Post #9604

For the newbies and lurkers.... A quick lessons in lithium batteries. LFP batteries are all the rage now. They will last 8000 daily charge cycles or 22 years. Sure they cost more than AGM or lead acid but once you replace the AGM battery twice, you have paid for the LFP. 1)Fire. When people think of lithium batteries they often worry about fire. A certain brand of cellphone was prone to burn and those 18650cells used in vaping devices too. Those are Lithium Polymer or they could be Lithium Nickle Cobalt Manganese. They are NOT what we use to power houses. Lithium Iron Phosphate (LiFePO4 or LFP) have a lower voltage per cell and do not catch fire. The remainer of this article will be about LFP since that is what we use to power most inverters. 2)Over and under charging batteries. Batteries are made of several "cells" in series. If you charge a LFP cell to over 3.65v or discharge to under 2.5v it will cause damage to the cell that adversely affects performance. If you over charge a 18650, LiPoly or NCM they tend to catch fire. The problem is you can't just monitor the entire battery voltage and assume that all the cells have the same voltage on them. In the case of 48v LFP batteries, there are 16 cells inside that have to be individually monitored. This is the job of the BMS. It constantly measures the voltage on all 16 cells and shuts off charging or discharging if any one cell is too high or too low. 3)Balancing. No two cells store exactly the same amount of energy. Some have a slightly higher resistance than others and convert power in/out into a minute loss of capacity. The common practice is to "balance" the battery when the cells are >3.45v. The way this works is that the BMS will monitor all 16 cell voltages and when any cell is higher than 3.45v the BMS will switch on a device that puts a small heater on that one cell, pulling it's voltage down in line with the other 15. Balancing is done when all 16 cells are between 3.45v and 3.60v and all the cells are within 0.010v of each other. Some BMS's use active balancing where they literally move energy from the highest voltage cell to the lowest voltage cell. 4)Temperature. LFP batteries like the same temperatures that you and I like. We should strive to keep them between 32F 0C and 120F (50C). A good BMS will shut off charging when a battery is too cold. 5)Voltage. LFP cells have 80% of their energy between 3.0v and 3.4v. Once they get below 3v they only have about 10% capacity left. Above 3.45v, they also have very little energy capacity per volt. This means that if you want to connect 2 batteries in parallel and one of them reads 3.45v per cell and the other reads 3.55v per cell, it's ok to connect them. There will be 100-200amps exchanged but only for a few seconds. On the other hand, if you connect two batteries at 3.2v and 3.3v, there could be >100amps for over a minute or two and it might cause damage to the BMS or wires. Long story short, the closer your battery is to 3.2v the more important two batteries are very close in voltage before you connect them together. 6)Communications. A BMS will measure voltage of individual cells and temperature. It will communicate requests to your inverter. For example, if the temperature is approaching 32F 0C, the BMS will advise the inverter that it does not want to be charged with more than 10amps. If the temperature continues to fall, the BMS will request 0amps. And if the temperature falls below freezing, the BMS will shut off the battery. If you do not use communication with a battery, never fear. The BMS will always protect the battery. Communication is nice because it allows some dog and pony stuff to happen and also communicates to the State of Charge to the inverter. Communication is not necessary.