
Dimmest screen setting: 6W
Running a virus scan, bright screen: 20W-30W
Watt meter is a Triplett 661
The attempted calculations of how much juice the radio uses neglect that power usage for driving speakers is not simple. Makers talk about 100 watts or a lot more as if more is always better, and it often is, but there are too many other factors involved. That rating is a maximum. when you hit that the amp starts clipping the signal. instead of the normal peaks and valleys you get a kind of plateau (not really, but it is a workable analogy.) That kind of output can damage speakers in no time. A high wattage rating lets you play loud music without clipping, though whenever I hear one of those humongous audio systems rolling down the highway they usually sound like theyâre clipping badly.
More power also reduces noise. However, to get back to the original situation, itâs almost impossible to know. The wattage rating on the audio system is just a maximum and tells nothing about the wattage used in typical listening, which is usually surprisingly low. Which is why little 5 watt computer speakers can sound all right and work fine at normal volumes. Your car radio will likely be using no more than a few watts most of the time. Also very significant is the speaker efficiency, and that varies quite a lot. If you can use a phone or MP3 player with the kind if earphone you stick into your ear canal youâll be using even less, though their efficiency is highly variable and I have a lovely pair of Etymotics that sound gorgeous, but canât be driven to sufficient volume from the headphone jacks of most portable devices. No, I wonât carry a headphone amp. I have others that also sound nice.
In a car the number of speakers also would matter, especially if there is a subwoofer. Low frequencies take far
more power than midrange and high frequencies. Most car audio systems are weak in the bass (it would be drowned out by all the low frequency noise in the car), but that means they use less power.
I would be far more worried about the computer than the radio. They use a lot of power continuously. The radio uses very little power most of the time, especially a radio like yours with skimpy bass output.
I had a good system way way way back, about 200W and 3 amps, and it kill the batt after about 4hrs, IIRC. I had a batt rated at 640 âcranking ampsâ (I forget the regular rating). Dunno if that useful.
Modern stereos still have an amp and still need watts to drive the stock spkrs, as many as 9 on a blah car. Dunno the avg total amperge they likely pull tho. that what should determine all of this. I also donât know how regular car batt ratings convert to Amp Hours. U see cell batts rated at 4000 mAh, and that determines how long a charge will last, but the draw is always changing bases on whar youâre doing so its a rough guess. With a car it be easier, but you have to know what it costs to start. Maybe that changes based on air temp or something. I dunno that either.
Anyone?
All I know is I recently killed a battery by running the blower and radio for several hrs. I jumped it and let it idle for hrs. Then just running the radio for hrs did it again. Supposedly car batts can take repeated full discharges like LiIon or NiCad can without damage. Dunno why. Can anyone explain how this works scientifically? Maybe then we can get hard numbers if we know our inputs.
So far all I know is I can get away with a few hrs and let it idle a couple 1st the next time (if undriven between) and its still alive.
Although, this 2005 VW has a tume-out function that turns off the radio after some time (havenât figured how long) if u turn the key off but dont take it out of the ignition. If only it did that with the key ON and for the blower and sensed the batts volts, vs a dumb timer.
Why would anyone run the blower âfor several hoursâ?
If you need heat, the heat would be depleted in just a few minutes when the engine is shut off, and then it would be obvious that you need to shut-down the HVAC system.
If you need cooling, you will get NO cooling from the HVAC system when the engine is shut off.
Please explain why you chose to run your blower âfor several hoursâ.
At the last drive in movie a couple of years ago, car was fine for 4 hours listening on the am radio, the blower is probably overkill as there is no more heat being provided by the engine. If you need the heat let the engine idle, otherwise open the windows
It DOES cool you a little. Itâs called a fan. I hear lots of people use them. I think Ive even seen new fangled AC versions in stores.
Its because I needed a little air movement but could stand it without A/C and window down, but there was no wind outside. It has a tad of heat but worth it. Interestingly, I felt heat off the engine (by hand under the hood) at least 6hrs after turn off. I didnât think it hold heat like that. Kinda a trip.
Thatâs about the fastest way to ruin a car battery, automotive batteries are not designed to be discharged that far.
Damut, stupid autocomplete screwup! It takes OFF the 't on canât if I manually type it vs autocomplete. I meant CANT take it.
So, why?
Because that is what people believe.
I worked at a Dodge dealer that replaced any discharged battery on new inventory because that battery would not recharge after 30 minutes of idling the engine. I would sometimes take these batteries home to use in my cars, trucks and camper. The battery in my car came out of a 2008 PT Cruiser and sat on my garage floor dead for 10 years.
OK, now THATâS interesting. I heard leaving a car batt on the ground without something under it will drain it. Then if it sits there too long (days, wks, dunno) itâll be trashed.
Did that 10yr dead batt h0ld
a chrg as well as one it didnât happen to?
Draining a car battery once or twice isnât going to cause a problem
Car batteries are specially designed to provide a short, sharp burst of power in order to crank over and start the vehicleâs engine.
Only a relatively small portion of the batteryâs capacity is used and this is restored by the vehicleâs alternator. Car batteries have thinner lead grids to maximize plate surface area, delivering greater starting power. However they are not suitable for providing long periods of power.
Deep Cycle batteries are constructed using thicker battery plates, to withstand repeated charge and discharge cycles.
Rather than providing high bursts of power for short periods of time, they are specially designed to deliver sustained power with lower current draw over extended periods of time. Repeated cycling does not cause the same level of damage that a car battery would sustain during the same usage.
If you want a battery that is designed to do both, get a marine starting battery.
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This saved my life a few time.You donât need to wait for AAA or another car to give you a boost. Get it out of your trunk,hook it up and start the vehicule. Good for 6-7 vehicule boost on a single charge.