Do I need a DC-DC charger?

[Zap-Ouch!]

I just bought a 2024 Jayco Jay Feather 21MML, tow-behind travel trailer. I'd like to replace the lead acid battery with a 200 ah lithium battery. The Jayco came with solar (200 watt panel) and a converter that are both lithium ready. So, is it as simple as just switching out the batteries or do I need a DC-DC charger? I've searched and searched and can't seem to find a definitive answer. Some have said that when the lithium BMS shuts down the charge, the "back EMF" can damage the alternator's diodes. Is that true?

Signed,
Newbie

 

[Rocketman]

Because you are in a trailer, you will not get much charging from the alternator. (The cables are small and the distance is great). Yes you will get some,but…

If you want good charging from the truck alternator you need to run new wires (probably #8 or#6 from the trucks power to the an Anderson power connector (for a good disconnect point) then to a Dc-Dc charger. That way it will charge correctly and at the rated power. If you are thinking about that -look at the Victron isolated Dc-Dc charger 30a (you need an isolated charger).

The main issue with charging lithium’s from the alternator is it never stops when the battery gets full. Charging a full battery at 14.?v for hours while you are driving is not good (and potentially very bad) for the battery.

In MotorHome’s where we have thick cables that can handle 200a, and a 200a alternator and a big lithium battery- the alternator gets ran at 100% all the time and can overheat and burn up.

If the bms kicks off when the alternator is running, you need a place for all the electrons to go, most units are also charging a lead starter battery and so the electrons go into the starter battery, if your alternator does not have another battery- that’s when the diodes get fried.

200w of solar is not a lot- I would look at expanding the solar first (better bang for the buck).

Also, I would install a switch somewhere so you can kill the power from the alternator charging the trailer. So if your battery is full you can stop it, but that trickle from the alternator is nice when needed. You probably need to measure the voltage and amps that the vehicle can charge the trailer.

One REALLY nice (in my opinion needed) piece is a shunt based battery monitor- like a Victron Smartshunt or BMV712. This way you know how much power you really have in the battery. It is also good for figuring out how much power different stuff takes to run.

Good Luck

 

[time2roll]

The DC-DC charger is not immediately necessary. Yes the DC-DC will get more amps into the battery with either lithium or lead-acid. If it has not been an issue to date it may not be needed. There is NO issue with the lithium BMS or the DC-DC suddenly adjusting the charge rate that will hurt the alternator. Can even pull the connector from the tow vehicle with everything running with no harm to anything.

If this is your first RV I recommend to take it out a couple times and get used to some of the systems before replacing a perfectly serviceable battery. A couple one or two night trips will help make some decisions. Also need to determine how much off-grid vs hookup camping will be done.

Probably need a lithium battery with both a heater and low temp shut down. Can add the DC-DC charger later if it seems to be needed.

 

[Reed Cole]

You might want post the model of converter and solar charge controller and links to their manuals so people can help you assess whether they are truly appropriate for lithium and what changes to their settings you may need to make.

 

[Zap-Ouch!]

You might want post the model of converter and solar charge controller and links to their manuals so people can help you assess whether they are truly appropriate for lithium and what changes to their settings you may need to make.

Thanks,
Here they are...

 

[Zap-Ouch!]

Because you are in a trailer, you will not get much charging from the alternator. (The cables are small and the distance is great). Yes you will get some,but…

If you want good charging from the truck alternator you need to run new wires (probably #8 or#6 from the trucks power to the an Anderson power connector (for a good disconnect point) then to a Dc-Dc charger. That way it will charge correctly and at the rated power. If you are thinking about that -look at the Victron isolated Dc-Dc charger 30a (you need an isolated charger).

The main issue with charging lithium’s from the alternator is it never stops when the battery gets full. Charging a full battery at 14.?v for hours while you are driving is not good (and potentially very bad) for the battery.

In MotorHome’s where we have thick cables that can handle 200a, and a 200a alternator and a big lithium battery- the alternator gets ran at 100% all the time and can overheat and burn up.

If the bms kicks off when the alternator is running, you need a place for all the electrons to go, most units are also charging a lead starter battery and so the electrons go into the starter battery, if your alternator does not have another battery- that’s when the diodes get fried.

200w of solar is not a lot- I would look at expanding the solar first (better bang for the buck).

Also, I would install a switch somewhere so you can kill the power from the alternator charging the trailer. So if your battery is full you can stop it, but that trickle from the alternator is nice when needed. You probably need to measure the voltage and amps that the vehicle can charge the trailer.

One REALLY nice (in my opinion needed) piece is a shunt based battery monitor- like a Victron Smartshunt or BMV712. This way you know how much power you really have in the battery. It is also good for figuring out how much power different stuff takes to run.

Good Luck

I like the idea of expanding the solar first. My hope is that it's just as simple as installing and plugging in an additional panel. I'll check out the forum pointers on that. In the meantime, if you want to share any links to videos, etc., I would welcome that!

I don't plan on doing much long-term boondocking, but I will be staying at Harvest Host spots from time to time where they might not want generators running all night, so I'd like to at least be prepared for an overnighter where I have to depend on battery power for the basics necessities (12v refrigerator, lights, etc.). My guess is that I would probably also invest in an inverter at some point as well.

Thanks for your reply and info. Much appreciated.

 

[chrisski]

I like the idea of expanding the solar first. My hope is that it's just as simple as installing and plugging in an additional panel. I'll check out the forum pointers on that. In the meantime, if you want to share any links to videos, etc., I would welcome that!

I don't plan on doing much long-term boondocking, but I will be staying at Harvest Host spots from time to time where they might not want generators running all night, so I'd like to at least be prepared for an overnighter where I have to depend on battery power for the basics necessities (12v refrigerator, lights, etc.). My guess is that I would probably also invest in an inverter at some point as well.

Thanks for your reply and info. Much appreciated.

Looks like you have a good adventure take some time and get to know your system.

I do recommend some sort of battery shutdown if not installed. Even if it’s taking a cable off. So many batteries have been ruined by leaving the battery on for weeks at a time. I go to RV places and see people pick their trailers up after long repairs and inevitably the battery is dead. The shop always seems to act as if its the first time a battery has died on them.

So much you can do with the build. My rule of thumb us guide is 350 watts of panels and 2 golf cart batteries will get you through a night. Better would be 600 watts of panels and 4 golf cart batteries will let you do two nights without worry, provided you don’t run an inverter. I find my big draw is a 120 watt blower motor for the propane heater.

There’s so much to it this and its so much more expensive than a generator that good planning helps so much.