RV House Battery to LiFePO4 conversion questions / concerns / advice

[M109guy]

Looking to convert house batteries to 280ah LiFePO4 cells. I was hoping to get some advice to make sure I don’t miss something in the conversion. Currently have 6 - 12v AGM’s. The RV has a full size all electric residential fridge. I’ve installed about 1400 watts of solar on the roof.

1. Any possibility of damaging the alternator? (2017 Cummins Diesel)
A. Have read the alternator can get damaged running lithium cells. Is this true with the new 280ah cells?
If so best way to prevent damage.

2. Batteries are charged 3 ways - Shore power (Magnum 2800 watt Inverter), Victron solar charger, and engine alternator.
A. Will the batteries get overcharged by the alternator or will the bms prevent this?

3. Any other tips would be appreciated.

Thank you!

 

[FilterGuy]

1. Any possibility of damaging the alternator? (2017 Cummins Diesel)
A. Have read the alternator can get damaged running lithium cells. Is this true with the new 280ah cells?
If so best way to prevent damage.

Yes!!! If you hook the cells directly to the alternator, there is a good chance you will burn out the alternator.
To prevent this you should put a DC-DC charger between the alternator and the batteries. I like the Victron Orion Smart... but there are outhrs out there. Note: The temptation will be to put the highest current one you can on it. I would not recommend going about ~40A... (It depends on the specs of the alternator.... Don't go anything more than 1/2 the rated current of the Alternator

2. Batteries are charged 3 ways - Shore power (Magnum 2800 watt Inverter), Victron solar charger, and engine alternator.
A. Will the batteries get overcharged by the alternator or will the bms prevent this?

The BMS would prevent it..... but that is not the way you want the system to opperate. The BMS should only cut out if the other chargers fail to shut off. Put the DC-DC charger between the alternator and the batteries and program it to cut out before the BMS does.

3. Any other tips would be appreciated.

* Don't buy anything till you have your full system designed. It is far to common for folks on the forum to get excited and start buying stuff only to learn they bought the wrong thing.

* A lot of lead acid based systems do not have a low voltage cut-off on the loads. I would definitely add some Victron Battery Protects to your system to keep the DC loads from draining the battery too far.

* install an SOC (State Of Charge) meter in the system.

• @Will Prowse likes the Victron Smart Shunt.... that is a great little device but I do not like the fact that you have pull out your phone and open an app to see the SOC. In a boat or an RV, I prefer to just glance at a display to know the battery status.
• Victron has a battery monitor with a display. This is also a great device but is very pricey.
• There are a lot of $30-$50 SOC meters available on Aliexpress. One that Will has favorably reviewed and I have used to good effect is the Aili battery monitor.

 

[M109guy]

Thank you so much for your reply. All good advice.

Adding lithium batteries is close to my last step.

Below is my current configuration
I have 1400 watts of solar, a victron 100 amp solar charger & a Victron battery monitor BMV-712 installed. Current battery setup is 6 group 31 agm’s for the house batteries and 2 large starter batteries.
2017 Cummins with around a 150 amp alternator.

Planning on 3, maybe 4 280ah lithium batteries. Hoping to build these myself.
When I talked to battleborn they said theirs were a drop in replacement and I didn’t need to worry about the alternator being hurt or over charging their lithium batteries.

(Victron Orion-Tr Smart DC-DC Charger Isolated - 30 amp output) is this the dc to dc charger you were talking about?
Seems like it would really slow down the charging process considering I may have up to 1000ah of battery storage.

I’m taking your advice on this step and going slow and not purchasing anything else till I know what design will work. I don’t want to damage the alternator or the new batteries.

This is where I really need some guidance and would appreciate design thoughts so I can do this right.
Thanks to this forum I’ve leaned a lot but I’m still not sure what to do to replace my AGM’s safely.

 

[Hedges]

AGM batteries don't mind the cold and are almost impossible to freeze unless deeply discharged.
They do require higher charge voltage at lower temperature, but charge controller with a temperature sensor takes care of that.

Lithium (most chemistries including LiFePO4) can't be recharged below 0 degrees C. At room temperature max charge rate is typically 0.5C, but this reduces to about 0.07C near zero degrees C.
BMS must disable charging below freezing. I suggest determining what "C" rate your charge sources could deliver, and setting BMS charge disconnect accordingly (could be higher than 0 degrees C.)

4, 12V 280 Ah batteries would be 15kWh. With 1400W PV, you have about 0.09C (ignoring PV PTC vs. STC etc.) so zero degree cutout should be fine.
Magnum 2800 charge rate is 125A, 1700W so slightly higher.
If it is possible to charge with Magnum and PV at the same time ... don't do that when batteries are cold.

Your inverter probably draws about 300A at full load (and lower battery voltage), so make sure the BMS can provide that much with appropriate margin. Some brands, people here say derate 50% from specs. Try for symmetry to balance current draw, but assume it will still be somewhat imbalanced.