question

Hi @RalphD

Have you read the chapter on DC Ripple in wiring unlimited?

That goes into the causes and then how to eliminate them.

It’s normal that you won’t ever see it on any DC supplies or loads like our DC MPPT. It’s a specific characteristic of an AC inverter or AC charger.

@Guy Stewart (Victron Community Manager) Yes, I have read it and all wire AWG and lengths meet or exceed the recommendations in the article. It is also strange that this occurs in Absorption when the DC current is very low. As a note, there were no AC loads on the Multiplus II during my test.

i just came across this article and was wondering if it may apply to my situation. Link below. What are your thoughts?

https://redflow.zendesk.com/hc/en-us/articles/360039311291-Additional-DC-Capacitors-required-for-MultiPlus-II-product-series-when-used-with-Redflow-ZBM2-ZCell-batteries

@RalphD

Some batteries when they are near full pwm the charge source a bit and it shows up as DC ripple.

If it is in absorption only lower the voltage a bit. It might be struggling to balance.

Hi @Alexandra , My initial thought that the BMS was disconnecting and reconnecting causing an oscillation, or ripple on the DC line. That is why I ran the test with a power supply on each battery to ensure that the batteries were balanced and the BMS was not shutting down the charge. Now, when the MPPTs are charging the batteries the DC voltage is stable (<50mV P-P ripple). When charging from the Multiplus II is when I see the high ripple using the exact same voltages as the MPPTs. I have lowered the absorption voltage to 14.1V so that the ripple riding on the 14.1V will not exceed the 15V BMS over-voltage limit. I agree with @Guy Stewart (Victron Community Manager) that the Inverter/Charger has different characteristics regarding the generation of noise and ripple - especially when under load, however, there was no load when I took the readings. I did come across the redflow article (link above). Not sure if this may apply to the ripple that I am seeing since the impedance of these LiFePo4 batteries is different than the AGM batteries that were initially in the system, but it can't hurt to check it out so I just ordered a 47,000uF, 100V, low ESR electrolytic cap to try. Your thoughts?? Stay tuned for the results....

Update: I added a 47,000uF, 100V low ESR capacitor across the DC connections on the Multiplus II. I mounted the capacitor in a plastic electrical box and connected it to the Multiplus II terminals using 6 AWG wire to keep the series inductance low. The wires are less than 16" long.

This eliminated the high frequency ripple and reduce the low frequency ripple (~90Hz-100Hz) of +/- 0.5V (1V p-p) that was riding on top of the DC voltage. (reference #5 on my post dated Jul 09 2024 at 6:34 PM).

The ripple is down to (<200mV p-p) during bulk, (~300 mV p-p) during absorption and (~100 mV) during float.

If there is an AC load on the Multiplus II the ripple reduces further. I have absorption set to 14.2V and float at 13.8V - there have been no high voltage warnings / alarms or BMS shutdowns since I made the change 2 weeks ago.

@RalphD , an interesting problem and one I am familiar with, although I must admit I liked your diagnosis better as you identified high frequency components on the supply cables which I did not do.

Please read this thread that provides my thoughts on the matter, from some time ago.

https://community.victronenergy.com/questions/129138/quattro-shutting-down-lithium-batteries.html

I hope it may help a bit,

Trevor

Good afternoon,

I just did the same with a 3 phase system adding capacitors on the DC side and with great improvements, see the topic VDC Ripple rules; https://community.victronenergy.com/questions/281098/vdc-ripple-rules.html

Read it carefully and check my Victron site, links are there and graphs can be seen live.

In your case it is first essential to reduce your ripple further as when you reduce your charge voltage that much with Lithium, then they are never "full".

Can you count the DC capacitors inside the Multi please (most probably those black Sancon ones), and how much mF each, all the rest info I have of that inverter.

Then I can calculate the correct size capacitor(s) you must mount on your BMS to reduce the ripple and to stay below the threshold of your BMS.

Regards, Jeroen.