Requirements for Solar Batteries

Typical requirements for the battery to be used in long term storage are:

  • low specific kWh-cost, i.e. the stored kWh during the whole life of the battery
  • long lifetime
  • high overall efficiency
  • very low self-discharge
  • low maintenance cost
  • easy installation and operation
  • high power

Specific kWh-cost

Usually it refers to a sum of investment and operation costs of the battery divided by the stored kWh (kWh) during its whole life. This cost is thus influenced by the battery's lifetime.

Lifetime

The lifetime of the battery should be long, especially in order to keep the specific kWh-cost and the installation cost low, particularly in remote areas.

Overall efficiency

The overall efficiency (nE) is derived from charge or coulombic efficiency (nI) and voltage efficiency (nV):

nE = nI n nV

The coulombic efficiency is usually measured at a constant discharge rate referring to the amount of charge able to be recalled from the battery (QD) relative to the amount put in during charging (QC). Self-discharge will affect columbic efficiency.

nI = QD / QC

The battery will usually need more charge than was taken out to fill it back up to its starting point. Typical average coulombic efficiencies are 80 – 85 % for stand-alone PV systems, with winter efficiencies increasing to 90 – 95 %, due to higher coulombic efficiencies when the battery is at a lower state of charge and most of the charge going straight to the load, rather than into the batteries.

The voltage efficiency is determined by the average discharge voltage (VD) and average charging voltage (VC). VC is lower than VD particularly by internal resistance of the battery.

nV = VD / VC

The overall efficiency (nE) should be as high as possible, to be able to pass the biggest proportion of the energy in the battery, which is generated by the PV generator, further to consumers.

Self-discharge

The battery discharges itself even without load connected. This effect is caused by secondary reactions at its electrodes and proceeds faster with higher temperature or in older batteries. Thermodynamic instability of the active materials and electrolytes as well as internal- and external short-circuits lead to capacity losses, which are defined as self-discharge. This loss should be small, particularly in respect of annual storage.

Maintenance cost

The maintenance, e.g. water refilling in case of lead-acid batteries, should be kept as low as possible.

Easy installation and operation Since batteries are often used also from non-experts, easy installation and operation are, therefore, favorable.

Power

In special cases battery must be highly loadable for a short time, e.g. at the start of diesel generators or in case of momentary power extension of PV systems. There are many types of batteries potentially available for use in stand-alone PV systems. Useful data of available batteries given in Table 1 shows approximated values and are provided as a guideline.

Table 1. Comparison between selection criteria of available batteries

Type Cycle life until 80%, DOD Coulombic Efficiency,  I Self-discharge, %/month Temp. range, °C
Lead Acid 500...1500 80 3…4 -15...+50°
NiCd 1500...3500 71 6…20 - 40...+45°
NiFe 3000 55 40 0…+40°

Since many values are dependent on charge and discharge conditions, they have not been standardized for PV applications and for test purposes until now. Therefore, the comparison between batteries and selection of the most suitable one for each application are not easy. Due to particular operating conditions with PV applications in practical operation, the cycle life given by manufacturer (and in Table 1) for cycling load can be reduced more than half.

According to Table 1 above, it follows that in most cases the lead-acid batteries would be the best choices for PV applications. The selection of suitable choices should be based on specific application.