Charging while towing

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Re: Charging while towing

Postby capnTelescope » Sun Dec 22, 2013 3:13 pm

ntsqd wrote:I've been toying with putting a battery back on our TrailBlazer (took them off when the then Optima's were DOA) and have been pondering a charging method.


Battery-inverter-charger-battery2 works. See above. :thumbsup:

Any opinion what caused the Optimas to die young?
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Re: Charging while towing

Postby GuitarPhotog » Sun Dec 22, 2013 4:09 pm

capnTelescope wrote:
ntsqd wrote:I've been toying with putting a battery back on our TrailBlazer (took them off when the then Optima's were DOA) and have been pondering a charging method.


Battery-inverter-charger-battery2 works. See above. :thumbsup:



But it is less efficient than tow vehicle generator -> battery1 -> battery2. Why give away 15-20% of your energy just for convenience and a couple of connectors?

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Re: Charging while towing

Postby ntsqd » Sun Dec 22, 2013 5:43 pm

The point of the large charging cable & connector isn't high current, it is for the low voltage drop that a large conductor will have at relatively low ampacities. See my post on the previous page.

My cheap 150W inverter pulls almost 20A. That is a ~20% loss right there. I agree that stepping up the voltage is a good way to transmit electrical power over a distance (power companies live by this), but the DC > AC and then AC > DC conversion losses are unacceptable to me. Expensive technology can reduce those losses, but there are other, more economical methods.

The Optima's didn't die young. They were dead and unrecoverable when we got the trailer, but had been on the trailer without much maint. for nearly 7 years. Infrequent charging and no solar on board killed them.
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Re: Charging while towing

Postby eamarquardt » Sun Dec 22, 2013 11:32 pm

E=IR. What this means is that the more current that is flowing the higher the voltage drop will be. When you first start to charge a discharged battery there will be significant current flow and the voltage drop will also be significant. However, as the battery voltage climbs the current will drop, and with it, the voltage drop will drop. So depending upon how long you drive it may or may not make much of a difference on how high your battery will get charged when using a wire that is, to one degree or another, undersized.

If you start out with 12 gauge wire, a twenty foot run, nominal voltage of 12 volts, and 20 amp current you get a voltage drop of 1.27 volts, 10.58%, and end up with 10.73 volts. That is significant.

I've never seen a battery take that much current for more than a few moments when connected to any battery charger I've ever had.

If you get the battery charged up a bit and the current drops down to one amp the voltage drop drops to 0.064 volts, 0.53%, and you end up with 11.936 volts. Hardly what one would consider significant.

The voltage drop for a 16 gauge conductor running 12 volts at one amp for twenty feet at one amp is .16 volts, 1.33%, and you end up with 11.84 volts. Again, IMHO, an insignificant voltage drop.

See: http://www.calculator.net/voltage-drop- ... &x=64&y=14

It's kinda like the wire is a voltage regulator which will allow the charging voltage to climb as the battery becomes more and more charged. Yup an undersized wire may slow things down a bit but I don't think the effect is nearly as important as some folks do. As as the battery voltage increases the charging current is gonna drop more because of the batteries state of charge and the maximum voltage of your tow vehicle's alternator/regulator than because of the voltage drop in the wires used to charge the battery.

You gotta remember that the battery in your tow vehicle doesn't normally require a lot of voltage to drive a high charging current because is isn't designed to be discharged more than small amount when starting the engine. My vehicles usually start with about one second of cranking time. If you assume yer starter draws 200 amps that works out to be 0.055 amp hours. That means at even one amp your battery will require about three minutes to replace the juice you used to start the car. Yup you have a 60, 80, 120, or whatever amp alternator but that current is never directed at the battery but to the electrical loads of your vehicle. The current the battery receives is typically a small fraction of your alternator's capacity even when the battery voltage is really low.

I kinda think one ought to have a good quality dedicated 120 volt charger for one's trailer. When you leave home your battery ought to be fully charged. Relying on your tow vehicle as your only means of charging when camping seems a bit inefficient and less than optimum to me. Better to have some method of charging your battery properly over the course of the day (not exceeding a current that is 10% of your battery's amp hour capacity). The best alternatives, IMHO, are shore power and solar power. A generator or tow vehicle will require a lot more run time than is, again IMHO, cost effective, convenient, and easy on the ears.

Just my two scents.

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Re: Charging while towing

Postby capnTelescope » Sun Dec 22, 2013 11:39 pm

Hi, GuitarPhotog and ntsqd. Yes, the Battery-*nverTer-CHarger-batterY2 (follow the caps) method is inefficient. But what are we doing here? Having an efficiency contest? If we are, I lose. I just want to charge my battery. AND I want to charge my $150 AGM deep cycle battery the way it wants to be charged, not by any old way to throw some watts back into it. It ain't what it used to be, but $150 is still a goodly chunk of change in my book. I want to take care of my expensive battery. Also the semi-expensive hunnert-buck TV battery, too.

There are a couple of convenience considerations here. One is the convenience of not having to remember to disconnect or turn something off when I turn off the TV. The second is the convenience of having the charger on board to plug in at a powered campsite or at home between trips, to give the Tear battery a nice big drink of electrons. Since I am going to have my charger on board anyway, I might as well use it while I'm driving.

So We're all working to the same end, but with different goals along the way.

The B*TCHY method works, (which is important) and works best for someone who's traveling all day and camping for the night before moving on. For a long-term boondocker who wants to throw some quick juice into the battery, the B2B method is just fine.

Important points I've learned about battery charging are: 1. Don't parallel batteries of different sizes/types/chemistries/ages/etc. So, for instance, it's fine to parallel two of the same batteries in your Diesel pick-up. It's even standard equipment on some makes. Otherwise, battery isolation is necessary to prevent over/under charging. 2. The average factory charging system on cars and light trucks is best suited to quickly recharging the starting battery, but is completely ignorant of how to charge an AGM, for example. Maybe there's an app for that, but it's not in your stock Engine Control Module. 3, Automotive batteries are not well suited to deep cycle camping usage. 4. Deep cycle batteries are not well suited to automotive charging systems. 5. I know from experience that a smart charger with a maintenance cycle will prolong the life of a battery that gets occasional, not daily, use. 6. There are battery isolators, and then there are Battery Isolators. The first kind are less expensive, but are lossy in their own right. The ones with FET power transistors are low-loss, except for your wallet.

So, in conclusion, You guys win the efficiency award, I get to use my battery for longer. You pays your money and you takes your choice. I hope I answered your questions.

I see Mr. Marquardt posted while I was expounding. I'm going to go see what he has to say. :)
Last edited by capnTelescope on Sun Dec 22, 2013 11:56 pm, edited 1 time in total.
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Re: Charging while towing

Postby capnTelescope » Sun Dec 22, 2013 11:50 pm

eamarquardt wrote:... Just my two scents.


Yeah, like he said. :thumbsup:
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Re: Charging while towing

Postby capnTelescope » Mon Dec 23, 2013 12:09 am

@ntsqd -- Yes, voltage drop at high current can be important. For example, the Chrysler 300/Dodge Charger & Magnum of the just past generation had their batteries in the trunk. The red wire going to the starter was as big around as my thumb. What that wire must have cost! $> :frightened:

Can't have low voltage burning up your starter motor. Leads to customer dissatisfaction.
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Re: Charging while towing

Postby dean_petley » Mon Dec 23, 2013 6:16 am

capnTelescope wrote:

@ Dean -- Your photog friend's setup is pretty ingenious, but I bet it really impacts gas mileage Something like the 4WD penalty you pay whether you use 4WD or not.



i honestly dont think old mate was worried about mileage his tow car was an OKA
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i also remember hearing the sweet sound of a big block as he drove out it was one hell of a set up
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Re: Charging while towing

Postby bdosborn » Mon Dec 23, 2013 11:17 am

You know, I charge while driving all the time.

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Of course I'm only about 16% efficient, does that mean I win? :lol:

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Re: Charging while towing

Postby ntsqd » Mon Dec 23, 2013 11:19 am

I don't have to remember to turn on or off anything either. By using a Voltage Sensing Relay or Automatic Charge Relay it makes or breaks the connection depending on if the alternator is charging or not. Simple to connect, efficient, and dedicated - no remembering to grab the charger out of the garage. Cost is about the same as having a second charger.

The above calcs forget two important numbers, minimum charging voltage for most batteries (13.6 VDC) and the typical alternator charging voltage (14.4 VDC). Apply those voltage drops to these numbers and notice that you're not always charging even though the alternator is trying to. By simply going to an 8 ga. or larger wire this problem pretty much goes away entirely. Strongly suggest reading the Handy Bob blog.
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Re: Charging while towing

Postby capnTelescope » Mon Dec 23, 2013 12:44 pm

@Dean -- Holy smokes! That is a serious TV. Does it get miles per gallon or gallons per mile? What do you metric people call gas mileage, gas kilometerage? :)

bdosborn wrote:You know, I charge while driving all the time.

Even at night? :lol: On the plus side, you don't even have to be driving. :thumbsup: This is the Cadillac way to go, IMO.

bdosborn wrote:Of course I'm only about 16% efficient, does that mean I win?

Since you don't suck any juice from the TV, you're more like 116%. I think you win the Most Efficient category, hands down. :applause:

ntsqd wrote:By using a Voltage Sensing Relay or Automatic Charge Relay...

... If you know such things exist and how to hook it up and where to get them. Maybe you could write this up and show us a schematic so someone who's interested could do it? I nominate this for the Most Highest Tech Solution. :applause:
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Re: Charging while towing

Postby bdosborn » Mon Dec 23, 2013 1:11 pm

ntsqd wrote: using a Voltage Sensing Relay or Automatic Charge Relay...


... If you know such things exist and how to hook it up and where to get them. Maybe you could write this up and show us a schematic so someone who's interested could do it? I nominate this for the Most Highest Tech Solution. :applause:


Just go back to page 8 of this thread. :D

Here's a thread where I posted some wiring diagrams:

Yandina Battery Combiner
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Re: Charging while towing

Postby ntsqd » Mon Dec 23, 2013 1:42 pm

capnTelescope wrote:
ntsqd wrote:By using a Voltage Sensing Relay or Automatic Charge Relay...

... If you know such things exist and how to hook it up and where to get them. Maybe you could write this up and show us a schematic so someone who's interested could do it? I nominate this for the Most Highest Tech Solution. :applause:


I don't know that they're all that high tech. They're certainly not complicated to install.
A couple links to ACR's/VSR's,
BEP Marine VSR's:
http://www.delcity.net/store/Voltage-Sensing-Relays/p_801489
Blue Sea ACR:
http://www.bluesea.com/products/category/Automatic_Charging_Relays
In either case I would recommend the dual sense version if solar is present and if the TV & TD are going to stay connected. The reason being that if/when the solar has brought the TD battery to full charge and there's more day to go it can then also bring the TV battery(ies) up should they need it. Otherwise the single sense version is most appropriate.

For basic service the wiring it simple. A wire in from the charge source and wire out to the battery, plus a small ground wire for voltage reference. Don't forget to include the necessary breakers. The first link above is a good source for a lot of low voltage electrical supplies.
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Re: Charging while towing

Postby rick e » Mon Dec 23, 2013 9:47 pm

Way to much information here for me. I read through most of the pages without nodding off. I am premature to be concerned about this but it has been in the back of my mind and recently discussed elsewhere.
My HHR has the battery in the back which is convenient. My plan has been to find a connector, preferably a 2-flat. Run about a 10 wire from the battery through the connector to the battery on the tongue of the tear. Not much distance there. This thread has told me I need a 30amp fuse near both batteries.
Am I missing anything? A switch maybe? Can I accomplish the charge with smaller gauge wire?
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Re: Charging while towing

Postby capnTelescope » Mon Dec 23, 2013 10:29 pm

bdosborn wrote:Just go back to page 8 of this thread.

and
ntsqd wrote:A couple links to ACR's/VSR's,


Both reference applications that combine batteries. Now, if you are ok with charging your deep cycle battery directly from an automotive system, then you are fine. If, as in my case, you want to use an AGM deep cycle battery and you want the correct charging method for that battery, the combiner strategy is a no go. Because I will have an intelligent charger/maintainer permanently installed in my Tear anyway, why not take advantage of it while on the road?

I've said before, we have different goals. The goals require different approaches. Costs are similar, unless you plan on not buying a charger. I'm giving up some efficiency for charge cycle. Y'all are giving up charge cycle for efficiency.
I'll burn that bridge when I come to it.

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