Electrical Energy Storage

Whilst I have a Tesla Powerwall on order and due fairly soon, I have been thinking generically about how batteries and energy storage would work best in my smart home. The Powerwall provides 13.5kWh 'usable storage' but, I could always use more.

All of the products on the market have limitations and little or no smart home features or support. Most use proprietary apps and cloud services for monitoring and control. There has to be a better way! This project aims to find that better way.

Requirements

Regardless of the underlying battery technology, the way I want to store and use electrical energy remains the same. I'm assuming an 'ac coupled' system that works in isolation of (but complements) 3^rd party technologies. This will give me complete freedom to store and use stored energy as and when I need to. My requirements are quite simple:

In normal operation the battery should charge using surplus generated energy and discharge to avoid pulling energy from the grid at times when demand exceeds generation capability.
There are times when you want to force charging a battery as fast as possible to store lots of energy in a short period of time. Typically this would be using cheap rate electricity over night.
There are times when you want to force charging at a very low rate, to avoid the battery discharging into a load, e.g. whilst charging an EV on a cheap rate tariff.
There are times when you want to force discharge into the grid at a set rate, to maximise your Return On Investment (ROI).
When the battery reaches 0%, it should stop delivering energy and wait to be charged.
This winter (2022) was an eye-opener for me, with many people complaining that their batteries were essentially useless in the cold weather. Like my Tesla Powerwall, my solution will use internal self-heating, to ensure it works in all weather conditions, whether it is installed inside or outside.

If you have enough storage, you can avoid using peak rate electricity. If you have even more storage, you can avoid importing electricity at all.

Battery Management System (BMS)

At the heart of any battery storage is a BMS. The BMS manages the charging and discharging, to ensure the batteries are protected and operate within their design parameters. It plays an essential part in ensuring their reliability and long lifetime. The BMS is specific to the battery technology used as it needs to know about the characteristics and the specifications of the component batteries.

One think I like about the Tesla Powerwall is the 'usable capacity' of 13.5kWh. This means you can safely charge it to 100% and discharge it to 0%, without worrying about minimum/maximum 'State Of Charge' (SOC). It actually has a 15.1kWh battery inside to enable this.

Energy Conflicts & Safety

With multiple battery systems, from multiple manufacturers, it is important that they don't conflict with each other or other equipment in my home. I don't want one battery trying to charge from another and I don't want a battery being used to charge an EV either. My contextual smart home will be responsible for ensuring these things don't happen.

There will be a manual isolation switch for the energy side of the battery and it will then report its state as 'Isolated', when this is enabled. This obviously means that the monitoring and control elements remain operational.

Monitoring & Control

My battery system will feature a real-time clock, to enable accurate monitoring and data logging.
It will monitor charge and discharge rates, with energy tracking over time.
It will feature an open API to enable control and smart home integration using my unified communications protocol.

Simplistically, I want my smart home to be able to set the battery 'mode' and 'rate' of charge/discharge. The mode will be one of 'Normal', 'Charge', 'Discharge' and 'Isolated'.