Sept 2020

Voltage Characteristics of Solar Rechargeable Battery

Battery Specifications

• Nominal voltage is 12 V
• 50% DoD
• Gel type
• Manufacture by JTE
• Rated capacity is 100 Ah
• Typical weight is about 33 Kg
• Maximum Charging Current Limit is 25 A

Circuit

Our project has three 250 W, 8V solar panels. We have two 12 V batteries connected in series giving a total voltage of 24 V as per the requirement of the system.

DC output from panels goes to the inverter to be converted into AC. Simultaneously, the DC output of panels is also supplied to batteries to get charged and then utilized at night. The output of batteries is given as input to the inverter so as to convert it into AC.

AC output from the inverter (directly from panels) is used in the daytime and at night the stored energy of batteries (after being inverted) is consumed by the load connected to the system.

We can conclude that batteries get charged in daytime and discharged at night.

Standard Graphs for Batteries

A 12 V, a 100 Ah battery stores 1.2 KWh and holds 13 V when fully charged. For a single battery we can refer to the graph below:

Calculations

12 V * 100 A * 3600 sec = 4.32 MJ

As we know that 1 KWh = 3.6MJ Stored KWh = 4.32/3.6 = 1.2 KWh We have two batteries in series so, Total KWh= 1.2 + 1.2 = 2.4 KWh

We can determine stored KWh in the batteries at a certain VOC using the graph below.

Precautions

Make sure that the battery doesn’t get discharged below 50% DoD. Batteries should always be in working condition i.e. continuously charging and discharging.

Problem Statement

“How much time the battery would take to get stabilized after discharging and what will be the stable voltage compared to the voltage measured just after disconnecting the battery from the system?“

Determining Battery Stabilization Characteristics (After Discharging)

An experiment was performed on the battery in order to determine the voltage characteristics of discharging the battery. Batteries were disconnected from the system and then their terminal voltages were measured and noted on a paper.

After every 15 minutes, these voltages were measured by using a clamp meter and noted until batteries got stabilized (giving constant voltages after continuous measurements).

Then the observations noted were plotted graphically in order to describe the voltage characteristics of the battery.

Results

Time	VOC (V)
11:51	25.72
12:06	25.54
12:21	25.50
12:36	25.48
12:51	25.46
13:06	25.45
13:21	25.44
13:36	25.43
13:51	25.42
14:06	25.41
14:21	25.41

Conclusion

• VOC of batteries after being disconnected drops rapidly in the beginning, but after some time (approx. half an hour) voltage drops gradually/slowly as batteries go to stabilize with time.
• So, it is to be concluded that when the battery is being discharged, stable voltage (i.e. voltage measured is to be constant) is achieved 2.5 hours after disconnecting it from the circuit.
• The stable voltage is 0.31 V lower than the voltage measured when the battery is disconnected from discharging circuit (i.e 25.72v-25.41v=0.31v).

Problem Statement

“How much time the battery would take to get fully discharged, and during this time how much KWh can be supplied to a 200 W load?“

Determining Discharging Characteristics of Battery and to Validate “KWh-VOC” Curve of Battery

A discharging test was performed on the battery in order to validate “KWh-VOC” curve. Batteries were disconnected from the system and then the DC Wattmeter (connected between batteries and inverter) was reset.

After every half an hour, KWh on the Wattmeter (KWh supplied by batteries to load) was observed until batteries got fully discharged i.e 23.7 V.

Then the observations were plotted graphically in order to represent the validity of the “KWh-Voc” curve.

• Equipment: DC Wattmeter & Irradiance meter
• Load: 200 W
• Irradiance level (When test was started): 942.4 W/m^2
• Solar panel’s output (When test was started): 32.4 V
• Age of batteries: 2 years & 6 months (approx.)

Results

Observations of the above experiment are as follows:

• VOC (before connecting load): 25.9 V
• Voltage (just after connecting load): 25.1 V

Time	KWh	Voltage (V)
08:30	1.6	25.1
09:00	1.4	24.9
09:30	1.3	24.8
10:00	1.2	24.7
10:30	1	24.5
11:00	0.9	24.4
11:30	0.8	24.3
12:00	0.6	24
12:30	0.5	23.9
13:00	0.3	23.7

Conclusion

• We found the “KWh-Voc” curve to be approximately valid.
• VOC of batteries after being connected to load drops suddenly, but after some time (approx. half an hour) voltage drops or KWh consumption from batteries occurs gradually/slowly as batteries go to stabilize with time.

Problem Statement

“How much time the batteries would take to get fully charged and during this time how much KWh can be consumed by batteries?“

Determining Charging Characteristics of Battery and to Validate “KWh-VOC” Curve Of Battery

Another experiment was performed on the battery in order to observe the time taken by batteries to charge from 26.1 V to 27.9 V.

Batteries were discharged by switching off the supply from solar panels and only batteries were supplying the load (bulb etc.).Before starting battery charging test, batteries were not fully discharged and were 60% charged.

Load was disconnected and batteries were left to get charged from solar panels. Total time taken by batteries to get fully charged was noted.

• Equipment: DC wattmeter & irradiance meter
• Load: 0 W
• Solar panel’s output (when the test was started): 27.7 V, 12.70A & 283Wh
• Age of batteries: 2 years & 5 months (approx.)

Results

Observations of the above experiment are as follows:

• Time taken: 2 hours
• Total KWh generated by panels (during above time): 0.63 KWh
• Total KWh consumed by batteries (during above time): 0.54 KWh
• KWh losses (during that time): 0.09 KWh

Conclusion

It was observed that initially, batteries charged rapidly but later on they got charged a little slower.

Problem Statement

“How much time the battery would take to get stabilized after charging and what will be the stable voltage compared to the voltage measured just after disconnecting the battery from the system?“

Determining Battery Stabilization Characteristics (After Charging)

Batteries were disconnected from the panel after being fully charged and then their terminal voltages were measured and noted.

Not a single load was connected to the battery until batteries got stabilized (giving constant voltages after continuous measurements).

Equipment used: DC wattmeter, clamp meter & irradiance meter Load connected: 0 W

Irradiance level (when the test was started): 979 W/m^2

Solar panel’s output (when the test was started):32.7V, 640Wh

Age of batteries: 2 years & 5 months (approx.)

Observations

• VOC of batteries after being disconnected from charging circuit(unstable voltage)=26.1volts
• VOC of batteries after being stablised=25.7volts

Conclusion

• VOC of batteries after being disconnected drops rapidly in the beginning, but after some time (approx. half an hour) voltage drops gradually/slowly as batteries go to stabilize with time.
• The stable voltage is 0.4v lower than the voltage measured when the battery is disconnected from the charging circuit(i.e 26.1v-25.7v=0.4v).

Problem Statement

“What are the storage characteristics of batteries compared to the Manufacturer’s Curve?“

Storage Characteristics of Battery

KWh of energy was withdrawn from the battery and the stable voltage was measured (Waited 3 hours after disconnecting the battery to let the voltage stabilize) at 23.4 Volts.

Calculations

So,

The storage characteristics of battery = 1.3KWh/(25.9 V-23.4 V)
Storage characteristics of battery = 0.52KWh/Volt

Conclusion

• The storage characteristics of the battery are 0.52KWh per volt compared to the manufacturer’s curve which has a slope of 0.56KWh per volt.
• So the storage characteristics are within 10% of the published value.

Next Steps

Appropriate steps will be taken to observe the efficiency of batteries in order to utilize maximum energy supplied by batteries or to minimize the losses.

After testing that whether these batteries are suitable for this project from a technical and economical point of view or not by comparing with other batteries, observing the inverter and panel’s efficiency, the decision will be made to install this project in needy areas.