"Redneck Generator": Why wouldn't this work?

[Martinjmpr]

On a camper/trailer Facebook group the question almost always comes up among newbie campers who own trailers with slide-outs:

"What if I'm camped in the boonies with the slide out and my battery dies? How do I get the slide back in so I can leave?" Most commonly, the answer is "plug it into your tow vehicle so it's getting power from the TV and close the slide."

But, really, have you seen how thin the wire is going through that 7 pin connector? If your battery was dead enough that the slide-out wouldn't go in, it would take a long time to charge through that little 7 pin port. My answer is always "reverse your tow vehicle so the hood of the TV is in front of the trailer tongue, and run jumper cables from the TV battery to the trailer battery. With the engine running and the alternator going, you should have plenty of power to retract the slide."

Anyway, that got me to thinking about my own "aux battery" setup with my F-150. My 12v fridge is powered by a 90AH FLA battery in a trolling motor box that I call my "power box." I keep the battery on a battery tender at home so it's always charged up and ready to go. I then went to the trouble of installing a Renogy 20A DC-DC charger connected to the battery. That in turn is connected to the Power Box and keeps it charged up as I'm driving (the DC-DC charger shuts off when the ignition is off to preserve the truck battery.)

But now I'm wondering: Was the 20A DC-DC charger even neccessary? Let's say I get a new vehicle and don't want to mess with the DC-DC charger. So I keep the "power box" in the truck, and just periodically check the voltage. If it drops below, say, 12.2v, I just take the box out, set it in front of the truck, connect jumper cables, start the engine and let the engine idle while the battery charges.

Or, as I like to call it, the "redneck generator."

Now, certainly, in these days of $4.50+ gas, nobody is crazy about the idea of idling a truck just to charge a battery, but is there any reason this WOULDN'T work? Assuming I have a 90AH FLA battery, and assuming that 12.2v means it's discharged close to 50%, wouldn't you think a modern truck with, say, a 130+ Amp alternator should be able to fully charge the battery in 20 - 30 minutes max? Heck, even if the alternator only puts out 100A at idle, that means it can push 50AH into the battery in just 30 minutes, right?

And while the DC-DC charger is more "elegant" (because it recharges the battery while I drive down the road with no further input from me), when you look at the cost ($120 for the DC-DC charger, $30 - $50 for the 8g cable, $20-$30 for connectors, wire loom, zip ties, etc) as well as the time it takes (the better part of a day for me, but then again I work very slowly) it seems to me that for a "casual user", just having a spare battery (any 12v battery will do, no need for a $500 "solar generator" ) and periodically charging it up with jumper cables would work fine.

Again, is there a flaw in my logic? In the Army we used to say "If it's stupid but it works, it isn't stupid." A "redneck generator" to keep your portable battery box charged may be kind of crude but I can't think of a reason why it wouldn't work. Something to consider for those of you with either limited funds, limited skills or limited time.

 

[DRAX]

Regarding the slide-out, they have a manual crank specifically for pulling the slide in if there's no power. People tend to overthink things, kind of like "How do I unlock my car if the battery is dead?" forgetting that there's a physical key on their fob that will unlock it.

Regarding using jumper cables to recharge your small battery, that really depends on the vehicle. Many modern vehicles have smart charging systems that aren't always putting out ~14v, they actually reduce or shut off alternator output if the BMS indicates the battery doesn't need to be charged and there's not enough electrical load to warrant running the alt, this is purely for fuel economy. Connecting your small battery may trigger a voltage drop that would kick the alternator into charge mode, you'd have to test and see.

As for the alternator being able to produce 100A at idle, that will never happen. You might get 30A if you're lucky, but how many amps are put out depends on the amp draw. The alternator doesn't push amps, the load pulls them.

 

[45Kevin]

My understanding of the need for DC-DC charger is if you have a lithium house battery. They require a different charge profile from a FLA battery and the vehicle alternator does not provide that profile. On your Renogy DC-D charger you had to set the charger for a FLA battery, right?

 

[86scotty]

It's not stupid, it's old school and it works. Go for it.

However, I'd just connect a wire to the power box all the time and let it charge while running without the DC to DC. Both FLA batteries, no problem.

 

[Martinjmpr]

My understanding of the need for DC-DC charger is if you have a lithium house battery. They require a different charge profile from a FLA battery and the vehicle alternator does not provide that profile. On your Renogy DC-D charger you had to set the charger for a FLA battery, right?

Correct, there are DIP switches on the charger that you set at different settings depending on what type of battery you have.

 

[DiploStrat]

Short answers:

-- Charging, especially lead acid, takes hours and hours and hours. This will explain a bit of it: https://cookfb.files.wordpress.com/2016/09/battery-charge-slides.pdf
-- If you design your system carefully, sizing your battery to your load, you will never have to worry about a dead battery. https://cookfb.files.wordpress.com/2016/09/plan-electrical-system.pdf
-- Many newer trucks have "smart" alternators what will not maintain the voltages necessary to charge a second battery. A battery to battery charger is the best option for most of us.

 

[Buddha.]

Regarding the slide-out, they have a manual crank specifically for pulling the slide in if there's no power. People tend to overthink things, kind of like "How do I unlock my car if the battery is dead?" forgetting that there's a physical key on their fob that will unlock it.

Regarding using jumper cables to recharge your small battery, that really depends on the vehicle. Many modern vehicles have smart charging systems that aren't always putting out ~14v, they actually reduce or shut off alternator output if the BMS indicates the battery doesn't need to be charged and there's not enough electrical load to warrant running the alt, this is purely for fuel economy. Connecting your small battery may trigger a voltage drop that would kick the alternator into charge mode, you'd have to test and see.

As for the alternator being able to produce 100A at idle, that will never happen. You might get 30A if you're lucky, but how many amps are put out depends on the amp draw. The alternator doesn't push amps, the load pulls them.

Out of the three kinds of slides typically used two have manual override and one of those is actually accessed without tearing the slide facia off.
I think op has an r pod which has the schwintec system and impossible for a novice to manually run in.

 

[jonyjoe101]

The alternator will quickly charge the battery to about 80 percent, the closer the battery gets to 14.4 volts the less amps the battery gets. As the lead acid battery gets full it won't accept high amps, maybe 3 or 4 amps. IF its an old battery that has lost capacity it will charge quickly to 14.4 volts. Like someone mention it will take hours to reach 100 percent and with lead acid you need to charge to 100 percent everytime, charging to 80 percent (undercharging) will reduce its lifetime. If the battery gets to 12.2 volts, you need to charge it up right away, if you let it sit too long at 12.2 volts or less it will reduce its lifetime.
The dc-dc charger is the best way to charge it since there is no voltage drop and the charger hits the battery with the appropriate volts/amps.

 

[TwinStick]

Yes, a Redarc dc to dc charger is the way to go, if you have the money. Everyone has different power needs, to be sure.

We rarely camp where there is no electric. We do have a Honda eu2000i that is incredibly fuel efficient. But we don't always bring it. From the truck battery, I have a 120 amp resetable circuit breaker, welding cable to the bed, to a manual blue sea on/off switch. Also have aux battery in bed to another blue sea on/off switch. Both switches to a blue sea battery selector switch, giving me the ability to run the 2000/4000 watt inverter on truck battery or aux battery or both or off. We rarely use aux battery for more than 2 nights without recharging. It has worked flawlessly for us so far. This is the 4th truck it has been in since 2000. We don't have a Redarc yet but they sure are the cats meow. When battery needs charging, I can charge it while driving down the road, from inverter. Certainly not efficient going 12v - 120v - 12v, but it works great and I used stuff I already had. I have a multitude of different battery chargers I can use while driving or at camp with electric. And a couple times a year, I hit the AGM batteries with 60+ amps to knock the sulfation off the plates. My first AGM battery is still going strong, I purchased it in 2006.