Ripple and voltage drop in photovoltaic systems
Ripple occurs in a system where the power supply is a battery (DC) and the load is an AC device.
This is the case for systems that have an inverter. The inverter is connected to the batteries but feeds an AC load.
The mechanism that causes the ripple is directly related to the voltage drop that occurs in the DC cables when a system has a load and the battery current is high. High current causes a significant voltage drop, which is even more pronounced if thin cables have been used.
The voltage drop of the whole system can be even higher, especially if too small, old or damaged lead-acid batteries are used. The voltage drop will not only occur in the cables, but also in the battery itself.
Ripple is related to the phenomenon that when an inverter feeds a large load, the DC voltage of the system drops. But the system voltage recovers once the load is switched off.
How is the ripple generated?
1. The inverter converts a DC voltage into an AC voltage.
2. The load connected to the inverter generates an AC current in the inverter.
3. This AC current produces (via the inverter) a fluctuating DC current in the battery.
4. The result of this fluctuating DC current is as follows:
• When the DC current reaches its peak, the battery voltage will drop.
• When the DC current drops, the battery voltage will recover.
• When the DC current peaks, the voltage will drop again.
• And so on and so forth.
The DC voltage will continue to rise and fall and will no longer be constant. It is now fluctuating. It will rise and fall about 100 times per second (100 Hz). The measurement of the fluctuation of the DC voltage is called the ripple voltage.
• When the DC current drops, the battery voltage will recover.
• When the DC current peaks, the voltage will drop again.
• And so on and so forth.
The DC voltage will continue to rise and fall and will no longer be constant. It is now fluctuating. It will rise and fall about 100 times per second (100 Hz). The measurement of the fluctuation of the DC voltage is called the ripple voltage.
How to measure ripple?
It is possible to measure the ripple. There are two options:
• With a multimeter. Select AC mode on the multimeter. Measure at the DC connections of the inverter. You are now measuring the AC component of the DC voltage. This AC voltage is the ripple voltage.
• You can use VEConfigure to keep track of the ripple.
When measuring ripple, remember that it only appears when the system is fully loaded. The same is true for voltage drop. Ripple can only be detected when the inverter is feeding a full load or when there is a charger charging with a high current.
There may be a small amount of ripple with no measurable effect. However, excessive ripple can have a negative impact:
• Inverter life will be reduced. The inverter capacitors will try to even out the ripple as much as possible and will age faster as a result.
• The life of other DC equipment in the system will also be reduced. They also suffer from ripple.
• Batteries will age prematurely, as each ripple is like a mini-cycle for the battery and due to the increased cycling, their life will be shortened.
• Rippling during the charging process will reduce the charging power.
• With a multimeter. Select AC mode on the multimeter. Measure at the DC connections of the inverter. You are now measuring the AC component of the DC voltage. This AC voltage is the ripple voltage.
• You can use VEConfigure to keep track of the ripple.
When measuring ripple, remember that it only appears when the system is fully loaded. The same is true for voltage drop. Ripple can only be detected when the inverter is feeding a full load or when there is a charger charging with a high current.
There may be a small amount of ripple with no measurable effect. However, excessive ripple can have a negative impact:
• Inverter life will be reduced. The inverter capacitors will try to even out the ripple as much as possible and will age faster as a result.
• The life of other DC equipment in the system will also be reduced. They also suffer from ripple.
• Batteries will age prematurely, as each ripple is like a mini-cycle for the battery and due to the increased cycling, their life will be shortened.
• Rippling during the charging process will reduce the charging power.
The inverter ripple alarm
Inverters or inverter/chargers have an integrated ripple alarm. There are two levels of ripple alarm:
• Ripple pre-alarm. The overload and low battery LEDs flash and the unit will shut down after 20 minutes.
• Full ripple alarm: The overload and low battery LEDs light up and the unit will shut down.
These are the ripple alarm levels for the different voltages:
12 V 24 V 48 V Ripple pre-alarm
1,5 V 2,25 V 3 V Full ripple alarm
2,5 V 3,75 V 5 V
Ripple will only appear when there is a voltage drop in the system. To fix the ripple, it will be necessary to reduce the voltage drop. This means that the resistance on the path from the battery to the inverter and back to the battery will have to be reduced.
• Ripple pre-alarm. The overload and low battery LEDs flash and the unit will shut down after 20 minutes.
• Full ripple alarm: The overload and low battery LEDs light up and the unit will shut down.
These are the ripple alarm levels for the different voltages:
Ripple will only appear when there is a voltage drop in the system. To fix the ripple, it will be necessary to reduce the voltage drop. This means that the resistance on the path from the battery to the inverter and back to the battery will have to be reduced.
How to solve the ripple?
To fix a high ripple in a system it will be necessary to do the following:
• Shorten battery cables that are too long.
• Use thicker cables.
• Check connectivity of fuses, shunts and battery isolation switches.
• Consult specifications for fuses, shunts and battery isolation switches.
• Check for loose terminals and cable connections.
• Check for dirty or corroded connections.
• Check for poor condition, old or undersized batteries.
• Always use good quality components.
• Shorten battery cables that are too long.
• Use thicker cables.
• Check connectivity of fuses, shunts and battery isolation switches.
• Consult specifications for fuses, shunts and battery isolation switches.
• Check for loose terminals and cable connections.
• Check for dirty or corroded connections.
• Check for poor condition, old or undersized batteries.
• Always use good quality components.
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