How long to idle engine?

[gatherer]

I need to charge my cellphone from my vehicle's starter battery, with the key in the accessory position. I'm camped in one location and not driving around at the moment, so roughly how long should I idle the engine to keep the battery topped up?

I'd like to use 10ah a day with the key in the accessory position (no radio or any other power draws). I know this will vary vehicle to vehicle and depend on lots of other factors like inefficiencies in the system, but can anyone give me a rough estimate? 

When I did the math based on my alternator output of 136a, I figure 5 minutes at a high idle would generate enough power to replenish the 10ah, but idk how much the starter actually drains the battery and what other factors to consider.

Yes I know this method is terribly inefficient, and I will be getting a better system in the future, but for now this is what I have to work with. Thanks for your input.

 

[Trebor English]

My Ford has other issues.  Having the key in the accessory position causes a couple of amps to be drawn even with everything off.  If you add a cigarette lighter socket to a circuit that is always on like the radio clock & memory circuit you may be able to bypass some issues. 

The alternator max current spec is almost useless in figuring this out.  The engine computer sets the alternator field current to get the voltage it wants to see.  After the engine start it will be high.  Quickly the battery will get to 14.4 volts.  That will be held for a short time.  Then the voltage will drop to 13.8.  At that point very little additional charging will happen.  Use a voltmeter to observe this. 

Look at your battery.  It has an RC, reserve capacity, rating.  That's how many minutes the battery can produce 23 (formerly 25) amps until dead.  Divide that by 60 and then multiply by 23 to calculate amp hours.  Then multiply by 0.05 to calculate the 5% limit.  Lead/calcium maintenance free batteries should not be discharged more than 10%.  A repeated 5% discharge, in my experience, will shorten the battery life but not terribly.  Over 10% is bad but I have no actual experience.  The 50% recommendation for lead/antimony deep cycle batteries does not apply to maintenance free starter batteries.  My Ford special extra small group size 59 battery RC was 100 minutes.  Divided by 60, times 23, times 0.05 gives 1.9 amp hours at 5%.  At 10 amp hours your proposed discharge would be is close to 25%, probably death of the battery in a month. 

If you add a small portable solar panel to your starter battery circuit you will be able to stay in one location indefinitely and charge your phone all you want while the sun shines plus a bit more at night.  A USB phone charger is about 10 watts.  Then you don't need to run the engine to charge.

 

[gatherer]

My 95 dodge cummins' batteries have a RC of 280 and not much in the way of electrodigicals. The ignition has a position for just the cigarette lighter. But I'd assume that the would be much higher for the 1 or 2 amp draw of a phone charger since batteries lose capacity as current increases.

 

[crofter]

I use a battery jump box to charge electronics. I try not to drain off anything from the van unless I am actually driving.
-crofter

 

[Spaceman Spiff]

I need to charge my cellphone from my vehicle's starter battery . . . 
I'd like to use 10ah a day with the key in the accessory position . . .
When I did the math based on my alternator output of 136a, I figure 5 minutes at a high idle would generate enough power to replenish the 10ah . . .

10 ah to charge a phone seems excessive; my deeply discharged iPhone will use about 2 ah.

There are many other ways to charge a phone, two good ones listed above:
 - get a small solar panel to charge the phone and/or to keep your van battery charged up.
 - get a jump pack and use that to charge the phone and charge the jump pack while driving.

The advantage of a jump pack:  If your battery will not start your vehicle you can use it to give your van a jump.

You will not get 136 amps out of your alternator at idle AND your battery will only accept a few amps (battery in absorption stage).  Probably take an hour to get to 100%.

 

[B and C]

Idling to charge a cell phone is probably the most expensive way to charge a cell phone.

 

[highdesertranger]

yeah but think of the bragging rights,

"I have a Cummins powered cell phone charger" or
"I have a diesel powered cell phone charger"

but seriously I would get at least a small solar panel.

highdesertranger

 

[Trebor English]

yeah but think of the bragging rights,

I have a nuclear fusion powered cell phone, not one of those crummy uranium fission piles of junk.

 

[B and C]

I'm working on a flux capacitor

 

[gatherer]

I appreciate all your input, and yes in the future I will be getting a better setup as I mentioned. But this is what I have to work with for now.

My cellphone has a 3100mah battery, but it needs several charges throughout the day.

 

[jacqueg]

Portable battery pack that you charge while driving with a USB adapter in your ciggy plug. And/or portable battery pack that comes with its own solar charging system.

 

[gatherer]

Thank you for the input. As mentioned before I will be upgrading soon. My question is, how long will I need to idle the engine to top up the batteries?

 

[tx2sturgis]

When you figure it out, let us know. It's going to involve some trial and error on your part. There is NO WAY that any of us can know your situation exactly.

How good are your batteries? How new or old? 

How much do you use your phone? How depleted is it when you are ready to recharge? 10% left? 90% left?

How long does your alternator stay in bulk mode? How cold or warm is it where you are parked or camped?

How much compression does your engine still have, which will affect how much current the starter pulls to start the engine?

How old is the starter and alternator? Are they new and fresh or about worn out? 

A good guess on my part would be to start the engine about every three days, and idle it above 1200 RPM for 10 minutes or so. You would need to adjust this routine to suit your situation. If your battery barely starts the engine on day three, then increase your time idling above 1200 RPM to 20 minutes or so. 

You will be burning a lot of fuel just to charge a smartphone. But you knew that already.

A small wireless device charger with a lithium pack will certainly cost less than one tank of fuel, and you could charge that when driving normally and then use it to charge your phone several times during a week, assuming you just need to top it off daily.

Good luck with this.