Thread: How long will the battery last?

The battery will last more than an hour but certainly a lot less than a day. So turn that system off when leaving the car for any significant amount of time.

I don't expect (nor have I noticed) any adverse effects on my battery. It's just a few extra amps of load on the electrical system, it can well handle it. As long as you don't allow the battery to discharge too much, it'll be all right.

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Fully custom Linux-based system on an '89 VW GTI 16V Wolfsburg Edition
Dual in-dash CD-ROMs, jogdial volume control, Sony joystick on the steering column, IR remote...
and still more HP than MHz

this has been discussed before, but i dont remember what conclusion we came to.

if i remember correctly:
watts = volts * amps
so, amps = watts/volts

you have 70 watts @ 12 volts, so that means you are drawing 5.83 amps if you're using every little bit of power that your PSU can handle.

it was theorized that a computer on SUSPEND draws approx. 1 amp.

the way that you find out how long your battery will last is you look at the amp rating on the battery. I have a 97 Chevy fullsize truck with a marine-grade battery that has 1200 amps. i need 600 amps to start, so as long as i have more than 600 amps, im good to go. this means that if i leave my computer on suspend, I can go up to 600 hours (25 days) without starting my vehicle and it will still start when i finally go somewhere.

please note that when you start your vehicle, all of your electrical systems run off of the alternator, and the alternator also charges your battery. I cant remember what size alternator an accord has, but its probably around 60 amps or less. that means you can only have 60 amps of accessories (including electronic fuel injectors, ABS brakes--anything that uses electricity) running at the same time without draining your battery while the car is running. (BTW, this is BAD! it happened to me once--the car just stopped running when i got to a stop sign and the battery was so drained that i couldnt get it started again)

I suggest ALL of you get better-than-factory alternators. warhammer, your accord is 11 years old so it probably needs a new one anyway. the first thing that i did when i bought my truck was go to autozone and buy a 100 amp alternator to replace the wimpy 65 amp model GM puts on their trucks.

so, the moral is:
1. check your alternator and replace if necessary
2. check your battery to see how many amps (or better yet, amp/hours) it has.
3. find out how many amps your car needs to start and how much it draws (stock--no extra electronics) while running.
4. subtract the cranking amps from the battery rating and divide by 5.83 for leaving it on or 1 for suspend. this gives you how many hours you can leave the system and still have enough juice to start up.
5. subtract the draw of your car's electrical system from the alternator rating (this number gives you a rough estimate of how many toys you can put in your car)

if you read this entire post, i hope it helped

No, that's not quite right, wizardPC. I have to correct you before you kill yuour battery

You've got the math right, a 70W power supply will draw about 6 amps max (well, maybe a bit more due to not 100% efficiency, but it doesn't matter). You're also right about the alternator. But the Amp rating of your battery has nothing to do with it. 1200 A is just the maximum sustainable current that the battery can deliver. It says nothing about how long your battery can deliver that current. The really important number is the storage capacity, rated in Amp-hours (Ah). Car batteries are usually in the 50-100 Ah range. So a 100 Ah battery can deliver 1 Amp for 100 hours, or 1200 Amps for 5 minutes. I'm sure your marine-rated battery is higher, but you don't mean to say it's got 1200 Ah?

And you certainly don't need 600 Ah to start the car. I don't have the figures handy, but a starter draws somewhere in the 1-10 KW range, Let's use your figure of 600 Amps @12V (= 7.2 KW). Say you crank it for 10 seconds, that's 1/600 of an hour, so the total capacity required is 600 x 1/600 = 1 Ah.

But the max sustainable current goes down as the battery discharges. At some point, it's still got quite a few Ah left in it, but it can't sustain the 600 Amps needed to crank the car. And as the battery gets older, it can store less Ah, and it also needs a bigger margin.

So the real moral is, take the ampere-hour rating of your battery, and cut that in half just to be on the safe side. For a 60 Ah battery, figure you can draw 30 Ah and still start your car. 30 Ah will power a 6 Amp (70W) system for 5 hours, or a 1 Amp system for 30 hours.

Hope this clear things up a bit. Or maybe not.

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Fully custom Linux-based system on an '89 VW GTI 16V Wolfsburg Edition
Dual in-dash CD-ROMs, jogdial volume control, Sony joystick on the steering column, IR remote...
and still more HP than MHz

[This message has been edited by Oms (edited 11-01-2000).]

yeah, i meant amp-hours. i knew that--why did i say amps.......

Be sure to factor in -everything- that is running. ie. If you are running your MP3 player, you most likely are running your head unit, maybe an amplifier, etc...

Just another factor...

I left mine on accidentally one time using a 140watt inverter for 4 hours. Started fine...suprisingly.

wizardPC: sure, I figured you meant to say ampere-hours. Still, it doesn't take 600 Ah to start a car! (Unless you're driving an M1 tank or something)

I drive a big *** truck with a big *** starter

Originally posted by wizardPC:
it was theorized that a computer on SUSPEND draws approx. 1 amp.

Sorry, but there is no way I can believe this. 1A at 120W is 120W, which is as much as a standard system (minus the monitor) draws in use. I'm sure the suspend current draw is much less.

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Aaron Cake
London, Ontario, Canada

Player: Cyrix 200, 32MB RAM, 10.2Gig Quantum HD, Onboard EtherNet/Sound/Video, Custom Lexan Case, Arise DC-DC, Win95 Kernal w/Custom Player
Car: '86 Mazda RX-7 w/Basic Performance Upgrades