Battery Basics #
First things first, batteries do not “create” energy, they simply store energy for later (be that from excess solar, wind or from being charged up from the grid overnight.) As such you do need an energy source to feed into them in the first place.
(A note to the nit-pickers: to an extent you can argue that they technically consume some energy through DC losses & internal circuitry /BMS power requirements but lets not get too far into that.)
As we have already mentioned, the energy stored is measured in kWh. The batteries output is measured as power in kW.
So, why do you need batteries. The short answer is you don’t have to have them, but it does enable you to store up your excess solar to use at a later date / boost your output from your inverter if you don’t have enough PV / charge them up from cheap overnight electricity and discharge through the day.
There are various different brands of battery, some work with numerous inverters (e.g Pylontech) others are a closed ecosystem and only work with their specific inverters (Givenergy, Alpha ESS and so on…)
As this is the Givenergy Wiki we will obviously focus on those but first lets dissect what is inside a battery:
What’s Inside #
Externally you have a nice shiny box with some cables going in & out. But what’s actually inside?
Each battery ultimately contains something called cells, these act similar to any other rechargeable battery (say a AA battery) but just on a much larger scale.
There are a few different chemistries & physical formats to be aware of (note the naming is VERY easy to confuse) but the most common types in home energy storage are:
Lithium Iron (LiFePO4 / LFP) – Nominal Voltage 3.2V
These are the most common type found in home energy storage. They often blue and rectangular (Prismatic) but can also come in cylindrical formats although these are much less common in home storage.
They offer a good energy density per cell (the amount of energy they can store) vs their size & weight, although this tends to matter more in EV applications as opposed to static storage where space & weight is less of a concern.
Acceptable charge / discharge rates (C-Rates) but lower than those of Li-Ion chemistry.
Often warranted for a higher number of cycles than Lithium-Ion
They also benefit from being generally very difficult to force into an unstable state or catch fire.
Givenergy & many other home storage brands use these type of cells.
Lithium Ion (Li-NMC / NCM) – Nominal Voltage 3.6V-3.7V
These are generally cylindrical (18650 / 2170) type cells which most will know from either laptop batteries, vapes or even Tesla EV batteries.
These offer a higher energy density given the same space constraints of an LFP & also weigh less per kWh of energy stored.
They are able to withstand higher charge & discharge rates (C-Rates) although to be honest the benefit of this is more useful to peak power output for EV motors than running your house!
They do also have some risk from thermal runaway & therefore some storage systems utilise coolant & sophisticated temperature monitoring to help combat this issue.
Tesla Powerwall & very few others utilise these type of cells.
No matter the chemistry, the cells are arranged in such a way that the total voltage of the pack is in the “nominal” voltage range for an inverter. This can be any voltage from 12V to hundreds of volts (for HV batteries)
For Givenergy (and most other LFP home storage batteries) this nominal voltage is 51.2V. So how do we end up at this voltage?
Well going back to the nominal voltage of each LFP cell we know this is a value of 3.2V. (Nominal Voltage is the average voltage the cell will output. Actual Voltage can be higher or lower depending on how charged up it is)
There are 16 cells inside, wired in series, which means the voltage increases by 3.2V for each cell in the chain.
16 x 3.2V gives you 51.2V.
Many popular brands are around this voltage although Pylontech are one of the few that are a little different. They have 15 cells inside so they actually run at a nominal voltage of 48V (so don’t try to mix and match with a 51.2V pack!)
So now the cells are all joined together we just attach some cables to the inverter and away we go?
Not quite: Lithium batteries have some specific charging & discharging characteristics that need to be managed to ensure they last as long as possible. Sitting on top (often literally) of the battery is a Battery Management System (BMS)
It’s the BMS’s job to act like the conductor of an orchestra, reading what the cells are doing and telling the inverter what’s going on so optimum conditions are met (such as voltages, how full the cells are, what current can be drawn from the battery….)
It’s basically a safety supervisor sitting between your inverter & cells making sure you don’t exceed pre-set limitations & giving you a reliable battery you can use for up to 10+ years.
They also carry out some other long term health functions that lithium batteries need called Cell Balancing.
Cell Balancing #
This is a function carried out by the BMS to ensure that all the cells in the pack (16 of) are as close as possible to being identical in respect to voltage.
In an ideal world, you would not need to perform this function but alas the world of batteries is not perfect. Each cell has it’s own minor discrepancies (capacity, resistance…) in it’s production and therefore will behave slightly differently than an adjacent cell when in service.
This is most commonly shown as a mismatch to adjacent cells in respect to voltage over a period of time & charge / discharge cycles. It can be a very small difference of mV.
It doesn’t sound like much but over time this can result in either a reduction in the cell capacity or damage to the cells depending on if the drift pushes cells beyond their specification (too low or too high voltages)
All is not lost however as the BMS has a few tricks it can pull to pull them all back to the same voltage by doing some called Cell Balancing.
When the battery is full, the BMS is able to individually bleed or charge cells by very small amounts (depending on if the cell is showing as under or over charged) such that they all become inline again. It’s a slow process but cumulatively over time this helps prolong the life of the battery by keeping the cells as close as possible to specification.
For Givenergy Batteries this process is entirely automatic so you don’t need to worry about performing it yourself through a setting.
Capacities & Depth of Discharge #
Work ongoing…
