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7/31/2019 About Battery Knowledge

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HOW TO SIZE YOUR BATTERY BANK

Here's a fairly simple formula for figuring how many batteries you need for your power requirements. Here's

how it goes:

A. First, you need to know how many Watt-hours per day you need for your power system;

B. Then from that figure [A] Wh x 1.3 [the fudge factor as batteries and other components aren't 100%

efficient] to get your "Adjusted watt hours needed per day".C. Next your adjusted watt-hours per day x the number of days of autonomy (we usually use at least a

minimum of 3days and as much as 5days);

D. From value of item C above x 2 (for 50% depth of discharge, you never want to take your batteries

lower than that);

E. Then value of item D divided by your system voltage (example 12V, 24V or 48V) and this equals to

the number of Amp-hours of storage you need.

Ex: Daily power consumption = 5,000Wh, Battery = 12V, Ah = 90Ah, Days of Autonomy = 3days;

a. Daily power Consumption = 5,000Wh

b. Adjusted power consumption = 5,000Wh x 1.3 = 6,500Wh;

c. Days of Autonomy = 6,500Wh x 3 = 19,500Wh;

d. Depth of discharge 50% = 19,500Wh x 2 = 39,000Wh;

e. Required Amp storage = 39,000Wh 24V (system voltage) = 1,625 Ah.

Now, if you need 1,625Ah of storage in a 24V system, Total Batteries in parallel = 1,625Ah 90Ah = 18.

Since it takes 2 pcs of 12V batteries in series to make a set of 24V system, therefore you need to multiply

18pcs x 2 = 36 to give you 5,000Wh a day for 3 days & 50% depth of discharge.

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Why Use a Fuse?With the positive and negative cables securely fastened to the

battery terminals and the solar panel outside and exposed to

the elements, any cable connection failure is most likely to

happen near the solar panel rather than at the battery. Should

the end of the negative cable touch any exposed metal of the

positive cable or visa versa, a short circuit will occur. Huge

amounts of electric current will flow potentially causing

sparks, melting the cable, and/or even causing the battery to

explode.

With an appropriately rated fuse fitted in the postive cable as near to the battery as possible, any short

circuit will be over within a split second before any serious damage can be done. The fuse chosen should berated at approximately 120% of the absolute maximum output current rating of the solar panels to ensure it

will immediately blow in short circuit conditions but will never blow in normal operation.

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A SHUNT (Aka as Current Shunt Resistor or Ammeter Shunt) is a

high precision resistor which can be used to measure the current

flowing through a circuit. Using Ohm's Law, we know that the voltage

dropped across a resistor divided by the resistance of that resistor is

equal to the current, therefore if we measure the voltage across a shunt

resistor in a circuit, we can easily calculate the current.

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For example, a typical 100 Amp Shunt Resistor can be used to measure

currents of up to 100 Amps although to prevent overheating it should

really only be used to measure continuous currents of not more than

60-70 Amps.

*If a shunt resistor overheats it can permanently change the resistance

of the shunt. This shunt is calibrated such that the voltage drop across

it is 100mV when the current flowing through it is 100 Amps.

Therefore we can calculate the resistance of this shunt to be voltagedivided by current = 0.1 / 100 = 0.001 Ohms (typically to within

0.25% accuracy).

Therefore if a voltage drop of 28mV [0.028V] is measured (using a standard multimeter or 0-100mV range

voltmeter), we know that the current flowing is 0.028/0.001 = 28 Amps.

The power wasted by the shunt resistor equals [W = A x V] voltage x amps = 0.028V * 28A = 0.78 Watts in

this example.

To save making this calculation manually each time, it is possible to

re-label a 0-100mV moving coil voltmeter so instead it reads 0-100

Amps. This would be achieved simply by sticking the word "Amps"

over "mV" on the face of the meter which would now be an ammeter

rather than a voltmeter.

Using a Shunt Resistor in a Renewable Energy System. It is very

important to know how much current is flowing in and out of the

battery bank in a renewable energy system.

When charging, the current flowing into the batteries should never be more than 10% of the battery

capacity - e.g. a 100Ah battery should not be charged with more than a 10 Amp current or it may be

damaged and/or overheat.

It is also very useful to know how much current is being generated by a wind turbine or solar panel, because

that information helps you to calculate how much power is being generated. For example a 12 Volt 15 Watt

PV Solar Panel may produce a voltage of 18 Volts when it is very cloudy and 21 Volts when it is verysunny.

When it is cloudy you may measure a current of just 0.1 Amps and when it is sunny a current of 0.8 Amps.

Power is equal to voltage multiplied by current, so the solar panel is generating just 2 Watts when it is

cloudy and almost 17 Watts when it is sunny.

Wiring

Current Capacity: Calculating required conductor size based on the current carrying capacity of the wire.

Voltage Drop: Calculating required conductor size based on voltage drop.

Wire Table: A quick wire size versus ampacity table for sizes up to 60 A.

Calculating conductor size in a PV system can be based on current capacity, or voltage drop, whichever results

in a greater size. Often, in PV systems operating at nominal 12V or 24V, the conductor sizes chosen to limit

voltage drop will have ampaciy ratings that far exceed the current they are required to carry.

Conductor Selection Based on Current Carrying Capacity:

Due to the facts that the service voltages of most PV systems are between 12 and 48 VDC, and that the

conductor voltage drop may have a major impact on the operation of a PV system, the actual voltage, rather

than the nominal values should be used.

Solar Source Current - The maximum current between the PV array and the battery is determined by the short

circuit current of all the PV power sources x 125 % divided by the temperature correction factor, or: (Module

Isc) x (# of Modules in Parallel) x 1.25 = Current Rating

Current Rating / Table 5A* = Wire Ampacity (based on ambient module temperature, usually 50 deg. C.)

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(*Table 5A shows a correction factor at 50 deg. C for Types R90, RW90, T90 to be .80)

Battery Source Current - The maximum current between a battery bank and an inverter will be when the

battery voltage is at its lowest. Therefore, in calculating current in this conductor we use the battery open

circuit voltage at a discharged state.

See the chart below:

State of Charge Specific Gravity Battery Open Circuit Voltage

12V 24V 48VFully Charged 1.265 - 1.285 12.72 25.44 50.88

75% 1.215 - 1.235 12.60 25.20 50.40

50% 1.180 - 1.200 12.48 24.96 49.92

25% 1.155 - 1.165 12.18 24.36 48.72

Discharged 1.110 - 1.130 11.70 23.40 46.80Remember: When close to the maximum rating for that conductor; always choose the next largest conductor.

To neutralize any escaped acid, wipe battery cases and terminals with a clean cloth or soft brush dipped

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in a baking soda and water solution (1 pound of soda to 1 gallon of water), make sure vent caps are on

and securely tightened. A bubbling solution is a sign that some acid is present; wait until bubbling stops

and then wipe with clean water and dry. Be careful not to let anythinginto the battery through the vent

caps.

Once the cases and terminals are clean, check the terminal connections and inspect the battery cables for

any wear or loose crimps, and replace if necessary. Clean and recoat terminals and lugs with a thin layer

of anticorrosion treatment (petroleum jelly works). Always leave your batteries clean so you can easily

see any future corrosion or acid leakage.

Flooded batteries need to have their electrolyte levels checked every one to two months. Even if batteries

only need to be filled every six months or so, checking water levels more often is recommended to ensure

the plates are never exposed.

Recording

One of the best ways to track your batteries health is to keep regular, precise records. During your

maintenance checks, measure individual battery or cell voltages, and check specific gravity for flooded

batteries. Ideally, readings should be taken after the batteries have been at rest for12 to 24 hours and are

fully charged, but this is generally impossible in an off-grid situation. Checking after 30 minutes of rest

(no loads, no charging) will still give you good information.These checks can alert you to bad cells, or let you know if the entire bank may be on its way out. Any

differences in cell voltages or SG indicate you may have a failing (or failed) cell, and checking your

readings against the expected SOC will tell you if they are losing capacity. The sooner you spot a

problem, the more likely you will be able to fix it.

Battery String Sizing

Fewer parallel battery strings in a bank means better performance over the batteries life. Slight

imbalances between strings within a battery bank can cause increasingly uneven performance, leading to

premature failure of part of a bank and early replacement of the entire bank. Larger individual cells allow

for fewer strings, as does higher nominal system voltage. A single series string is a wise choice, and two

strings in parallel are considered acceptable. Three is the maximum number of parallel strings, but should

be avoided if possible.

Battery-based systems are generally wired at 12 V, 24 V, or 48 V. Systems have progressively moved

toward inverter-based AC loads, so 12 V system advantages have largely disappeared and the strong

disadvantages of high current and large wire sizes discourage the use of 12 V for all but RV and portable

applications and the smallest cabins with minimal loads.

For a 48 V system, a single string of batteries of the proper Ah capacity is recommended. If a single cell

fails, it can usually be temporarily bypassed until a replacement cell is installed, and the system can

remain in use. Even with the temporary bypass of three cells (an entire battery), as would be the case with

a string of eight 6 V batteries, set points can often be adjusted to allow the 48 V system to operate at 42V.

Twenty-four volt systems are often designed with two strings, as the failure of either a cell or a 6 V

battery requires only that one of two strings be temporarily disconnected from the system.

Conductor Selection Based on Voltage Drop.

In PV systems, the maximum voltage drop allowed between the PV array and the battery is 5%, with no more

than 3% allowed on any individual portion of the circuit. DC load circuits should be treated similar to AC load

circuits with a 5% maximum voltage drop allowed from the battery to the load, with no more than 3% drop on

any individual portion of the load circuit. As above, always calculate the voltage drop using the maximumcurrent that may exist in the conductor. When the maximum current is not known, then use 80% of the rating

of the breaker or fuse protecting the conductor.

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We have found that there are many on-line calculators for determining conductor size based on voltage drop,

but the one we like best is from Southwire and is linked here:http://www.southwire.com/voltagedropcalculator.jsp

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http://www.southwire.com/voltagedropcalculator.jsphttp://www.southwire.com/voltagedropcalculator.jsp


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