Axpert MPPT Charger

[PaulF007]

With all the back and forth with regards charge controllers and how good or bad they are. I decided to do "test" into how the Axpert MPPT is doing in general. 
Keep in mind that this is not a LAB test and there is quite a few Variables that could still make a difference but this is giving a reasonable idea as to how it performs. 

Method of testing:

Used the BMV 702 with logging that data every 20 sec on emoncms.
I am away for the long weekend after some advice , I decided to give the batts a "rest" for the weekend. The houses base load is quite low at night (as low as 60 w total) so it won't break the bank if I run on eskom overnight. 
So I set the inverter on SOL with Solar and Grid Charge priority together. I will use the batts only early morning and late afternoon as the sun rises and sets. The batteries was almost fully charged yesterday evening and the last bit was pushed in by the grid by about 21h00 the bats was in full flout with only 0.6 - 0.7 a going into the batts. 
This morning the system used about 1ah before the MPPT could start charging the batts and by 9 it was in full flout with 0.6 a gain. 

The interesting thing is that the whole system will only draw about 250w max and with 3kw of panels and a near perfect clear sky day there should not be much difference in the charging of the batts but I have found that there is almost a 1v difference in charging between Grid charging en Solar Charging. Maybe some of you can do the same test so that we can compare.,,

 

Here is the graph:

[Coulomb]

So I see utility charging at 54.1 V and very little ripple, and solar charging at 54.2 V with more ripple and spikes up to 55.2 V. I assume that these spikes are the volt difference you are talking about. 

The SCC charger connects to the battery via some 500 mm of wire (two sets of 250 mm).  These wires carry tens of amperes as loads come on and off. The tens of milli-ohms of resistance times the tens of amperes of current results in these sometimes thousand millivolt differences, and is why the utility charger has relatively low ripple; the voltage it is measuring is the voltage that is being controlled.

To improve the situation, the Axpert could use thin voltage sense wires as well as the thick current carrying wires. That way the voltage drops due to the current surges would not affect the voltage control nearly as much. But that would add complexity and cost.

I wonder if the Blue MPPTs have such sense wires. 

[Guest]

6 minutes ago, Coulomb said:

I wonder if the Blue MPPTs have such sense wires. 

FWIW. Morningstar PWM controllers have sense wires, if you want to connect them. Also has a battery temp connection to adjust charging based on temp. So it is pretty flexible.

Victron MPPT's don't have a sense wire that I am aware of.

[___]

31 minutes ago, Coulomb said:

I wonder if the Blue MPPTs have such sense wires. 

The top-end ones do, the VE-Can controllers (150/70 and 150/85). Well, the option is there, it doesn't ship with the wires. The lower end ones don't have it.

It's something that is being worked on too. At the moment it uses the "vebus voltage" for most decision making, but that is the voltage seen by the inverter. It comes as no surprise that the voltage seen by any particular MPPT, the inverter and/or the BMV could all be a little different depending on the voltage drops on their particular cables. The best source for voltage should in fact be from the BMV, but I don't think that is what they use yet. In theory however ,if you have the CCGX and a BMV... then the CCGX can combine info of a temperature sensor (which could be on the CC, the BMV or the Inverter) with voltage sense info (likely from the BMV) to control everything.