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Pellet Stove Battery Backup Power
see also: Pellet Stove Backup Power: Battery or Portable Generator?
A typical pellet stove battery backup system is comprised of a variation
on the 12V deep cycling battery, an inverter to convert DC battery power to
appliance friendly AC current, and a battery charger.  

At the heart of the system, and also the most expensive component is the
battery.
A pellet stove backup system requires a battery that functions
differently from the familiar automotive battery.

An automotive battery is designed to provide a large amount of amps for a
very short period of time. A deep cycling battery is designed for low amp
discharge over longer periods.

An example of a true deep cycling battery is the type used to power an
electric fork lift. The smallest of these batteries weighs in at almost 600
pounds and has a price tag to match. A cost effective alternative for pellet
stove backup power is the 60 pound "hybrid" deep cycling battery used for
RV and marine applications.

Another difference between true deep cycling and "hybrid" deep cycling
batteries is the DOD, or depth of discharge. A true deep cycling battery can
be discharged by as much as 80%, while the DOD for a "hybrid" battery is
not recommended to exceed 50%. I'll explain how the percentage of
discharge figures into battery sizing in a moment. The following are the 3
major hybrid battery types:

1. Flooded acid batteries have their place, but your living room or den isn't
one of them. Safer alternatives are the gelled acid and AGM types.

2. Gelled batteries or "Gel Cells" contain acid that has been gelled by the
addition of Silica Gel, turning the acid into a solid mass. The advantage of
these batteries is that it is impossible to spill acid even if they are broken.

Some disadvantages of gel cells are they must be charged at a lower voltage
than flooded or AGM batteries. If overcharged, voids can develop in the gel
which never heal, causing a loss in battery capacity. In hot climates, water
loss can cause premature battery death.

3. AGM batteries have several advantages over both gelled and flooded.
Since all the electrolyte (acid) is contained in the glass mats, they cannot
spill; even if broken. This also means that shipping costs are lower because
they are non-hazardous. In addition, there is no liquid to freeze and expand,
so they're practically immune from freezing damage. For in home use, the
AGM type battery is recommended.

Backup Battery Sizing  The primary reason for installing a battery backup
system is to provide power to the blower motor and pellet auger of your
pellet stove. The battery size will be determined by the number of amps
needed to run the stove and the duration of your "average" power outage.

The backup battery system discussed here will provide power to the average
pellet stove for approximately 10 hours. Power outages exceeding 10 hours
would require additional batteries. Since the cost of an AGM  type battery is
around $250, the cost of two or more batteries would equal or exceed the
price of a portable generator. If your typical power outage is measured in
days, not hours, you should consider a backup portable generator instead.

The storage capacity of deep cycle batteries is rated in Amp hours (Ah). This
is the maximum sustained amperage drawn from a fully charged battery over
a certain time period to a point where the battery is at 100% depth of
discharge (DOD), or dead.

Since the recommended maximum discharge is 50%, the backup battery
should have twice as much power as needed. Here's an easy formula to size
a backup battery.

Amps x Hours x 2 x 1.10 = Battery size in amp hours
3 x 10 x 2 x 1.10 = 66 amp hours.

  • 3 amps/hour is what most pellet stoves draw (check your owner's
    manual).
  • 10 hours is the estimated length of the power outage.
  • Multiplying by 2 doubles the battery size to prevent discharge beyond
    50%.
  • Multiplying by 1.10 (adding 10%) compensates for power loss from
    wire resistance, and other DC system inefficiencies.

If the result of your calculation falls between battery sizes, choose the next
largest battery as measured in amp hours (AH). Although some batteries are
described as solar batteries or sump pump batteries, any 12 volt AGM type
battery will do.

and converts DC battery power to appliance friendly AC power. One end of
the inverter connects to the battery with cables; the other end has one or
more receptacles to plug in your pellet stove or other appliance(s).

Inverter Sizing  Since inverters are sized by wattage you need to convert
the amps your pellet stove draws to watts to determine the correct size
inverter. Most pellet stoves draw 3 or 4 amps, but if your stove has
electronic ignition, it could draw twice as many amps on start up.

Although inverters have surge, or peak power capability for appliance start
up, I would suggest using the ignition amps requirement of your stove as
the baseline for calculating inverter size. Another reason to slightly oversize
the inverter is to compensate for the 10% loss of power during the
conversion of DC battery power to AC. The formula for converting amps to
watts is: amps x 120 = watts.

For instance, if your pellet stove needs 8 amps to ignite, multiply 8 x 120 to
arrive at the inverter size of 960 watts.

Battery Charger  The final component in the pellet stove battery backup
system is the battery charger. More batteries are damaged and prone to
premature failure due to improper charging and overcharging than by any
other means.

If batteries are charged at a constant amperage and low voltage, charging
time is significantly longer and battery capacity is effectively reduced because
the battery is never completely recharged.

To avoid this, many individuals opt for a higher charging voltage which
accelerates the rate of charge but can ultimately harm the battery if the
charge is continued for any length of time after the battery reaches capacity.
The best charging solution is a 3 stage or
multi-stage battery charger.

More Than Just A Pellet Stove Backup System  The components of a
pellet stove backup system have numerous other applications in addition to
their intended purpose.

The battery charger can also be used to diagnose and charge any 12 volt
battery. The inverter could be hooked up to your car battery to run small
appliances outdoors. You could take the inverter and charged battery
camping to run or recharge your lap top or portable TV. You could even
make the entire system portable by adding a solar panel to keep the battery
charged no matter where you are.