Charging while towing

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Re: Charging while towing

Postby capnTelescope » Wed Jun 10, 2015 3:56 pm

Just for giggles, here's a fresh post in my build thread showing how I did it. It's all Plug & Play. No switches to flip or "Don't forget to"'s.

Enjoy! :beer:
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby bdosborn » Wed Jun 10, 2015 6:58 pm

Is there a reason you didn't just bring the 12V DC straight to your battery?

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Re: Charging while towing

Postby tony.latham » Wed Jun 10, 2015 8:17 pm

bdosborn wrote:Is there a reason you didn't just bring the 12V DC straight to your battery?

Bruce


Sheeeeesh. I just read through this entire thread. Forty-five minutes? I dunno. Maybe an hour. It's kind of a fog. Maybe it was a dream. Perhaps Einstein could 'xplain to me how time is relative and I could look back and understand what just happened to me. Maybe he could warp-drive string theory into my mushed brain while he's at it. And I thought AC-DC was a band. :thinking:

And then you, Bruce, lay the one-liner down like (Dirty) Harry Callahan back in '71: "Do I feel lucky?" Well, do ya, punk? 8)

I need another beer. :beer: I think I'll stick to my "factory disconnect when the engine is off" charging wire.

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Re: Charging while towing

Postby capnTelescope » Wed Jun 10, 2015 8:57 pm

bdosborn wrote:Is there a reason you didn't just bring the 12V DC straight to your battery?

Well, Bruce, you know me. Gotta do it the dc-ac-dc way. It's the same old "is the voltage drop too much/is the wire big enough/is it right for the battery" thing. I'm somewhat surprised you had to ask. :)
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby bdosborn » Wed Jun 10, 2015 9:04 pm

Ummm, wow. I just went back and re-read the thread too. I must be getting old as I forgot all about the discussion previously. :?

I know there are a lot of different ways to skin this cat but lets don't loose sight of the fact that you can charge your trailer battery straight off the TV without using an inverter/smart charger. I think newbies read threads like this and decide it's too complicated to charge your trailer battery while driving, when it actually can be really simple.

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Re: Charging while towing

Postby capnTelescope » Wed Jun 10, 2015 9:22 pm

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

Ah-HAA. :lol:

bdosborn wrote: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. :oops:

Kiss and make up? :lips:
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby tony.latham » Wed Jun 10, 2015 9:35 pm

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

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Re: Charging while towing

Postby KennethW » Wed Jun 10, 2015 10:13 pm

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.
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Re: Charging while towing

Postby capnTelescope » Thu Jun 11, 2015 1:01 am

KennethW wrote: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.
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby bdosborn » Thu Jun 11, 2015 7:47 pm

KennethW wrote: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...

capnTelescope wrote: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...

capnTelescope wrote: 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?

capnTelescope wrote: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.

capnTelescope wrote: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... :frightened: ;)

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Re: Charging while towing

Postby capnTelescope » Thu Jun 11, 2015 11:37 pm

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.

bdosborn wrote: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?

bdosborn wrote: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?

bdosborn wrote: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.

bdosborn wrote: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.

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

It's all about pampering the battery.

bdosborn wrote: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.

bdosborn wrote: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. :D
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby H.A. » Fri Jun 12, 2015 12:03 pm

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Last edited by H.A. on Wed Feb 03, 2016 9:03 pm, edited 3 times in total.
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Re: Charging while towing

Postby H.A. » Fri Jun 12, 2015 12:05 pm

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Last edited by H.A. on Wed Feb 03, 2016 9:03 pm, edited 2 times in total.
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Re: Charging while towing

Postby KennethW » Fri Jun 12, 2015 2:21 pm

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. 8) And there would be no wiring in the TV.
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Re: Charging while towing

Postby capnTelescope » Fri Jun 12, 2015 3:37 pm

H.A. wrote: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. :D

H.A. wrote: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.

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

Beer fridge! :beer:

KennethW wrote:How about using a $90 HF generator...

Sure, why not!? Charge while towing, or anytime you're not in a hospital quiet zone. :thumbsup:
Mega beer fridge! :beer:
I'll burn that bridge when I come to it.

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