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Mecer Battery charging on solar not as good as on Utility


Vijen
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I have a Mecer 5kVA/4kW invertor with 8x200Ah Vision bateries and 3000W of PV. Item 32 is set to Auto in the Mecer setup and I have the patched firmware. My system is set to run loads of solar during the day (appox 600W) and continue on batteries until it battery voltage drops to 49V. It then switches back to Eskom and battery charging is only via solar.

I find that when charging the batteries by solar they do not receive as much charge as when charged by utility.Can anyone explain how the Mecer/Axpert charging work?

I have a feeling that this is because of the fact that on PV the charging current sometimes drops away completely when a cloud passes or a big load kicks in - the Mecer still counts this as charging time but battery current may actually be negative. The voltage drops to 54V and then approx 4 A is supplied to battery even though there is surplus solar available.

Any recommendations on whether I should set 32 to manual and if so what time should I select.

Regards

Vijen

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21 hours ago, Vijen said:

Can anyone explain how the Mecer/Axpert charging work?

I'll have a go. Charging continues till it sees for 30 seconds with no exceptions both of these conditions:

  1. The battery voltage but be at minimum the bulk/absorb setting less 0.5 V;
  2. The battery charge current must be strictly below the current in setting 02 (maximum charge current) divided by 5, with a minimum of 5 A.

So let's say your maximum charge current is set to 0.15C = 200 x 0.15 = 30 A. So the current threshold will be max(30/5, 5) = max(6,5) = 5, so the current reading has to be 4 A or less.

If the absorb voltage is the default for AGMs, that's 56.4 V, so the battery voltage has only to reach 56.4 - 0.5 = 55.9 V.

According to my post here, if you change setting 32 to a number, then the current threshold falls to 2 A, which is hard to achieve, so if you choose say 30 for setting 32, you'll likely get a bulk charge time of 30 minutes [ edit: was seconds ]. With the patched firmware (73.00e?), time spent below 55.9 V does not count towards those 30 minutes. So that might be worth a try if you can't get anything else to work.

I note that if you have a ridiculously high value for setting 02 (perhaps because your panels are going to limit the charge current anyway), then the threshold current could be 20 A or more, which would be too early in the charge process. But if utility charging works properly, and you really have patched firmware, then I don't get why your PV charging is still going to float too early.

Edited by Coulomb
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Thanks Coulomb

So, if I understand correctly, charging occurs as follows when set to solar only. The charge circuit directs any surplus current (not used by load) to the batteries. This max current is limited by setting 02. This first stage of charging occurs until the voltage reaches the bulk/absorb voltage is reached (in my case it is currently set at 57.6V). The charge circuit then tries to maintain the bulk charge voltage by controlling current directed to batteries. ie If voltage increases current is decreased until voltage is back at bulk voltage.

If the charging current drops to max current in setting 2 divided by 5 (in my case max current is set to 30A so this should be 6A?) and the battery voltage is greater than 57.1V then charging is considered complete and the charge circuit sets the voltage set point to the float charging voltage of 54V which is basically a trickle charge.

If this is the case then its weird that I experience "better charging" when charging on utility. When the charging current drops below 6A on solar because of clouds or larger loads coming on I am sure that my voltage drops below the 57.1V so it should not see this as charge complete. I will look more closely at this transition when charging on solar.

Just to confirm, I am on 73.00e. Before this firmware change the unit would reach the bulk charge voltage and almost immediately drop to 54V. Now it seems to stay there for about an hour including time spent when charge current drops. If I leave on utility charge however it appears to charge longer - 90 minutes to 2 hours. It also definitely takes longer to reach the 49V changeover from battery to utility that I have set. Normally around midnight compared to around 10/11 pm when charged on solar.

Strange...

 

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Hi Coulomb

I read your previous post on the charging time referred to above. It helps me understand this system much better - thanks.

However I am not sure why you said this

1 hour ago, Coulomb said:

if you change setting 32 to a number, then the current threshold falls to 2 A, which is hard to achieve, so if you choose say 30 for setting 32, you'll likely get a bulk charge time of 30 seconds

Did you mean 30 minutes instead?

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I have just had the charging stop early again. have a look at the attached screen shot from ICC. The battery voltage drops to 54V while the current is around 9A - this should not be occuring until the current drops below 6A.

Capture.JPG

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56 minutes ago, Vijen said:

The battery voltage drops to 54V while the current is around 9A - this should not be occuring until the current drops below 6A.

That's weird. It's as if the battery is misbehaving somehow. Is the 9 A actual net current into the battery, or output current from the solar charge controller? If the latter, it could be due to loads.

It's not unusual for a lead acid battery to have a lower internal resistance as the state of charge increases (up to a point); that can mean the battery voltage falling despite the same or higher current going into the battery. But that ~7 minute gap looks different.

I wonder if you might learn something from utility charge one day in the middle of the day (disable PV charging to eliminate a variable), to see if that misbehaves the same. If so it could be some weird battery effect, perhaps due to temperature in the middle of the day.

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The 9A is actual net current into battery.

You could be onto something there (regarding battery internal resistance and temperature. Up to now I have only done the utility charging in the late afternoon - will try it around midday and see if it reacts similar to PV charging.

Thanks Coulomb

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Hi Coulomb

Just an update. I measured my batteries and noticed one of them 0.8V below the rest. Forced the system to charge to 58V and left it there for an hour to equalize. The system then ran until midnight before it swapped back to utility. The charge current seems to behaving correctly again and the last few nights I have run on batteries until roughly midnight before the changeover to utility power.

 

I am thinking that the battery resistance may have been a bit high and somehow it affected the charging so that it stopped early? Still a bit confused but happy that solar charge is now appearing to last as long as charge by utility.

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