| Fuel Tank of the Solar Electric System
Batteries store electricity for night time in off grid solar systems. An off
grid home will have 4 to 6 days of storage in the batteries, while grid tie systems
may have no batteries at all. Batteries also help loads to run at a constant voltage,
while allowing the solar modules (or other power sources) to still charge as much
as it can. In On Grid homes, batteries are only used to power loads if there is
Batteries are the "weakest link" in a modern solar
system. They're heavy, full of lead and sulfuric acid, require regular maintenance,
and will wear out sooner than any other component. Most complaints on older systems
are due to old batteries.
So how do we deal with this integral, but unpleasant
side of solar power systems?
Other Battery Types
and energy storage machines :
First, what about Lithium Ion (Li+), Nickel
Metal Hydride NiMh, NiCad, fuel cells, fly wheel storage, perpetual motion machines,
We've been waiting (100 years now) for a better energy storage method.
Li+ looks like the next step followed by fuel cells and flywheel storage. Charging
large banks of Li+ batteries is proving troublesome compared to traditional chargers
for Lead Acid batteries. Nickel based batteries are just not enough of an improvement
to justify their extra cost. Fuel Cells are available now, but reasonably priced
Hydrogen to run them is not. Flywheel storage is still too heavy, bulky, and expensive.
Perpetual motion machines? Windmills on the roof of a car to power the car's electric
motor? Soon we'll have a section on energy and efficiency that will explain why
those don't work.
For the time being, we'll just say that every time energy
changes form, some of that energy is lost to friction, heat, etc. How much energy
is left after each conversion process shows the efficiency. Extra energy always
comes from somewhere. Solar panels and wind mills are the closest thing to free
energy machines you will ever find. Only the machine costs money. The energy is
free for the taking after that!
How a Lead Acid battery
When a battery is connected (you turn on a light for instance), this
starts a chemical reaction between its sulfuric acid and the lead in the plates
submerged in the acid. This reaction produces electricity (a flow of electrons
from the chemicals) to run the light. The reaction also depletes the sulfuric
acid and builds up a crust of sulfate crystal on the battery plates. When the
battery is recharged (by PV modules, or other source), it reverses the reaction,
electrons are introduced back at the plates, the sulfate is dissolved into acid
again and the battery water becomes more acidic. At full charge, the acid quits
increasing in concentration, and further charging releases hydrogen gas. This
battery gassing, in small amounts, stirs the electrolyte, knocks sulfate loose
from the lead plates, and brings the battery to fuller charge. Too much gassing
can boil the battery water dry, or cause overheating both of which shorten a battery's
It's a Chemical Reaction:
usually slow down as the temperature drops, so this means a colder battery can't
produce as much energy as a hot one. However, colder batteries will last much
longer than ones that overheat (over 115°F) Also, we know that the greater
the surface area of the lead plates, the greater the number of electrons that
will be allowed to react at once, creating more current. But if the plates are
thin, they can't react for very long before being drained or even damaged. This
is how a relatively small starting battery can start a car. By increasing the
surface area of its lead plates, a starter battery allows for a rapid flow of
high amperage -- but they can't last as long because of their thinner plates.
A deep cycle battery, the type used for energy storage in renewable energy systems,
is the opposite of the starter battery -- it has much thicker plates with less
surface area, and thus will produce power much more steadily over a longer period
battery cell that completes its own chemical reaction, in its own acid bath only
produces 2 volts. So smaller batteries are usually sets of cells wired together
internally. Notice a 12 volt car battery will have 6 separate cells. Many different
voltage batteries are available (2, 4, 6, 8, 12) and can easily be wired for whatever
the nominal voltage of your system. Go to our Battery Wiring Diagrams for more
Amp Hours (AH):
energy storage capacity -- literally, how many amps can be taken from a battery
in how many hours (amps x hours). A 100 Ah rating could mean different things
-- 1 amp for 100 hours or 100 amps for 1 hour -- the difference lies in the rate
of discharge (see side bar).
Only true deep
cycle batteries are rated in Amp-hours. (Cold Cranking Amps is a rating
for starter batteries -- this is the measurement of how many amps can be produced
by a new, fully charged battery at 0°F for 30 seconds, while maintaining at
least 60% of its nominal voltage.)
Batteries with thicker lead plates can withstand more cycling.
A battery can only store, then release,so much energy before it needs to be replaced.
By multiplying a battery's cycles x its energy storage rating, gives the total
energy capacity of the battery for its entire life. Cycle life depends on depth
of discharge (DOD). For example, a Deka Golf Cart battery can be discharged
and recharged many more times at 20% DOD(3500 cycles) than at 80% DOD (900 cycles).
Steps to Success:
1) Start with Deep Cycle batteries,
and compare cycle life. For small to medium systems, we generally recommend
deep cycle Trojan T105 RE batteries. They can be drained completely (to
0%) and recharged an amazing 1000 times. A car battery can only do this
maybe 5 times before it's dead. A marine RV battery might say "deep
cycle", but it can only be fully cycled 100 times. So a marine battery
could last a year or so, while Trojan RE batteries regularly last10 years.
Compare this to the HUP Solar One that can be cycled 2100 times and can
last over 20 years!
Get as large a battery bank as is reasonable. You need enough
batteries to store electricity for at least 3 days of usage for cloudy weather.
We recommend 4 to 5 days of storage, and some systems have 10 to 14 days of capacity.
Spending more money now, means less trouble, and longer battery life. Deep cycle
batteries are rated in Amp Hours AH, the amount of energy that can be stored in
the battery. 100 Amp- Hrs means that the battery will deliver 1 amp for a 100
hrs or 100 amps for 1 hr. (see side bar) Get a battery that lasts longer (has
more cycles), in the long run you'll be happier.
3) Don't put more than 4 strings of batteries in parallel.
The batteries begin to struggle among themselves if you have too many in
parallel. The weaker cells drain current from the stronger batteries, and
you have power being burned up as heat before you ever get to use it! If
you need more capacity, use bigger batteries. Industrial cells like the
HUP Solar-One batteries used in larger systems are rated at more than 2100
cycles at 80% DOD and may last 15 to 20 years. They also need less maintenance,
since they have almost 3 extra inches of water over the battery plates.
Go to out satellite site, HUPSolar.com, and use this battery conversion calculator to find out which HUP would be equivalent to your current set
of other batteries.
# of 6v
Don't mix new and old batteries, or different types of batteries.
Just as too many batteries causes problems, mixing new batteries with ones that
are older than 6 months will cause problems. Basically, the newer cells are cycled
more than the other cells, so the whole set only lasts as long as its weakest
5) Keep the batteries at 40 to 80°F, with
55°F being optimal. A Basement is perfect! Batteries last longer
if kept cool, but conversely they have more capacity if kept warm. They don't
need to be inside, but in cooler climates outside they do need an insulated box
with ventilation. A fully charged battery is filled with strong sulfuric acid
and can't freeze till 30° below zero Fahrenheit. A completely discharged battery,
however, is filled with very weak acid (almost water) and can freeze at 25°
6)Take Care of them! Check the water
level regularly -- we recommend doing so every 3 months. Only use distilled
water when adding water to the batteries, and don't overfill them. Clean the terminals
with baking soda water, and recoat all connections with petroleum jelly. The HUP
Solar-One industrial batteries have much more water over the plates, so they don't
need to be maintained as often. We now even carry an automatic battery watering
system, that keeps all the cells filled from a central 5 gal. tank of distilled
Comparing Battery Types
The Table Explained:
(to 80% Depth of Discharge) is how many times you can drain most (80%)
of the energy from the battery and recharge it completely. Eventually the battery
will lose its storage capacity and need to be replaced. In a normal solar system,
the battery is only discharged about 10 to 20% a day, so it can last many more
cycles than if it is drained totally every night (as in an Electric Vehicle).
is the real world life expectancy of the batteries based on the experience of
ourselves, our customers, and other installers. We have overlayed all this experience
with the laboratory test results for cycle life. Actual lifespan depends on proper
maintenance. If you don't check the water in your batteries, they could die in
less than a year!
Price per kWh : This is the bottom
line -- how much you are paying over the years for each type of battery.
While initially more expensive, the Industrial Batteries are a good buy
in the long run . When you figure the cost of extra maintenance, power
outages, swapping out sets of heavy and corroded batteries, they become
an even better choice.
Total cost is calculated by figuring
total kWh that can be cycled from the batteries over their lifespan. Maintenance
cost assumes a half hour (@ $50/hr) to check water and terminals every
2 months for golf cart batteries, every 4 months for L16s and every 8
months for the HUPs. Installation cost are included. Since the Industrial
cells don't need to be replaced as often, their life cost is better.