Hello, I’m speccing out an off grid vehicle system with solar and generator inputs.
I plan to use a Multiplus II 12v/3kw since that best suits the load requirements.
The system will have ~7500wh of LiFePO4 batteries, and I would like the option to charge from the generator at a full 0.5c, so roughly 300 amps @ 12v.
But, the multiplus maxes out at 120 amps DC charging.
I can’t find definitive information from the datasheets/manuals for whether any of the Victron AC-DC chargers support DVCC and can coordinate with the multiplus internal charger (other than a second multiplus, which isn’t viable for space/weight reasons and still could only charge at 240a total).
Do any of the Victron AC->DC chargers work in this application supporting full coordination with the multiplus internal charger?
If not, my fallback plan is to program one of the MP relay outputs to turn on during bulk phase, to enable an external, dumb, fixed-voltage AC->DC charger during bulk.
But that’s not an ideal solution.
Any input is much appreciated, including any suggestions totally separate from the two options described above.
2.3.5. CAN-bus connection (VE.Can) The VE.Can sockets (RJ45) provide access for monitoring and control. For example with a Skylla-i Control GX panel, a GX device such as the Cerbo GX, or to connect the charger to a NMEA 2000 network.
From the above, I would expect a Cerbo / GX device to be able to control the charger over the VE-CAN bus system. This is from the manual of the Skylla-i Victron Chargers.
Thanks Mike, though that is pretty light on details.
Does anyone know for sure whether the VE.Can input can control voltage levels, charge phase, algorithm and so on?
Also I suppose Skylla is the only candidate? Slightly disappointing since the cost is much higher than the other chargers.
Also interested to hear if anyone has experience controlling either the Skylla, or an external “dumb” charger via simple relay output is effective for charging an LiFeOP4 bank.
You have to be careful as some older chargers have comms connections ut only for monitoring, e.g. The IP43 has VE Direct but no DVCC.
See Section 11.4.1 subnote 2 of the DVCC section of the Ekrano manual, Cerbo is the same.
This mechanism only works for Victron inverter/chargers including Inverter RS, Multi RS, Solar chargers incl. MPPT RS and Orion XS DC-DC battery chargers. Other chargers, such as Skylla-i’s are not controlled and also their charge current is not taken into account.
So I guess the only option is to cobble something together?
My thinking is that the simplest approach is to detect when the inverter is in bulk charge phase, and set up a simple on/off trigger to activate an external charger with a fixed bulk-charge voltage that matches that of the multiplus.
Theory being that bulk phase will see the most gains, and the multiplus probably has enough current during absorption phase that the external charger isn’t even necessary at that point.
But the voltage match would have to be pretty exact. Both chargers would need to be resilient to reverse current flow to avoid damage.
And in theory at least even a tiny voltage difference would cause the charger with the higher voltage to bear all of the charging load.
Does anyone know if this works in practice? a super exact voltage match for bulk phase across two different chargers seems pretty difficult to achieve.
Bulk phase is before the charger has reached absorption voltage, so exact voltage accuracy is of no consequence at this stage. If both are set to 14.2V say, if the voltage is 14.0V both will be in bulk at full current. The issue is the end of bulk, one will hit its absorption voltage first, this will vary based on exact accuracy of the measurement and different voltage drops in charging cables. What you do is a bit of trial and error bit you set the extra charger to have an absorption voltage perhaps 0.05 or 0.1V below the Multiplus, then as the batteries come off bulk, the extra charger automatically ramps down first.
I used to run my solar, alternator and Multiplus like this so the solar was the last charger to leave bulk to maximise solar.
My concern is actually the voltage difference between the multiplus charger and the external charger, even during bulk phase.
Say they’re set to 14.0v nominal, but one charger ends up at 14.0v, and the other @ 14.05, since no voltage regulator is perfect.
In theory, the charger @ 14.05 would assume all of the charging load, and actually attempt to backfeed current into the multiplus charger (which I’m guessing has an output diode to prevent this.. but still, it would mean that only one charger is actually contributing to charging the batts).
But hopefully my EE101 understanding of what happens in this situation is too simplistic
The phase you are describing is absorption, where the chargers are operating to a target voltage. At that point the batteries are limiting what current is required so it does not matter about one taking all the load as absorption for lithium batteries can be quite short. And that is where as described in my previous post you can choose the primary charger by setting that one a bit higher and overcome any inaccuracy.
Lithiums mainly charge when the chargers are flat out operating at their design limit and the voltage is below target so the voltage accuracy is irrelevant.
Chargers do not back feed, as noted I have 3 independent chargers and charged lead acid without any overall DVCC control with all of them having different voltage set points so I could end up with solar and Multiplus on float at 13.5V and the alternator at 14.4V because it does not have float and the current did not back feed.
Yes,
Mostly it may be more economical to use additional inverter/chargers to boost the charging capacity, this also gives reserves and back up devices. then there is no need to worry about whether the charger will follow the DVCC correctly…
