Charging TT Batteries with TV

[Omnial]

Hi,

I know this overlaps with many other threads but I want to verify my understanding of limitations and potential solutions with regards to charging TT batteries with your TV.
Note: I loath to run my generator due to noise.

I have a 2019 Globetrotter (love it!). Soon I'll start upgrading various components with the goal of increasing my amp hr capacity and ability to charge.
I have the factory solar option installed (160W, I think). My plan is to pursue the following steps over the next few seasons:

1. Add a 90W panel to the roof; replace factory solar charge controller to Victron MPPT 100/30, reroute the external zamp plug by the battery box to run parallel with the roof panels, into the new MPPT, and use my existing 100w portable panel (no controller) in that zamp plug when needed. Also, install Victron BMV-712 Smart battery monitor.

2. Some day... replace AGMs with lithium (e.g. Battleborn, Relion, etc.). Possibly upgrade charger too.

Now for my question: it seems like charging from a TV really does not work all that well, which seems like a shame since it should be a source of lots of free power, especially when in motion. I have a 2018 F150 which has some brains to decide when it wants to send charging power through the 7 pin connector, but even when it's flowing, it's only a few amps, mostly due to inadequate wire size. Also, i understand from reading the forums that the alternators will cut back on the amps once the TV battery is sufficiently charged, again meaning the TV is not a reliable source for charging through the existing 7 pin connector.

Am I missing some other solution that would make the TV a good, efficient source of charging power both when towing and when parked (e.g. boondocking)? I've read about installing second alternators, which I'm not interested in doing.

I've also read about dc-dc converters, which seems like it should work, right up until the point the alternator cuts power because the TV battery is full.

At one point I was considering as simple solution of purchasing a 2000 watt inverter which I would connect to my TV battery with jumper cables and plug in the trailer shore power cable into it. But that seems very inefficient and is only workable when parked. And even then, I don't see how this would work if the alternator cuts power when the TV battery is full.

I'm a newbie but am eager to learn! What say you?

Thanks.

[Al and Missy]

If you really want to do this, I see two ways - DC to DC converter, and a medium sized inverter connected to the TV battery posts.

While TV alternators are large, I think I recall that a wet cell battery should be charged somewhere between .1 and .25C, where C is the amp-hour capacity. Making the assumption that a Group 27 TV starting battery has the same C as a deep discharge RV battery (it probably has less) that is at most about 100 Ah. So it should be charged at around 25 amps. All those remaining amps are to run the loads in the vehicle - lights, blowers, pumps, and let's not forget the computers. My point is if you are not running a lot of accessories, there should be ample power left over to run an inverter that you could drive the TT converter with. If you only need, say, 25 amps at 12 volts, that is 2.5 amps at 120 volts or 300 watts. The inverter could be placed under the hood temporarily and a standard extension cord run to the trailer shore power input. This beats having to run large wire battery to battery or DC-DC converter to battery. All the above numbers ignore efficiency losses, but using a TV to charge the TT batteries is clearly not the most efficient approach.

I'm definitely not recommending this approach, just pointing it out if you are dead set to use the TV to charge the TT.

My preferred approach would be a Honda EU 2000 or 2200 (or even a 1000 if you can find one at a good price and just want to charge batteries).

Al

[wh500special]

I’m new to the RV world, but I’ve been wondering myself if either a Voltage Sensative Relay (“combiner”) or a marine on-board alternator charger and an extra set of quick-connect leads between the TV and trailer would be a good way to harness extra amperage from the alternator.

https://www.amazon.com/BEP-Digital-V...89767946&psc=1

https://minnkotamotors.johnsonoutdoo...ard-alternator

These things are compact and can be mounted under hood (or elsewhere) and power cables routed to the back of the TV. When underway and once the TV battery is topped off, the module would switch and connect the trailer battery to the TV and charge it. When the vehicle is stopped, the batteries would decouple.

The wiring in the trailer might need to be modified to isolate the original power feed in the trailer connect from charging the TT batteries.

Maybe use Anderson connectors or
Something similar to connect between TV and trailer.



I have a setup like this in my fishing boat to help rejuvenate the deep cycle trolling motor batteries when the outboard is running. It works well, but the available current from my outboard isn’t huge.

The advantage of the VSR is that you could get all of your TV’s alternator’s available current to the trailer. The disadvantage might be that you could overwork the alternator.

The onboard alternator charger advantage is that you can limit the current somewhat and provide more robust isolation when parked.

I don’t see why these options wouldn’t work. But I’ve not seen it mentioned before.

Parked, I don’t have a solution for you other than a dedicated 120v charger.

Steve

[Shermy1987]

You use something like this.
https://battlebornbatteries.com/shop...wered-charger/

Alternators do a terrible job of fully charging a battery.

[skido23]

Two summers ago I took a trip cross country for a month. I dry camped at state parks. I found that if I backed my truck close to the trailer after a days outing and reconnected the 7 pin connector that the battery would receive enough recharge from the truck battery to enable it to run the fridge all the next day. The trailer battery never ran down and the truck battery started the truck easily the next day. I had a backup 800 watt honda generator but never used it.

When driving down the road you need to run the fridge on LP and have the trailer switch put on "in use". Otherwise the fridge won't work since these newer fridges have a circuit board which controls the thermostat in the fridge.

I understand that some of the older fridges did not have these circuit boards and therefore did not need a constant source of 12 volts to run properly.

[azflycaster]

Originally Posted by skido23

Two summers ago I took a trip cross country for a month. I dry camped at state parks. I found that if I backed my truck close to the trailer after a days outing and reconnected the 7 pin connector that the battery would receive enough recharge from the truck battery to enable it to run the fridge all the next day. The trailer battery never ran down and the truck battery started the truck easily the next day. I had a backup 800 watt honda generator but never used it.

When driving down the road you need to run the fridge on LP and have the trailer switch put on "in use". Otherwise the fridge won't work since these newer fridges have a circuit board which controls the thermostat in the fridge.

I understand that some of the older fridges did not have these circuit boards and therefore did not need a constant source of 12 volts to run properly.

While that might be a good solution for some. Many vehicles have a relay in the charge line that shuts off power to the trailer when the truck is not running. My Tundra is designed that way.

[larry9000]

Originally Posted by Omnial

Hi,

2. Some day... replace AGMs with lithium (e.g. Battleborn, Relion, etc.). Possibly upgrade charger too.

...

I've also read about dc-dc converters, which seems like it should work, right up until the point the alternator cuts power because the TV battery is full.

Thanks.

Yes- you will need some other technique other than a straight wire from your TV to charge your TT lithium battery. I measured current flow between my lithium-upgraded Bambi and my Toyota 4runner going the wrong way - the TT was charging the TV!. This is due to lithium's naturally higher cell voltage than lead acid. I solved this with my lithium system by installing a Victron Orion TR 12/12- 18A. It is an isolated DC-DC converter that supplies up to 18 amps of charging current to the lithium battery. A screwdriver is used to tune the converter to the correct charging voltage for lithium. It was a modest cost compared to the rest of the system. It will work regardless of the state of the TV alternator, since it is drawing from the TV battery directly. The alternator will keep the TV battery charged. I don't have this running all the time, as I have plenty of solar power on top. I have a switch in the Bambi to enable it if I need it. After evaluating all the alternatives, I chose this path and I'm glad I did.

[GM Airstream]

Oh wow... you have actually a couple of problems using the EXISTING PU TV as a charger.
First the factory wiring is WAY to light... i.e wire size... for charging the RV bats. at the distance it has to go. You need to run a No. 8 or 6 wire from a NEW Solenoid at the TVs battery to the rear 7 pin plug. (and from the plug to the RV bats if not No. 8 or larger)

This then will help reduce the resistance of the distance to the RV bat for charging. (check the etrailer video about wiring your PU for towing)

While the factory does wire it up.. as the tow package... it is at best WEAK. If you have all day run'n it will eventually charge the RV bat but if their is any electrical load on at the RV it won't ...due to what is called IR loss in the charge line.

Most of the time changing this and the wire size will then allow the RV bat to charge up.

why a new solenoid? The factory tow package has it built into the vehicle but its only rated for about 20 amps. now while 20 amps is a current... it becomes too light when the RV bats have been depleated. You can see most normal bats draw about 35 to 40 amps for the first several min of recharge time (at the battery post).

So again the factory wiring is way too light to handle this current. You will lose the current and voltage in the TVs factory wiring due to size and distance before it gets to the RV battery post.

The new solenoid that is common used... is rated for about 80 amps. It is hooked up to the 'run' side of the ignition key so that when the TV is not run'n you don't depleat the TVs start battery.

Once you start the TV , the solenoid then closes and you get charge current /voltage back to the RV bats. We also fuse (100 amps) from the charge line at the source... i.e from the TV battery tie in point... to the solenoid for short circuit protection.

inverters on 12 volts don't work for boosting the 12 volts up to recharge the RV bats.

Good low loss wiring is your solution to charging the bats while under way. (we have measured over 50 amps ... which is the limit for No. 6 wire at 12 volts dc....to the RV bat when first started charging from the TV.)

As to run'n the refrig on 12 volts while run'n down the road... DON'T...
most of them draw about 15-20 amps 12v. DC.

While the tow vehicle can produce it... it takes HP from the engine to drive the alternator. This puts a load on the drive belt on the engine as well as makes the alternator work harder all the time and shorten its internal brush life. Not to mention lowering your gas mileage.

One other thing that you cause when run'n on 12 volts... is that when the TV is off or id'ling... your drawing from the RV bats if you don't have the cut off wired in... so instead of charging them your actually discharging them through the refrig...

Best to run the refrig on propane... but, remember you will destroy the boiler section in the refrig if you don't keep it level when you stop.. and its illegal in some states to have the propane tanks open while on the road... let alone risky of a fire in the RV or at the gas station.

Most of the time if you 'flywheel' the refrig when on shore power of overnight.. it will keep the innards cold enough without being run while your under way... That is the way we do it... full on overnight.. off for the 4-6 hours of towing a day... as the box will remain closed during that time and will maintain a cold temp inside.

Again... the solution to your ability to recharge the RV bats from the TV is to get the charge line re-wired. The rest of the wiring except for the brake line...is OK because we have gone to LED lights on the RV which doesn't add much to the current draw from the TV.

The Adventure Contenues...

[Omnial]

Originally Posted by larry9000

Yes- you will need some other technique other than a straight wire from your TV to charge your TT lithium battery. I measured current flow between my lithium-upgraded Bambi and my Toyota 4runner going the wrong way - the TT was charging the TV!. This is due to lithium's naturally higher cell voltage than lead acid. I solved this with my lithium system by installing a Victron Orion TR 12/12- 18A. It is an isolated DC-DC converter that supplies up to 18 amps of charging current to the lithium battery. A screwdriver is used to tune the converter to the correct charging voltage for lithium. It was a modest cost compared to the rest of the system. It will work regardless of the state of the TV alternator, since it is drawing from the TV battery directly. The alternator will keep the TV battery charged. I don't have this running all the time, as I have plenty of solar power on top. I have a switch in the Bambi to enable it if I need it. After evaluating all the alternatives, I chose this path and I'm glad I did.

Thanks.
Where did you install the Orion? In trailer or truck? And did you upgrade the wiring to handle the current load as other's have suggested in this thread? If so, can you use the existing 7 Pin connector or is there some other way to get the power from Truck to TT? Finally, do you run your solar feed and the Truck/Orion feed into the TT batteries simultaneously or does that 'confuse' the Solar charge controller?

[larry9000]

Originally Posted by Omnial

Thanks.
Where did you install the Orion? In trailer or truck? And did you upgrade the wiring to handle the current load as other's have suggested in this thread? If so, can you use the existing 7 Pin connector or is there some other way to get the power from Truck to TT? Finally, do you run your solar feed and the Truck/Orion feed into the TT batteries simultaneously or does that 'confuse' the Solar charge controller?

No, I didn't change anything in the TV. My 4Runner has a 30A limit. The Orion has an 18A limit, so it will not exceed the limit of the 4Runner. If you are worried about the IR drop (which equates to a voltage drop), you can make the calculations. Estimate the length of the wire going to the 7-pin plug, and then to wherever the Orion unit is mounted. Then calculate the resistance of the wire by looking up the Ohms/foot of that gauge. Then multiply that value by 18, the maximum number of Amps out of the Orion. Maybe 20 for the Orion inefficiencies. That calculation is how much the voltage will drop before getting to the Orion. Remember, V=I x R. The Orion has a range of input voltage it can operate over. See if this voltage drop takes it out of range. Example: if you have 20 feet of 10 AWG wire, which has a resistance of 1 Ohm per 1000 feet, you have 0.02 ohms of resistance. Times 20 amps = 0.4 volt drop. But the Orion can convert voltages as low as 8 volts. A 0.4 volt drop in the wiring won't hurt it at all.

I mounted the Orion in the TT, under the dinettes seat in my Bambi, alongside the lithium battery. Before I installed the Orion, I had disconnected the charging wire going to the battery in the TT, and just used the solar charging from the panels on the roof. Otherwise, the lithium battery would start charging the TV battery after a few minutes. (For the first few minutes, my truck idles quickly and the alternator voltage was high enough to charge the lithium battery. Once that period is over and the RPMs decrease, current flowed the wrong way, even at highway speeds). Your friend here is the BMV-700 series of battery monitors. It is so easy to monitor the direction and amount of the charging current. For initial operation, I had my iPhone in the cab of the TV so monitor the current. This is how I found out I had a problem before installing the Orion. It's also great for testing the Orion and whether the TV can charge the battery while going through it.

[larry9000]

Originally Posted by Omnial

Thanks.
Where did you install the Orion? In trailer or truck? And did you upgrade the wiring to handle the current load as other's have suggested in this thread? If so, can you use the existing 7 Pin connector or is there some other way to get the power from Truck to TT? Finally, do you run your solar feed and the Truck/Orion feed into the TT batteries simultaneously or does that 'confuse' the Solar charge controller?

Whoops, I didn't answer the last part of your question. I actually have two solar chargers in addition to the Orion. One for the panels on top, and another for optional suitcase panels. Both are PWM controllers. No, I don't have any issues, except near full charge when one controller may think the battery is fully charged as it starts taking less charging current. The others keep charging. But this is not a problem when the battery is not near fully charged- all controllers add current simultaneously. The entire reason I have an Orion is for the situations where I need an emergency charge from the TV, so this works well for me.

[GM Airstream]

a SECOND solution for the charging ckt... is to run TWO wires from the charging point.. i.e battery through the new selo... By using two smaller wires you can achieve the same results as one large one... saving in cost.

So if their already is a 30 amp charge line... that is along the frame of the TV... you can add a second No. 8 wire and get a larger charging current... by not having the voltage drop at the battery that it needs.

(you start off with 14.7 volts at the TV battery charging... but due to the length of wire going to the RV... you are luck to get 13 volts due to IR of the wire and length... which is not enough to recharge a depleated battery. )

but remember... nothing is free.. your power demand at the source is going to be greater than what is consumed at the end of the line.... its called IR loss... and what in effect your doing is developing resistance in the wire when you demand more current at the end. So where does the extra power go... easy.. its heat disapated in the wire along the length...

What you want is not to have much more than a volt or two loss along the WHOLE line coming from the TV to the RV... so you need to tape measure out how long that is.. and then go to the WEST MARINE web site and plug in the numbers... their it will tell you how big a wire size for the current needed... in the RV.

I'd figure that the initial charging current drawn through the charge line would be about 50-100 amps.. depending on the size and shape of the bats... (lets assume you have 2 100 amp bats.. that were drawn down to 50% ... about 100 amp limit... Its a good starting point.. as once the bats initilize... they will drop the demand for high current... and fall back off to 25-30 amps.. and then when they get about a hour or so.. will drop again... if they are in good shape... to about 10 amps... and remain their no matter how long you charge them...

*DISCLAIMER: This test represents this slightly used AGM and may not be representative of a brand new AGM or a battery with more use and in a worse state of health.
DEFINITIONS:
“C” Rate – .2C & .4C – Also called Point Two C and Point 4C. This is just represents a percentage of Ah capacity in either charging or discharging current. For example .2C = 20% of Ah capacity or 20A for a 100Ah battery and .4C = 40% of Ah capacity or a 40A charge or discharge current for a 100Ah battery.
SOC = Sate of Charge – SOC is a percentage of charge based on battery Ah capacity.
DOD – Depth of Discharge – Is the depth to which the battery has been discharged. For example 80% DOD is also 20% SOC or 80Ah removed from a 100Ah battery.
Ah Capacity = Ampere Hour Capacity – The total ampere hours a battery can store and deliver at a discharge rate that yields a 20 hour discharge time at 77F before hitting a terminal voltage of 10.5V.
20 Hour Discharge Rate – Also the “20 hour rating”. The discharge current at 77F that will yield a 20 hour run time before hitting 10.5V

How long does it take to charge from 50% SOC to 100% SOC?
I have long known that a higher charge rate, with AGM batteries, does not necessarily translate to *significantly faster charge times from 50% SOC to 100% SOC and thought it would be good to test this and measure the actual differences. For the first part of this test, 50% SOC to 100% SOC, I compared a charge rate of .2C with a charge rate of .4C on the same Lifeline GPL-31T battery.
“*Significantly Faster” – Please understand, & take with a grain of salt, what I am comparing “significantly faster” to. I was once told by a quite vociferant expert on AGM batteries, “expert” defined here as a boat owner who had AGM batteries and whom had read only the glossy marketing materials, (grin) that with his 150A alternator he could recharge his 440Ah bank of AGM batteries from 50% to 100% in less than an hour and a half! Apparently the marketing departments don’t have prerequisites of chemistry 101, Ohm’s law or physics 101?

While the owner is fully capable of basic math, 1.5 hours times 150A = 225Ah’s, he fails to take into consideration that his .34C 150A alternator could not put out 150A for 1.5 hours and his bank, at anywhere close to .34C, would hit absorption voltage rapidly and begin limiting alternator current. He also failed to include for a declining CAR or what is referred to as the charge acceptance rate. Even if his alternator could run at at a face value of 150A it is simply impossible to return 225Ah’s to that bank in 1.5 hours from 50% SOC.
I will repeat this, IMPOSSIBLE!
One can’t really can’t blame most of us for the rather misguided expertise because most had been rather grossly mislead by nearly everyone in the industry on the expected charge times from 50% SOC to 100% SOC with AGM batteries. Suffice it to say it is physically impossible to recharge a 440Ah bank of AGM batteries at 50% SOC to 100% SOC in 1.5 hours with a .34C charge current.
Charge Rate Explained:
0.2C = 20% Charge Rate of the Ah Capacity
0.4C = 40% Charge Rate of the Ah Capacity

Also remember that you need 1.5 times the current to charge the battery than what you took out of the battery... 1.5:1

most flooded and AGM batteries... will not accept a brute charge i.e 100 amps for 1 hour to fully recharge... If you push too much you will over heat the cell and damage it. (the only way to push more current it to raise the voltage above the recomended manufacture charging voltage... which is most common... A typical 12-volt lead-acid battery must be kept to approximately 14.2-14.7 VDC

Note: once you achieve a boiling point inside the cell... it does no good to increase this action. Not only will you lose the electrolyte... but you will start to HEAT up the battery and cause the plates to warp. It gets more serious for AGM type bats as they have no way to vent. (SOME JUST SIMPLY EXPLODE FROM THE PRESSURE INSIDE)

for AGM type batteries.... It is recommended to keep current limited to 20% or 0.2C of the batteries capacity at a charging voltage of between 14.6 – 14.8V then lowered after deep cycle use, 13.6V – 13.8V for standby use. Again.. if you feel heat in the battery.. you are damaging it.

thus you can do the math... them batteries don't charge up well when forced with higher voltage such as a inverter would.

Your Tv probably has a computer control on the alternator.. which limits it to 13.8 to 14.7 volts which when used for charging is not too bad... but, will take some time at the 0.2C... thus your 100 amps needed to fully recharge the bats.. is at a rate of 20 amps for 1.5x or 5 hours plus 2.5- 7.5 hours at 20 amps...
Forcing the batteries to charge more quickly will only shorten their life... and have less cycles.
So by hyping up the voltage by using a inverter... your not going to gain anything but a boiled out overheated warped plate battery..... sorry.
however, limiting the current to the battery by IR loss... cuts your charging rate down to about 0.1C or so... doing the math...by thumb appears to extend the charging time to complete recharge ... out to twice what it should... i.e 15 hours...

Good luck... the adventure contenues

[Omnial]

Thanks for the info!