Question ❓Riddle me this Batman (LiFePo4 vs. Vehicle Alternators)

[ADR1RU]

I’ve read about various tow vehicles destroying their alternators across the internet when trying to charge LiFePo4 Batteries in route to/from campgrounds. To allievate such supposedly* a DC-To-DC Converter is required, so the LiFePo4 batteries are isolated from your vehicle’s alternator.

My LiFePo4 batteries set at ~13.5 to 13.8 VDC at rest in a 100% SOC. Meanwhile my Jeep’s 7-Pin Trailer receptacle rests at 14.6 VDC while idling. That said, what would trick the Jeep’s alternator into thinking it had to charge so extensively (with LiFePo4 batteries in my TT) as to cause it to somehow fry itself

*supposedly, because I have yet to see a warning about this on the various LiFePo4 battery vendor websites.

 

[Richpatty]

Lithium batteries will pull a lot of Amps, which can overload an alternator. It depends on how many Amp hours of lithium’s you have and the size of your alternator.

 

[redbaron]

I just setup my JL Wrangler to tow a small trailer with a LiON 208Ah battery.

My jeep did not have a 7 way, so I had to install the Mopar 7 way battery. Doing the research, I found that the voltage coming from the 12v on the 7 way was only a 10Awg wire, and with the distance I was going, anything over 10amps would burn up the wire.

The battery will pull as many amps as allowed, so it would put a constant drain on the wire and potentially stress the wire.

I also was not happy with just 10amps, as I had converted the RV to a 12v refrigerator and wanted it running all the time along with charging the battery.

My solution was to also run a 4Awg wire to and Anderson connector. This wire connected directly to a 200Amp Solenoid, and then to a 40amp breaker before going to the battery. I am using the solenoid to turn on the charge line with the ignition, thus when parked, the trailer does not pull from the Jeep battery.

I also installed a 30amp DC-to-DC charger in the RV, directly in line with the 4Awg wire before the battery. Now I can charge at 30amps and use a profile that will allow the smart alternator to cycle and not overheat.

This was a test for my next upgrade which will be on my parents 2019 Canyon Star, and then finally I will upgrade my 2022 LADP.

 

[ADR1RU]

Thanxx for the info. In doing a deep-dive search on the LionEnergy.com website I did turn up this tidbit:

For Motorhomes​

Most RVs will charge your batteries from the engine’s alternator. If you have this setup, you need to have a DC-to-DC adapter (not sold by Lion Energy) to prevent backflow.

 

[Chuggs]

I believe the problem is not one of voltages. It is in the internal resistance of the batteries. The LiFePO4 battery has a low internal resistance…tag a bunch of them together and it’s almost like dropping a metal bar across your battery terminals. The amperages are driven up and the poor alternator can overheat to the point of failure. If you regulate the amperage (dc-dc charger) you can mitigate it. The LiBIM uses a duty cycle algorithm to allow a charge for a set number of minutes and then stop to allow the alternator to cool back down. Many of the LiFePO4 batteries can accept a 1C charge rate…a 100aH battery can basically handle a 100a charge. If you have a 30 or 50a dc-dc charger that’s probably adequate for a 100aH bank…many folks have 600 to 1200 aH banks…so, you play a game of maximizing what your alternator can safely handle…whilst speeding your banks recovery while driving to the next location.

The nice thing is… they have a pretty constant and slightly higher voltage which makes the amps your loads need not be as high. And they don’t waste the energy going in or coming out to heat loss like lead acid batteries. If you are familiar with the Peukert effect…. an example would be a 100aH battery. At the standard rate of 20 hrs…your load would have to be 5amps figuratively. If you drew twice that…or 10 amps your battery would last 10 hours, right? Well no…because of the internal resistance and the Peukert effect…it might only last 9…You lost some to heat. The Lithiums allow you to run higher discharge rates without loosing as much. Very nice problem to have.

 

[ADR1RU]

I believe the problem is not one of voltages. It is in the internal resistance of the batteries. The LiFePO4 battery has a low internal resistance…tag a bunch of them together and it’s almost like dropping a metal bar across your battery terminals. The amperages are driven up and the poor alternator can overheat to the point of failure. If you regulate the amperage (dc-dc charger) you can mitigate it. The LiBIM uses a duty cycle algorithm to allow a charge for a set number of minutes and then stop to allow the alternator to cool back down. Many of the LiFePO4 batteries can accept a 1C charge rate…a 100aH battery can basically handle a 100a charge. If you have a 30 or 50a dc-dc charger that’s probably adequate for a 100aH bank…many folks have 600 to 1200 aH banks…so, you play a game of maximizing what your alternator can safely handle…whilst speeding your banks recovery while driving to the next location.

The nice thing is… they have a pretty constant and slightly higher voltage which makes the amps your loads need not be as high. And they don’t waste the energy going in or coming out to heat loss like lead acid batteries. If you are familiar with the Peukert effect…. an example would be a 100aH battery. At the standard rate of 20 hrs…your load would have to be 5amps figuratively. If you drew twice that…or 10 amps your battery would last 10 hours, right? Well no…because of the internal resistance and the Peukert effect…it might only last 9…You lost some to heat. The Lithiums allow you to run higher discharge rates without loosing as much. Very nice problem to have.

Great info, Thanxx for sharing.

In thinking more about it, I realize an overload could take place (for the vehicle alternator) upon leaving camp with several depleted batteries.

For an occasional camper (w/o a DC-DC converter installed), could one just disconnect their LiFePo4 batteries in the TT when heading home afterwards? I assume the vehicle’s 7-pin power/lights plug supplies adequate power to activate the brakes.

 

[Chuggs]

If your rig is setup to charge, you could modify it. Just try to make sure the a battery of some sort is wired thru the breakaway switch for the trailer brakes. Don't think you have to worry about the alternator being over taxed if the breakaway cable has been pulled.

 

[Kevin D Pem]

Chuggs is spot on! To add a little bit, the DC to DC converter is used more as a current limit circuit than a voltage converter. It will take care of any voltage drop in the run as well! If the voltage is to low charging can be limited.