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Topic ClosedCharging your battery from your tow vehicle

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offgrid View Drop Down
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Direct Link To This Post Topic: Charging your battery from your tow vehicle
    Posted: 04 Sep 2018 at 11:36am
Charlie, you are correct, i wouldn't attempt to run a 3kw inverter on the Highlander starter battery with the engine off. Especially running the air conditioner which takes 1750 watts so will consume about 170Adc at 12V when running when you figure in inverter and conductor inefficiencies. Even with the TV charging I don't expect to get more than about 80-90A out of the alt so the remaining 80-90A would need to come from the starter battery. 

i would be depending on the duty cycle on the aircon and buffering by the TV batt to deal with the peak demand and hoping the aircon cycles off before the start battery dies. IIRC when I measured this the aircon was on about a 50% duty cycle, 6 mins on, 6 off. And the darn thing never really shuts off, the fan keeps running continuously consuming about 350 watts. That was with ambient conditions of about 85F/85% humidity, a typical summer day here in coastal NC. RV roof aircons are really inefficient, not to mention extremely noisy. 

6 minutes with an 90A deficit is only about 9AH so the start battery capacity should be OK. But if the duty cycle goes much beyond 50% then I'd be in trouble pretty quickly.  So, I'm leaning back to placing the inverter in the rPod connected to the rPod batts and directly charging the rPod battery from the TV. More available battery capacity to buffer duty cycle variations by the aircon. I still would be fighting a losing battle after more than a couple of hours at anything over about 50% duty cycle on the aircon. At 100% duty cycle I'd only have about an hour even while charging, I have 220AH but don't like taking my batteries lower than about 50% SOC. 

Not sure the idea of connecting two vehicles together with solid copper busbar works?  Might need to add blocks and leveling jacks to the TV to get everything to line up Big smile. Think I'll stick with my anderson connectors and 1/0 welding cable between the two vehicles and 4/0 between the inverter and the trailer battery. 
 
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Direct Link To This Post Posted: 04 Sep 2018 at 11:47am
Originally posted by offgrid

Not sure the idea of connecting two vehicles together with solid copper busbar works?  Might need to add blocks and leveling jacks to the TV to get everything to line up Big smile. Think I'll stick with my anderson connectors and 1/0 welding cable between the two vehicles and 4/0 between the inverter and the trailer battery. 
 


Maybe some soft drawn 3/4" copper plumbing pipe - with the ends hammered flat and a hole drilled to fit the battery posts????   

At the risk of speaking for Charlie, I think he is saying to use the biggest/least resistive conductor possible.   

Great conversation, all! It makes me do something I rarely do....think!   

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Direct Link To This Post Posted: 04 Sep 2018 at 12:05pm
There is, take a look at the Victron Argofet 200-2. Its FET based so very low voltage dops.

https://shop.pkys.com/Victron-Energy-Argofet-200-Amp-Diode-Battery-Isolator-for-two-batteries_p_3636.html?gclid=CjwKCAjw2rjcBRBuEiwAheKeL4NfSkOX3nhFHss2dc-bUVLC7fDr1CBkZdsTLpFT5gxg3mBo5p172BoCxEcQAvD_BwE

Of course its up to you and will depend on your actual daily amp hour usage, but I'd suggest never letting your lead acid batteries drop below about 50% SOC. Lower than that will kill their lifetimes really quickly. 

To get an idea what your daily consumption is, 50% should be right around 12.2V if you completely disconnect the battery from any loads or charging sources and wait about 30 minutes for it to reach chemical equilibrium. So you could start with a fully charged battery, run your normal house loads for 24 hours, then disconnect and wait half an hour, then measure voltage. Should be about 12.4V at 75% SOC, 12.2 at 50%, and 12.0 at 25%. 

In my case, DW is driving the air conditioning requirement. The only campgrounds near here with hookups are private ones which she doesn't like, she wants to stay at the national seashore campgrounds which are quiet and right on the beach. And she refuses to camp in this area in the summer w/o air conditioning. Can't blame her for that.  

Happy wife, happy life so I'm working toward a solution to the air conditioning problem. The good news is that the Highlander is whisper silent when idling, so the generator noise issue at least would be solved. The other approach I'm considering is major surgery: replace the roof air with a high efficiency inverter mini split, replace the 12V 200AH lead acid batteries with a 24Vdv, 250AH lithium battery pack from a wrecked Tesla. 24V inverter, 24 to 12V step down dc/dc converter to run the trailer 12V loads, 12 to 24 boost dc/dc converter to charge the Tesla batt from the Highlander, plus 1kW of solar modules, which can now fit because the roof air is gone....You get the idea. 

A full time RVer with a 5th wheel has done most of this, link is here if anyone is interested. 

https://www.mortonsonthemove.com/toms-tech-stuff







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CharlieM View Drop Down
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Direct Link To This Post Posted: 04 Sep 2018 at 12:21pm
Originally posted by offgrid

Not sure the idea of connecting two vehicles together with solid copper busbar works?  Might need to add blocks and leveling jacks to the TV to get everything to line up Big smile. Think I'll stick with my anderson connectors and 1/0 welding cable between the two vehicles and 4/0 between the inverter and the trailer battery. 
 


Yes, some leveling would be required. The TV must be slightly higher to let the electrons run downhill. Tongue
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Direct Link To This Post Posted: 22 Sep 2018 at 9:27pm
Originally posted by offgrid

As others here have said, unless you're using some type of advanced battery technology, using a solar charge controller to regulate voltage from the TV alternator to your trailer battery is unnecessary. Modern alternators are internally regulated to 14.4V anyway, so at best they will provide the same bulk charge regime as your WFCO.

The reason for using the solar charge controller has nothing to do with regulating voltage.  It regulates amperes (amps).  Deep cycle batteries are not the same as starting batteries.  Deep cycle batteries want lower amperage when charging and for longer periods than do starting batteries, which want a high amperage charge for a short time.

By using jumper cables, you're dumping raw current from the TV battery (not just the alternator) into the Pod's battery.  Yes, this will charge the Pod's battery, but it's not the 'healthiest' option for your battery.  But if you aren't doing it often, it probably doesn't matter too much.  If you're using your TV as a charging source often (beyond charging while driving, obviously), the solar charge controller is better.

Remember that deep cycle batteries do best with multi-stage charging (which is what your WFCO does but jumper cables do not), not by dumping raw current into them.
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Direct Link To This Post Posted: 23 Sep 2018 at 7:53am
I'm sorry WillThril but it is not correct to say that chargers regulate current. 

Chargers work by regulating voltage. For example, the WFCO bulk mode is regulated at 14.4V, absorption at 13.6V, and float at 13.2V. This is clear from their literature. The current the battery takes in each mode is what it takes to maintain that voltage, up to the maximum capacity of the charger in bulk mode. High quality PV PWM charge controllers do the same thing, up to the available current from the PV module, except some add battery temperature compensation to adjust the setpoints for each charge regime.

You can consider that directly connecting the TV system to the trailer is the equivalent of bulk charging, because the alternator limits the voltage in the TV circuit to 14.4V. However in practice series resistance in the connection will limit the voltage that the trailer batteries can be charged to to well below 14.4V. 

The battery chemistries in SLI (starter, lights, ignition) batteries and deep cycle golf cart batteries are not different. Consider that pretty much all class A,B, and C RV's, as well as cruising boats all charge their hours batteries this way with no ill effects.   If someone were to say that they were going to leave their TV idle for 24 hours straight to try to float charge their trailer batteries I would agree with you that that is not a great idea. Its not great for the SLI battery either, both would require frequent addition of water if run that way. 

But no one is going to want to do that.  The purpose of TV charging is the same as most generator charging, to quickly recharge a discharged battery to roughly 80% while on site. Charging beyond that is a waste of fossil fuel and should be left to either the on board ac charger or a solar charging system. But if neither of those are available at the campsite thats ok for a few charge cycles, which is all most of us would ever need this for. 

You are correct that by connecting to your TV, you are connecting both the alternator and the TV battery in parallel to the trailer battery. But the actual charge will be limited to the net available current from the TV alternator after it has supplied the requirements of the TV itself. Which is why you should of course shut off as many electrical loads on the TV as possible when charging this way. This current will vary based on the vehicle and its alternator capacity at idle (typically much lower than its rated charge capacity). Probably for most of us this will be tween 50-100 amps or so. Mine is about 70-80A.

There is going to be a short period of higher current flow when the initial connection is made, just like you see when you jump start a car. This is caused by the two batteries being at different voltages and is again limited by the series resistance in the circuit. This initial current might be as high as 200-300 amps for a short period. 

And that leads me to the concern I have with adding a solar charge controller into this circuit. PV modules are current limited devices. They can't put out more current than their rated short circuit current (Isc) even when the leads are directly shorted while the module is in full sunlight. PV charge controller designers know this and design their controllers for a maximum solar array size/maximum current. So the FET's (field effect transistors) they use to do the PWM switching function have specific current limits.  The controller's maximum current limit is clearly stated in the specifications of each controller. 

Batteries are not current limited. If you connect one of these charge controllers to a battery that can provide a much higher initial current than the FETs in the controller can handle either they will fail or internal circuit protection will engage to protect them. Either way it isn't good. I would be very surprised if a solar charge controller manufacturer would honor their warranty with this usage. 

That's why using a PV charge controller this way is both unnecessary and not within the parameters the controller was designed for. 










 







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Direct Link To This Post Posted: 23 Sep 2018 at 10:05am
Originally posted by offgrid

I'm sorry WillThril but it is not correct to say that chargers regulate current. 

Yes, they do.  The article linked to below from Battery University explicitly says so.  This also depicted in this graph.  Chargers regulate both voltage AND current.



The battery is fully charged when the current drops to a set low level. The float voltage is reduced.  Float charge compensates for self-discharge that all batteries exhibit.
Courtesy of Cadex

The switch from Stage 1 to 2 occurs seamlessly and happens when the battery reaches the set voltage limit. The current begins to drop as the battery starts to saturate; full charge is reached when the current decreases to 3–5 percent of the Ah rating. A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to end the charge.

https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

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Direct Link To This Post Posted: 23 Sep 2018 at 11:19am
That is not inconsistent. The chargers are acting as constant current devices initially in bulk charge mode because as I said they are limited by the maximum current capacity of the charger initially. So they will put out their rated current in that mode until the battery voltage reaches 14.4V, at which point the current will taper off because they are regulating voltage to 14.4V.  Then when the charger determines its time to go into absorption mode the voltage will drop to 13.6. The current will drop off and will be what it needs to be to maintain 13.6. Ditto for float mode at 13.2V. 

If the chargers didn't work that way you would need to change out or reprogram your charger when you added to your battery capacity. Obviously a larger capacity battery needs more current to achieve the same state of charge, but your charger doesn't know that you increased the size of your battery bank. But since the voltage setpoint for each stage remain the same with the larger battery and that's what the charger is controlling, its not a problem, other than the initial bulk charge duration will be longer due to the lower charge rate the larger capacity battery is getting. 

A car alternator does essentially the same thing as charger bulk mode, it attempts to regulate voltage to maintain 14.4V but generally can't initially because it can't supply sufficient current depending on battery SOC and loads on the system. 

Anyway, I'm glad your TV charge method works for you and obviously you haven't had any problems with burnt up solar charge controllers as of yet, so I would be the last person to say you should change it. My interest here is only to clarify the facts and so everyone who reads the thread can make up their own minds what they want to do. 


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Direct Link To This Post Posted: 24 Aug 2019 at 10:19pm
To CharlieM
When using the TV as a charger is it connected as if giving one a jump start  --- plus to plus and minus to minus?
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Direct Link To This Post Posted: 25 Aug 2019 at 6:55am
Yes. Same procedure and safety considerations as when jump starting one vehicle from another using jumper cables. . 
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