The maths of solar setups

[clinton brits]

I bought a 6kw devel inverter that has an input voltage of 48V and a 60 amp charge controller for my panels.

The sun came up and my charge controller said the batteries were flat on 47 odd volts but it was charging at 10 amps eventually.

The batteries were new at 105 amp.hour and i said 105 amp.hour/10 amps needs 10 hours to charge.  I understand it doesnt discharge completely but im rounding figures.  The batteries charged in less than an hour and my voltmeter agreed with the charge controller.

 

The inverter showed 6% output current.  The batteries ran flat in less than an hour.  Assuming you use 10amps it should run for 10 hours assuming 100% efficient but 1 hour?

 

The battery supplier put the battery on the load tester and 2 were replaced but still running the same.

 

Am i mssing something?

[PaulF007]

Hello Clinton 

Just to make sure , how many batts do you have? As this is a 48 v system you will need at least 4 batteries. It is also a good idea to give us your total system specs , total solar size , bank size , type of batteries as well. 

Then batteries isn't nearly as straightforward as you might hope . @Chris Hobson will be able to give you the more technical details but the short is if you draw 10a from a 105 ah battery for 10h you will kill it in a very short time! As your battery goes flat the internal resistance will increase as well so not a good idea by any standard. 

When you charge the batts the same applies as well and in the beginning you will get as much amps as what the charge controller can give into the batts but as they are getting "full" they will take less and less. Also looking at the volts is a poor indication as to if the batts is charged or not. 

Have a look at this thread as there is a lot of good info considering batteries on there. 

 

[clinton brits]

Thanks.  Many similarities to what im seeing but mine is a bit lower budget from theirs.

 

My solar panels i bought are 30v and 250 watts.  I was advised to buy 4 panels and connect 2 panels in series and connect this in parallel to another 2 in series.

 

The batteries are delkor royal deep cycle batteries 12v 105 amp.hours 1150K. To get the 48v i have 4 identical batteries in series.

As you say it charges at 10 amps to start and when its "full-ish"  it drops to 1.3amps but as i say it happens in a short time.  

If i look at the SOC window it is bright green but im not sure if this is a good indication because all the batteries showed this as the faulty batteries they replaced showed the same.

 

When full the inverter shows 55v but one you turn on the inberter it drops tp 49v and then rapidly drops to 47 and then the inverter shuts off to prevent excessive discharging.

[PaulF007]

@clinton brits I think you will need to spend some time reading through a lot of the content here on Power Forum , there is some really good info there. A couple of notes to consider is 

• It is really bad to draw or batts down to 0 and what your are explaining seems like your batts might be damaged 
• Your panel open voltage seems low on your scc (60v) but we won't be sure unless we see the speck sheet on the SCC.
• Lastly how accurate is the SOC indicator on the inverter? Most of us are using a BMV  battery monitor to see what the batts are doing so you might have to look into it as well.

The best advice I could give you is to break your system down and check that everything matches optimal. So look at the panels and match them as best to the SCC then the SCC to the batts (Bulk charge , float extr.) Then your load to your inverter and make sure that you know why you have setted up something the way you have. It is a steep learning curve but well worth the effort.  

[Chris Hobson]

Hi Clint

Lets try start at the beginning or some logical place in dealing with your setup. 4 by 12V 105Ah batteries roughly holds  5000Wh of power - a little over 5 Eskom units. General convention is that one does not discharge ones batteries below 50%. So you have roughly 2.5 Eskom units available stored in your batteries. These batteries are not REAL solar batteries and will not last long. They are rated for 260 cycles at 80% discharge. That tells me they are standby batteries.

That is a sizeable inverter and unless it was bought with a large solar setup in mind is too big for  your panels and batteries. The quiescent current of your inverter might account for a sizeable portion of the draw on your batteries overnight.

Your panels could generate about 3 to 4 kW which is not enough to charge your batteries. Not knowing your setup It is difficult to say exactly what is going on. I suspect that as a beginner you have vastly underestimated the size of battery bank and the PV array you need to power it (Don't worry I did that too). What solar charge controller do you have?

[ghatikar]

9 minutes ago, Chris Hobson said:

Hi Clint

Lets try start at the beginning or some logical place in dealing with your setup. 4 by 12V 105Ah batteries roughly holds  1260Wh of power - a little over 1 Eskom unit. General convention is that one does not discharge ones batteries below 50%. So you have roughly ½ an Eskom unit available stored in your batteries. These batteries are not REAL solar batteries and will not last long. They are rated for 260 cycles at 80% discharge. That tells me they are standby batteries.

That is a sizeable inverter and unless it was bought with a large solar setup in mind is too big for  your panels and batteries. The quiescent current of your inverter might account for 90% of the draw on your batteries overnight.

Your panels could generate about 3 to 4 kW which is more than enough to charge your batteries and do a tiny bit extra. Not knowing your setup It is difficult to say exactly what is going on. I suspect that as a beginner you have vastly underestimated the size of battery bank and the PV array you need to power it (Don't worry I did that too). What solar charge controller do you have?

12 x 4 x 105 is  5KWH - His panels are not enough He needs atleast  1.5 KWp  to 2 KWp assuming he consumes also in daytime and the batteries needed top up to full charge 

Also assuming C10 rates ( sorry did not check up on battery)  of 10 amps he can draw max 500 watts continuously ( less if you factor in inverter efficiency /losses) anything more and Phukerts law will get him the 105 Ah may Deliver only  70 AH 

[Chris Hobson]

2 minutes ago, ghatikar said:

12 x 4 x 105 is  5KWH - His panels are not enough He needs atleast  1.5 KWp  to 2 KWp assuming he consumes also in daytime and the batteries needed top up to full charge 

Thanks @ghatikar for correcting my error. I am tired and should not answer questions late at night.

[___]

1 hour ago, clinton brits said:

The batteries ran flat in less than an hour.

Others have already said this, but it sounds to me like you're using an inaccurate means of determining the state of charge, most likely voltage. There is a relationship between voltage and SoC, but it usually only applies when the battery has no load. The moment it's charging, the real SoC will be lower than the associated voltage, and when discharging, it might show a low voltage with plenty of charge left. It has to do with the health of the battery, the size of the battery, and so on. A smaller/older battery has a higher internal resistance and so the voltage drops much lower under discharge, in other words, on an older battery voltage is an even worse measure of SoC.

Chris has already covered some of this, but there are several little caveats and thieves in the equation. Just float-charging that bank might take as much as 1kwh. You have 1kwp of panels, so on a good day you might make 5kwh (though, as we frequently point out on these forums, with off-grid systems you might make as little as 3.5kwh a day. Minus the 1kwh you need to float-charge that bank to 100% (this is my own empirical results, charging becomes really inefficient as you approach 100%). So you can use maybe 2.5kwh out of that bank, which incidentally is also around 50% of its capacity, but then all your PV has to be used for charging the next day. I therefore agree with those who said you need more PV: I'd say you at least another 500W of PV (scrolling up, I see ghatikar agrees :-) ).

Now, about charging and discharging. The batteries are 100Ah, which naively interpreted means that it can give you 1 ampere for 100 hours, or 100 ampere for 1 hour. I say naively, because lead acid batteries will be damaged if you discharge them in a single hour. They generally prefer to be discharged over no less than 5 hours, so the maximum prolonged draw you should allow is 20 ampere. You can go higher for short periods, but this is the rule of thumb. If you discharge a battery at rates higher than C5 (ie, over 5 hours), the voltage will drop a LOT... it could go down to 46V. This doesn't mean the battery is empty and might not even be a cause for concern. Keep this in mind: When a car battery is cranking an engine, it will drop to 10V or even 9V for those few seconds!

Conversely, when charging, you also cannot recharge it in a single hour. The preferred rate is 15% of the capacity, or 15 amps in your case. Lower is better (it is more efficient), so 10 ampere over 10 hours is also good. But... (there are always several of these buts), as the battery approaches full charge it starts to accept less and less charge, later dropping to less than 1% of the capacity (in your case, 1 ampere). This means that to get the battery from 98% to 100% can take two hours, whereas getting it from 50% to 80% can also be accomplished in 2 hours (it charges faster lower down).

In other words, batteries are a bit like sponges. When almost fully saturated with water, it is slow to absorb more. When it is dry, it absorbs water fast. Conversely, when full it discharges water easily, but as it becomes drier it takes more work to get the last bit out.

Finally, Peukert's law. This simply states that the usable capacity of a battery is lower if you discharge at high rates. Most batteries are rated at C20, in other words, when discharged at a constant 5 ampere over 20 hours, the battery will yield a total of 100Ah and will be empty afterwards. Sometimes manufacturers get a little cheeky and rate their batteries at C100 in order to get a higher number for marketing purposes. Real solar batteries are often rated at C10. The long and the short of that: When your batteries are working at 5 ampere, it will last 20 hours and you'll get the full 100Ah. When discharging at 10 ampere, it might only last 8 hours and you will get only 80Ah. If you want to understand this better, head over to youtube and find a guy called knurlgnar24. He has a series of videos where he shows that 1 + 1 = 2.7 (putting two batteries in parallel, strangely, gives 2.7 times the capacity). This is Peukert at work.

Soooo... the long and the short of it, is that 100Ah pseudo-solar batteries really shouldn't be used with such a big inverter. That bank will be happiest at around 500W.