nixfwc said:
Thanks for the info. Rando, on your set up using the ACR, do you get a full charge from the alternator. Alternators for FLA batteries usually charge at about 13.9 volts whereas lithium requires 14.6 volts. is there also a bleed off from the higher lithium battery voltage to the lower voltage truck battery? Or does the ACR take care of that? I would like to be able to keep using the ACR and not have to buy a DC-DC charger. The BLUE SEA folks did not think ACR would work with lithium. I am using 10 gauge wire in the ACR circuit. Should the wire size be larger? This is sure helpful. Thanks
I am not sure where the idea that lithium batteries need 14.6V to be fully charged came from. It is absolutely untrue. There are very interesting (albeit quite technical) results from some actual test that Battery Stream performed on LiFePO4 batteries with their state of charge vs the charge voltage:
https://www.powerstream.com/lithium-phosphate-charge-voltage.htm
Note this are all for a single cell, so you need to multiply by 4 for a 12V battery, but to summarize the capacity vs charge final voltage for an LiFePO4 battery is:
12.0V = 2% SOC
12.4V = 4% SOC
12.8V = 8% SOC
13.2V = 31% SOC
13.6V = 99% SOC
14.0V = 99% SOC
14.4V = 100% SOC
14.6V = 100% SOC
What this tells us is that once the battery gets to 13.6V it already at 99% SOC. Now there can be some voltage drop in the wires, so maybe going to 14V to be totally safe is worth it. Going up to 14.6V is just increasing the risk of the cells becoming out of balance and shortening the life of you battery and there is no extra benefit. For reference, I set my solar charger to 13.4V when in storage and 13.8V when we are actively using the camper and want the battery to be full. On most all days the battery gets back to 100% using 13.8V from solar alone.
As for the ACR, yes it will stay on most of the time, and in effect the starting battery will be always float charging off your house battery. There is not really a down side to this, particularly if you have solar to charge things back up. If for what ever reason, your solar is not producing, then the ACR will disconnect the lithium battery at about 20% SOC, which is actually a great SOC to store it at. Unlike lead, lithium batteries prefer to be stored at a low state of charge. There is no risk of the truck battery being drained by the camper.
The reason to get a DC-DC charger is if you need maximum charging current from your alternator. Others suggest it is required to stop your alternator burning out from the load of charging the batteries, but with the long run of 10awg wire that you have, there is no way this will happen. Given the size of your solar system, I am guessing you don't need much from your alternator, so a DC-DC is likely not required, but you could add one if your alternator is not providing enough current. It may actually be cheaper to add the DC-DC then to upgrade the wire.
