TV drawing power from trailer

[dznf0g]

So, I'm on my first trip after converting from fla to agm. I think I see a need for a dc to DC converter. With my solar active and programmed to lifeline specs, I note at the end of the day's drive that my batts are down about 10%. Per app, solar is charging as it should, and the tv charge line is all good. I believe I am back feeding to the truck, as solar and agm voltage is higher than fla truck batt and changing system. Am I correct?

[lsbrodsky]

I thought you were a Chevy guy? I know that in my Chevy with Tow-Haul engaged the alternator output is such that my Lithiums charge.
Larry

[dznf0g]

Originally Posted by lsbrodsky

I thought you were a Chevy guy? I know that in my Chevy with Tow-Haul engaged the alternator output is such that my Lithiums charge.
Larry

Mine does until it runs through an "absorption " phase.
I took out the charge line fuse for tomorrow's run.

[ROBERT CROSS]

Originally Posted by jsmall01

We have a 2016 19ft flying cloud and pull it with a 2019 Jeep Grand cherokee. I replaced the batteries with lithium (200amps) and added 200 watts of solar. I notice that when we drive, the battery is not charging much from the TV. Assume it's not really meant to.

Also, when Jeep is off and still plugged in to 7pin connector, I notice about 1.5-2 amps of draw on the trailer batteries. Why is Jeep drawing power from the TV batteries? Is that normal behavior? I thought power was supposed to flow to TV, not from it.

Sorry for the basic question, but still new to pulling a trailer.

I just un-plug the 7pin when not moving....That lesson learned years ago when both the TV & Safari were dead as a doornail
POI We do have lithiums, and no DC-DC. SFSG.

Bob
🇺🇸

[jsmall01]

got it! Black wire is positive charge wire, white is neutral.

I will connect victron to those as input and send output back to the original locations on the bus bar from victron output terminals.

Sounds like that is all I need to do.

[lsbrodsky]

Originally Posted by dznf0g

Mine does until it runs through an "absorption " phase.
I took out the charge line fuse for tomorrow's run.

Rich, I am having trouble thinking through what you are saying. On my truck, in Tow-Haul, I have noticed that the alternator voltage stays high, period. It never drops down as it does without T-H, as observed by the truck's voltmeter. Since the charge line goes directly to the batteries, not through any charger/controller, that should be sufficient voltage to always charge the batteries. The voltage is not high enough to do any damage, certainly not for the period of time one drives in a day. I have trouble seeing how there could be any backfeed to the truck, except when the engine is off and the 7-way is still connected. Sure pulling the fuse eliminates any possibility and if you don't want TV charging the trailer while driving, it is a fine solution.
Certainly without T-H or in other makes of trucks that do not behave this way, when the truck regulator drops the voltage, there would be backfeed from Lithiums, and any fully charged LA battery.
Larry

[dznf0g]

Originally Posted by lsbrodsky

Rich, I am having trouble thinking through what you are saying. On my truck, in Tow-Haul, I have noticed that the alternator voltage stays high, period. It never drops down as it does without T-H, as observed by the truck's voltmeter. Since the charge line goes directly to the batteries, not through any charger/controller, that should be sufficient voltage to always charge the batteries. The voltage is not high enough to do any damage, certainly not for the period of time one drives in a day. I have trouble seeing how there could be any backfeed to the truck, except when the engine is off and the 7-way is still connected. Sure pulling the fuse eliminates any possibility and if you don't want TV charging the trailer while driving, it is a fine solution.
Certainly without T-H or in other makes of trucks that do not behave this way, when the truck regulator drops the voltage, there would be backfeed from Lithiums, and any fully charged LA battery.
Larry

Larry, I'll pay closer attention tomorrow, when we move on, but I'm sure I saw (and always have) voltages, by the dash needle) down around upper 12s or low 13s.
I am always in th mode when trailering.

[dznf0g]

Larry, what voltage do you have your controller set to? I use a custom charge routine with absorption at 14.3V, which is what lifeline calls for in their technical manual.
I haven't measured the Silverado output on TH for an accurate voltage. Online all I can find is anecdotal 14V.
If, indeed, my solar output is higher than the alternator voltage, the solar (and a completely saturated agm batt) will backfeed to the TV.

[lsbrodsky]

Which controller? I have Lithiums, Victrons, with the older separate BMS where I cannot set anything. My TV charging line is essentially connected directly to the batteries through the BMS. My solar controller is also Victron set to whatever profile they recommended for Lithium. I have routinely seen just over 14v on the truck voltmeter when driving although I have never hooked up a separate meter to measure.
However, using my phone app, I can see the battery charging from the TV while I am driving. It stops taking a charge when the BMS tells it to. I feel certain solar can back feed until the controller drops down to float.
Larry

[dznf0g]

Originally Posted by lsbrodsky

Which controller? I have Lithiums, Victrons, with the older separate BMS where I cannot set anything. My TV charging line is essentially connected directly to the batteries through the BMS. My solar controller is also Victron set to whatever profile they recommended for Lithium. I have routinely seen just over 14v on the truck voltmeter when driving although I have never hooked up a separate meter to measure.
However, using my phone app, I can see the battery charging from the TV while I am driving. It stops taking a charge when the BMS tells it to. I feel certain solar can back feed until the controller drops down to float.
Larry

Ok, thanks. I was asking about the solar controller, but with lithium, its apples and oranges for comparison purposes.

[lsbrodsky]

Right, cannot really compare settings but the principle of backfeed or not is the same. You said your trailer batteries were discharging to feed the TV battery and I just cannot see why that would happen with T-H engaged. Solar will be the highest voltage but the controller will drop that to float. Even then, the trailer batteries will quickly drop to a voltage lower than the TV output and the TV should keep the trailer batteries charged, since so little current will be required. If there is no sun, the TV should still have higher voltage.

Larry

[dznf0g]

Originally Posted by lsbrodsky

Right, cannot really compare settings but the principle of backfeed or not is the same. You said your trailer batteries were discharging to feed the TV battery and I just cannot see why that would happen with T-H engaged. Solar will be the highest voltage but the controller will drop that to float. Even then, the trailer batteries will quickly drop to a voltage lower than the TV output and the TV should keep the trailer batteries charged, since so little current will be required. If there is no sun, the TV should still have higher voltage.

Larry

Yes but agm resting voltage is 13.1 and tv flooded is about 12.7v. The dissimilar chemistry equalizes to tv voltage, solar and AGMs feed truck, and I arrive with AGMs at 12.7ish. Basically, if I turn on headlights and TH mode, truck stays at 14.xx and i arrive with full batts.
Owners manual sort of mentions this, but totally neglects anything other than flooded in both tv and trailer.

[dznf0g]

Correction, new lifeline AGMs resting voltage, fully charged, is 13.2V.

[drbrick]

Like "Sealevel1" we have 200w solar on the roof and have no need to charge from our tow vehicle so we simply pulled the fuse in the truck - adding the DC-DC charge control may not solve the issue see video on YouTube by LongLongHoneymoon -

[psy1]

To the original question:

Most people get a DC-DC charger so they can use the existing power from the TV alternator to rapidly charge batteries. Lead acid and AGM batteries benefit less than lithium batteries in this case because lithium batteries can accept all the charge you can throw at them and the lead acid batteries are limited in their charging characteristics.

In your case, you are talking about a relatively small DC-DC charger hoping to be able to isolate your TV battery from the Li batteries; charge the Li batteries faster than the 7-pin connector; and not alter the TV for concerns about voiding warranty.

Most of the larger DC-DC chargers require beefy wiring from the alternator to an Anderson connector at the rear of the TV and then a corresponding Anderson connector from the trailer hitch to the DC-DC charger in the trailer. You are trying to get way with using the small gauge existing wiring. You need to be careful about that and be certain that the wiring is up to specs for the amperage involved.

[GeorgeR]

Instead of a DC-DC charger to isolate and control the voltage between the batteries, I opted for installing a Victron Cyrix-li-ct "battery combiner". It's a essentially a relay with a microprocessor. The relay control is the voltage coming from the tow vehicle. Maybe not as smart as a DC-DC charger, but it works. It disconnects the batteries when the tow vehicle's engine is off (low voltage), and connects the batteries when the tow vehicle's engine is running (high voltage). It also has an override function that can be used to jump start the tow vehicle by sending current to the tow vehicle from the trailer battery (have not tried that yet).

Some folks use the Cyrix in conjunction with a DC-DC charger and I may do that in the future.

Got the idea from other posts (search for "Cyrix").

Bought from Amazon: https://smile.amazon.com/dp/B07LCN3KCZ/ref=cm_sw_em_r_mt_dp_UCDdGbZB7WX66?_encoding=UTF8& psc=1

[gcarter]

All you really need is a diode big enough to handle the current. It gives you one way movement. TV to trailer. Not trailer to TV.

[suncoasteng]

All overkill, just need simple Hopkins diodes

https://www.amazon.com/Hopkins-48955...a-448727961086

[dznf0g]

Originally Posted by suncoasteng

All overkill, just need simple Hopkins diodes

https://www.amazon.com/Hopkins-48955...a-448727961086

Diodes would cure the backfeed situation, but does nothing to help the tv to charge the AGMs which require a higher voltage.
I just want though 2 days of very heavy overcast, drizzle and thick fog. Solar didn't do much.

[uncle_bob]

Originally Posted by suncoasteng

All overkill, just need simple Hopkins diodes

https://www.amazon.com/Hopkins-48955...a-448727961086

Hi

Ok, diodes. Normal silicon diodes have a 0.7V forward drop. You are trying to get max voltage from the alternator to the Lithiums and you just took out 0.7V. Simply disconnecting the wire would be cheaper and just as good for topping up.

Same 0.7V goes up a bit with current. Best bet is around 40A or so you are at a volt or more. You would be dissipating 40W in your diodes and it could be for a while. Just how long depends on how fast they melt. The silicon is likely rated at a max of 125C junction. Just how many cubic feet of heatsink did you attach to them?

(The diodes in the Amazon listing are rated at 3 amps so no need to worry about 40A, they will be long gone by then)

Obviously those are two ends of the use spectrum. The first is a nearly full battery in the trailer. The second is a dead trailer battery. One happens a lot, the second one wold be pretty rare. Equally rare is anybody heat sinking those diodes with a < 1C/W heatsink.

Next up would be the surge from a short in the trailer. This is getting into the "very rare" end of things. Diodes blow very fast when significantly over current. A standard fuse is "protected by the diode". There are very fast acting fuses out there, they aren't cheap and the only work sometimes ( I have data on that ...).

Bob