Charging while towing

[capnTelescope]

I must be getting old as I forgot all about the discussion previously.

Ah-HAA.

there are a lot of different ways to skin this cat

Yes, there are, and none of us really has the hotline to the Ultimate Truth. It comes down to each individual's priorities.

I hope we didn't scare anyone off.

Kiss and make up?

[tony.latham]

Hey, it's a good thing. It allowed me to bring up Einstein's Theory of Relativity and quote Dirty Harry in one paragraph.

T

[KennethW]

I can see how your dc-ac-dc set up would charge your TD battery a lot faster. A TV alternator will only put out the voltage required to charge the TV battery regardless of what the TD battery needs.

[capnTelescope]

I can see how your dc-ac-dc set up would charge your TD battery a lot faster. A TV alternator will only put out the voltage required to charge the TV battery regardless of what the TD battery needs.

You're on the right track, Kenneth. What the TV alternator sees in DC-only is actually the TV battery paralleled with the TD battery, but it's not smart enough to realize it's being fooled. So it might put out a voltage that is too much for one battery and not enough for the other, depending on the two battery's differences. BTW, most smart chargers would probably be fooled too, if they tried to charge 2 different batteries at once.

Dc-ac-dc hides the TD battery from the alternator. The TV charging system then only has to contend with the TV battery's needs plus all the system loads, which it is used to doing and is quite capable of with headlights, etc.

As for charging the TD battery, the inverter is tolerant of high or low voltages and so offers a certain amount of output voltage regulation . Then the smart charger (at least the Noco, but surely most others) is likewise tolerant of variations in input voltage and can output the right charging voltages and currents for the TD battery under varying conditions.

Dc-ac-dc may or may not charge faster. I couldn't say for sure. Too many what-ifs. I see the main advantage is that the charging method can be set to best match the TD's battery. That keeps the TD battery happier, longer.

[bdosborn]

I can see how your dc-ac-dc set up would charge your TD battery a lot faster. A TV alternator will only put out the voltage required to charge the TV battery regardless of what the TD battery needs.

Yeah, but that's how all battery chargers work, they hold a constant voltage and vary the charge amps to the battery. Smart chargers do it at 3 different voltages. You only have a problem if the charge voltage is too low by the time it gets to the trailer battery. More to follow...

You're on the right track, Kenneth. What the TV alternator sees in DC-only is actually the TV battery paralleled with the TD battery, but it's not smart enough to realize it's being fooled. So it might put out a voltage that is too much for one battery and not enough for the other, depending on the two battery's differences. BTW, most smart chargers would probably be fooled too, if they tried to charge 2 different batteries at once.

The charge profile for a lead acid isn't that different from an AGM. So close that most chargers don't have a different setting for AGMs, which includes the nice PD charger that a lot of us like so much. Realistically, the battery spends minimal time being charged by the TV. There really isn't a need for a different charge profile for the AGM, even when it's being paralleled with a lead acid battery. 99% of the time your trailer battery is being carefully maintained by your smart charger on shore power. A gel cell battery is different, they are notoriously sensitive to charge voltage and an automotive alternator would ruin them. Don't use a gel cell...

Dc-ac-dc hides the TD battery from the alternator. The TV charging system then only has to contend with the TV battery's needs plus all the system loads, which it is used to doing and is quite capable of with headlights, etc.

Not at all, you've just added quit a bit of load to the alternator. The inverter is 90% efficient at best and the smart charger is even worse at about 70% efficient. That's a 40% increase over the equivalent DC amperage, meaning a 40a smart charger (the size I have) is going to pull 56 amps from the alternator. How big is your alternator, have you run the numbers on what your setup will draw for a deeply discharged battery? It's probably not an issue with a 10A or less charger. But then again, why bother with a 10A smart charger when you can usually much get that from the TV?

As for charging the TD battery, the inverter is tolerant of high or low voltages and so offers a certain amount of output voltage regulation . Then the smart charger (at least the Noco, but surely most others) is likewise tolerant of variations in input voltage and can output the right charging voltages and currents for the TD battery under varying conditions.

This is a very good point and one that I've mulled over quite a bit. I was having voltage drop problems due to the skinny little charge wire that came stock with my truck. Even though the alternator was at 14.2V, I was getting effectively zero charge amps at the trailer battery since the voltage was barely at 13V by the time the wire got there. I ended up changing out the wire on the truck from a #18 (!) to a #6 and now I get 13.8V at the trailer battery. Still not as high as I would like but it will charge now. Voltage drop is a real issue and it's usually the problem when people have trouble charging their trailer batteries while towing. Manufacturers are notorious for under-sizing stock charge wires.

Dc-ac-dc may or may not charge faster. I couldn't say for sure. Too many what-ifs. I see the main advantage is that the charging method can be set to best match the TD's battery. That keeps the TD battery happier, longer.

You'll have to keep us posted on the long term effectiveness of this approach. My personal feeling is it's quite a bit of added complexity and cost for a small increase in performance. But then again, I don't remember things like used to...

Bruce

[capnTelescope]

I think we can boil this all down to how I want to treat my batteries vs. how you want to treat your batteries. I don't care how you treat your batteries. You've evaluated the facts and decided you're okay. I've also evaluated and decided.

You'll have to keep us posted on the long term effectiveness of this approach.

That's a deal. If dc-ac-dc turns out to be a mistake, I don't want other people to follow. So far, no problem. BTW, the jump starter/compressor that started all this died a few months ago from a shorted cell. It lasted about 7-8 years. It still holds enough charge to run the compressor. Is that good, bad or indifferent?

There really isn't a need for a different charge profile for the AGM

The AGM manufacturers recommend the different charge profile for better performance and longevity. Whether the differences are noticeable or not, I can't say. An AGM charged directly by the TV will charge, discharge when used, and charge again. Without a side by side comparison, who really knows?

Realistically, the battery spends minimal time being charged by the TV.

That's true in the same sense that we all spend minimal time camping. The difference is that when we camp is when we discharge our batteries. This seems to me to be the time the smart charger should be on the job. At home there's no discharging for long periods, so a simple maintainer could do the job.

you've just added quit a bit of load to the alternator.

About 140 watts max peak, using my charger and your numbers. (((7.3A * 12V) / .7) / .9) That's roughly equivalent to driving with your headlights on: (55W * 2) + non-LED running lights. A (small) 100 amp alternator would be 1200 watts. I don't think this is going to hurt a thing.

why bother with a 10A smart charger when you can usually much get that from the TV?

It's all about pampering the battery.

now I get 13.8V at the trailer battery. Still not as high as I would like but it will charge now. Voltage drop is a real issue and it's usually the problem when people have trouble charging their trailer batteries while towing.

I think this is the strong point of dc-ac-dc. I haven't measured voltage at the battery, I don't have to. The charger's lights blink normally, and the charge indicator shows progress. The charger is obviously getting enough voltage to do its thing, or it will show an error condition. In 2-4 hours of driving, the battery is fully charged. It just works.

My personal feeling is it's quite a bit of added complexity and cost for a small increase in performance.

The complexity issue is subjective. You're adding an inverter. The relays and solenoids are optional. They keep the operation simple.

Start the truck, the solenoid connects and the TD battery gets charged. Switch off, no more charging and the solenoid disconnects the TD from the TV. Plug in shore power, the TD battery gets charged. If I start the TV while shore power is plugged in, which I don't recommend, the relay safely switches the charger to TV power.

The added cost isn't much either:

• Wiring: 10ga ($0.25/ft) vs. 6 ga($0.61/ft)

• Inverter: $40 vs. $0

• Charger: same/same

(slightly off topic)
You piqued my curiosity, and I had to go refresh my memory (you aren't the only one) on the PD series. With the Total Charge Management System, does the PD only charge from A/C or will it charge if hooked to the TV's 12V power? I'd let the PD do the charging. You wouldn't draw 56A for long, just at first. Charging at a constant 40 amps (480 watts) would tend to boil the battery before long. An 800 watt inverter should take care of you.

[H.A.]

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[H.A.]

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[KennethW]

Wild ideal. When towing noise is not a big problem(unless in a park or____) How about using a $90 HF generator (mounted on the tonge,in a dog house when not in use) and battery charger when on the road. The generator will take very little gas after the battery is charger. And there would be no wiring in the TV.

[capnTelescope]

Clever !
I like this charging scheme.

Thank you, H.A. The first time this worked, I darn near threw my shoulder out patting myself on the back.

Better yet for efficiency at the inverter, Place the inverter in the car nearer to its supply, Running then 120vac back to the smart charger within the trailer...

That's true. I just wanted it all in the trailer. Also, I was a little squeamish about putting higher voltages out there in the open.

Just think,
This could also operate your electric shaver whilst driving with or without towing your trailer along.

Beer fridge!

How about using a $90 HF generator...

Sure, why not!? Charge while towing, or anytime you're not in a hospital quiet zone.
Mega beer fridge!

[bdosborn]

(slightly off topic)
You piqued my curiosity, and I had to go refresh my memory (you aren't the only one) on the PD series. With the Total Charge Management System, does the PD only charge from A/C or will it charge if hooked to the TV's 12V power? I'd let the PD do the charging. You wouldn't draw 56A for long, just at first. Charging at a constant 40 amps (480 watts) would tend to boil the battery before long. An 800 watt inverter should take care of you.

It works on AC only. Here's a link to the charge profile. I have an older model where the charge wizard was an external dongle. The newer series have the charge wizard built in.

PD Charge Wizard

My solar panel controller has a more aggressive profile as it will charge the batteries to 100% in a day. The PD charge takes a couple of days to bring a battery gently back to 100%. A 90% charge is fast, the last 10% takes forever.

Bruce

[Hal W]

Nicely done and well thought out. One of your pics didn't load (my comp not your upload) where you were talking about shore power but I'm assuming that you have a receptacle you can plug a shore powered extension cord into that makes live your 120V side?

[capnTelescope]

I'm assuming that you have a receptacle you can plug a shore powered extension cord into that makes live your 120V side?

Correct. You can see the inlet in my signature picture below. It's the small thingy below the larger white water fill on the driver side wall back by the galley. Even harder to see is the 12VDC plug-in just aft of the shore power receptacle.

ParkPower 150BBIW.RV 15 Amp Power Inlet, White

[OverTheTopCargoTrailer]

Charging while parked

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[capnTelescope]

Where the campground at?