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Topic ClosedCharging your battery from your tow vehicle

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WillThrill View Drop Down
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Direct Link To This Post Topic: Charging your battery from your tow vehicle
    Posted: 03 Dec 2017 at 4:30pm
Whether we are boondocking (aka 'dry camping) or just moving from place to place, Podders quickly learn that charging their Pod's battery (or batteries) is very important.  Most of the time, people focus on charging their battery from shore power, a generator, or solar panels.  All of these are viable options that can work, but there is another option for charging batteries that is not discussed often: your tow vehicle.  Some people try to do this by just using the trailer light and brake assembly, but the electrical lines that do this are often small, resulting in minimal flow of power to your Pod's battery.  Also, this is not the multi-stage charging that is recommended for deep cycle batteries; your Pod's onboard converter/charger is a multi-stage charger.  The other problem with this method is your Pod's battery is not isolated from your tow vehicle's battery.  This means that if you leave them connected and aren't running your tow vehicle's engine, the Pod can and will eventually drain both its battery and your tow vehicle's battery.

There is a very inexpensive and simple means, however, of achieving all three of your desired goals, (1) adequate charging capacity, (2) multi-stage battery charging, and (3) isolating the Pod's battery from the tow vehicle's battery.  It is a solar charge controller.

Solar charge controllers are designed to be used in conjunction with solar panels and lead acid batteries.  They normally take power from solar panels and then regulate the voltage and amperage that is sent to the batteries.  They are multi-stage chargers that will not under- or over-charge your batteries.  They also prevent power from flowing from the batteries to the solar panels.

But solar panels are not the only power input that charge controllers can handle.  They actually work perfectly well with alternator systems used in tow vehicles' engines.  When the tow vehicle is running, the voltage output of the alternator is around 14.4 volts.  The charge controller senses that it is receiving power and begins charging the Pod's battery using a multi-stage charging process.  When the tow vehicle is not running, the voltage of the tow vehicle's battery quickly drops to below 13.0 volts, and the charge controller stops.

All you need is a set of wired alligator clips adequate to handle the amperage from your tow vehicle's battery to the charge controller (16 gauge is fine for a 10 amp controller, 6 gauge is needed for a 30 amp controller), a charge controller (30 amps is the maximum you can use with most vehicles based on their output when idling), and another set of wired alligator clips to connect the charge controller to your Pod's battery (for a 30 amp controller, 16 gauge wire would be good for lengths up to 7 feet, while 6 gauge wire would be good for up to 23 feet).  You clamp on a short run (two feet is fine) of wire with alligator clamps from your tow vehicle's battery to your charge controller, and you then attach a longer run with the other set of clamps to your Pod's battery.  Once it's connected, you just start your vehicle and let it idle.  The charge controller will then charge up your Pod's battery.  Note that if your Pod has a Zamp port, you could just connect your charge controller directly into that port with a universal two-wire connector (sold everywhere) and not need a second set of alligator clamps.

AllPowers has a highly rated 20 amp charge controller for sale on Amazon for $18 that would be great for this job.  A set of Battery Tender 6 gauge wired alligator clamps (2' long) sells for $6 each on Amazon.  So the cost for this setup would be just $30, plus however much additional wire you need to run from the charge controller to your Pod's battery, perhaps $10 worth.

This method of charging your Pod's battery has some definite advantages over other charging methods.  First, it works day and night.  As long as you are able to idle your vehicle, you can charge your Pod's battery; this is obviously not the case with solar panels, which don't work when it's cloudy or dark.  Second, the charging capacity for this setup is very good.  A single 100 watt solar panel can only output a maximum of 8.3 amps of charging capacity, and even that is only under ideal conditions, which you will not usually experience if you aren't camping in the desert.  Third, this method is very inexpensive.  For $40, you can get 20 amps of charging capacity (30 amps would run you about $50-55); doing that with solar power would easily cost you $350 or more (and only under ideal conditions).  Fourth, your tow vehicle probably makes less noise than does pretty much any non-inverter generator.

This method does have some cons.  First, your vehicle cannot produce electricity as efficiently as a generator can.  An idling vehicle capable of towing a Pod will probably consume a gallon of fuel every 1-2 hours.  A 2,000 watt inverter generator can easily run 6 hours on a gallon of fuel and output five times more electricity than your tow vehicle.  Second, you cannot idle your vehicle in some campgrounds at certain times, though during these times you are usually prohibited from running a generator either and it's too dark for solar to work.

I'm not saying that this is the best solution for everyone, but for those who don't need to run the Pod's air conditioner or microwave but still need to charge their battery periodically, this may be a much simpler, more cost efficient method than purchasing a very expensive generator or solar panels, both of which take up much more space as well.

And for the record, an electrical engineer I know by the name of Steven Harris showed me how to do this.
"Not all those who wander are lost." Tolkien

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CharlieM View Drop Down
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Direct Link To This Post Posted: 03 Dec 2017 at 7:41pm

An interesting idea and one that I have thought of too. However there are a few problems. The output at the battery terminals of most new vehicles is limited to about 13.6V. By the time you take the line losses of #16 wire you don’t end up with much to charge with. Most inexpensive solar controllers don’t increase voltage; they just regulate by a process called pulse modulation. They are designed to work with solar inputs around 17V.  A better solution would be an MPPT controller but they are much more expensive and I don’t believe they boost available voltage above the solar input either. A third disadvantage, as Willthrill notes, is the TV is very inefficient. With the very limited charge current it will have to run a long time to recharge the battery.

However, there is another way to use the TV as a charger. Connect the TV and TT together with high quality jumper cables. Relatively inexpensive, much easier and much more effective. Using 20 foot copper #4 jumpers I have observed 40A charging current to a partially depleted battery. The current falls of off as the battery charges but the process can be much more effective than even the onboard converter. An hour at idle will generally bring the battery back to 80% and will burn about ½ a gallon of gas. I find that a lot easier an quieter than hauling gas and a generator. It won’t run the A/C or microwave but it certainly recharges the battery after a few days boondocking.  The important thing is using very good jumper cables. Cheap small gauge aluminum cables won’t cut it.

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WillThrill View Drop Down
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Direct Link To This Post Posted: 03 Dec 2017 at 9:00pm
Originally posted by CharlieM

An interesting idea and one that I have thought of too. However there are a few problems. The output at the battery terminals of most new vehicles is limited to about 13.6V. By the time you take the line losses of #16 wire you don’t end up with much to charge with. Most inexpensive solar controllers don’t increase voltage; they just regulate by a process called pulse modulation. They are designed to work with solar inputs around 17V.  A better solution would be an MPPT controller but they are much more expensive and I don’t believe they boost available voltage above the solar input either.

"At idle, most charging systems will produce 13.8 to 14.3 volts with no lights or accessories on (although some may charge at a slightly higher voltage depending on temperature, engine RPM, type of battery, and the battery's state of charge)."

http://www.aa1car.com/library/charging_checks.htm

I've seen this technique used with many vehicles, and it's worked every time.  If the alternator's output voltage is enough to activate the charge controller, the latter can and will charge.  And yes, an MPPT controller would be more effective, though I wouldn't purchase one just for this use.  If you had one already for a solar panel setup, you could use it in this context if you were in a pinch (e.g. solar panels were damaged, you need a charge when it's cloudy or dark).

I would recommend anyone doing this to check the voltage of their alternator while running.  If it's below 14.0 volts, then you should go with heavier gauge wire (e.g. 6 or 8 gauge) and/or a shorter run of wire.

Originally posted by CharlieM

However, there is another way to use the TV as a charger. Connect the TV and TT together with high quality jumper cables. Relatively inexpensive, much easier and much more effective. Using 20 foot copper #4 jumpers I have observed 40A charging current to a partially depleted battery. The current falls of off as the battery charges but the process can be much more effective than even the onboard converter. An hour at idle will generally bring the battery back to 80% and will burn about ½ a gallon of gas. I find that a lot easier an quieter than hauling gas and a generator. It won’t run the A/C or microwave but it certainly recharges the battery after a few days boondocking.  The important thing is using very good jumper cables. Cheap small gauge aluminum cables won’t cut it.


I've used this method before, and it does work.  It's just not as good for your tow vehicle's battery nor your Pod's battery.  When you make the connection, your tow vehicle's battery will start dumping raw current into your Pod's battery, causing the TV battery to discharge very quickly (not good for it on a regular basis), and this type of charging isn't good for the Pod's battery either.  But in a pinch, they will work.  On a regular basis, I wouldn't do this though.  The charge controller option could be used as often as you desired, just requiring you to pop your hood, clamp on to the battery, and hook up to your Pod's battery (30 seconds).
"Not all those who wander are lost." Tolkien

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Direct Link To This Post Posted: 04 Dec 2017 at 5:14pm
CharlieM is right. The better way to do this is with some heavy duty jumper cables. You will avoid the TT battery discharging the TV battery if the TV is running before you connect the jumper cables. many vehicles these days have alternators capable of producing over 100 amps of charging current (our F-150 will do over 200 amps).

There would not be enough power coming through smaller gauge wires as you decribe, and as CharlieM pointed out, the voltage will be too low anyway.
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Direct Link To This Post Posted: 04 Dec 2017 at 8:49pm
Originally posted by GlueGuy

CharlieM is right. The better way to do this is with some heavy duty jumper cables. You will avoid the TT battery discharging the TV battery if the TV is running before you connect the jumper cables. many vehicles these days have alternators capable of producing over 100 amps of charging current (our F-150 will do over 200 amps).

Stock alternators cannot produce more than about 30-40 amps when the vehicle is at idle.  The amperage rating of alternators is when the engine is spinning at high RPM, not at idle.

Originally posted by GlueGuy

There would not be enough power coming through smaller gauge wires as you decribe, and as CharlieM pointed out, the voltage will be too low anyway.

I know for a fact that this does work; again, an electrical engineer showed me how this works and does it himself all the time with a battery bank he keeps in his truck's toolbox.  You just have to get appropriately sized wire.

If the voltage were too low to charge your Pod's battery, it would be too low to charge your vehicle's battery.  Over a short run and with appropriately sized wire, the voltage drop is tiny.

Again, the problem with jumper cables is that they are just dumping raw current into your Pod's battery, which isn't healthy for it and can actually cause it to explode (though this is very unlikely).
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Direct Link To This Post Posted: 05 Dec 2017 at 8:51am
Originally posted by WillThrill

I know for a fact that this does work; again, an electrical engineer showed me how this works and does it himself all the time with a battery bank he keeps in his truck's toolbox.  You just have to get appropriately sized wire.

If the voltage were too low to charge your Pod's battery, it would be too low to charge your vehicle's battery.  Over a short run and with appropriately sized wire, the voltage drop is tiny.

Horsehocky. I have worked with several solar charge controllers, both PWM and MPPT, and none of them will boost the available voltage.

Originally posted by WillThrill

Again, the problem with jumper cables is that they are just dumping raw current into your Pod's battery, which isn't healthy for it and can actually cause it to explode (though this is very unlikely).
They don't dump "raw current". They will produce a current-limited output factored (primarily) by the state of the batter(y/ies) attached. The voltage output will not go above the absorption voltage of a typical car battery. That is a fact.

Of course, you are welcome to try it. I am fairly certain you will be disappointed, but I've said my peace.

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Direct Link To This Post Posted: 05 Dec 2017 at 9:19am
Originally posted by GlueGuy

Originally posted by WillThrill

I know for a fact that this does work; again, an electrical engineer showed me how this works and does it himself all the time with a battery bank he keeps in his truck's toolbox.  You just have to get appropriately sized wire.

If the voltage were too low to charge your Pod's battery, it would be too low to charge your vehicle's battery.  Over a short run and with appropriately sized wire, the voltage drop is tiny.

Horsehocky. I have worked with several solar charge controllers, both PWM and MPPT, and none of them will boost the available voltage.

So you're just calling me a liar?  I won't respond to you further then.
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Direct Link To This Post Posted: 05 Dec 2017 at 9:37am
Originally posted by WillThrill

Originally posted by GlueGuy

Originally posted by WillThrill

I know for a fact that this does work; again, an electrical engineer showed me how this works and does it himself all the time with a battery bank he keeps in his truck's toolbox.  You just have to get appropriately sized wire.

If the voltage were too low to charge your Pod's battery, it would be too low to charge your vehicle's battery.  Over a short run and with appropriately sized wire, the voltage drop is tiny.

Horsehocky. I have worked with several solar charge controllers, both PWM and MPPT, and none of them will boost the available voltage.

So you're just calling me a liar?  I won't respond to you further then.
Let's not go off the rails here; I wasn't calling anyone a liar.  Fact is I am an engineer, so I don't have to consult with one. I have also worked with many different solar controllers, and I understand how they work. What I am saying is that there is something that you (or we) are not understanding here. That's not calling anyone a liar.

Let us know how your experiment works.
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Direct Link To This Post Posted: 05 Dec 2017 at 10:25am
Originally posted by GlueGuy

Originally posted by WillThrill

Originally posted by GlueGuy

Originally posted by WillThrill

I know for a fact that this does work; again, an electrical engineer showed me how this works and does it himself all the time with a battery bank he keeps in his truck's toolbox.  You just have to get appropriately sized wire.

If the voltage were too low to charge your Pod's battery, it would be too low to charge your vehicle's battery.  Over a short run and with appropriately sized wire, the voltage drop is tiny.

Horsehocky. I have worked with several solar charge controllers, both PWM and MPPT, and none of them will boost the available voltage.

So you're just calling me a liar?  I won't respond to you further then.
Let's not go off the rails here; I wasn't calling anyone a liar.  Fact is I am an engineer, so I don't have to consult with one. I have also worked with many different solar controllers, and I understand how they work. What I am saying is that there is something that you (or we) are not understanding here. That's not calling anyone a liar.

Let us know how your experiment works.

You said "Horsechocky."  That's equivalent to calling me a liar.  I've already told you that I've seen this work.  And an electrical engineer is the one who showed me how to do it.
"Not all those who wander are lost." Tolkien

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Direct Link To This Post Posted: 05 Dec 2017 at 1:02pm
Will, I thought about this as well. My intention is to add a power outlet in the bed of my Tacoma. One reason is the hopes of running a solar charge controller on it for recharge when traveling.

My case, I will probably run a 12g wire back (should be over-kill) and see just what I have back there for voltage.

I have both PWM & MPPT charge controllers at the house from my house solar experiments. Which I use will depend on the voltage I read.

I have to dis-agree with using 'jumper cables' battery to battery. Yes through a charge controller. I personally don't like un-controlled charging.

MHO on the matter.
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