What should be considered when Connecting Batteries in Parallel?
The capacity of a battery installation can be increased through parallel connection. In theory, an unlimited number of strings may be connected in parallel, however, a maximum of 4 strings in parallel is recommended.
NOTE: Contact the Battery manufacturer for specific recommendations if more than 4 strings must be connected in parallel.
The following should be considered when sizing a battery system with multiple parallel strings:
· Batteries must be of equal age, voltage, and capacity.
· Parallel cables must have equal resistance (usually the same length with looping to accommodate the extra lengths for some strings) and should be sized to maintain a safe temperature increase in case one or more strings fail causing the remaining strings to support a higher load than normal.
· During discharge, a weak battery string may cause the other strings to support more of the load resulting in reduced back-up time. In addition, the weak string may discharge below the specified end-voltage (over-discharged).
· When connected in parallel, the current from a charger will tend to divide almost equally between batteries in good condition. However, during charge, a weak battery string may draw most of the charge current and prevent other strings from being fully charged.
· Check that the system power plant (rectifier/charger) will be capable of maintaining the load and recharging the batteries at the same time without overloading.
Below are a list of several frequently asked questions that pertain to subjects from batteries, power plant systems and properly sizing. Each question is bolded while the following answer is listed below it. If you have a question or comment that isn't found on the list below, simply shoot us an email via the contact form over at the Contact Us page.
What are the recommended Inspection and Record Keeping guidelines for my battery system?
To prevent premature battery failure, the following inspection and maintenance schedule based on IEEE Std 1188 is recommended.
NOTE: Maintenance records will be required for warranty claims. Some maintenance procedures may require special tools or skills. Voltage readings may require special probes or techniques, consult the battery manufacturer's literature for guidance.
Initial Inspection - After the battery has been on float for one week, measure the following data and record on the installation report:
· Ambient temperature in the battery room or area should be measured. If the temperature is not 77°F (25ºC) or less, environmental controls should be used to control the temperature. If temperature controls are impractical, the float voltage must be compensated for temperature (Refer to manufacturer's requirements).
· Charger output current and voltage
· Overall float voltage measured at battery terminals.
· Charger imposed AC ripple current and or voltage.
· Condition of ventilation and monitoring equipment.
· Condition of battery (Appearance, Cleanliness, Accessibility).
· Cell number, Float voltage.
· Internal Ohmic Value.
· Negative terminal temperature.
· Intercell Connector Resistance.
· Visually inspect each battery for signs of wear to the case, cover and terminals, electrolyte leakage, and corrosion at the terminals, connections or racks.
How are Battery Capacities properly determined?
The most common battery rating is the AMP-HOUR rating. This is a unit of measurement for battery capacity, obtained by multiplying a current flow in amperes by the time in hours of discharge until a specific end of discharge voltage is reached. (Example: A battery which delivers 5 amperes for 20 hours to a final voltage of 1.75 volts per cell delivers 5 amperes times 20 hours, or 100 ampere-hours.) The amp-hour rating varies with the discharge period and final voltage. The longer the discharge period, the higher the rating. Batteries used for telecommunications are generally rated over an 8-hour period, those used for solar electric systems are rated over 100 hours. For this reason Amp-Hour Ratings and discharge period are both considered when evaluating a battery's capacity for selection purposes. A good way to understand this is by thinking of a runner. A runner may be able to run at a slow speed for a long period of time, however if they were to run faster, their distance run will decrease. The amp-hour rating also varies with temperature, most ratings are at 25°C/78°F. Lower temperatures decrease the rating while higher temperatures increase the ratings.Some batteries are rated on a constant power basis rather than amp-hour basis because the application is with electronic equipment that draws a constant power even as the voltage decreases during discharge (current increases during discharge to keep the power level constant). These batteries are rated in watt-hours, again to a specific end of discharge voltage.
What are some of the basic considerations that apply to installing new batteries?
· Keep a copy of the applicable battery manual and layout drawings (if any) near the battery installation at all times.
·Only allow properly trained personnel to perform battery installations and servicing.
·Batteries contain sulfuric acid which is harmful to skin and eyes. In the event of contact, flush immediately with water and obtain medical attention.
·Batteries contain lead and lead compounds which are toxic materials, wash thoroughly after handling.
·Use protective equipment, such as acid resistant rubber gloves, protective aprons, safety shoes, safety glasses and insulating tools when working with or around battery systems.
·Batteries are capable of high voltage and current which can cause injury to personnel
.Do not lay any metallic objects on the battery as it may cause a short circuit. Do not wear metallic objects, such as jewelry, when working around batteries.
·Neutralize static buildup just before working on battery by contacting the nearest effectively grounded surface.
·Use caution when lifting batteries. Use proper lifting devices. DO NOT lift cells by the terminal posts.
·DO NOT smoke, use an open flame or create a spark in the vicinity of the battery.
· DO NOT install cells in sealed (airtight) enclosures. Each cell is fitted with a vent through which hydrogen gas may escape under all operating conditions. Provide adequate ventilation in accordance with local, state or federal building and fire codes.