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MPPT query


Tony1

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This is a bit of (mainly) idle curiosity to be honest, and one for the solar afficionados, or general experts who might have an intellectual interest in the subject, and are willing to educate me in matters solar:

I have a pair of victron 100-50 MPPTs (one for the front panels and one for the rear panels), and my understanding is that the maximum current from each is 50 amps at around 13ish volts. 

 

My front panels don't tilt, and even in high summer I haven't seen them yield more than about 35 amps.  

However, the rear panels will tilt up to about 40 degrees, so when facing south they yield more. For example, even this morning, in early March, when the sun is out and clear of cloud the rear panels are yielding around 45 amps. 

The panels are rated at 760 watts in total (2x385 watts), and at a charging current of say 13.5v they are theoretically capable of giving current in the high 50s, right? Perhaps around 58 amps?

 

Now I thought it was almost impossible in the UK for panels to even get close to their theoretical maximum, but maybe its not? If they are giving 45 amps in early March, it seems possible they will give well over 50 amps on sunny days in June/July. 

 

So how does the MPPT handle that situation? The 'excess' current (i.e. more than its 50 amp rating) couldn't be more than 5-10 amps, so it doesn't seem like a dangerous scenario for the MPPT, but I've no idea really, so I'm interested in how the MPPT itself manages things when it is getting more power/current than it is rated for? 

 

I can obviously bring the charging current under the 50 amp rating by laying the panels flat, but if its a day when I want to heat up the water for some reason, I might be tempted to tilt the rear panels to get maximum power in quickly, but I also don't want to put the MPPT unit at risk of damage. 

 

MPPTs rated higher than 50 amps are quite expensive so that idea's not even on the table. If I have to proactively manage things and keep the MPPT below 50 amps that's more than adequate, I just wont tilt them after, say, the end of April.

 

Edited by Tony1
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A decent MPPT unit will simply limit the current to its allowed maximum value  and  will probably dump the excess power into the heat sink.  My Epever unit says it is happy indefinitely in these conditions.  It will switch off entirely if it gets too hot though.

 

If you are getting 45 A at 13.75 V that is about 650 W from your panels, allowing for losing a little in the MPPT and the cables.

 

N

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1 hour ago, BEngo said:

A decent MPPT unit will simply limit the current to its allowed maximum value  and  will probably dump the excess power into the heat sink.  My Epever unit says it is happy indefinitely in these conditions.  It will switch off entirely if it gets too hot though.

 

If you are getting 45 A at 13.75 V that is about 650 W from your panels, allowing for losing a little in the MPPT and the cables.

 

N

Generally agree but I don’t think the excess power is dumped as heat, it will simply not be extracted from the panels by raising the panel voltage as required (to above the Vmp).

To the OP it’s worth bearing in mind that cold panels produce more power than hot panels. And in summer, whilst it can appear “hot and sunny” in fact haze from water vapour (warm air holds much more water vapour than cold air) can reduce the energy reaching the panels. So it is likely that mid summer current won’t be much more than it is now.

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1 minute ago, nicknorman said:

Generally agree but I don’t think the excess power is dumped as heat, it will simply not be extracted from the panels by raising the panel voltage as required (to above the Vmp).

To the OP it’s worth bearing in mind that cold panels produce more power than hot panels. And in summer, whilst it can appear “hot and sunny” in fact haze from water vapour (warm air holds much more water vapour than cold air) can reduce the energy reaching the panels. So it is likely that mid summer current won’t be much more than it is now.

But the sun hits the panels from a higher angle in summer🤔

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Thanks folks, so I wont have an issue anyway with going over 50 amps, because of haze, and because the panels will be a lot hotter in summer. 

 

I was pretty confident that if it did happen, the victron MPPT would be have a system to handle it, as they generally seem very well designed- but its always nice to be sure.

 

 

Edited by Tony1
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15 minutes ago, XLD said:

But the sun hits the panels from a higher angle in summer🤔

 

If its a decent day and they're facing south I will tilt the rear panels, so in the summer the angle of the sun might not be that much different to what it is now.

 

My front panels dont really tilt, but even when the sun was its highest last summer, I didnt see them getting beyond mid-30s amps, so the 50amp MPPT will be able to cope ok. 

 

 

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2 minutes ago, Tony1 said:

 

If its a decent day and they're facing south I will tilt the rear panels, so in the summer the angle of the sun might not be that much different to what it is now.

 

My front panels dont really tilt, but even when the sun was its highest last summer, I didnt see them getting beyond mid-30s amps, so the 50amp MPPT will be able to cope ok. 

 

The light would be good in the summer, but heat and higher resistance reduces the current. 

 

 

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6 minutes ago, Higgs said:

 

The light would be good in the summer, but heat and higher resistance reduces the current. 

 

 

 

Thanks- I wasn't really paying much attention to the solar last Spring as I was on hookup, and I didnt get the rear panels done until early Sept, so I dont know yet how they perform in high summer, what with the tilting etc.

It seems like the answer is that they'll give similar yields in July to what they do on a sunny day in March- which is plenty for my needs. 

 

One great bonus of the extra panels is that I can be in shade for most of the day during the heatwaves, and the panels only have to see about two hours of direct sun to top up the batteries.  

 

 

Edited by Tony1
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28 minutes ago, XLD said:

But the sun hits the panels from a higher angle in summer🤔

Yes it does, but that isn’t quite the point because one can tilt the panels to match. It’s more that the sun passes through more atmosphere (slant range vs directly down) but as I said, atmospheric conditions make a big difference, as does the panel temperature.

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Just now, Tony1 said:

 

Thanks- I wasn't really paying much attention to the solar last Spring as I was on hookup, and I didnt get the rear panels done until early Sept, so I dont know yet how they perform in summer with the tiling etc.

It seems like the answer is that they'll give similar yields in July to what they do on a sunny day in March- which is plenty for my needs. 

 

 

If you could keep your panels cooler in the summer, you'd see an increase in the current. 

 

 

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4 hours ago, BEngo said:

A decent MPPT unit will simply limit the current to its allowed maximum value  and  will probably dump the excess power into the heat sink.  My Epever unit says it is happy indefinitely in these conditions.  It will switch off entirely if it gets too hot though.

My Tracer MPPT says it will handle input power up to twice the rating, but output is limited to the rated current. So I could double up on the panels fitted to get more output in winter, but with the same maximum output in summer.

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On 04/03/2022 at 16:36, David Mack said:

My Tracer MPPT says it will handle input power up to twice the rating, but output is limited to the rated current. So I could double up on the panels fitted to get more output in winter, but with the same maximum output in summer.

I think Tracers handle 'overpower' better than Victron?

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37 minutes ago, robtheplod said:

I think Tracers handle 'overpower' better than Victron?

 

There are probably pros and cons for both manufacturers, but my personal preference is very much for victron. The app they provide is just brilliant- intuitive, easy to understand and to use, and really clearly laid out.

I can disable/enable my victron MPPTs remotely using the phone app, if I feel the batteries are full enough.

So if I'm at the tiller and cruising, I dont have to dash inside, or pull over to the bank, I can do it with the phone).

They also have a BMS comms connector, so that I can use a BMV712 to switch them off automatically when the batteries reach their daily target SoC%, which is handy.  

I did have a 40 amp Tracer unit, and they do seem really well made and good quality units, but I used their solar station software to configure the unit, and it was not half so easy to set up or to understand for a novice. Even their bluetooth app is pretty crappy-looking compared to the victron offering. 

 

The overpower thing was a red herring really- the folks here are pretty sure that a 50 amp MPPT can handle anything that a 760 watt panel set can throw at it (within UK conditions).  

 

 

Edited by Tony1
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On 05/03/2022 at 18:51, Tony1 said:

 

There are probably pros and cons for both manufacturers, but my personal preference is very much for victron. The app they provide is just brilliant- intuitive, easy to understand and to use, and really clearly laid out.

I can disable/enable my victron MPPTs remotely using the phone app, if I feel the batteries are full enough.

So if I'm at the tiller and cruising, I dont have to dash inside, or pull over to the bank, I can do it with the phone).

They also have a BMS comms connector, so that I can use a BMV712 to switch them off automatically when the batteries reach their daily target SoC%, which is handy.  

I did have a 40 amp Tracer unit, and they do seem really well made and good quality units, but I used their solar station software to configure the unit, and it was not half so easy to set up or to understand for a novice. Even their bluetooth app is pretty crappy-looking compared to the victron offering. 

 

The overpower thing was a red herring really- the folks here are pretty sure that a 50 amp MPPT can handle anything that a 760 watt panel set can throw at it (within UK conditions).  

 

 

I assume here, that talking about basically switching off the solar panels, you have Lithium batteries, which do not handle overcharging well.

Flooded lead acid batteries will themselves control the amount solar charge they will accept, thus not needing to be "switched off", indeed  dependant on the accuracy of your SoC% meter, undercharging is a real possibility, with manual control.

 

Bod

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17 minutes ago, Bod said:

I assume here, that talking about basically switching off the solar panels, you have Lithium batteries, which do not handle overcharging well.

Flooded lead acid batteries will themselves control the amount solar charge they will accept, thus not needing to be "switched off", indeed  dependant on the accuracy of your SoC% meter, undercharging is a real possibility, with manual control.

 

Bod

 

You're right, mine are lithium batteries. I never used to have to worry about what the solar panels were doing when I had the lead acid batteries, but with the lithiums I had to put in place a variety of systems to disable the solar charging at a point when the batteries are sufficiently charged. 

 

Most folks seem to use battery voltage as a guide to decide when the batteries are sufficiently full that the charging should stop, but I use the SoC (as measured by a BMV712). 

Because I will occasionally allow friends/family to use the boat for a few days, I wanted a system that was automated, so I use the BMV712 relay function to switch off the MPPTs (and also the engine charging) via a small control wire, when the batteries get to 85% full. 

 

 

Edited by Tony1
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3 minutes ago, Tony1 said:

Most folks seem to use battery voltage as a guide to decide when the batteries are sufficiently full that they should to stop the charging, but I use the SoC (as measured by a BMV712). 

Because I will occasionally allow friends/family to use the boat for a few days, I wanted a system that was automated, so I use the BMV712 relay function to switch off the MPPTs (and also the engine charging) via a small control wire, when the batteries get to 85% full. 

 

 

 

This I find quite worrying. The BMV712 is hardly accurate in its AH counting is it? Does it not drift without regular re-calibrating? Or does it use some other method of determining 85% SoC?

 

 

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Just now, MtB said:

 

This I find quite worrying. The BMV712 is hardly accurate in its AH counting is it? Does it not drift without regular re-calibrating? Or does it use some other method of determining 85% SoC?

 

 

 

I charge them to 100% (or close) about every 3 weeks, which means the SoC is roughly accurate most of the time, but I do have a backup plan. 

If the voltage gets above 14.6v at any time, the high voltage disconnect (which is a victron Battery Protect unit) will be activated by a second BMV712, and that will phyiscally disconnect the B2Bs and the MPPTs from the batteries.

I've tested it a couple of times and It works ok, but its a last ditch thing that I hope will never be needed. 

 

My thinking is that if the SoC was reading low, and the batteries started getting above 95%, the battery voltage would rise to a point that the chargers would go into float mode anyway, so the charging would stop. 

 

Its one of the downsides of my crazy system that I can have up to four B2Bs and two MPPTs putting charge into the batteries.

In reality that never happens (i.e. I rarely need the four B2Bs for 8 months of the year), but the times I've run it with everything on, the charging voltage goes up to 14v, and the various chargers start to go into float mode earlier than I would want them to. 

 

As a result, I set the bulk charging voltages to 142v, which is a bit higher than I would ideally want, but it ensures they dont go into float too early and carry on charging. To be honest I'm still fine tuning the settings, and finding the ideal values for charging voltage.

  

 

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I fail to understand why people don't run batteries with a working BMS. At home my solar hybrid inverter and lithium battery have it built in,  it cycles between 98% and 8% SOC it just works. Surely someone must manufacture something similar to work at 13v rather than the 120v plus that this system works on.

ETA the hybrid inverter will also charge the battery from the mains supply  and you can set the charging time and % as well.

 

Edited by Loddon
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2 hours ago, Loddon said:

I fail to understand why people don't run batteries with a working BMS. At home my solar hybrid inverter and lithium battery have it built in,  it cycles between 98% and 8% SOC it just works. Surely someone must manufacture something similar to work at 13v rather than the 120v plus that this system works on.

ETA the hybrid inverter will also charge the battery from the mains supply  and you can set the charging time and % as well.

 

Tony does have a working BMS but it is rather opaque, not liking to communicate with the outside world much! That is the penalty of having that type of battery, they are good value in many ways but not in the context of the BMS, without the additional external switching module. And the BMS is "proprietory"

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