Installation manual

7.1.4. What is electricity and what is a battery?

Electricity is the flow of electrons along a wire. Metal is a good conductor of electricity as the

electrons in each atom of metal are free to move from one atom to another.

Consider how difficult it is to store the energy of a car that is moving. Understanding the fact

that electricity is the flow of electrons helps us to understand that electricity is also difficult to

store as it is energy in motion.

Batteries do not store electricity as such but use the flow of electrons to alter the number of

electrons in the chemicals inside the battery. Then when the battery is discharged the

chemicals return to their original state. However, the chemical process means that batteries

degrade with use and time.

Renewable energy systems normally use batteries based on lead-acid chemistry as they are

still the most cost effective and readily available type. Lead-acid batteries are made from

plates of lead in a solution of sulphuric acid. While the discharging and recharging of lead

acid batteries is a reversible process all lead acid batteries lose health when not charged.

The car battery is a lead-acid battery. A car battery is designed for starting a car’s engine

and so has thin plates to provide as much surface area as possible, allowing the chemical

reaction to occur in a short time. This type of battery can provide large currents to meet the

high power demands of starting an engine. As the duration of engine starting is very short

the total amount of energy is not that great. However, automotive batteries suffer when

significantly discharged. The thin plates are quickly damaged and may even disintegrate.

The plates also have a high resistance, so lose energy, making a car type battery less

efficient as an energy storage device. They can be employed in some hydro situations where

there is plenty of power to meet the base load of the home, with the battery merely providing

storage for short-duration peak loads.

A deep cycle battery designed for standby energy systems has heavy plates that are much

more robust against deep discharges. However, a deep cycle battery has limited surface

area and cannot convert stored energy as quickly. Thus deep cycle batteries must not be

subjected to heavy currents or there will be damage to the battery.

7.1.5. Battery bank sizing with the 10:10:10 rule of thumb.

For a 10 year life:

 Cycle batteries through no more than 10% of their capacity each day.

 Limit the maximum sustained power draw in W to 10% of battery Wh.

 Limit the maximum charge current in A to 10% of battery Ah.

For example for a hydro turbine generating 500 W (0.5 kW) into a 48 V DC battery bank that

consists of two banks at 200 Ah each:

 Depth of discharge (DOD) each day = 10% x 2 x 200 x 48 = 1920 Whrs.

 Maximum sustained draw of 10% x 200 x 2 x 48 + 500 = 2420 W for a time not

exceeding 1 hour.

 The charge rate is 500W/48V = 10 A.

Maximum allowable = 10% x 2 x 200 = 40 A.

This 40 amp limit is therefore only a concern when backup charging from a gen-set.

Average daily draw from the battery bank (allowing for 10% battery loss and 10% inverter

loss 500W x 0.9 x 0.9) is 400 W = 9.6 kWh/day (0.4 kW x 24 hrs/day) total consumption.

This is normally adequate for an energy efficient home using a 3 kW inverter. If you wish to