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Off-grid PV system: ''Axpert'' 2 kW power consumption and battery cut-off voltage


Matt.Sun

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Greetings solar enthusiasts;

I have small off-grid solar system for summer house which was/will be in use from May till October (the whole system is 6 months old). It is used mainly for little fridge (380W in 24h), led lamps and small TV. It’s on during our absences (2,3 weeks periods) too.

Parts:

1x ‘’Axpert’’ Inverter 2KW – MPPT (U1 0952, U2 0334)
Inverter Mode: DC Input: 24VDC, 95A
AC Charger Mode: DC Output: 27VDC, 20/10A
Solar Charger Mode: Rated Power: 1000W
Settings: MAX charging current 10A; Bulk charging V 28,6; Floating charging V 27,6

2x PV modules Pmax: 300W, Vmp: 32.5V, Imp: 9.24A, Voc: 39.7V, Isc: 9.83A (in series)

2x Sonnenschein SOLAR BLOCK SB12/100A (C100h) batteries + 24V battery equalizer + 100A breaker

My questions/ problems:

I have currently the system at home (inverter and batteries). I have charged (24h) the batteries to 100% over AC Charger Mode (10A) and unplugged it from the grid. The batteries voltage has dropped in 24h to 25.48V (inverter), 25,40 (multimeter) with no load attached to the inverter. Is that voltage drop normal? This is about 20% (480Wh, C100h) of the batteries capacity. Is it possible that 2kW inverter uses so much energy (ca. 25W, battery discharge current: 1A)? The seller has said to me that there is no way that it consumes that amount of energy...

GEL batteries (12V) supposed to have 12,85+V at 100%, 12,65V at 75%, 12,35V at 50%, 12,00V at 25%. Is this correct? I couldn’t find anywhere in the manual capacity/voltage/load information. The problem is too, that I can’t set on inverter higher batteries cut-off voltage settings than 24V – so is it actually not suitable at all for those batteries - 75% DOD - 900 cycles, 50% DOD – 1500 cycles? I also have to turn off all alarms, otherwise it begins to beep constantly whenever the voltage drops under 25V.

I will get during next week DC current clamps and will measure actual power consumption of the inverter when it is in off-grid mode.

I have increasing feeling that I should have bought Victron SmartSolar 100/20, Victron Phoenix V.E. direct 1200 or 800 /24 or 12, have peace on my mind,5 years warranty and used ECO function in our absence… What are your thoughts about that? Would be this future proof (li ion) or is there better option for such small off-grid system?

In attachments are some info files: inverter specifications, settings; battery manuals; table of data collected from inverter with Watchpower…

Thank you for any help.

Inverter specifications .jpg

Overview.jpg

Settings.jpg

Inverter data log table.xlsx Sonnenschein_gel_handbook.pdf Sonnenschein_Solar_Block.pdf Sonnenschein_Solar_Operating_Instruction_Jul_2015.pdf

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4 minutes ago, Matt.Sun said:

GEL batteries (12V) supposed to have 12,85+V at 100%, 12,65V at 75%, 12,35V at 50%, 12,00V at 25%. Is this correct?

Welcome Matt. 

Start by getting a Victron BMV battery monitor if you want to know the state of charge of the batteries, of what goes into them and what is drawn out of them.

Using volts is a pretty useless measurement from my perspective because unless the batteries have "rested" for a while (not used), you cannot fathom the correct SOC (State of Charge) ever.

Other could help more on the inverter side and settings.

8 minutes ago, Matt.Sun said:

I have increasing feeling that I should have bought Victron SmartSolar 100/20, Victron Phoenix V.E. direct 1200 or 800 /24 or 12, have peace on my mind,5 years warranty and used ECO function in our absence… What are your thoughts about that? Would be this future proof (li ion) or is there better option for such small off-grid system?

Yes, I would have done the same, just added a BMV too the mix unless it was lithium batteries, then you won't need it.

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Thank you The Terrible Triplett for quick reply and suggestions. Would be Victron Phoenix V.E. direct 800 enough for the fridge (50W) and other small consumers or would be better to take 1200 version. Is there any difference between 12/24V system in this case? Would be this combo compatible only with Victron lithium batteries (BMS) or others too?

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3 minutes ago, Matt.Sun said:

Thank you The Terrible Triplett for quick reply and suggestions. Would be Victron Phoenix V.E. direct 800 enough for the fridge (50W) and other small consumers or would be better to take 1200 version. Is there any difference between 12/24V system in this case? Would be this combo compatible only with Victron lithium batteries (BMS) or others too?

For fridges /freezers the startup current is the issue. I would go with a 1200va min. You can try a 800va and it may work. My 1600VA handled a A++ fridge but took exception at times when the fridge also started same time whilst powering a average of 600w load during the startups. When all was started, the 1600va ran happily, unless it tripped. 🙂 

12/24v makes no difference bar thicker cabling, which I got tired of, so I installed 50mm2 cables as a default. Be gone worries of 12v high amps.

And if you need a large ah bank, get bigger batts, not more. 48v is nice, solves a lot of issues for high loads like a house, smaller systems are 1000% ok with 12/24v.

Far as I know all Victron equipment is compatible with most if not all lithiums. The MPPT the driving force to charge. Phoenix has no charger in any case. Even my Mutligrid can charge lithiums.

All lithiums banks needs a BMS, either built in or added afterwards, to protect the lithium batteries against the inverter running them to hard.

Just shop right for the right prices for the Phoenix VE.Direct and MPPT!!! 🙂 

Ps. Just make 100% sure the Victron SmartSolar 100/20 is ok for future proofing. 

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13 hours ago, Matt.Sun said:

Is it possible that 2kW inverter uses so much energy (ca. 25W, battery discharge current: 1A)? The seller has said to me that there is no way that it consumes that amount of energy...

I don't know the smaller models, but the 5 kVA / 4 kW models certainly draw 50 W, so it seems perfectly reasonable that a 2 kW model would draw half of that, or 25 W, or 1 A. So that's 24 Ah per 24 h.

13 hours ago, Matt.Sun said:

GEL batteries (12V) supposed to have 12,85+V at 100%, 12,65V at 75%, 12,35V at 50%, 12,00V at 25%. Is this correct?

That sounds about right. 24 Ah is 24% of the 100 Ah battery's capacity, so close to 75% SOC, so about 12,65 x 2 = 25,3 V. You measured 25,40 V, about right, assuming no load at all.

100 Ah is quite small for a 2 kW inverter, although the manual has suggested cable for 100 Ah and 200 Ah batteries. Until you're ready to add another string of 100 Ah battery modules, you'll have to live with the fact that the idle power consumption is quite significant. Or of course replace it with a Victron. If you do, please report the power consumption you measure for the two. Don't forget to zero your meter just before each measurement, if you use a clamp meter.

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21 hours ago, The Terrible Triplett said:

12/24v makes no difference bar thicker cabling, which I got tired of, so I installed 50mm2 cables as a default. Be gone worries of 12v high amps.

I already have 35mm2 cables from Axpert which should be more than enough for 12V system. The + point from 24V system is perhaps that Victron SmartSolar 100/20 would be able to charged batteries (Lithium SuperPack 12,8/100Ah) with 580W while 12V system with 290W. But in case with my batteries I think I shouldn't charged them with more than 300W? Would I need battery balancer for 12V system in parallel too?

22 hours ago, The Terrible Triplett said:

Just shop right for the right prices for the Phoenix VE.Direct and MPPT!!! 🙂 

It is hard to get good discount on Victron since they have almost everywhere the same and static pricing... ;(

12 hours ago, Coulomb said:

If you do, please report the power consumption you measure for the two. Don't forget to zero your meter just before each measurement, if you use a clamp meter. 

I should get the current clamps tomorrow and will of course report the actual power consumption.

Is it reasonable to expect that will those two batteries last more than 5 years (light use, 1/2 year in operation, 20-25°C)?

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5 hours ago, Matt.Sun said:

But in case with my batteries I think I shouldn't charged them with more than 300W?

Sealed lead acid 100 Ah (per your first post this topic) should not be charged at more than about 15% of Ah capacity, so 15 A @ 25 V ≅ 375 W.

Quote

Would I need battery balancer for 12V system in parallel too?

No. Balancing is only needed for series strings of battery modules.

Longer answer: of course, each 12 V module is made up of 6 nominally 2 V cells, so ideally there should be balancers for each cell. But lead acid is somewhat forgiving of short strings of cells in series, and you rarely get access to the connections between the cells. Having said that, it's a fairly common failure mechanism for one cell for fail short circuit (zero volts); arguably, failures like that might have been prevented by a cell-level balancing system.

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I have measured current (A) with the clamp meter and result was as we expected:

In the first hour it was about 0,939A x 26V = 24,4W

After an hour it was constantly about 0,760A x 26V = 19,76W

In my case this means that during night inverter consumes about 25% of the available energy (more than fridge).

The second problem is, it would stop at 25% DOD because I can't set higher cut-off voltage than 24V.

Does anybody have experience if ECO function on Victron Phoenix V.E. direct works as it should?

20190326_194416_1280x720.jpg

20190327_222053_1280x720.jpg

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11 minutes ago, Matt.Sun said:

After an hour it was constantly about 0,760A x 26V = 19,76W

The 24V 1600VA Multi has a no-load draw of around 18W. So this sounds about ballpark.

13 minutes ago, Matt.Sun said:

Does anybody have experience if ECO function on Victron Phoenix V.E. direct works as it should? 

I only tested this on the small 250W model I use for testing, and it appeared to work as designed. I didn't measure the draw, but it is advertised to be quite low.

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2 hours ago, Matt.Sun said:

The second problem is, it would stop at 25% DOD because I can't set higher cut-off voltage than 24V.

The cutoff voltage is meant as a last resort, and will turn the inverter off, no longer powering the loads.

If you want to stop discharging at a higher SOC, use the back to grid setting (setting 12). It will probably go up to 25.5 V. Then the inverter will switch to line (bypass) mode, powering the loads from the AC input.

It's still not ideal for LFP, because the Axperts were designed for lead acid, despite the marketing about being lithium and LFP compatible. This is fixed in patched firmware, but there is no patched firmware for 24 V models.

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Thank you plonkster for the info.

On 2019/03/28 at 1:37 AM, Coulomb said:

If you want to stop discharging at a higher SOC, use the back to grid setting (setting 12). It will probably go up to 25.5 V. Then the inverter will switch to line (bypass) mode, powering the loads from the AC input.

I have tried it, but since it is off-grid system it didn't happen anything. Axpert was still consuming energy from the batteries.

I have sent all the facts and measurements to the shop where I have bought the Axpert, and they will took it back and give me full refund, which is really fair from them.

Does Victron BMV play any role at battery health or is it just for the battery status info? I am planing to set Phoenix inverter cut off voltage at 12,35V (12V system, 50% DOD) and that should be it.(?)

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