Page 10 - PS Battery Guide May 18

Battery guide For PowerSpout

Specifying the right size of battery
The battery must be large enough to do its job, but buying an oversized battery
can be a waste because batteries are costly and have limited life. A larger battery
will last longer (if correctly cared for) because it is less heavily cycled, but any
battery will eventually need to be replaced, even if it has been underused and
kept fully charged.

Your budget is inevitably a factor in the sizing of the battery, but make sure it is
not too small because it is unwise to add new batteries to old in the event you
change your mind and wish to expand the capacity a year later.

It may be prudent to patch a failing old battery bank with a few new cells in
order to keep it going and defer the cost of a new bank for another year or so.
This is often true when the owner needs time to organize finance and such a
short term patch is a small percentage of the cost of a new bank.

Energy storage
With a hydro power system that generates all day and night there is much less
need to cycle energy through the battery than there is with a solar PV system. In
both cases your battery size will be based on your daily energy usage.
 With a hydro system, one day's energy usage is a minimum working
capacity. You will not need all of that every day, but it allows for turbine
downtime. It is unrealistic to expect batteries to carry the system through
periods of drought when the flow is not sufficient.
 Solar PV system batteries are often sized to meet three days usage to
allow for changeable weather.
 If you have a hybrid system then perhaps two days is adequate. Much
depends on the seasonal variation in flow and how willing you are to use
your back-up generator.

Many well designed hybrid solar PV/hydro systems do not need any support
from a generator set. Maximize the hydro resource for the wet season then
upgrade the solar PV array until fuel generation is no longer needed.

Power handling
The battery has to be capable of delivering and absorbing energy fast enough
without suffering. This rate is expressed as power in Watts. A good rule of
thumb for deep cycle lead acid batteries is to restrict the power (kW in or out) to
not exceed 10% of the nominal energy capacity (kWh).

Example
Your turbine produces 500W output along with a 2kWp PV array. Your house
uses 12 kWh of energy per day. You plan for a 48V battery and a 3kW inverter.

(Note that the hydro will produce 0.5 x 24 = 12kWh when all goes well.
Your solar maybe produce 2kW for 5 hours = 10kWh in suitable good
conditions. The excess can go to heating water. It is good to design a

© 2018 EcoInnovation Ltd (NZ) Page 10
Last reviewed 19/6/18