AC ripple on batteries

glenq · May 2015

Hi, im new on here, i have had a little look around and cant really find what i am after.
The setup i have is : 24 x 2 Volt 1000 Ah batteries as 48 V connected up to a MPPT PIP-4048MS with 6 x 300w LG mono solar panels connected. Also i have 4 KW wind turbine and charge controller connected up to the batteries. There is a 60A DC breaker between solar panels and inverter and a 100A DC breaker between inverter and consumer unit.
The issue i have found is by checking across the battery pos and neg for AC voltage i am getting 114.5 AC volts (ripple) how can i get rid of this or at least lower it to an acceptable level, or have i missed something out thats causing this. Cheers in advance.

Vic · May 2015

Hi glenq, welcome to the Forum,

Am not familiar, at all, with this Inverter (I guess).
Is the unit that you have?:
http://www.mppsolar.com/v3/pip-ms-series/

Is it a True Sinewave (or Pure Sinewave) Inverter? ... The "MS" at the end of the part number MIGHT mean Modified Squarewave ...

How is it that you are measuring that 114 VAC? And what is the system doing when you see that voltage?

You might try shutting off the PV input breaker, and the Wind Turbine to try to isolate things a bit.

Later, Thanks - Vic

glenq · May 2015

Hi Vic, thank you for your input. Yes it is a high frequency pure sinewave inverter. I made an error in the first post the 100A DC breaker is between the batteries and inverter. to measure the VAC i used my multi meter set to AC and put black probe on the battery negative and the red probe on the battery positive, i had both the wind turbine and solar panels off. So the ripple i am getting is from the inverter for some reason.

Vic · May 2015

Hi glenq,

Thanks for the updated information.

It can be difficult to measure AC voltages that are riding on a DC voltage. You might try placing a capacitor between the positive lead of the DMM, and the battery positive.

The approximate value of the capacitor depends on the frequency of interest, to some extent. If you are looking for the AC ripple at the line frequency, or in this range, perhaps a 0.1 microfarad capacitor. If you are more interested in the high frequency components, perhaps a bit smaller cap would be a bit better.

Ideally, an Oscilloscope would be by far the best. FWIW, Vic

JoshK · May 2015

glenq wrote: »

...my multi meter set to AC

Only a RMS multi-meter would read correct, true?

BB. · May 2015

Digital Multi-Meters can have very different responses to measuring DC voltage with an AC component.

The "cheap" DMM when set to DC, will measure the DC Voltage of the battery (sampled a few times a second). No problem.

When the C-DMM is set to AC, frequently they have a blocking diode that blocks all DC current. They measure the "peak to peak" AC Voltage and divide by sqrt(2) (multiply peak AC reading by 0.707)--The sqrt(2) conversion factor is the mathematically correct number to convert from Peak to Peak to RMS (root mean square) reading for AC power.

With an expensive True RMS reading DMM, they sample the waveform upwards of 50,000 times per second and "do the math" to calculate the true RMS power of the signal.

if you have a 48 volt DC battery running at 52 VDC with a 10 VAC signal, I would guess you will read about (52 VDC + 10VAC*0.707 RMS conversion=) ~59.07 VAC RMS.

If you want to measure the true AC voltage noise signal, as Vic says, you will need a non-polarized capacitor (not an electrolytic capacitor) to block the DC component if using an RMS reading meter.

You may need to experiment with your meter. Try setting the meter to VAC and put it on a 12 VDC car battery--See what you get (0 volts, 12 volts, etc.).

The 114.5 VAC reading sounds wrong... The battery bank is actually quite a good filter "capacitor"... And it should be near impossible to impose a 114 VAC signal on a 48 volt battery bank.

So--To have a reading like that--Something is probably not right. It could be a bad meter, near dead meter battery, or even "ringing" in your wiring harness (the AC inverter or even the solar charge controller can have very "sharp" wave form edges (fast rise time)--Which can cause wiring harnesses to "ring"--An L-C oscillator).

You may need an oscilloscope to see what is really happening. Be very careful, many/most standard oscilloscopes have grounded leads for their test probes--And you can create a short circuit through the scope and its power cord (grounded plug). A large 48 volt battery bank is a wicked force of nature (i.e., your probes/scope may not survive a short circuit from high current flow through the scope).

-Bill

john p · May 2015

Sometimes you get ac on batteries from inverter. .You shouldnt but you do, but its usually at such low current its not worth worrying about.. You get it a lot from poor designed mains battery chargers..12 or 48v makes no difference... Its actually very hard to get pure DC from a battery charger or in fact from any mains converted ACto DC. charger/power supply..

Darryl · May 2015

Yes I have measured AC ripple on batteries, your ripple seems to be higher than I recall seeing. I think it is like 100 mV, but it has been a long time since I measured. The inverter draws power from the batteries, in spirts usually at 60 or 50 Hz, I have been told this reduces battery life. If you have a clamp on current probe try looking at the current to and from the battery, I have proposed to some equipment people cures but they respond it is expensive to fix this problem. The obvious is capacitors, but internal resistance is the killer of this solution.

inetdog · May 2015

114VAC riipple on a 48V DC battery is simply not a credible number. So what you are really reading is the result of your meter not filtering out the DC when you set it to an AC range.
If your meter is the averaging type and uses a half wave bridge, and then converts that approximately to what would be the equivalent RMS voltage for the same average, I would expect to read around 115 volts when connected to pure 50V DC with no ripple at all.
What happens when you reverse the meter leads? Zero?

Take Bill's advice and put a suitable capacitor (~1microfarad, 200V non-polariized) in series with your meter and check again.

AC ripple on batteries

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