Battery Information and Sizing

All stand-alone and batter backup PV systems require battery storage.  Photovoltaic modules charge the batteries during daylight hours and the batteries supply the power when it is needed, often at night and during cloudy weather.  Utility intertie systems supply power directly to the utility grid; no battery storage is needed.  the two most common types of rechargeable batteries in use today are lead-acid and alkaline.  Lead acid batteries have plates made of lead, mixed with other materials, submerged in a sulfuric acid solution.  We do not list nickel-cadmium batteries in this catalog because of their high cost and environmental problems related to disposal.  Nickel-Metal Hydride and Lithium Ion batteries look promising for the future, but at this time their price is much too high for the size needed for all but the smallest of remote lighting systems.

Lead-Acid Batteries
Lead-acid batteries are the most common in PV systems because their initial cost is lower and because they are readily available nearly everywhere in the world.  There are many different sizes and designs of lead-acid batteries, but the most important designation is whether they are deep-cycle batteries or shallow-cycle batteries.  Shallow-cycle batteries, like the starting batteries in automobiles, are designed to supply a large amount of current for a short time and to stand mild overcharge without losing electrolyte.  But they cannot tolerate being deeply discharged.  If they are repeatedly discharged more than 20% their life will be very short.  These batteries are not a good choice for a PV system.  Deep-cycle batteries are designed to be repeatedly discharged by as much as 80% of their capacity so they are a good choice for PV systems.  Even though they are designed to withstand deep cycling, these batteries will have a longer life if the cycles are shallower.  All lead-acid batteries fail prematurely when they are not recharged completely after each cycle.  Letting a lead-acid battery stay in a discharge condition for days at a time will cause a permanent loss of capacity.  Sealed deep-cycle lead-acid batteries (gel cells and absorbed glass mat) are maintenance free.  They never need watering or an equalization charge.  Sealed batteries require very accurate regulation to prevent over-charge and over-discharge.  Either of these conditions will drastically shorten their lives.  We recommend sealed batteries for remote, unattended power systems.

Caring For Lead-Acid Batteries
Always use extreme caution when handling batteries and electrolyte.  Wear gloves, goggles and old clothes.  "Battery acid" will burn skin and eyes and destroy cotton and wool clothing.  
The quickest way to ruin lead-acid batteries is to discharge them deeply and let them stand "dead" for an extended period of time.  The positive plates change fro lead oxide when charged to lead sulfate when discharged.  If they remain in the lead sulfate state for a few days, part of the plate does not return to lead oxide when the batter is recharged.  The parts of the plates that become "sulfated" no longer store energy.  
Batteries that are deeply discharged and then charged partially on a regular basis can fail in less than one year.  Check your batteries on a regular basis to be sure they are getting charged.  Use a hydrometer to check the specific gravity of your lead-acid batteries.  If batteries are cycled very deeply and then recharged slowly, the specific gravity reading will be lower because of incomplete mixing of electrolyte.  Check the electrolyte level in we-cell batteries at least four times a year and top-off each cell with distilled water. Do not add water to discharged batteries.  Electrolyte is absorbed when batteries are discharged.  If you add water at this time and then recharge the battery, electrolyte will overflow and make a mess.  Keep the tops of your batteries clean and check that cables are tight.  Do not tighten or remove cables while charging or discharging.  Any spark around batteries can cause a hydrogen explosion inside, and ruin one of the cells, and you.  It is a good idea to do an equalizing charge when some cells show a variation of 0.05 specific gravity from each other.  this is a long steady over-charge, bringing the battery to a gassing or bubbling state.  Do not equalize sealed or gel-type batteries.
With proper care, lead-acid batters will have a long service life and work very well in almost any power system.  With poor treatment lead-acid battery life will be very short.
We strongly recommend the use of an amp-hour meter with all battery systems.

Battery State-of-Charge
Battery state-of-charge (SOC) can be measured by an amp-hour meter, voltage or by specific gravity.  Some care and knowledge is required to interpret state-of-charge from voltage or specific gravity readings.  We recommend amp-hour meters for all systems with batteries.

Amp-Hour Meters
An amp-hour meter is like having a "gas gauge" for batteries.  It gives the user all the information they need to keep their batteries charged.  At a glance the user can see system voltage, current, and batter condition.

Measuring Battery State-of-Charge
Battery voltage will vary for the same state-of-charge depending on whether the battery is being charged or discharged, and what the current flow is in relation to the size of the battery.  The charge below will give you an idea of state-of-charge for various battery conditions in flooded cell lead-acid batteries.  Voltage varies with temperature.  While charging, a lower temperature will increase battery voltage.  Full charge voltage on a 12 volt battery is 0.9 volts higher at 32°F than at 70°F.  While discharging, a higher temperature will increase battery voltage.  There is little temperature effect while a battery is standing.  (This information courtesy of Ralph Heisy, Bogart Engineering.)

Hydrometers
A hydrometer is very accurate at measuring battery state-of-charge if you measure the electrolyte near the plates.  Unfortunately, you can only measure the electrolyte at the top of the battery.  When a battery is being charged or discharged, a chemical reaction takes place at the border between the lead plates and the electrolyte.  During charging, the electrolyte changes from water to sulfuric acid.  The acid becomes stronger and specific gravity rises as the battery charges.  Near the end of the charging cycle gas bubbles rising through the acid stirs the fluid to mix it.  It takes several hours for the electrolyte to mix so that you get an accurate reading at the top of the battery.  Always try to take readings after a period of no charge or discharge.

Hydrometer Readings
The chart below shows batter state-of-charge at various specific gravities.  these readings are correct at 75°F.

Battery Sizing Worksheet

Battery Wiring Diagrams