Off The Grid

Here is a real world example of what a properly functioning MPPT does. It's taking 184v @ 5amps from the solar panels and converting to 56.8v @ 15.8amps for the batteries.

Solar Battery Chargers. These come in two varieties. PWM are the older ones and MPPT are the newer ones. 1)PWM. In short, don't use these. They are not efficient. If these were a car, you would have a one speed transmission and would have to slip the clutch to make the car move. Because of the way they work, solar panels end up not operating in their ideal voltage range. Back about 15 years ago they are all that was available. Most commonly seen in 12v systems like the Harbor Freight solar kits. 2)MPPT. If MPPT chargers were cars, these would be the CVT (constant variable transmission) version. These have a computer on board that finds the sweet spot in the solar panel's power output and keeps it running at that level. It converts that power down to the voltage of your battery. An example is you have 4 solar panels making 2000watts at a specific voltage and amps. if you reduce the load on them, they "rev" too high and get out of their power band. if you load them down too much they drop to a lower "RPM" and make less power. The MPPT's job is to figure out what "speed" these panels make the most power at and convert that down to the battery voltage. Let's imagine that the panels are making 10amps at 200vdc but the battery is at 50v. In my car analogy, we need a 4:1 ratio transmission.... and so the MPPT ends up converting 200v down to 50v and the amps goes from 10amp from the solar panels to 40 amps into the batteries. 200v x 10amp = 2000w.... and 50v x 40amp = 2000w. It does this with a 99% efficiency.

7)Charging methodology. When charging an LFP battery our goal is to try to get all cells over 3.45v and balanced. But once the cells are balanced and very closely match each other's voltages there is no need to keep the cells at such a high voltage. The typical charging profile is that the BMS will ask for 56.8v and if the temperature is above freezing advise the inverter or MPPT chargers to send all the power they can. This is called "bulk charging". When the battery voltage reaches 56.8v (3.55v per cell), the BMS will usually wait 2 hours. This is called "absorption" (the word is a hold over from lead acid battery days, in our case it should be called "balancing"). After two hours, we assume the battery is balanced and there is no need to maintain such a high cell voltage. The BMS will advise the inverter or MPPTs that the battery now wants to stay at 55.2v for the rest of the day. The inverter will draw down the battery to 55.2v and maintain it at that voltage. 8)State of Charge. Because LFP cells have such a flat discharge voltage profile, it is impossible to tell state of charge by simply measuring the pack voltage. Good BMS's will measure amps in and out of the battery to keep an "odometer" of sorts. When a battery is full, the BMS will reset this "odometer" to say 100%. This is subject to small errors so the more days you run without hitting 100% the more uncertain the BMS will be about the actual SoC. We like to have a battery hit 100% every week or so if the BMS is measuring the amps in / out. An exception to this is if you use an external Victron Smart Shunt. This device is so accuate that you could go a month without hitting 100% and still be within 5% of actual SoC. Take aways.... LFP don't catch fire. BMS keeps battery cells within a safe range. Try to design a system that can fully charge your batteries at least once a week, weather permitting.

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.

Four kids added to the family. EDIT: we moved mama and kids to the "birthing apartments" this morning. yesterday when her water broke, she was not interested in following us. So this morning we carried all four to the small pens in the barnyard and she's settled in there nicely. All the kids look good and they are all female.

Made a mini greenhouse for seed starting

We got a mama goat in labor! Her water just broke. Will we have baby goats soon? Details at 11.

Thermal Mass Greenhouse Construction https://t.me/OnlyFaithOverFear

Bringing the wood-fired flavor home! This ingenious DIY barrel oven is perfect for those crispy crusts and artisan loaves right in your own backyard. https://t.me/OnlyFaithOverFear

✨👁️✨ We love cutting watermelons… Rusty Cage