I have been battling a high voltage ripple on the DC charge voltage from my multiplus II when charging three 280Ah LiTime LiFePo4 batteries configured in parallel. The worst case ripple occurs in the absorption phase and can be up to +/- 0.5V. This ripple on top of the absorption voltage can be enough to trigger the BMS to stop accepting charge. Plus that much ripple is concerning.
First I will go over the steps that I have taken to find out what it is NOT -
1) My first thought was that the cells in one (or more) of the batteries may be out of balance and the BMS may be triggering a stop charge due to an over-voltage on a cell. This does not appear to be the case. I disconnected all of the batteries and hooked a power supply the batteries one at a time. I then slowly raised the voltage while monitoring the current. I brought each battery up to 14.6V, left it for an hour and watched the current slowly drop to < 3mA.
2) I reconnected the batteries with the Multiplus II off - the voltage was stable.
3) There are three MPPTs in the system and I turned them on. The voltage was stable with a 13.8V float charge.
4) Using the MPPTs, I initiated a equalization at 14.1V - The voltage was still stable.
5) I disabled the MPPTs and set the Multiplus II absorption to 14.1V (low enough so the ripple would not trigger an over voltage in the BMS) - When the the Multiplus II was turned on, it entered Bulk quickly followed by Absorbtion there was a lot of noise on the DC output. Viewing on an O-Scope there is a high frequency component ~125 MHz which is <100 mV p-p, and a much lower frequency component which causes most of the ripple (+/- 500mV)
6) When the Multiplus II entered the float stage the voltage was stable at 13.8V. (<100mV p-p)
As a note there are two 1/0 cables in parallel between the multiplus II and the bus bar for the batteries for both positive and negative.
Also, a Smart Shunt is used as the battery monitor.
Based on the above it does not seem to be related to the BMS or the voltages since the MPPTs do not have an issue with ripple. It only seems to happen in Bulk and Absorption with the higher ripple in Absorption.
Any thoughts of what could cause this and how to mitigate it would be appreciated.
