From 2018 onward, a range of new models called the MultiPlus-II Series were released by Victron Energy.
At the time of writing, there are two relevant models in this range, the MultiPlus-II 48/3000 VA and the MultiPlus-II 48/5000 VA.
Potential "High DC Ripple" shutdowns when using MultiPlus-II units with Redflow batteries
MultiPlus-II units are physically smaller and use a redesigned main system board, compared to the other MultiPlus and Quattro product series devices.
As in-field experience was gained with the use of these new products with Redflow batteries, some sites demonstrated occasional unstable operation that can in turn lead to the system shutting down unexpectedly.
The shutdowns are typically associated with 'High DC Ripple' warnings from the MultiPlus-II unit(s) concerned, and (if sustained) result in the unit protectively shutting down.
These events can occur either when the Redflow batteries are offline to undertake their periodic maintenance cycles, and/or when a Redflow battery goes offline unexpectedly for any reason.
Victron Energy have determined that this issue is an unintended consequence of the new MultiPlus-II product range having less internal capacitors installed inside the unit, compared to the prior models.
Where deployed on a DC battery bus, a capacitor 'smoothes' the operation of the bus, by absorbing sudden transient changes in DC voltage and current. In effect, on a DC battery bus, capacitors are 'shock absorbers' for the system, helping it to handle sudden changes in the DC voltage/current conditions on the bus/
While the reduced level of internally installed capacitors is not an issue for conventional batteries, it is an issue for some customer installations using Redflow batteries.
To resolve this issue - and to protect against the issue occurring in general - it is recommended that external capacitors are installed onto the DC battery bus for all installations where Victron Energy MultiPlus-II series products are used with Redflow ZBM2/ZCell batteries.
Sizing the capacitors required
The requirement is to install capacitors to a total size of:
- at least 39,000uF per MultiPlus-II 48/3000 unit installed on the DC bus
- at least 65,000uF per MultiPlus-II 48/5000 unit installed on the DC bus
The capacitors should have a voltage rating of 80-100V DC (or higher).
The requirement is that the total of the capacitor ratings used meets or exceeds the minimum values noted above. It is ok to achieve that minimum with one big capacitor or with multiple smaller ones - it all works out the same.
If you have multiple MultiPlus-II units, remember the total capacitance required is per unit - so if you have (say) 2 MultiPlus units installed, you need 2 x the total amount of capacitance installed (and so on).
It is ok to add more capacitance in total than this - more 'shock absorbers' don't hurt - just don't install less than those totals, in order to be effective.
Other sizes or brands fine too - such as the 47,000uF/100VDC unit pictured below.
Just make sure you wire in at least the minimum required capacitance in total, and that the capacitors are DC rated to at least 80 volts.
Simply wire as many capacitors as are needed (to add up to the total capacitance required), in parallel, directly to the terminals of one of the MultiPlus-II units in the system.
The wires used should be of at least 4mmsq diameter, and should ideally be kept to a total length of 50cm or less.
Where multiple capacitors are required, just connect all the positive capacitor terminals together - and run that positive lead through to the DC positive terminal of the MultiPlus-II.
Do likewise for all of the negative capacitor terminals, through to the DC negative terminal of the MultiPlus-II.
No fuses or disconnects should be present - just hard-wire the capacitors, in parallel, directly across the MultiPlus-II DC terminal pins.
These are photographs from a typical installation.
This installation happens to contain 3 x MultiPlus-II 48/3000 units, connected (in this case) to 3 x 39,000uF capacitors, hard-wired in parallel to the terminals of the master unit of the cluster.
This box contains the capacitors, mounted in a jiffy box beside the MultiPlus-II installation.
This image shows the wiring of the box of capacitors straight across the DC battery terminals of the MultiPlus-II cluster concerned - in parallel to the DC cables going off to the battery cluster.