Batteries

Batteries are applied to store energy for times of little or no sunlight. They are the most sensitive component of any photovoltaic system and require a certain amount of care. They are not easygoing like batteries in cars.

Only lead-acid batteries are used in the field. They come in different types: 

• Automotive batteries (regular type, used in cars, also called SLI batteries)

-   very short service life, high maintenance, only shallow charging

+  very low price, good availability

• Automotive batteries (heavy- duty type, used in trucks and buses)

-   short service life, medium maintenance, shallow discharging

+  low price, good availability

• Solar batteries (different designs)

-   medium discharging, limited availability, medium price

+  better service life, limited maintenance

• High-quality batteries (diff. designs)

-   expensive (4-6 x autom. battery price)

+  high service life, low maintenance, rather deep discharging

 

Characteristics of battery types Figures depend on type and operating conditions
	automotive	solar	quality
Energy efficiency	70%	75%	80%
Self-discharge/month	20%	4%	2.5%
Costs per kWh	70 €	120 €	300 €
Maintenance	4x/a	2x/a	1x/a

Regular batteries have to be filled up with distilled water in certain intervals. A few are maintenance-free. Only those of the gel-type are suitable in solar systems.

Automotive batteries are unsuitable if any alternative exists. Heavy-duty ones may be used in smaller systems with severe budget constraints or if others are not available. High-quality, deep-cycle batteries are optimal for solar applications. Solar batteries are a compromise between quality and costs.

The service life of batteries, which is expressed in cycles of charging and discharging periods, increases with: 

• the quality of the battery,
• the quality of the charge controller,
• low temperature,
• a low depth of discharge and
• careful operation and maintenance.

 

The depth of discharge (DoD) is that portion of the battery capacity which is removed during discharging. The capacity is indicated in ampere hours (Ah) or sometimes in days of autonomy. This is the number of days a fully charged battery can satisfy the energy demand. The common 12 V battery includes six cells of 2 V in one housing.

Example: A 12 V, 60 Ah battery can store 12 V x 60 Ah = 720 Wh. At a 75% depth of discharge, 540 Wh have been removed
and the remaining capacity is 180 Wh / 12 V = 15 Ah. If the daily energy demand is 270 Wh, the battery has 2 days of autonomy. 

As a rule of thumb, reducing the DoD by half will double the number of cycles. Furthermore, batteries have to be protected against overcharging and deep discharging by a charge controller.

As with modules, batteries are connected in series for higher voltage levels or in parallel to increase the capacity. Batteries that are connected should be identical in type, capacity, age and electrolyte density. Parallel connection should be avoided if possible.