How long to idle truck to charge battery?

[fiddypal]

Not sure if there is general info on this or if i need to find out specifics like what type of alternator my truck has, but about how long would i need to idle my truck to "top up" my 2nd battery?

I have a 70ah house battery in my truck, connected to the starting battery with an isolator. The only thing i will have constantly hooked up to the house battery is a Dometic CF50 fridge, if we are using the battery for charging laptops/phones and other uses through the night i would definitely want to idle the truck the next morning just to make sure the battery is happy.

Most of the time we will be driving so the smallish battery size should be fine, max time we would be camping is 2-3 days at a time and i wouldn't expect we leave the truck for more than a 24 hour period which i know the battery can handle with just the fridge before needing a charge.

 

[fiddypal]

Thanks for not tearing me apart on the vague question

My 2nd battery is a Die Hard AGM, same type as the stock battery 2014 Silverado 1500 V6 (not sure if the stocker is a die hard probably some GM factory brand).

What i really need to do is get a voltage meter for my battery so there is no guess work involved. Any options on the cheap?

Also, if my battery is really drained and i turn the truck on to charge, is it going to suck all the power from my starting battery since its fully charged or does the alternator put out enough power to charge both batteries and keep them from pulling power from one another?

 

[AndrewP]

Your plan overall can work for short duration trips. Just realize you are not really "fully" charging your battery since that takes hours to get the last bit of charge in there. But I would not worry too much about a less that full charge unless it's for an extended period.

Most alternators don't put out that much amperage at idle, so you're needing a way to "idle-up" to about 1500 rpm to make meaningful power.

It's likely in a modern vehicle, your alternator can make plenty of power to keep one battery from discharging into another. They will both be receiving current (charging) and thus no discharge is possible.

When measuring voltage, to reflect state of charge, the battery needs to be "at rest" at least 4 hours, and more optimally, overnight.

 

[comptiger5000]

With a decent size alternator and an AGM battery, you can put a good bit of power back into it pretty fast, especially if you have a way to idle the engine up a bit. I've seen AGMs charge at more than 1C off a big enough alternator. It's getting that last 10% or so of charge that takes a while. You could run the truck for 20 - 30 minutes and get a decent chunk of power back into it. As long as you're not going more than a day or 2 without driving for a couple hours, it shouldn't be too hard to keep things charged up.

 

[tarditi]

For this reason I got a jump pack... can also power items off the 12v port if needed.
I would like to go to a full dual-battery solar charging setup, but it's not in the cards right now.

You can get a solar battery tender to help keep up with the draw, too.
Inexpensive ones are available at harbor freight if on a budget. You do get what you pay for, but it may suffice.

 

[Buddha.]

Prop open the throttle with something like a dime or book of matches. You can keep track of voltage with a DVOM or hard wire a volt meter. 12.6 is full 12.0 is 50% charged

 

[fiddypal]

Can i pickup any of this information over the OBD port and a bluetooth dongle by chance?

 

[Martinjmpr]

Can i pickup any of this information over the OBD port and a bluetooth dongle by chance?

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No need to get that fancy, just get a simple electronic volt meter and check the battery periodically, that way you eliminate any voltage drop that may come through an OBD port or 12v socket.
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A fully charged AGM should read about 12.7 or maybe 12.8. I have a chart at home that shows the charge level and voltage, I want to say 50% charged for an AGM is about 12.39 and 25% charged is about 12.2.
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My house battery will be a deep cycle marine or RV battery, that should be a little more resistant to discharge/recharge cycles.
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EDITED TO ADD: Make sure you take the load off the battery before you check it.

 

[Buddha.]

Can i pickup any of this information over the OBD port and a bluetooth dongle by chance?

I have a scan guage2 that displays battery voltage among a lot of other things. It does not show any information when the truck is not running which you would need to see true battery voltage. Not sure if the item you refer to shows info with the car not running.

 

[Ducky's Dad]

A fully charged AGM should read about 12.7 or maybe 12.8.

Depends on the battery. Odysseys are full at 12.86 OCV. Optimas are full at 13.1 OCV or 13.2 OCV, depending on whether you have a starting or dual purpose Optima. Check your battery manufacturer's website for details. The cheapest way to test battery voltage is to get one of the free-with-coupon multimeters from Harbor Freight. I have a couple and have tested them against readings from a quality meter, and the voltage readings (at least) are dead on with the HF meter. Make sure you test resting voltage after the battery has sat for a few hours (8 preferred) after coming off the charger, or flash off the surface charge before you test voltage. The National Luna hardwired battery monitors are not very accurate, but they are directionally correct, so handy to wire into the truck to give you an early warning of low voltage, but don't trust their calibration.

Using OBD-II port for voltage readings will depend on your specific vehicle. Not all systems support things like system voltage or trans temp.

 

[Martinjmpr]

Depends on the battery. Odysseys are full at 12.86 OCV. Optimas are full at 13.1 OCV or 13.2 OCV, depending on whether you have a starting or dual purpose Optima. Check your battery manufacturer's website for details. The cheapest way to test battery voltage is to get one of the free-with-coupon multimeters from Harbor Freight. I have a couple and have tested them against readings from a quality meter, and the voltage readings (at least) are dead on with the HF meter. Make sure you test resting voltage after the battery has sat for a few hours (8 preferred) after coming off the charger, or flash off the surface charge before you test voltage. The National Luna hardwired battery monitors are not very accurate, but they are directionally correct, so handy to wire into the truck to give you an early warning of low voltage, but don't trust their calibration.
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Using OBD-II port for voltage readings will depend on your specific vehicle. Not all systems support things like system voltage or trans temp.

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All good points but I think the OP was looking for a 'quick and dirty' answer to his dilemma which is simply: How do I know when I need to idle my truck to charge the battery and how long should I idle it?
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If it were me, I'd just take the load off (unplug the fridge) for maybe 5 minutes, then check the battery with the multi meter. If it's below about 12.4 I'd probably reconnect the fridge and then idle for maybe 30 minutes max. Since his truck has a separate house battery, he doesn't have to worry about flat battery leaving him with a dead vehicle so he can allow it to discharge further than would be wise for a starter battery.

 

[DiploStrat]

...
What i really need to do is get a voltage meter for my battery so there is no guess work involved. Any options on the cheap?

Voltmeter won't tell you much - when the engine is running, it tells you the voltage of your charging system; when the engine is off, it may give you an artificially low reading because your battery may be under load. (Voltmeter readings are only accurate for "resting" voltage.) That said, a voltmeter that reads the second battery only, can be useful, as long as you realize that it is not very accurate.

Also, if my battery is really drained and i turn the truck on to charge, is it going to suck all the power from my starting battery since its fully charged or does the alternator put out enough power to charge both batteries and keep them from pulling power from one another?

The short answer is, "no." The longer answer is that this is an old wives's tale and the longest answer is that what will actually happen depends on a lot of things, ranging from your alternator output and engine speed to wire size.

Final important point, the greatest limiting point is the rate at which your battery will accept a charge. Assuming a lead-acid AGM battery, some 50 Ah down, you can expect that the charge rate will drop by 50% about every 30 minutes to an hour. This is because the battery's internal voltage will rise as it charges and as the battery's internal voltage and the voltage of the charge source get closer, the charge rate drops. This why many of us stress that you probably cannot drive long enough to charge a deep cycle battery bank. 50Ah will probably take two to five hours to fully charge, but that is a SWAG. All depends on your battery, alternator voltage (Chevrolet does very well here), wire size, etc. For example, a second battery at the end of a 10 AWG wire will charge very slowly.

 

[AaronK]

I have a pair of these in my rig. One for starting battery, one for the house battery. Very handy.
https://www.amazon.com/dp/B00BZPH6KQ/ref=cm_sw_r_cp_apa_2QpOxbN5ZQ6M0

Sent from my A0001 using Tapatalk

 

[Kerensky97]

Is topping up the battery going to be a common issue or is this a once or twice thing that will rarely happen?
If you're driving between campsites every day the hours driving will obviously recharge the batteries for the next night. But if you're settling in to one camp for days on end, running the vehicle for a few hours to charge a battery is pretty inefficient. If this is a common problem it might be time to get a portable generator for charging. Then power at camp is no longer a limitation and you can haul all kinds of electric gear.
Personally I'd get the Honda EU1000 http://amzn.to/2avWhHk

Although I'd rather go solar if anything were possible so it all depends on your needs.

 

[rayra]

few stock alternators generate their rated amperage at idle. You'll have to hold your RPMs at least twice that.