Adding LFP to an AGM bank. The AGM is a healthy single 12V 100Ah battery located under a dinette seat in the trailer. I will be acquiring a 100Ah LFP bank. This will increase my Ahs from 50 for the AGM alone, to 130 by adding 80%, or 80Ah from LFP.
T he dangers are:
1. Protecting the LFP:
a. The BMS must shut the LFP off for all the over and under limits such as too hot or cold, etc.
b. The AGMs need to be the same Volts before connecting in parallel or a burst of current could cause damage.
2. Protecting the AGM:
a. AGMs over/under Volts or temps are more forgiving.
b. The LFPs need to be the same Volts before connecting in parallel or a burst of current could cause damage.
Charging devices:
1. Magnum MMS-1012 must be connected through the LFP’s BMS. The best available charge and float Volts are 14.5, followed by a float at 13.5V. The only other control is to use the remote to turn the charger on/off. This device could be a problem.
This process must be performed without the Battery Temperature Sensor connected because the charge Volts will be raised higher when the battery is cool. The 14.5V is barely lower than the allowed max of 14.6V for LFP. In cell terms that is 3.625 compared to 3.65. The LFP must be well balanced so that no cell exceeds 3.65V during charging.
2. Morningstar Corp MPPT charger must be connected through the LFP’s BMS. This is very programmable and can avoid BMS many shutdowns. Per the specs, it does require the AGM in parallel to know battery Volts(12/24) in case the BMS shuts the LFP off.
This device will be set to charge = 13.8/3.45 Volts and Float = 13.5/3.375 Volts for AGM /LFP. It will have the BTS connected and the max charge Volts, or HVD = 14.4Volts.
Operation:
After a charge each chemistry will settle to different resting Volts; AGM to 12.8 and LFP to 3.375. This happens within a few hours and both figures represent fully charged. What you want, is to begin each night with is sufficient power to last until morning.
At the end of daily charging, batteries in parallel will be at equal Volts. While supplying power, the battery with the most Volts will supply Watts to the loads until the two are equal again. Even without a load, they will try to stay equal by passing current to the lower one. The LFP will reach resting Volts first. At that time it will supply current to the AGM. The AGM needs to be healthy or it could draw excessive current. The LFP will continue to carry loads until the AGMs resting Volts are reached.
Until now, the AGM has not contributed much to the load. Now, for the rest of the night, 50% of the AGM capacity is available plus whatever is left in the LFP .