Photovoltaics - Understanding Renewable Energy Systems - page 167

Figure 4.46 Operating Points of a Solar Module Connected to Battery
Storage with a Blocking Diode and 0.1

Ω Cable Resistance without Load

irradiances, and it varies with the battery state of charge. If a photovoltaic
generator is connected directly to a battery, a good operating point is achieved
for a wide irradiance range (see Figure 4.46). Therefore, DC-DC converters
and MPP trackers are very rarely used in battery systems. In some cases, the
power consumption of the additionally required electronics is higher than the
possible energy gain. Only for inhomogenously irradiated solar generators do
MPP trackers have some advantages.
Rechargeable batteries in simple battery systems with the photovoltaic
generator and load directly connected to the battery are not protected against
deep discharge or overcharging. Such a simple system should be chosen only if
negative operating conditions for the battery can be avoided with certainty;
otherwise, the battery can be damaged very quickly.
Therefore, most battery systems use a charge controller (see Figures 4.47
and 4.48). Charge-controller lead-acid battery systems usually work on the
basis of voltage control. The charge controller measures the battery voltage
V Bat . If it falls below the deep discharge voltage (11.4 V for a 12-V lead-acid
battery), the switch S 2 disconnects the load from the battery. When the battery
is charged again, i.e. the battery voltage rises above an upper threshold, the
switch reconnects the load. If the battery voltage rises above the end of charge
voltage (about 14.4 V for a 12-V lead-acid battery), the switch S 1 stops
charging. The series charge controller (see Figure 4.47) and the parallel or shunt
charge controller are two principal charge controller types (see Figure 4.48).
Power semiconductors such as power field-effect transistors (power
MOSFETs) are normally used as switches. Continuous forward losses at the
switch S 1 are a disadvantage of the series charge controller. The forward
resistance of good MOSFETS is less than 0.1 ; nevertheless, for a current of

6 A, the field-effect transistor BUZ 11 with a forward resistance of 0.04

Ω

Ω