Alternator 'Ripple too big' and battery keeps running flat


I am trying to get to the bottom of a problem I am having.

Battery keeps running flat, its only a few months old so took it back to where I bought it and they did a voltmeter test (?) that showed that the battery was good and that the alternator had a ‘ripple too big’. They told me it was an alternator issue but the alternator was replaced at the same time as the battery so I got a second opinion that said the same thing. I went to the garage that fitted the alternator and they checked the battery seperately and said it was losing its charge so it wasn’t the alternator. However I have also been having various electrical issues e.g. alarm going off randomly that I have read can be due to a faulty alternator and I nearly always get an electrical hum / murmur when just starting or turning off the engine. I tried another battery and still have these electrical problems, and am thinking it is just a matter of time before it starts running flat. Could it be something else? Are there any tests I can ask for to identify the problem? Would an oscilliscope test determine one way or other if the fault was with the alternator? Should I get a parasitic draw test?

Any help would be appreciated, thanks

how long ago was the alt. replaced ? “Ripple” means your alt. is not putting out pure D.C. its like its bleeding A.C.and that is not good for your battery. an oscilliscope will show you what the alt. output looks like. far as the alarm going of, not sure about that.what kind of car is this?

+1 for @BigMarc. A very small amount of ripple can be expected but if it exceeds specs then a new alternator is needed. A third opinion is not a bad thing but I think the results are going to be like the first two.

+2 for Marc.

The regulator portion of the alternator assembly contains a rectifier, made up of diodes that are arranged to take the bottom portion of the sine wave that the alternator produces and flip them over, creating a pulsing DC. It also includes a filter, which is basically a resister and a capacitor, that “cushions” the waves peaks and smooths the highs and lows out. By virture of the capacitor’s storing and releasing voltage at a rate controlled by the resister, the DC is smoothed out into something more consistant that’ll be between the peak and the low spot. It’ll never be a perfect filter, there’ll always be some ripple, but if there’s a problem with a diode or with the fiilter, the output will vary too greatly, creating excess variation. If your battery is charged to 12VDC and the bottom of that ripple goes below 12VDC, it will act like an alternator (AC) rather than a generator, at least around the limit. The total voltage stored in the battery will be low.

Fix the voltage problem, reinitialize the alarm system, and you should be fine.

Yes, a scope is the way to go.

“Rebuilt” alternators FREQUENTLY have quality control issues…A/C “ripple” means it has a bad or weak diode, it’s output will be limited, and your car’s electronics will not like it. If you try to listen to your car radio on the AM band, you can probably hear the A/C buzz from the defective alternator…Request a new alternator…

I agree that it is the alternator and that the alternator has a shorted diode. The alternator is a three phase AC generator. It uses either 6 or 8 diodes to form a full wave rectifier for the output of the coils of the generator. A filter is not needed because the highs and lows of the ripple are within about 11%, the battery will act as the filter.

If one diode was open, then the ripple would be greater, but probably not enough to cause any problem. But shorted diode would actually put an opposite polarity voltage onto the output wire periodically and that would cause a large ripple, enough to do some damage.

The shop that is saying the alternator is testing the alternator with a DC voltmeter. At the frequency that this ripple occurs, the DC voltmeter will not pick it up, but the alternator will have an adequate DC voltage level to look good to them. The alternator may not be providing enough current to keep the battery charged,or it maybe it will.

The bad thing about a shorted diode is that when the engine is off, that diode will drain the battery, but it will show up in a parasitic test. On older vehicles, a bad diode would light up the battery light when the ignition switch was in the off position, but now everything goes through a computer so when the power is cut to the computer, it may not light the battery light anymore.

From what you say about the trouble you could have two problems. It definitely sounds like you have an excessive AC ripple problem and while a scope is nice to have to look at that with you really don’t need one. A good digital meter will do the testing just fine. Set the meter to read AC volts and measure the battery while the engine is running around 1,500 RPM and you have a good load on the electrical system (headlights and blower on). If there is more than .1 volt of AC ripple then the alternator is bad. One thing you need to know while doing this test is that the meter you are using blocks DC voltage while in the AC mode. Check the battery voltage with the engine not running and see if the meter reads voltage while in the AC mode. It should show no voltage if the DC is blocked. If it doesn’t block the DC you could add a .5ufd capacitor in series with the probe lead and that will block the DC.

You may also have a parasitic current draw that could be due to the alternator or something else in the electrical system. Normal current draw should be less than 40 milliamps, depending on the accessories installed in the car. Things like alarm systems will cause extra current draw.