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
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.
Inverter The inverter is the second component of a battery backup system 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.