DIY LiFePO4 zero-export apartment-scale PV

Living in Germany we are facing sky rocketing primary energy prices due to a neglected renewable energy transformation in the past. However, with more and more attractive pricing of PV modules and LiFePO4 batteries, even small scale systems on your balcony can help alleviate the pain of your monthly utility bill.

What’s possible in Germany

As of May 2023, it is allowed to feed up to 600W into the energy grid without much bureaucracy. You only need to notify your utility infrastructure company and the German Bundesnetzagentur.

Currently you are supposed to use a special Einspeisesteckdose (directional power socket), due to a VDE guideline. It was already announced, that this will be changed to the standard Schuko socket in the next iteration of the guideline.

The PV subsystem

Depending the situation and mounting options, we are currently using either two 400 Wp panels or multiple smaller 50 Wp panels with a total of 200 Wp at the moment. Using smaller panels has the advantage of more modular setup, which can be dual used as a sun shade on your balcony. Stay tuned for a new post, as we’ll try to have it extend and retract automatically. The mechanics are still to be figured out though. 😉

The easiest way to integrate the solar panels into your home grid, is to use a micro inverter like the Hoymiles HM-600. If you look for it, you’ll find multiple different offers of ready made sets, including panels and the suitable micro inverter.

Just plug in the panels into the micro inverter and the micro inverter into your power socket and you would be done. But that would be too easy.

The energy storage subsystem (hardware)

Instead we paired the panel with an additional energy storage system, using a LiFePO4 battery to storage excessive power. Otherwise the generated power, we don’t use ourselves at the time would be lost to us, and given away to the energy supplier without financial compensation.

The advantages of using LiFePO4 batteries instead of lead-acid batteries (or the more advanced AGM batteries) is the higher cycle count (in the order of 3000 vs 300) and the more usable long term capacity. If you’d choose lead-acid batteries, you shouldn’t discharge them by more than 50% of their nominal capacity. However, fully discharging a LiFePO4 batteries repeatedly is not an issue.

The normal advantages of LiIon batteries like higher energy density and higher (dis)charge rates aren’t as relevant in this stationary use case. And LiFePO4 aren’t known for exploding spontaneously, like their LiIon counterparts. 😉

There are are a couple of gotchas you have to be aware of, when using LiFePO4 batteries like the requirement to use a battery management system (BMS) and that they don’t like to be charged in environment below 5°C. Nowadays, BMS are typically integrated in the batteries.

All in all, while the LiFePO4 has the higher initial investment, in the long run, it will be much cheaper, due to the longer life span. We chose a 24V system, for example the 100Ah system by LiTime or 60Ah by Kepworth. There are a bunch of different more or less trustworthy resellers online. Choose your own, with the best shipment conditions depending on your location.

To charge the batteries from the energy from the PV modules, we use the Victron SmartSolar MPPT 75/15.

Putting everything together

Normally micro inverters feed in all the power they get from the panels into the power grid, as it has no idea about the actual power consumption in your home. What we want is a zero-export system, so we can use the sun’s harvested energy, when we need it, not when we generate it.

To achieve this goal, we need to be able to monitor our current power consumption in the home, as well as regulate the power output of the micro inverter. Coincidentally, the Hoymiles HM series does allow for output regulation.

Using a Shelly 3EM in the electrical distribution box, we can know about the current current draw[sic] from the public power grid.

Already having a smart home controlled by Home Assistant, it is an easy task to feed in all data provided by the subsystems, combine them and regulate the inverter accordingly – no energy wasted!

The Hoymiles HM series does have a proprietary 2.4 GHz radio interface, which can be used with Hoymiles way too expensive gateway. Or we just use an ESP8266 from espressif, an nRF24L01+ transceiver from Nordic and some open source software (AhoyDTU or OpenDTU for an ESP32) to communicate between Home Assistant and the micro inverter.

Writing some simple automations we are now able feed in just enough energy from our system into our home. Let us know if you are interested in an Home Assistant blueprint.

Considerations when designing your system

Using 24V battery system to be compatible to input range of the micro inverter

Don’t forget to choose the panel’s open-circuit voltage (Voc) and short-circuit current fitting the needs of the MPPT charger. The Voc must be significantly higher then the battery’s end of charge voltage.

Limiting LiFePO4 to Depth of Discharge (DoD) of 80% and the State of Charge (SoC) to 10% to 90% increasing the lifespan considerably

To achieve this we came up with two different solutions. The expensive way is to use the Victron SmartShunt, a smart shunt[sic], which monitors the in- and outgoing current and the voltage from the battery to calculate the actual State of Charge (SoC).
Without additional hardware you can estimate (or more like guesstimate) the SoC simply by measuring the battery voltage alone. In case of the Kepworth 24V 60Ah battery an end of charge voltage of 27.1V and end of discharge voltage of 25.2V at 0.1C seem to achieve similar results. Keep in mind, that the voltages are heavily dependent on the current load (and probably temperature).
For accurate voltage readings of the battery, use the Victron device. You can either use its Bluetooth interface and the Victron BLE project based on an ESP32 or the VictronMPPT project connecting to the serial interface.

Thanks to the battery buffer, you don’t need to plan the micro inverter according to the peak power of the solar panels

For example, even though you have solar panels with a peak power of 800W, you can get away with micro inverter of far less than 800W (in our case 300W), as additional power will be stored in the battery instead.

Future improvements

  • Heating system for the battery using the solar power in environment under 5°C (winter, outside)
  • Adding AC charger (e.g Victron Blue Smart) to take advantage of variably priced energy tariffs
  • ???
  • Profit!

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