DC Voltage is usually thought of as constant – You have a 12V battery, it outputs a steady Voltage, only changing gradually as it is charged and discharged.

DC ripple occurs when the Voltage of a DC system oscillates, usually only on a scale of milivolts. It can happen for a number of reasons.

• Volt droop – As you discharge a battery, the Voltage drops. Volt droop refers to the phenomenon by which the voltage drops disproportionately low under a heavier load. Say if you discharge a 12V battery from 13V down to 12V. If the load was heavy for the battery’s size and you turn it off once the battery reaches 12V, you will see the voltage come back up – maybe to 12.1, maybe to 12.5 – depending on how heavy the load is. This is not DC ripple on it’s own, but DC ripple occurs due to Voltage droop when loads are uneven – and most loads are. A 240V battery inverter will pull more power at certain points in the wave, which causes the battery Voltage to oscillate at a rate of 50Hz – too quickly to show up on a multimeter, but can be observed on an oscilloscope.
• Charging methods – Most methods of charging a battery do not provide a truly constant voltage to the battery. On the extreme end you have switch mode power supplies and PWM solar controllers which let through high Voltage pulses. Even high quality charging equipment tends to allow small ripples and smart chargers use high Voltage pulses to sense battery Voltage (still go for a 3 – stage charger if you can – the benefits far outweigh the small disadvantage).
• Rapid charge – discharge cycles. So you’re charging your batteries from solar at the same time as you’re using them. Output power isn’t constant, neither is input power, and the load includes an inverter. You have DC ripple due to the inverter power, an uneven draw, and uneven input power all at once.

So why does this matter?

Firstly, at high enough voltages, DC ripple carries similar risks to AC current. This isn’t worth worrying about with a simple 12 or 24 Volt system, but at higher Voltages it is something that professionals need to be aware of. Battery life – DC ripple basically charges and discharges your battery very fast. Whilst only at the scale of mili – or – micro – volts, these mini – cycles do add up and long term shorten the usable life of your battery. How much will depend on the type of battery and the severity of the DC ripple. Batteries with a higher internal resistance experience a greater degree of DC ripple – So lead batteries typically fare worse than lithium batteries of the same size. On the flip side, the high voltage spikes put out by PWM controllers and other cheap chargers are especially damaging to lithium batteries.

What can be done to minimize DC ripple?

• Quality charging – Avoid PWM’s if possible and go for an MPPT, use quality 240V chargers as well if possible.
• Ensure your battery bank is properly sized. Lead batteries shouldn’t be discharged faster than 0.2C, or 20% of the batteries Amp – hour capacity, i.e. if you have a 100Ah battery, you shouldn’t pull more than 20Amps from it. Lithium and lead – Carbon batteries can be discharged faster, so go for these if running heavy loads.
• You knew I was going to talk about supercapacitors. These have a much lower internal resistance than either lead or lithium batteries, and have millions of charge – discharge cycles in stead of mere thousands. With a supercapacitor in parallel with a battery bank, all of those micro – cycles go through the supercapacitor first, buffering or eliminating the DC ripple effect. This is not necessarily a cost effective solution for a small AGM battery, but on larger banks it’s a no–brainer.