Isolating TT's Lithium from TV's Lead Acid Batteries

[cruz-in]

Hi Team,

I am installing Lithium batteries on our TT. To provide isolation between TT's Lithium and TV's Lead Acid batteries, I plan to install a Renogy DC-to-DC converter. It is kind of neat. Has three stage charging and supports Lithium. Here is a link to them:

20 Amp and 40 Amp DC-to-DC charger

TV has 30 AMP fuse for the power line to the Trailer Plug. I assume I need to stay below 30 Amps so am I stuck with the 20 Amp Model?

Would be nice to use the whole available 30 amps.

Thanks
Dan

[grizzzman]

BFL13 wrote:
If they take bulk at 14.4 but can be overcharged at 13.9, I assume that is after they are full and charging should be cut off?

What was that about the Lis having some sort of protection attached so they can't be overcharged that is part of that balancing thing? Or (very possible) I got that all wrong from the other thread on Li batts?

If they are that close to full before heading down the road, why charge them up at all? They like to be at 80% SOC

My problem with the premise of this thread's OP post is the idea that the Lis in the trailer will be supplying current to the truck while driving down the road and thus lose some AH. I find that notion peculiar.

If charging is wanted driving down the road, and the 7-pin isn't enough (I don't see how it can be too much, but waiting to hear more about that), then the DC-DC gizmo is known to work. We have had other threads on those by guys who have them.

Time at voltage plays a real roll when it comes to overcharging LFP.

[CA_Traveler]

OP,

300Ah is a potential 240Ah at 20% charge needed from the TV to full charge. Solar helps when there is sun of course. My 220Ah discharge at 50% is handled by the nearby 200A alternator.

[NRALIFR]

I can’t speak for the Renogy DC-DC charger, but my Redarc unit does not back feed voltage from the TC house battery to the truck starter battery. It effectively functions as an battery isolator as well. I also have a isolation solenoid on the truck that is only energized when the engine is running to keep the charger from running the starting battery down when the engine is not running.

:):)

[BFL13]

I will get to that later, no time right now. I trust cruz-in from before when he had the twin L15 (ISTR) set-up, which I learned from, so I expect to learn something here despite my doubts.

[cruz-in]

CA Traveler wrote:
OP,

I don't recall your intended battery size or usage. A 100Ah discharged battery needs up to 80A charging when driving - ie 20A for 4 hours. Double that for 200Ah.

Since your interested in the Renogy that suggests that you are not a pedestal to pedestal camper starting with a fully charged battery.

Are you planning on any TT charger changes for Li when plugged in?

It is two 100AH LI batteries with plans to add a 3rd.

In addition to the Renogy 20 Amp DC to DC LI Isolation/charger (referenced in the OP), I have also changed to a Renogy LI compatible solar controller/charger and a Progressive Dynamics LI compatible converter.

The LI batteries are situated in a compartment where they will be more/less the same temps as the interior of the TT.

Should be done with the mod tomorrow.

[cruz-in]

Below is a link to an extensive thread on this subject. Includes real world measurements and acknowledgement from Sean of Battle Borne of the issue.

If you want to cut to the chase, after reading the OP, go to post number 70 & 81. A gentleman used his BVW Battery monitor and bluetooth to monitor current flow while driving. He saw 1 to 2 amps going from the TT to the TV.

Then go to post 84 where Sean from Battle Borne says:

"Dear SilverHouseDreams (the OP of the thread)

Your observation of the problem with keeping your Li-ion trailer batteries hooked in parallel with your tow vehicle starter battery is spot on. The Li-ion battery operates on a higher and flatter voltage curve and therefore will continue leak charge into your lead acid starter. That is why a battery isolator is imperative in this situation."

Extensive Thread on Isolating LI Batteries from TV. Even real world measured data and confirmation f...

[time2roll]

Need to review battery specs to know if 13.9 float will overcharge the specific lithium battery. If the lithium is charged to 14.4 to 14.6 range like most I have my doubts 13.9 would be an issue. Lithium resting voltage does not drop like lead-acid from 14.8 to 12.7 just for resting.

[BFL13]

If they take bulk at 14.4 but can be overcharged at 13.9, I assume that is after they are full and charging should be cut off?

What was that about the Lis having some sort of protection attached so they can't be overcharged that is part of that balancing thing? Or (very possible) I got that all wrong from the other thread on Li batts?

If they are that close to full before heading down the road, why charge them up at all? They like to be at 80% SOC

My problem with the premise of this thread's OP post is the idea that the Lis in the trailer will be supplying current to the truck while driving down the road and thus lose some AH. I find that notion peculiar.

If charging is wanted driving down the road, and the 7-pin isn't enough (I don't see how it can be too much, but waiting to hear more about that), then the DC-DC gizmo is known to work. We have had other threads on those by guys who have them.

[grizzzman]

BFL13 wrote:
Do you really have to isolate the Lis from the TV batteries? Why? Going down the road, you might be getting 13.9 volts from the TV and 14.x from solar, so in that case the solar will do the charging.

If no sunshine, who cares if the Lis are charged at 13.9 volts for a while? When you get there, you unplug the 7-pin and now you are isolated anyway.

I can see it if the 7-pin were charging at a higher voltage than the Li spec calls for, but who cares if they are getting less voltage than they can handle?

You can overcharge a LFP at 13.9 volts.

[CA_Traveler]

OP,

I don't recall your intended battery size or usage. A 100Ah discharged battery needs up to 80A charging when driving - ie 20A for 4 hours. Double that for 200Ah.

Since your interested in the Renogy that suggests that you are not a pedestal to pedestal camper starting with a fully charged battery.

Are you planning on any TT charger changes for Li when plugged in?

[CA_Traveler]

OP,

Your linked Renogy manual seems short on Li battery information to me. It may be adequate for you. I'd want to better understand the Li charging.

1. Does it time out after 4 hours?
2. Or does it sense the charging voltage and stop charging when the current drops to the right amount? Apparently not since I didn't see any mention of a shunt since otherwise it could only detect house + battery amps.

3. What about low temperature charging restrictions?

And likely more questions if I dug deeper as I saw other information like an ignition wire to the charger - ie another wire from the TV to TT, etc.

[CA_Traveler]

IHMO High dollar lithium batteries should be properly charged and a flooded battery alternator does not do that. Plus the TV battery/alternator voltage is not the voltage at the end TT end of the wires. Often TV and TT wiring is adequate for small loads like a propane refer and inadequate charging.

Typically the starting battery is quickly charged and the voltage drops to 13.6V or so regardless of the TT battery charge on the end of those long wires.

[BFL13]

cruz-in wrote:
I think the point is two fold:
1) When driving the TV/TT combo, current will flow from the higher voltage LI battery (assuming it is fully charged) to the lower voltage Lead Acid system.
2) Having a DC to DC charger for the LI batteries will allow the TV to charge the LI batteries (albeit in limited fashion) when traveling down the road.

1. The resting voltage of a full 12v Battleborn is about 13v and it wants 14.4v for bulk charging, 13.2 for float if I have that right. So when can the TT battery be higher in voltage?

Any alternator charging will be above that 13v, starting at 14.x when the vehicle engine is turned on and tapering quickly to 13.8 or so soon after, once the alternator warms up and the vehicle battery is recharged.

When parked, if the TV is a Chev, just disconnect the 7-pin. With a Ford you have the relay, so no worries.

2. You just want one of those DC-DC gizmos! (And why not?) 🙂

[cruz-in]

I think the point is two fold:
1) When driving the TV/TT combo, current will flow from the higher voltage LI battery (assuming it is fully charged) to the lower voltage Lead Acid system.
2) Having a DC to DC charger for the LI batteries will allow the TV to charge the LI batteries (albeit in limited fashion) when traveling down the road.