Solar Ammeter readings in/out

[Dave 2000]

Hi all just registered and looking for an answer to an issue with my installation.

I will give you the spec first.

Two X BP solar panels each one:

Nom peak power 75.00
Peak power voltage 17.00
Peak power current 4.45
Short circuit current 4.75
Open circuit voltage 21.40

Victron energy 75/15 MPPT controller

I have made a power distribution board and been using it in the car for a couple of years now with the above panels and batteries with no issues. Something I wanted to do was monitor the amps coming in from the panels and the amps coming out form the controller to the batteries. What I see is the amps coming in mimic those going from the controller, so if it is bulk charging using just the one panel the current from the panel shows around three or four amps depending on battery SOC which is identical to the output from the SC. Now I get it makes sense but when I connect the second SP the input amps do not go up. I wanted to measure the input from the second panel as it is a 'roaming' panel and wanted to watch amps coming in to ensure I had good panel/sun correlation. Can I assume that this will not be possible due to the SP simply matching input versus demand from the SC? Presently the two panels are connected to the single ammeter but I can switch each one independently and the result is the same but by switching it does allow me to see that each panel is contributing to demand. Either way the input from the panels always mimics the output.

I hope that all makes sense, I have a reasonable grasp on vehicle electrics as I have been a mechanic for more years than I can remember.

TIA,

regards

Dave

 

[marconi]

Have you connected your two panels in series? The Victron 75/15 can accept an open circuit input voltage up to 75V. So two of your panels would provide 2 x 21.4 = 42.8V so that is fine for the solar charger. By wiring the two panels in series, they act like two batteries in series, albeit one may be producing less voltage than the other because of the different amount of sunshine each captures. Nevertheless, the current through each panel is the same - which is what the solar charger will use together with the the combined panel voltage to achieve maximum power output from them using the MPPT algorithm. So if they are in parallel now, put them in series instead. I think this will then achieve the outcome you desire.

 

[Dave 2000]

Many thanks for that @marconi.

I do understand the series versus parallel etc, but the question is about the input amps that show through the ammeter. If the controller only 'sees' the batteries needing say for example 2 amps then the demand from the controller on the panels will be 2 amps right? Having the two panels in parallel is essential to my usage. If going off road for a couple of days then I only take the single roaming panel. If more than a few days then the roof rack goes on the car and the second panel comes into play as it slides out from under the rack, I appreciate the thought about going in series but as you can see it is not practical for my setup, I need to have a fridge.

So this is more about being able to monitor the amperage from each panel. As a thought, perhaps a volt reading from each panel would be more suitable, the volts would climb depending on the varied exposure? I have three meters in the power distribution board, ammeter measuring panel input, another reading controller output, and a volt meter for battery readings, perhaps I should have a volt meter for each panel and one ammeter for the controller output?

The install works perfect and never gives me a problem with the fridge running all night etc, I was hoping I could wire the system to optimise SP input by moving the roaming panel around.

Perhaps I am just being picky?

Thanks again for your input.

Regards

Dave

 

[marconi]

I do understand the series versus parallel etc, but the question is about the input amps that show through the ammeter. If the controller only 'sees' the batteries needing say for example 2 amps then the demand from the controller on the panels will be 2 amps right?

No that is is not the way the charge controller works. Without too much technical detail, the charger controller (CC) aims to maximise the power transfer between the panels and the battery so that at any instant and particular level of sunshine on the panels the most energy is being converted from photons into electronic current and then stored as chemical energy in the battery. The voltage of the battery does not vary much albeit the small changes in voltage do indicate the state of electro-chemical charge of the battery.

So let us say on a particular day there is a 75W of sunlight falling on both panels. The aim is to convert all of this into energy. In your case the open circuit voltage is 42V when no current flows -so since power is current x voltage there is no power transfer P + 42 x 0 = 0

If the panels were short circuited then all the power would be absorbed and dissipated in the panels as heat - so no transfer to the battery. Between the open circuit condition and the short circuit condition is a combination of current and voltage, which when multiplied together produces the maximum power. This is what the MPPT algorithm continually searches for by varying the current, measuring the voltage, calculating the product to track the maximum power.

With equal insolation on both panels in series the MPP is at 2 x 17V = 34V and 4.45A, which produces 34 x 4.45 = 150W (ie 2 x 75W from each panel).

But you cannot directly connect 34V to a 12V (or 24V) battery to charge it because the battery is such a dominant producer of electromotive force with low resistance. The job of the CC is to convert the 34V and 4,45A = 150W into 12V x 12.5A = 150W or it was a 24V battery 24 x 6.25A. So you can see the current from the panels is different to current to the battery - but the power is the same and maximised.

A device such as this will enable you to see the instantaneous power in to or out of the battery and thus do the optimisation you seek:

https://www.cclcomponents.com/victr...UBRQx5JCK1jxugIy6_FS-7rlT0N4Cw38aAsbsEALw_wcB

 

[Dave 2000]

I do understand the series versus parallel etc, but the question is about the input amps that show through the ammeter. If the controller only 'sees' the batteries needing say for example 2 amps then the demand from the controller on the panels will be 2 amps right?

No that is is not the way the charge controller works. Without too much technical detail, the charger controller (CC) aims to maximise the power transfer between the panels and the battery so that at any instant and particular level of sunshine on the panels the most energy is being converted from photons into electronic current and then stored as chemical energy in the battery. The voltage of the battery does not vary much albeit the small changes in voltage do indicate the state of electro-chemical charge of the battery.

So let us say on a particular day there is a 75W of sunlight falling on both panels. The aim is to convert all of this into energy. In your case the open circuit voltage is 42V when no current flows -so since power is current x voltage there is no power transfer P + 42 x 0 = 0

If the panels were short circuited then all the power would be absorbed and dissipated in the panels as heat - so no transfer to the battery. Between the open circuit condition and the short circuit condition is a combination of current and voltage, which when multiplied together produces the maximum power. This is what the MPPT algorithm continually searches for by varying the current, measuring the voltage, calculating the product to track the maximum power.

With equal insolation on both panels in series the MPP is at 2 x 17V = 34V and 4.45A, which produces 34 x 4.45 = 150W (ie 2 x 75W from each panel).

But you cannot directly connect 34V to a 12V (or 24V) battery to charge it because the battery is such a dominant producer of electromotive force with low resistance. The job of the CC is to convert the 34V and 4,45A = 150W into 12V x 12.5A = 150W or it was a 24V battery 24 x 6.25A. So you can see the current from the panels is different to current to the battery - but the power is the same and maximised.

A device such as this will enable you to see the instantaneous power in to or out of the battery and thus do the optimisation you seek:

https://www.cclcomponents.com/victr...UBRQx5JCK1jxugIy6_FS-7rlT0N4Cw38aAsbsEALw_wcB

Hi again, sorry I think I should have prepared myself before the original posting to convey that I do understand the MPPT system but not the fact that the two ammeters one from the panels and the one going to the battery constantly show the same amperage.

If it is amps that charge the battery 'pushed' by the voltage then why can I not see a difference. If I run the batteries down a little and point my roaming panel at a full bright sun I see circa 4 amps coming from the solar panel and the same amperage going into the batteries. Move the panel away from full exposure and of course the amperage goes down on both ammeters, which I expect. By carefully tracing the sun by manually moving the panel the amperage stays fairly constant and after a couple of hours the amperage to the batteries starts to fall, again as expected, but despite the still full sun exposure the ammeter for input also drops. So what I am seeing is identical input and output from both meters regardless of battery charge and solar exposure. So perhaps the MMPT is faulty? I do know that the MPPT is limiting voltage and the volt meter shows this. I also know my batteries are kept charged and never give me issues. Curiosity made me wonder why if the batteries only need say 1 amp for say the float setting and yet two panels are in bright sunlight, the input amperage shows just 1 amp. So the CC has to shed that amperage via a heat sink or simply block it so it is lost through the heat dissipated in the panel as you mentioned earlier?

I really do appreciate you taking the time to explain or clarify the system function, but at the end of the day if the input from the panels will always mimic that leaving the CC I then I will have to stay with the system I have.

EDIT: I do have the Bluetooth app from Victron (which I begrudged being forced into buying it) and IIRC it shows the same as input/output as my gauges.

regards

Dave

 

[marconi]

Without knowing the actual logic of the software controlling the CC I suspect it is a feature of the way the current and voltage are controlled to charge the batteries in the optimum way in terms of speed and available pv energy while maximising their life. I am familiar with charging AGM batteries and these have three phases of charging which this diagram illustrates - constant current/varying voltage for bulk, constant voltage/varying current for absorption, constant voltage and current to a trickle amount for float:

AGM Battery Charging - https://www.chromebattery.com/blog/post/battery-care/agm-battery-charging/

The CC is being 'smart' to make the best use of the available electrical power input to apply a voltage and current which maximises the power input to the battery for storage - subject to some limits to avoid damaging the batteries by attempting to charge them at too high a rate which would cause excessive heating inside them shortening their life. The decision on when to move from one phase to the next is decided on by the electrochemical state of charge as measured by the instantaneous voltage of the battery (and perhaps time elapsed too). In the Iconica hybrid chargers I have used one can set up the type of battery eg AGM, its capacity in Ah and a limit on the maximum charging current as advised by the battery maker. It then works automatically to make best use of the pv energy to store it in the battery 'knowing' the nature and state of charge of the battery.

Alas then, there is no simple relationship between between the instantaneous current in and current out to guide you. I admire your curiosity.

 

[marconi]

Simple Voltaic Cells batteries copper zinc cell gcse chemistry KS4 science igcse O level revision notes - http://www.docbrown.info/page01/ExIndChem/electrochemistry10.htm

Lead-Acid Batteries Chemistry Tutorial - https://www.ausetute.com.au/pbbattery.html

Fast and Ultra-fast Chargers - Battery University - https://batteryuniversity.com/learn/article/ultra_fast_chargers

Here are three links for you to delve more into the electrochemistry of batteries of different types. In a nutshell, batteries are complex, non-linear devices a phrase I will leave you to research the meaning of.

Enjoy
M

 

[Dave 2000]

@marconi Thanks for that. Being under the bonnet of a car for 50 years I have the battery bit covered. Anyway to put this to bed I have a solution. I did pose the same question on the Solar Panel Talk forum and I am sure it is the way I pose questions as the replies from there were somewhat vague and not really directly aimed at a solution, and that has happened here in the past as well anyway, the solution was simple.

Today I had some time to myself and came up with an answer. We know the open voltage of a panel rises when not connected to a load, so I altered the wiring a little. As soon as any one of the panels is plugged in I get a voltage reading, as the rack mounted panel is fixed I get what I am given so to speak, but it is the roaming panel I want to make sure is giving me its best. One of the Moderators ask a question about the panel Voc voltage perhaps to test the roaming panel I was plugging in? Anyway it struck me that the better the exposure to the sun the higher the Voc would be, of course that is not true if you apply a load. So, I wired the panels one at a time to the volt meter. The roaming panel first with no load, I moved the panel into the sunlight and the voltage hit 21 volts as per the panel specs, and then I switched in the load i.e. the Victron and everything worked as expected. The ammeters read as per my earlier install but small amounts of movement circa 0.25 of an amp are virtually impossible to read on an analogue meter, whilst voltage swings from 0 - 21 are super easy to see. So, plug in the panel, move to get highest open Voc reading and then switch in the load, super simple. I can switch out the load and immediately see if the open Voc has fallen thus a panel movement is needed.

Many thanks for all that contributed.

regards

Dave