« on: August 13, 2022, 10:16:51 pm »
I have a few trips under my belt after swapping my AGM batteries for two 100Ah Battle Born LiFePO4 batteries. I thought I'd share what I've learned (or at least think I've learned) thus far. Hopefully, this will help anyone else considering the swap, and also documenting what I've learned, in case I forget! There are also some excellent threads on this board with great info that have helped me tremendously. First my equipment (all factory installed): 2022 2552 with Progressive Dynamics PD4575TB (PD told me the TB stands for Terminal Board) converter charger and easy access switch for Lithium or Lead Acid charging profiles, two 150w solar panels, Samlex SCC30AB 30-amp solar charge controller with both Lead Acid and Lithium charging profiles, no dc-dc charger between the alternator and the coach batteries. I added a Victron Smart Shunt to monitor the batteries and charging.
Before I swapped batteries, I called the PC factory, and they told me Battle Born had visited the factory and concluded no modifications were necessary to change the AGM batteries to lithium. In fact, they said that when someone orders a new coach with the lithium battery option, they don't change any equipment, except the batteries. (This applies obviously to newer coaches.) The batteries charge in one of three ways: 1) From the alternator when the engine is running, 2) From the PD4575 converter/charger when hooked to shore power or if the generator is running, 3) Solar. I'll address each separately.
1) Alternator to coach batteries: There has been some excellent discussion on whether a dc-dc charger was necessary to keep the alternator from providing too much current due to the very low internal resistance of lithium batteries. Using Ohm's Law, V=IR, or I=V/R, the lower the resistance, the higher the current for a given voltage. Since the voltage in the equation is the difference between the supplied voltage (measured at the batteries) from the alternator and the present voltage of the lithium batteries, it is typically only going to be somewhere around 1-2 volts. I've never measured the exact voltage supplied by the alternator at the batteries, but due to the smaller wire gauge (hence higher resistance in the wire and some voltage drop), the voltage supplied from the alternator isn't much above the battery voltage (which of course varies depending on the batteries' State of Charge or SOC). I don't know for sure what wire gauge PC uses from the alternator to the batteries, but I suspect it is small enough that the wire resistance plays a small role and somewhat makes up for the low internal resistance of the batteries.
The upshot of all of this is that I did not install a dc-dc charger and the most I've seen supplied via the alternator is approximately 20 amps - no problem. I suppose if I let the batteries drop to an SOC=0, resulting in a very low voltage, I could see a significant spike in the amps, but since I have solar, I don't anticipate this. If, for some reason I did let the batteries drop to a very low voltage, I could just run the generator a bit, to get them to a reasonable voltage before running the engine, to prevent a possible over amping situation. A dc-dc charger would eliminate any concerns, but I monitor the battery parameters and charging via my Smart Shunt on the Victron Connect app while driving, and again, I've seen no reason to install one at this point.
2) PD4575 converter/charger: I switched the charging profile on the PD4575 to the "Li" position for the lithium profile. This has caused me problems. The Li profile supplies a constant current until the batteries reach 14.6 volts. However, when the batteries are not quite topped off, this resulted in the PD4575 supplying almost 55 amps. Unfortunately, PC installed a 50 amp auto-reset Type 1 mini-breaker, and this kept tripping and resetting. When the batteries approach a high SOC, the current drops off and the problem resolves. However, in the Li charge profile, the PD4574 remains at a constant 14.6 volts. I watched a video with the CEO of Battle Born and he said "floating" the batteries at 14.6v for a week or so, is not a problem for the batteries, but some DC devices in the coach might not like constant 14.6v.
To keep the 50-amp breaker from tripping, and to avoid the constant 14.6v float issue, I tried the LA, or lead acid position. It has three modes PD calls Boost, Normal, and Storage. Boost mode (which I believe is a constant current mode, commonly called Bulk mode) supplies a constant current until the batteries reach 14.4 volts. This resulted in a constant 48 amps and did not trip the 50-amp breaker. After approximately 4 hours at a battery voltage of 14.4v, the charger switches to Normal mode and supplies a constant voltage of 13.6 volts. This is ideal for a float voltage for the Battle Born LiFePO4 batteries. However, if there is no "significant" battery usage for 30 hours, the charger switches to Storage mode and supplies 13.2 volts. It then increases voltage to 14.4 volts every 21 hours for a 15-minute duration. (Battle Born states that the batteries should be charged at 14.2-14.6v for approximately 15 minutes for each battery in the bank, or in my case 30 minutes, to balance the batteries.) The problem is that the LA profile will not switch back to the Boost mode until the batteries reach 11.9 volts. This works well for lead acid batteries but is far too low for LiFePO4 batteries and it is not adjustable. Fortunately, the PD4575 has a built in "Charge Wizard" and you can hold the small blue button on the front, until the LED is solid green, and it will put the unit in Boost mode for up to 4 hours. Not sure whether it drops back to Normal or Storage after the 4 hours.
3) Solar: The installed Samlex SCC30AB solar charge controller is a PWM controller and has two options for lithium charging. They are both very similar and only change voltage by .1 volts. In the lithium profiles there are three phases, Bulk, Absorption, Float. Bulk is a constant current until the batteries reach 14.3/14.4v. The charger then switches to Absorption, or constant voltage of 14.3/14.4v for 30 minutes. According to Battle Born, this is perfect for "balancing" the cells. It then switches to Float of either 18.104.22.168v. Again, ideal Float voltage. Unfortunately, it does not switch back to Bulk mode until the batteries drop below 13.0v, which is about a 30% SOC. This is not adjustable. You can put a heavy load on the batteries, by disconnecting shore power or the generator, and running the inverter and some heavy load devices, to drop the voltage below 13.0, but that is a bit kludgy.
I have decided to switch to a Victron 100/30 SmartSolar charge controller. It will allow me to check parameters via the same Victron Connect app that I use to monitor the Smart Shunt connected to the batteries. I currently have to crawl on the floor to see the display of the Samlex if I want to see any info. It will be nice to just use the app for the Victron. The biggest advantage is that it is much more programmable and has a "re-bulk" mode. You can specify an offset voltage, below the Float voltage, to trigger the bulk mode. For example, if the Float voltage is 13.6 and you set an offset of .2v, the unit will re-enter the bulk mode when the batteries reach 13.4 volts. This would allow me to keep the PD4575 in the LA mode (to avoid the constant 14.6v float in Li mode) and control entering the bulk mode much more precisely with the Victron solar controller. I'll eventually figure out a proper replacement for the 50-amp mini breaker (and wiring if necessary), since even in the LA mode, the PD4575 pumps 48 amps. I believe the rule of thumb is to have a breaker that gives you a 20 percent buffer.
As for the lithium battery performance, I'll just say, I'm glad I switched. Much less voltage drop under load, more actual capacity because you can drain them further without worry/damage, they don't lose charge when sitting in storage, and I don't have to remove them in the winter. Long-winded...sorry, but perhaps this will give someone considering the swap to lithium a starting point for their research.