Use MPPT controller as DC-DC charger

[shadowsteve]

I'm changing out my 6 yr old AGM batteries (fifth wheel trailer) for Lithium and this means that my onboard DC charger likely won't be able to properly charge the batteries as it has no profile for Lithium.

The system is currently 640w solar into a Rover 40A MPPT to 4x Fullriver AGM and I have a Xantrex 2kw PSW inverter. At some point I may change to an inverter/charger but as the 2kw supports our needs I'm in no rush. I thought about using a DC-DC charger to connect to the existing trailer DC charger and then support the Lithium that way and then I figured that an MPPT controller is basically a DC-DC charger anyway. I have a spare EPSolar (Renogy) 40A MPPT CC and thought maybe I could feed the 12v input from the existing DC charger into it as if it came from panels and the CC would then boost it for charging?

Does this sound reasonable?

 

[fratermus]

Some thoughts:

Most MPPT including the Renogy/Epever will only buck (downconvert), not boost. If you can provide higher voltage then the MPPT can downconvert it. Examples: I've done this with a 24v power supply for shore power charging a 12v bank (link was proof of concept stage), and @rickst29 has done it from the tow vehicle.

Lack of a lithium profile does not necessarily rule out the onboard DC charger. A single-stage DC charger that puts out 13.4v or something would be peachy.

 

[MisterSandals]

thought about using a DC-DC charger to connect to the existing trailer DC charger

I am pretty sure powering a charger with a charger is not a good idea.

Do you know what your existing trailer charger's charging profile is? That would be the place to start; understanding the problem you are trying to fix.

I am using my old PD4645 (not lithium aware) with my LiFePO4 bank. It seems to work pretty well but its not very often that i plug into shore power since i have 330W on the roof. The new lithium aware PD equivalent charges to 14.6V which is way too aggressive for what i want to subject my batteries to (and its not adjustable).

So understand your problem, it will help find a solution you like.

 

[shadowsteve]

Thanks. The onboard power center is a Progressive Dynamic PD4500. It has a fixed profile as described below. I've not used Lithium batteries before so I'm not sure what their actual charge profile should look like.

From the manual:

The Charge Wizard Operation

While the built-in Charge Wizard automatically determines which operating mode is best suited to recharge or maintain optimum battery condition, the Wizard Mode Button allows for manual override and has indicator light(s) to indicate the mode of operation.

BOOST MODE – Indicated by green LED remaining on. The output voltage is 14.4VDC to rapidly recharge the battery up to 90% of full charge.
NORMAL MODE – When the battery is between 50% and 90% charged, the green LED will flash once per second. When the battery has reached 90% of full charge the green LED will flash 2 - 3 times per second. In this mode the output voltage is 13.6VDC and the converter is safely completing the charge of the battery.

STORAGE MODE – Indicated by green LED flashing every 6 - 8 seconds. In this mode the output voltage has been lowered to 13.2VDC, the RV
battery is fully charged and the converter is maintaining the charge.

MANUAL BUTTON - The manual button has been provided to allow the operator to temporarily override the converter (not recommended) or to verify the converter is operating properly. For manual operation, Press and Hold the button. The indicator light will soon remain “ON” indicating Boost Mode. Continue to hold the button and the light will blink rapidly indicating the converter is in the Normal Mode. Continue to hold the button until the light blinks slowly indicating the converter is now in the Storage Mode. After the manual button is released the converter will stay in the selected mode. When the battery charge status changes, the converter will return to the automatic mode of operation to prevent damage to the battery.

 

[fratermus]

The 13.2v storage mode to provide a "floor" might be the ticket. It's common for people to quasi-float Lithium at 13.2-13.6v to preserve capacity without stressing the cells.

Also see this article on PD's site:

Can I Use My Present Progressive Dynamics PD9200, PD4000 or PD4500 Series Lead/Acid Units With the Charge Wizard to Charge My Lithium Battery?​

Yes, if you are willing to live with an expensive battery that is only partially charged. The Normal Output voltage of the PD9200, PD4000 and PD4500 Series voltage provides only 13.6-volts and a full charge voltage for Lithium needs to be 14.4 – 14.6 Volts. The PD9200, PD4000 & PD4500 Series Charge Wizard will initially jump to 14.4-volts in the Boost Mode when first connected to 120 VAC power and will remain there until the battery voltage reaches 13.8-Volts, then automatically drops down to the Normal Mode of 13.6-Volts. The faster charge rate of Lithium means that in the Boost Mode it will reach this 13.8-Volt point after only a few minutes of recharging and then the charging current will drop to ZERO AMPS and will not add any additional charge to your Lithium Battery. This lower charge state will not damage your battery, but will eliminate most of the advantages you paid for.

 

[shadowsteve]

The majority of the time the batteries will be charging from the solar. The only time they'd be charging from shore power is if we happened to be in a park and there had been no sun for an extended period. A DC-DC charger might be the better option as I can switch it to the 7pin input to charge off the truck. Even then the solar puts in more than the truck does but the DC-DC unit supports Lithium and if it will boost then it's a usable solution. Just looking to avoid buying a Lithium charger when I'll eventually get a Li compatible charger/inverter (or just trade RV's and start all over )

 

[Smokin]

Diode

 

[NVS]

The majority of the time the batteries will be charging from the solar. The only time they'd be charging from shore power is if we happened to be in a park and there had been no sun for an extended period. A DC-DC charger might be the better option as I can switch it to the 7pin input to charge off the truck. Even then the solar puts in more than the truck does but the DC-DC unit supports Lithium and if it will boost then it's a usable solution. Just looking to avoid buying a Lithium charger when I'll eventually get a Li compatible charger/inverter (or just trade RV's and start all over )

So what did you decide on. I'm putting a DC-DC charger on my class c motorhome for the alternator to charge the lifepo4 battery. I'm running the bot the alternator and the converter through the DC-DC charger. I'm about 3 quarters done with the installation. Just need to put wire loom on the wires and connect the battery, which I'm moving underneath the dinette seating.

 

[shadowsteve]

So what did you decide on. I'm putting a DC-DC charger on my class c motorhome for the alternator to charge the lifepo4 battery. I'm running the bot the alternator and the converter through the DC-DC charger. I'm about 3 quarters done with the installation. Just need to put wire loom on the wires and connect the battery, which I'm moving underneath the dinette seating.

I've been charging with pretty much just the solar. A couple of times we were on hookups and before I left I used the onboard converter to top off the batteries. I'm not using the truck connection at all (fuse pulled). So far I haven't had any need for DC-DC charging