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I have an offgrid system with a Victron mppt charger, and a Victron inverter, connected to a 12V battery. The mppt power is limited by the maximum current the battery can absorb, even when I have loads connected and their loads should be added to the limit. I also have a BMV meter that could send the actual current absorbed by the battery to the mppt controller.
Is there any way to implement this feature?
Hi Jcabetas,
I believe the feature you are requesting is implemented by Victron via a Venus-device and the Distributed Voltage and Charge Current (DVCC) feature.
In this arrangement, with all components connected, the system has full visibility to the loads from the inverter/charger, the charger and the battery current and is able to set charge current limits that are applied dynamically to the charge controller and inverter/charger.
This can also be used if there is a generator in play for example, and the generator is charging the batteries, and then the sun comes out and the MPPT will also try, potentially providing current beyond battery specifications or fuse/wire size.
This realtime processing, instructions and balancing of multiple components requires the memory (for programming) and CPU power of the Venus-device.
Hi JC. Apologies if I'm reading this incorrectly, but your mppt already has a link - the batt V. It will simply continue on attempting to reach it's programmed batt absorb/float targets regardless of loads.
So add 300W of load, it will increase 'charge' by 300W (providing it has enough available pv). That 300W won't go through the batts, just the 12V bus.
If you've some way of seeing A from your mppt, watch what happens when you add a load. It will compensate virtually instantaneously, maintaining V and adding A.
Thanks for the comment, but I think is not the case: Pb batteries have a current charging limit of 10% of the C12 capacity (depends on the model). You have to adjust the maximum regulator current to that value, otherwise you could exceed that charge. As the regulator current is not the battery current (you have to add the load), the regulator has no way to know how much is the battery current unless adding other sensor.
I can see the current output from the regulator. When there is enough sun, it reaches the limit and remains in that value no matter the load (in my case 23A). The rest of the current that goes to the inverter came from the battery. If for example the inverter current is 20 A, the battery will be charging at 23-20=3A, much lower than the charging capacity of the battery. In your example, if add 300W load, the limit is not increased (that is the problem I'd like to solve).
Of course, if the absorption voltage is reached, then it's true that the limit is now the voltage and then the regulator can add more current (still limited to the maximum).
There are regulators with inverter integrated that can do this trick, but either they have a huge noload consumption or they are very expensive.
Maybe VictÅon has a solution, but I haven't been able to identify it.
Ok, I can see where you're at now. I don't think there's a direct solution for this (I could be wrong). This may too have been discussed here before, and you could try scanning the Topic index.
I tend to take a somewhat pragmatic view of that '10%', and don't believe it is 'cast in stone'. Indeed I recall 10-20% suggested by a Victron staffer somewhere (don't ask me to find it). Of course battery-dependent would have been a proviso.
Pb's tend to be self-limiting in this regard too, and on a typical well balanced system one may rarely see 10%, and even then only for a short time.
Then too, a stickler battery man will suggest the '10%' also applies to the draw on the batts as well. The odd loads I sometimes apply, usually only for like 10 min or so, suggest a tripling of my bank size. Yeh, that won't happen..
That all said, I use flooded Pb's so that I 'can' exceed 10% if I must. I don't think 'sensitive' batts are necessarily the right choice for daily-cycling offgridders.
Although going beyond the topic subject, I'd say there isn't sensitive and not sensitive batteries, each technology has its limitations and we should respect manufacturers specifications. In my case the batteries are Trojan, and they recommend between 10-13% of C20 capacity for flooded battery, and 20% for AGM. LiPo can be designed to much higher charging, but each one has a specific value. Exceeding those values could affect battery life, and for Lipo could even lead to severe incidents.
With current PV prices, it's a pity to limit FV power to avoid overcharging batteries. In my case summer power is twice the winter time, so at that season we have even more spare FV power that could be used.
Thanks a lot, yes, it seems that Venus devices are what I was looking for...
