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Hi Guys,

I have spent all of today reading forum after forum trying to understand and figure out the issue I'm facing with my installation.

Apologies in advance for the long winded post, but I'm going to try outlay as much information upfront and test results to avoid back and forth.

Before I explain my installation/setup let me give you all a bit of history into it which will lead up to my problem and come to the point of my installation that has me completely perplexed.

Last year Jan (2017) I bought a RCT/Axpert 3kV 24V inverter/charger/ups, I already had a modified sign wave inverter running off 2 x 180Ah batteries that came out of a cell tower we decommissioned the year before (2016). I then thought I would buy 2 x 320Wp PV panels since the inverter could handle 600W solar input and create myself a little system.

So for a year my system ran fine, 2 x 180Ah batteries connected in series for 24V connected to 640Wp solar (never seen it go above 520W) and this is wired into my DB to provide power to all the lighting in the house and ceiling fans as well the TV in the lounge. So we average about 300W draw out of the system.

For the most part I left the RCT/Axpert settings as default - except for option 01 which I changed to SBU.

Fast forward to this year Feb and my batteries are buggered - OK they were installed in the cell tower in 2010 and I removed them 2016 and ran them for a year on a modified sine wave with a PWM charger and 250W panels, then a year on 640W panels and the MPPT Axpert - so i wasn't really too cut up about the batteries as I felt they had a good run of 8 years give or take.

Now the issue and confusion part!

I made the mistake of getting the 180Ah batteries refurbished - it just didn't work... 2 weeks and they were dead dead! I then made mistake number 2 and traded them in for 2 x 240Ah refurbished batteries and paid the difference that the battery shop swore were brilliant batteries for solar - again 2 weeks and the failed to provide 500W power under load for more than a few minutes. After a long fight they agreed to take them back and I bought 2 brand new Delkor Royal 1150K 105Ah UPS batteries (I just can't afford to lay out for anything more expensive right now)

I installed these batteries on Friday (29/03/2018) and I still do not seem to have the backup time that I think I should have.

By calculation, 300W on 105Ah battery at 50% DoD should give me 4.2 hours, which would be enough time once solar disconnects at sunset to the following morning if you average it all out - about 180W for 2.5 hours then 60W for rest of the night generally.

So I set up a load test, fully charged the batteries to 27V and connected a 312W load to the system - results are as follows:

1. hour zero - battery 26.9V no load

2. hour zero - battery 25.4V loaded at 312W

3. hour one - battery 23.9V

4. hour two - battery 23V (empty)

Some 2 hours short of the 50% DoD target calculation...

What am I doing wrong? have a missed the boat completely that I am not getting the expected run time out of brand new batteries? I don't want to stuff them up and throw money away - so I'm even considering setting the Axpert to SOL priority and the charger to Solar only, and running the house on utility after sunset to save batteries.

Am I potentially damaging 105Ah batteries with the charger minimum at 20A? Even though minimum recommended battery size is 100Ah? - I read all the forum posts on the battery charger and I still do not understand how its set up when there is no max charge setting for solar but 2 settings for utility charging essentially.

Is there possibly a fault with the inverters battery measurement that its possibly under reading the value and disconnecting sooner than the 23V default?

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those royals are useless batteries. won't last more than 1 year...... charge rate on them around 6amp and do not discharge more than 20%

if you commision cell towers etc, get yourself 2 x 12v Narada or similar, that will help you a lot.

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Plus one on Mike's comment. Your problem starts with the batteries.

Also, the 2 x 320Wp PV panels are fine to charge the batteries daytime, but not charge and power a load at the same time, assuming you are powering a load to the panels max. You may find that they are not fully charged once a week, which then brings the SOC at night lower.

520W that you say you get at times seems about right. Could go higher on a very cold day under full load.

Step 1: Invest in a BMV battery monitor, start recording what goes in and out of the batteries. 

May sound callous, but you could use the batteries better, learn more from them, if you know exactly what is happening with them.

Or you can buy better batteries, and not a BMV, and we have the conversation again in a few months. :D

Get a BMV, then we go further. 

This one, 700 or 702: https://www.victronenergy.com/battery-monitors/bmv-700

Or the new one: https://www.victronenergy.com/battery-monitors/bmv-712-smart

@Camel can maybe get you a good price. 

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Thanks for the responses.

First up - the cell tower story was a once off, so wont be coming across any more big "free" batteries anytime soon again.

nothing runs off the solar during daylight hours, no one is home from 8 to 5 every day, so the shared load on solar is pretty much the TV system standby current... I think once a week the garden service mow the law on the solar using a 1200W mower for about 40minutes but thats it... The idea was to use the power generated during the day to power the most used items at night - TV and ceiling fans...

 

I will look into the battery monitor and see what we can do from there, I have been trying to quantify power generated versus power consumed for a while now and been going in circles

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6 hours ago, Nic Holmes said:

By calculation, 300W on 105Ah battery at 50% DoD should give me 4.2 hours

300W plus the self-consumption of the inverter (around 50W) at 24V is roughly 15 amps. That's a discharge rate of more than C/10 (capacity at ten hours). Good lead acid batteries are rated at C/10, but those delkor's most assuredly are not... I'd bet they are 105Ah over 100 hours (I kid you not... the marketing people do this sort of thing), but let's be generous and assume it's a C/20 rate (which is very common). Because of the Peukert effect, at higher discharge rates your usable capacity is affected, so you won't get the full 100Ah. The Peukert factor for a lead acid is typically 1.25.

If you plug that into the Peukert equation (borrowed from Wikipedia here), you get around 5 hours. Your 105Ah battery is good for around 80Ah under these conditions.

Selection_102.png.66e8d70817905f071c7bbd67049cfb0b.png

At 23V it's not empty yet. Empty is only at 21V... open circuit. Your battery is under load. If it was open circuit, it would have a good 20% left in it. Under a C/10 kind of load, chances are it is actually at 50%, it's just working too hard.

I tend to agree that you are a little under-batteried here.

Note: I edited this post a few times.

Another edit: If the 105Ah rate is a 100h rating, the expected time to empty is just 3.6 hours. Your numbers are almost in the ballpark here.

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Found the spec sheet. It's 105Ah rated over 20 hours ;-)

Selection_103.png.cdb5cd7e215d2e179d0af51f704f8852.png

I'm also laughing a little as the spec sheet has a lot to say about lead-calcium being good to prevent thermal runaway... while the people who had batteries explode on them on this forum were precisely those with lead-calcium batteries :-)

And here is a discharge curve chart from a similar though slightly better battery (Deltec Lead-Crystal). There is something odd about the hour-scale at the bottom, it seems to be doubled (0.05C should empty out at 20 hours). It is good enough for illustrative purposes.

Selection_104.png.bd162192a436cefd69375aa09917399c.png

The curve you need to follow to get the full 100Ah is the 0.05C one. The one you are following is about a quarter of the way between 0.2C and 0.1C. Note how a terminal voltage of 1.9V (around 22.8V) is reached much sooner (8 hours vs 40).

So this is all speculation so far, and I'm probably beating a dead horse, but since Royal/Delkor don't publish a discharge curve this is what I've got: What you're getting might be exactly what you can expect.

Note: I may have slightly overestimated the quiescent draw of the 24V inverter. The 48V inverter has a quiescent draw of around 50W. The 24V is a little better, I think around 30W.

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On 4/3/2018 at 10:28 AM, plonkster said:

So this is all speculation so far, and I'm probably beating a dead horse, but since Royal/Delkor don't publish a discharge curve this is what I've got: What you're getting might be exactly what you can expect.

Note: I may have slightly overestimated the quiescent draw of the 24V inverter. The 48V inverter has a quiescent draw of around 50W. The 24V is a little better, I think around 30W.

Thanks! I also could not find any discharge graphs for the Royal batteries, they publish other curves that are useless in my opinion...

I was told that all deep cycle batteries are rated over a 20hr discharge and all automotive/high cycle batteries rated over a 10hr discharge? Is this another BS story spun by salesmen?

Well, I have set my inverter to switch over to main again at 23V, so then am I right in saying that my DoD is more like 30% at 23V than 50%?

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On 4/3/2018 at 10:28 AM, plonkster said:

So this is all speculation so far, and I'm probably beating a dead horse, but since Royal/Delkor don't publish a discharge curve this is what I've got: What you're getting might be exactly what you can expect.

Note: I may have slightly overestimated the quiescent draw of the 24V inverter. The 48V inverter has a quiescent draw of around 50W. The 24V is a little better, I think around 30W.

Thanks! I also could not find any discharge graphs for the Royal batteries, they publish other curves that are useless in my opinion...

I was told that all deep cycle batteries are rated over a 20hr discharge and all automotive/high cycle batteries rated over a 10hr discharge? Is this another BS story spun by salesmen?

Well, I have set my inverter to switch over to main again at 23V, so then am I right in saying that my DoD is more like 30% at 23V than 50%?

Edit: I do agree I am under batteried... These batteries need to last me til end of the year while I save up to install something big and decent... They cost R1300 each, so if they only last me 8 months, and they save me R2600 in utilities Im happy. I am concerned that the utility charger is going to overcharge these batteries with the minimum set current being 20A, so Im thinking about setting it to charge from solar only, this will also save costs as if it switches over during the night it wont be charging off mains for 4 hours...

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