DC OUTPUT RIPPLE is a common and important specification for stationary battery chargers. The term “ripple” usually refers to the ac voltage measured at the battery terminals, but it may also be measured at the charger output terminals, if the battery is disconnected from the dc bus for maintenance.
If you’re unsure about the causes and effects of ripple, then start with “Ripple,” below. A discussion of dc filters, for reducing output ripple, starts in SECTION 3.4.
A Play in One Short Act
E, an engineer
S, a salesman
PA, a purchasing agent
cs, a customer service agent. He speaks in lower case because he doesn’t know how to use the shift key.
The lunchroom in a nondescript office building.
E is fumbling with a soda can, trying to open it. He’s not sure which way to pull the tab. PA is seated in a chair in one corner, dozing. Every few minutes, he awakes with a snort, moves around a little, then falls back to sleep.
S enters. Seeing E, he immediately starts grilling him about a rumor that E is changing the design of the company’s premier product, a stationary battery charger.
E: “Well, yes, we have something, but it isn’t a change across the board. Call it an option.”
S: “An option? What does it do?”
E: “We call it a filter. It takes all of the noise and ripple out of the dc output voltage.”
S: “Wait a minute. Whoa up there. First of all, what is ripple? And what’s wrong with our dc output that you need to screw with it?”
E: “Nothing’s wrong with it, as far as it goes. But up to now, our only product offering has been a bare-bones charger, with no filtering. There’s a lot of ripple in it.”
S: “There you go again with that ripple. What is it?”
E: “Think of ripple as an unevenness, sort of a wiggle, in the dc output voltage that we deliver to the battery. Officially, it’s a periodic variation in the dc output voltage.”
E: “Yeah, that just means that it’s consistent in its frequency content.”
S: “I don’t get that at all.”
E: “Read Under the Hood: Sources of Ripple.” [Section 3.2
S: “OK, I’ll do that later. But for now, what’s the big deal with ripple? We get the battery charged, don’t we? I never heard anyone complain about uneven voltage.”
E: “You probably won’t, as long as the battery is connected. The battery itself acts as a pretty good filter. But it’s taking a bit of a beating doing it. And if the user ever disconnects the battery, say to change a cell, or just to clean the connections, all hell could break loose in the substation.”
S: “Why is that?”
E: “With no battery, and no other filter, the ripple voltage could be high enough to damage equipment connected to the dc bus. That’s especially true for a single-phase charger.”
PA: (stirring slightly) “Ripple? Never drink it.” (Goes back to sleep.)
S: (Ignoring PA) “So where does this ripple come from?”
E: “It’s in the nature of how we create the dc voltage in the first place. A perfect charger would deliver pure, noise-free dc current to charge a battery.”
S: “You mean just like the voltage we get from a battery?”
E: “Right. But the power for the charger comes from the ac power line, and has to be converted to dc. That’s what the rectifier in the charger does. And you know that ac voltage is constantly changing, first positive, then negative, and on and on, 60 times a second.” (E finally gets the soda open with a snap, spraying some of it on S’s shirt.)
S: “Thanks, I needed that. Now, where does the ac voltage go, after you change it to dc?”
E: “That’s the core of the problem. We have dc voltage, and can use it to charge a battery, but we also have a lot of the ac voltage left over, and that gets delivered to the battery, too. The ac voltage doesn’t contribute to charging, but does raise the battery’s temperature.”
S: “I don’t see that as a big deal.”
E: “It isn’t, for a large flooded battery with lots of excess electrolyte. But in a VRLA or gelled electrolyte battery, it can shorten the life of the battery. And too much internal heating could actually cause thermal runaway, which can destroy a battery.”
S: “OK, that could be a big deal.”
He had been standing in the doorway, listening.
cs: “hey, i just had a call yesterday from a guy at the fifth of may substation.he said they had to replace the whole battery because 3 of the units were all melted inside. is that what you’re talking about?”
E: “Could be. But it probably just got too hot in the substation, or they had some other problem.”
S: “I’m almost convinced. But how do you get rid of the ripple, and how much is it going to cost, and who pays? Us or the customer? I’m going to have a hard time selling this, if it increases the price.”
cs: “c’mon, you know the answer to that last one. we always end up paying.”
E: “Well, not necessarily. The filter option might add 15% to the cost of a charger. But in most places, the battery costs more than the charger, and extending its life will more than pay for the option. Customers who can see five years into the future won’t have a problem with it.”
PA is sitting up straight now.
He’s been listening to the conversation for the last few minutes.
PA: “This filter option. What’s in it? What do I have to buy now? And where are we going to store the stuff? The stockroom’s full as it is. The boss keeps telling me to reduce inventory.”
E: “Good point. Yeah, you’ll need some space to stock more inductors and some big capacitors.”
cs: “people are going to ask me what this filter thing actually does. can you give me some specs?”
E: “Thought you’d never ask. Take a 130 V charger in a substation. If the charger is unfiltered, the ripple voltage on the battery might be 1 or
2 volts. But take away the battery, and the ripple voltage could shoot up to a hundred volts. And the peak voltage, as I said before, could be high enough to damage the substation electronics.” (Note that E said ‘as’, not ‘like’. My kinda guy.)
cs: “so, how much ripple do you get with the filter in the charger?”
E: “We’ll have two levels of filtering. With the standard filter, the ripple would be only 0.1 volt when the battery is connected, but might rise to a volt or so when the battery is disconnected.”
S: “That sounds a lot better. You said you had two levels. What’s the other?”
E: “We’ll call it enhanced filtering. This would keep the ripple below 0.1 volt even if the battery is disconnected.”
S: “You can call it anything you want, but we’ll call it Battery Eliminator filtering. How much is that going to cost?”
E: “It’ll add about another 10% to the charger. But customers that have really sensitive equipment, especially communications stuff, will want it.”
cs: “a lot of customers won’t want to pay extra for any filtering. how about if i just tell them to be sure to turn off the charger first, if they’re going to disconnect the battery?”
E: “Sure, that’ll work. You want to take the chance that they’ll always do that?”
E finishes his soda and leaves the room.
S looks down at his shirt, and shakes his head slowly.