Another DC to DC Question

[JDT]

I started to ask my question in Rarebear's post but decided I didn't want to hijack it. We've ordered a Grand Design Imagine tt after selling everything last fall and thinking we were out of the rv life, until we weren't. No more long trips for us, maybe up to 200 miles to Branson on occasion, but mostly local use.

To keep weight down for the sake of our F150 I am going to use a lithium battery in the 200ah range, instead of the 2 group 27 la's the dealer would install. The dealer said they would put the lithium in my storage bay. Looking at a LiTime which says it must have a DC to DC charger installed with it. I'm looking at the same Victron 30 amp that Rarebear has. I know with short runs it will not do much charging so it seems the main function is to protect the battery from the truck alternator/charge voltage. In this case I'm thinking a smaller amp charger would serve the same purpose?

Next question is about hook up. My understanding is the way Rarebear is doing it is isolated. Correct me if I'm wrong but non isolated would be using the charge line from trailer umbilical for the charge input to the charger and grounding it to the trailer chassis. If that is correct and all I'm wanting to do is protect the battery non isolated would be the easiest way for me to accomplish it.

Ok, for clarification I'm far from being an expert. Basically all I know is what I read on the internet and you know how that goes so any advice/corrections are appreciated.

Editing with another question. Since the charging I would get is negligible would it be okay just to disconnect the charge line and not worry about it?

[Gravelette]

Having an Li battery in your trailer won't hurt your truck and your truck won't hurt the battery. You only need the DC-DC if you want more charging while driving than the stock 7 pin provides. Kind of surprised LiTime says differently. Maybe they sell chargers as well? Not what you asked about but maybe helpful.

[JDT]

Originally Posted by Gravelette

Having an Li battery in your trailer won't hurt your truck and your truck won't hurt the battery. You only need the DC-DC if you want more charging while driving than the stock 7 pin provides. Kind of surprised LiTime says differently. Maybe they sell chargers as well? Not what you asked about but maybe helpful.

Here's what the manual on the 230ah bluetooth LiTime says: "If the alternator/generator supports DC output, a DC to DC charger needs to be added between the battery and the generator." That's all I know. Our truck is a hybrid with the built in generator but I don't think it does anything when towing.

[powercat_ras]

One difference between Lead-acid and Lithium Iron (LiFe04) is that a depleted Lead Acid Battery still will only draw a limited amount of current thru the 12 V charge line from the F150 and won't blow the 25 amp trailer 12V power feed fuse in it.

A depleted LiFe04 can try to absorb as much as 100 A and may trip the 12V power feed 25 amp fuse. A 20 amp max output (or less) DC-DC charger will limit the current. You can mount it in a dry location like where they put the LiFe04 battery. However it has to be wired in between the 12V Aux pin on the towing connector and the rest of the 12V TT system, not directly between the battery and the 12V system.

All that said, based on how you said you use your TT, assuming you camp plugged in you your LiFe04 should not deplete enough to get to that depleted state where it would blow the F150 Fuse. That said you might want to buy a few of the 25A fuses to carry as spares just in case.

[JDT]

Originally Posted by powercat_ras

One difference between Lead-acid and Lithium Iron (LiFe04) is that a depleted Lead Acid Battery still will only draw a limited amount of current thru the 12 V charge line from the F150 and won't blow the 25 amp trailer 12V power feed fuse in it.

A depleted LiFe04 can try to absorb as much as 100 A and may trip the 12V power feed 25 amp fuse. The 20 amp max DC-DC charger will limit the current.

All that said, based on how you said you use your TT, assuming you camp plugged in you your LiFe04 should not deplete enough to get to that depleted state where it would blow the F150 Fuse. That said you might want to buy a few of the 25A fuses to carry as spares just in case.

We never boondock, never. So you are saying also that running without a DC to DC charger is fine as long as the battery is not depleted? We have an rv garage at home with a 50 amp service so keeping it charged shouldn't be an issue.

[d23haynes57]

If your travels are post to post and you don't need significant power or you battery can cover the needs for the time traveling, for fridge operation or other needs, no need for tow vehicle charging.
If you do need to support some loads while traveling, you can do some management. If you have your batteries charged before connecting, then that charge line really only has to support the connected loads. If you do want to charge the batteries while traveling a separate charge line would be best and controls so trailer battery does not back feed into the towing vehicle. Dc to DC charger or relay controlled by alternator.

[Sunnyfalls]

The other option is to not allow your tow vehicle to charge your RV LiFiPO4 i battery at all...

This is a viable option as long as your RV battery can power everything needed in the RV when traveling and as long as you have the means to replenish the RV battery when camping. The benefit of this option is that the RV doesn't increase the stress on the tow vehicle alternator at all and you get a tiny winny improvement in MPG.

This is what we do since we can use solar panels, propane generator or shore power for battery replenishment when we are camping and we have an absorption cycle refrigerator that uses propane when we are traveling.

In any event, we would suggest holding off on getting/using a DC-DC charger until you gain a better sense of your specific situation...and in the mean time, carry some spare fuses too. As long as you are using the proper fuse, you can't harm anything, but without the DC-DC charger you won't be charging using the optimum charge profiles.

Gayle & Bob
"Los Gatos Casita"

[NoviBill]

The DC-DC charger does two separate things:
1) Limits the charging amperage - this helps battery life and alternator life *1
2) Takes the alternator voltage (13-14) and steps it up to the correct voltage for charging the Lithium batter (if memory serves 14.4). This is required to fully charge the battery.

Make sure you are getting a LifePo4 battery with BMS. Far less risk then other Lithium Ion.




*Iif the battery current draw is high enough it can damage your alternator - not major with 20 foot of 10 gauge wire from alternator that will never draw 100 amps but restricting to 30 or 40 amps (DC-DC charger) will keep the wires from heating.

[JDT]

It is a LifePo4. I just remembered the trailer does have a small solar panel as standard so I guess it could provide some charging. I think my best option is to disconnect the charge line from the truck. The only thing that should be running is the 12 volt 10cf fridge which a 230 ah battery should handle for quite a while.

[SafariBen]

Originally Posted by NoviBill

The DC-DC charger does two separate things:
1) Limits the charging amperage - this helps battery life and alternator life *1
2) Takes the alternator voltage (13-14) and steps it up to the correct voltage for charging the Lithium batter (if memory serves 14.4). This is required to fully charge the battery.

Make sure you are getting a LifePo4 battery with BMS. Far less risk then other Lithium Ion.




*Iif the battery current draw is high enough it can damage your alternator - not major with 20 foot of 10 gauge wire from alternator that will never draw 100 amps but restricting to 30 or 40 amps (DC-DC charger) will keep the wires from heating.

Realistically, you'll be lucky to get 20A charging through the wire and connections all the way to the trailer. Alternator protection is more of an issue with motorhomes with short 2/0 cables connecting the house and chassis, needed for lead, not for Li.

[Sunnyfalls]

Originally Posted by JDT

It is a LifePo4. I just remembered the trailer does have a small solar panel as standard so I guess it could provide some charging. I think my best option is to disconnect the charge line from the truck. The only thing that should be running is the 12 volt 10cf fridge which a 230 ah battery should handle for quite a while.

Right...

And ask yourself this fundamental question... Why would anyone want a RV that is designed such that it needs battery charging from the tow vehicle while traveling? One isn't always traveling...so the RV needs to be fully self-sufficient...

One needs to design the RV such that the daily battery depletion can be replenished in 4 hours or less using solar panels or generator power when dry camping or using shore power when it is available. A proper RV design minimizes the RVs DC and AC power needs by using lower power electrical appliances that are only used for short periods (e.g., coffee/tea makers, hairdryer, microwave oven, toaster oven, vacuum cleaner, etc.) and have electric and propane options for those appliances that are used for long periods (e.g., air conditioner, hot water heater, furnace, refrigerator, etc.). We can dry camp for as long as we desire using only a 100AH LIFEPO4 battery, 3000 Watt Inverter, 200 Watt solar panels and a propane generator.

Gayle & Bob
"Los Gatos Casita"

[B-Man]

Originally Posted by JDT

Since the charging I would get is negligible would it be okay just to disconnect the charge line and not worry about it?

You have to answer 1 question:

Will I drive far enough, with the refrigerator running, to deplete the on-board battery before I get to my next destination?

If the answer is No, you don't need to worry about charging from your tow vehicle.

If the answer is Yes, you should probably look into a method for the tow vehicle to supply enough power to at least run the refrigerator so the battery isn't depleted.

Everything else is a moot point if you always camp with some type of shore power available.

[JDT]

Like I said, we never boondock. We don't have the trailer yet as it is on order. I looked at the features as I knew it had solar but wasn't sure how. It comes with a 200 watt system. The only thing I can think would need power is the 12 volt fridge. I ordered a 230ah LiTime battery which should not need much in the way of charging as our longest drive is around 5 hours. Then we have full hook ups.

[Sunnyfalls]

Originally Posted by JDT

Like I said, we never boondock. We don't have the trailer yet as it is on order. I looked at the features as I knew it had solar but wasn't sure how. It comes with a 200 watt system. The only thing I can think would need power is the 12 volt fridge. I ordered a 230ah LiTime battery which should not need much in the way of charging as our longest drive is around 5 hours. Then we have full hook ups.

You are all set. Happy trails!

Gayle & Bob
"Los Gatos Casita"