Can Lithium back charge TV?

d_v

If you are using LI batteries in your trailer and they are fully charged will they back charge the LA batteries in the tow vehicle when the 7pin is connected? Can that harm the tow vehicle battery?

Horigan

I'll be interested in the responses.  Due to this very concern I pulled the power fuse from my tow vehicle so the lithium battery wouldn't over charge my tow vehicle battery.

Grumpy_G

Yes, and no. Modern vehicles frequently run with over 14V of charging voltage so car batteries can take it. 

pthomas745

You need to see if your tow vehicle has an "isolation relay".  This subject comes up when owners discuss whether or not to unplug the 7 pin from the tow vehicle when parked.

A comment from the thread: ....."I do not, but I have an isolation relay that stops the connection from the truck to the T@B when the ignition is off. How to check. Turn off your T@B battery, plug in your 7-way. Crank your vehicle. Turn on a light in the T@B. If the light comes on, you have a charge line from your vehicle. Turn off your vehicle and remove the key. If the light goes out, you have an isolation relay, so no need to unplug. If the light stays on, you do NOT have an isolation relay, and you will need to unplug when stopped."

This "the trailer drained my tow vehicle battery) or vice-versa is one of the legendary trailer things that "used" to happen all the time, that may not be as common as it used to be.  The lithium battery is a new addition to the worry about this.

Here is the thread. 

https://tab-rv.vanillacommunity.com/discussion/10956/do-you-disconnect-the-electrical-plug-from-your-car-truck-when-stopping-for-an-hour-or-so-to-shop

Very common conversation across many trailer forums.  You can install your own isolation relay if your vehicle is not equipped with one.

https://www.google.com/search?client=firefox-b-1-d&q=isolation+relays+in+tow+vehicles

d_v

Thanks! I should have checked the cars first. Sequoia battery at rest was 12.3v, with car running was 14v. I checked my Model 3 and it was 12.9 (uses LA for 12v). I found this chart for LiFePO4, so the lowest the car could take it is still 10% and the highest the LiFePO4 could take the LA is below the alternator voltage. Not going to worry. 

Grumpy_G said:

Yes, and no. Modern vehicles frequently run with over 14V of charging voltage so car batteries can take it. 

Jilong

From this Renogy article, it seems to say that if you have a DC-DC charger then you are safe from this issue.

https://www.renogy.com/product-faq/Battery-Chargers/More-DC-DC-FAQ-s

SLJ

When hooked up, my alternator charges both the lead acid and my two 100AH at 14.7 volts. My lithiums get around 5.5 amps from the tow vehicle. I don't use a DC to DC converter. Two years and no problems. Just don't leave them plugged in when not towing.