pilotfish

Without being able to see the scale it is tough to know what is going on, but my best guess is as follows; The graphs are ICC output using SOC as measured by Axpert (based on battery voltage) and not Victron BMV (based on amps in/out) Your batteries are at 100% with with very low other loads so PV output is low, You have a load which is cycling at regular intervals (like a pump or fridge compressor), When the load kicks in the MPPT is unable to respond fast enough so the batteries are initially supplying the power required, As the battery current increases your battery voltage drops, The drop in battery voltage is reported by Axpert as a drop in SOC (which is nonsense), The MPPT then starts to pick up the slack, by which time the pump/compressor has either stopped naturally or stalled because it is too high for the inverter. If the load is running as it should then there is nothing to worry about.

This is a discharge chart for a 2v cell. If you look at the 1st and last last square in the 2nd data line of the top chart it tells me that a fully charged cell will deliver 5amps for 20hr before reaching 1.8v, so this tells me that the cell has a C/20 rating is 100Ah. Looking at the chart I am guessing that it is probably a Ritar battery, but they come in many shapes and sizes. Battery chemistry and life expectancy would form a very important part of your analysis - A 100Ah/C20 battery with a life expectancy of 1000cycles to 50% SOC is not the same as a 100Ah/C20 battery with a life expectancy of 2000cycles to 50% SOC.

Once a month during winter only in Joburg, rain will generally do the job in summer. I see a difference of about 5-10%, so useful but not spectacular.

Yes they should, not only same wattage but preferably same spec exactly. In parallel you would force them to operate at the same voltage, in series you would force them to operate at the same current - even though this may not be ideal for the the panel in question. If you really feel that you need to do it then make sure that they have the same operating voltage and then put them in parallel (which would be easier to match in dissimilar panels).

You are correct about the "ex vat", I never picked that up. I August I paid about R2250 ex vat (R61k inc vat for the bank), I guess I got lucky!

The current price from Mantech for 24 x OPZv2-490 R2679.97 x 24 = R64319.28 for a 550Ah bank which is R2473/kWh. The Trojan T105RE from Re-Volt is R3100 for a 6v 225Ah battery which works out to R2296/kWh (both numbers inc vat). The prices /kWh are very similar, so you need to look at battery specs to see which gives better value. I think the OPZv2 wins that one by a huge margin - at 200amps average consumption per night (which I believe is your approximate usage) you could expect about 3000+ cycles to 80% capacity for the OPZ, @Chris Hobson will know more but I think the Trojans will be about 1200cyc. Bear in mind that if you go the Trojan route you will require 2x parallel strings for a 450Ah bank which is a similar capacity to your current setup. This is 48 watering points that will require regular maintenance, also you must deal with the whole "Axpert cant equalize Trojan batteries" story.

I used a 25mm x 5mm solid copper busbar rated at 250amps that I got from ACDC. It comes as a 3m length which I cut to length and drilled as required. Part Code: CUB025X5 The busbar is also very handy for extras like metering wires and HA-02 wires, which I have drilled into the busbar with small screws and ring lugs rather than compromise the battery terminals with additional lugs etc

I have put 750w on a MKS 3k and it worked well, but never 900w. For safety sake I reckon go with @plonkster.

I was thinking of the MKS 5k when I said 3 panels in series may exceed max open circuit voltage, in the MKS 3k it will definitely exceed MOCV. You can go with 3 panels in parallel and you should be fine (36V and 25A), if you want more panels than that then I would recommend upgrading to the PLUS version or the MKS 5k.

MKS = MPPT system KS = PWM system

The 3kVA MKS is an MPPT system. You can add the additional panel capacity without causing damage as the MPPT will only draw what is can handle. You will get better results in early/late times of the day and in less than optimal conditions, but not more than the system can handle (probably a little more than 600w) during optimal PV hours. Note that putting 3x 300w 72 module panels in series will come very close to (and possibly exceed) max system voltage on a cold clear day or when cloud edge effect comes into play - you may be ok but just giving you a heads up.

I looked at both options before buying and found the OPZv2 to be better value even before the Trojan maintenance issues are taken into account - but this relies on the OPZv2 coming close to their advertised spec. If I had a golf cart I would definitely go with Trojans

The batteries are doing a great job, performing exactly as I had hoped for now. My system only has 1 day autonomy so is not truly "off grid". I am off grid as long as the weather is decent, but If I have a cloudy day then the system will switch to grid when the batteries reach 50% SOC (normally between 8 and 10am with no PV charge). I have 2 geysers, 1 oven and 1 aircon which are grid only, but I have solar heating on the geysers and a gas hob so the grid usage is very low (around 100kWh per month, down from around 1000kWh pm before PV). I charge my battery bank [solar only], so after switching to grid the system will remain on grid until the SOC reached 65% when it will switch back to PV mode. To be fully off grid I think I would need to have 3 days autonomy tripling the size of my battery bank, I dont see the value in that. The battery bank is 26kWh total capacity and my average daily discharge from the batteries is 250Ah (12kWh), you can compare this to your own requirements. (Note that the OPZv2-490 is 490Ah at C10, but 540Ah at C24 which is a better approximation of my discharge rate). The OPZv2 is a proper deep cycle battery with excellent cycle life at 50% SOC and 25"C - I have taken steps to keep within these parameters and hope to get about 8 years from my battery bank (to 80% capacity). If this dream comes true then my cost /kWh from the batteries will be around R1.50exv which is much better than all the LiPO rates that I have seen quoted. Of the 30kWh daily consumption from the system only 12kWh is provided by batteries and 18kWh is provided directly by PV during the day, so at R1.50 x 12 / 30 = R0.60 /kWh - that is pretty good and I reckon my system will provide a decent return in the long run, but in the end this will be decided by the batteries so it is not the place to skimp on cost.

The only solution is to get a 24v inverter or a 12v charger, all these other "almost solution" are going to trash something - it would be nuts to damage 4 batteries trying this workaround.

Have you considered that the fault might be with the multi-meter that you are using and not with the inverters? It is very unusual for 3 systems to have the identical fault.

It is not safe and you will convert your charger to smoke, unless it is auto voltage sensing in which case it will settle at 12v. Your inverter and charger are effectively connected to the same terminals which cant be 12v and 24v at the same time.

The operating voltage of these 60 module 250W panels is 30.3V, so 2 panels in series will give 60V which is the minimum PV voltage of the Axpert MKS 5K (which is too close in my opinion). I would recommend 3 of the above panels in series, or 2x 300W (72 module) panels which will probably have a 36V operating voltage, 2 of which would give 72V which will be a better match to the Axpert.

De Lille will hunt you down!

The Axpert above is actually 24v not 12v - the problem with it is the maximum PV array capacity of 600w which is very limiting. It is ok if you have a very specific function in mind, but it is no good in a system that you may want to expand in future. This is a versatile system that is very easily upgraded in future for not much more starting money than #1, make sure that you are getting the MPPT version if you decide to go this route. This inverter with 4x 300w panel and 4x 100Ah battery would be a good starting point if your budget can reach there. You will not be able to run a 0.75kVA pump on a 0.5kVA inverter, so this would not be a good choice for your stated goal. Pump starting currents can be quite high so I would suggest not dipping much below 3kVA inverter in order to run the pump.

Yes. The geyserwise unit comes with a thermostat that will plug into the Kwikhot element. Note that you get several Geyserwise versions - one that is a "smart timer" with no solar heating function (I think called the TSE), then you get the MAX which works with a pumped solar thermal system, the DUAL works with the PV heating system, etc I have limited experience with Geyserwise equipment, if you mention what you are trying to achieve and I may be able to advise further.

https://www.bidorbuy.co.za/item/311826675/5A_12V_POWER_SUPPLY.html From Bid-or-Buy, R80 for power supply, around R100 for courier depending on location.

The Axpert will switch between Battery/Solar mode and Grid mode on it's own without any external device, but it does this based on State of Charge [SOC] of the batteries which it determines based on battery voltage which it massively inaccurate. To remedy this you can add an external device from Victron called a BMV-702 which measures SOC very accurately, and then add an external PC/Rasberry-Pi etc with software called ICC which interfaces with the Victron and the Axpert, and will send the Axpert accurate information when to switch. The reason for this is to look after your batteries better which is the major cost in off-grid systems, but ICC will also provide you a ton of other very useful logging info relating to your system which is great to have.

I used info in City Power documents for the above response, I don't think Eskom smiles on grid-tie

Hi Chris, Yes and no to the above... My system is set to charge SOL (solar only), and if the SOC drops below 50% then ICC will switch the system to supply loads via the grid until the bank reaches 75% SOC. The effect of this is that my home runs Off Grid almost all of the time, with the battery bank getting down somewhere between 55_65 % overnight and then charging up again via PV the next day (the battery bank is only designed to carry 1 night load). When the dark and dingy days come around then the system will switch to grid if the bank drops below 50%, which will allow all loads to run off the grid BUT all available solar power will go to charging the batteries (my bank can accept 80amps = 4kW, it is not likely to reach this level on the dark days when this situation arises). This system works for me in that very little of my PV production goes to waste, but note that I have ZERO interest in feeding back into the grid - if anyone out there truly believes that the feed-back model works in a domestic environment then Axpert is definitely not for them.

The fish only read during the day The client uses 100kWh a day requiring 20kWp to be "virtually off grid" - if you want a system to reduce the bill a bit then just about anything will do that to some degree. If the grid is reliable, the fish not too fussy and authorities not too difficult then a grid tie system of whatever size you like and gennie backup is the way to go.