Batteries need DC for charging and produce DC when they are discharged. A so-called battery inverter transforms their output energy to AC. On the input side some battery inverters take 220V AC whereas others take a DC voltage which is derivated from panel strings. On the output side the discharging electronics always delivers 220V AC as a single phase or in three phases – exactly matching the grid.
In an ES-Micro system the battery charger/discharger is a dedicated piece of hardware called the “battery inverter”. It takes 220 AC input:
grid (220 AC, 50Hz, 1phase or three phases)
| ^
| AC AC |
| |
v /--battery inverter--\ |
(charging module) (discharging module)
| ^
| DC DC |
+-------> battery -------------+In an ES-Hybrid system the battery charger / discharger is an integral piece of the solar inverter hardware. On its input side it takes high voltage DC derived from the panel strings:
solar inverter (battery charging module)
| |
| | DC HV
| +-------------> battery
| |
| +------------------+
| | DC HV
v v
solar inverter (battery discharging module)
|
v
grid (220 AC, 50Hz, 1phase or three phases)The word “hybrid” in “ES-hybrid” denotes the double functionality of the inverter.
Battery Cells and BMS
All batteries are made of cells with a very low voltage (2 .. 3V). Several cells are connected in series to produce the nominal voltage of a battery, say 12V for example. A battery will need a somewhat higher voltage to be charged (say, 14V) and its voltage may drop down a little bit below its rated voltage at the end of the discharging process (say, 11.5V).
Some fairly primitive battery types (which can only be combined with ES-Micro systems) use a voltage of 12 or 24V. Many others use 48V. The latter batteries come with a special piece of hardware and software (the BMS = battery managemet system) which controls the current for charging and discharging the batteries in a way that extends their life expectance. The BMS knows the current state of charge (SoC) of the battery, it monitors the temperature of the cells and can inform you about the general health state (SoH) of the cells.
ES -Micro systems usually operate with 48V batteries which have a BMS. The battery inverter communicates with the BMS via a special control link which often is labelled as RS485 or CAN (an abbreviation for the technical communication protocol). All batteries up to 48V are called “LV” (low voltage) batteries.
High Voltage Batteries
Large LV batteries handle high currents which means that they need very thick cables for connection. Generally speaking it is technically more efficient to use higher voltages because you need smaller current and thinner cales if you have higher voltages. This led to the development of HV batteries in recent years. They start at ~ 150V but some types go up to 600V and more.
HV batteries are composed of the same cells as LV batteries and they also have their (specific) BMS. That BMS has to do a lot of tricky things so that long rows of serially connected cells can then be connected in parallel to enlarge battery capacity. This is beyond the scope of our explanation, however. You only need to know that HV batteries are more efficient, somewhat more expensive and (at the moment) mostly proprietory – as opposed to LV batteries which have a widely standardized interface for their BMS.