Solar MPPT Charge Controllers

[mutleee]

I'm in the process of fitting a solar system to my narrowboat. I have 4 x 100w mono panels and want to connect them through a MPPT Charge controller to my leisure batteries. (12v system)

 

I have 2 questions:-

 

1. What size mppt charge controller do I need, 30amp or 40amp? all the 30amp controllers I've seen have a max PV Wattage of 390w so that would suggest I need a 40amp controller, but I've seen places advertising 400/420w packages which come with a 30amp controller.

 

2. Does anyone have any recommendations for a brand of controller? There seem to be the expensive ones MorningStar, Outback, Soltronic etc and the cheaper Chinese/Taiwanese brands, EP Solar being one that I've noticed. I've also read lots of stories of people buying a Chinese brand only to find it's not an MPPT but a PWM one labelled up as an MPPT.

[Justme]

You would be fine with a 30amp one. The slight loss you might get at the top end is minimal.

 

I would personally go for the 40a one if you might ever want to add extra panels.

 

Also remember to check the controllers max voltage. That is far more critical to the system.

[by'eck]

MPPT controllers dissipate a lot of heat when running.

 

Its bad policy to run any electronic device constantly on its design limit.

 

Use the 40 amp one as a minimum.

[Justme]

MPPT controllers dissipate a lot of heat when running.

 

Its bad policy to run any electronic device constantly on its design limit.

 

Use the 40 amp one as a minimum.

 

 

It wont be running at its limit for very long each day. Even in summer the peak will only be for a short time.

 

The heat dissipated should only be equal to the efficiency losses.

 

So a 400w array with a 90-95% controller should only be giving off 20 to 40 watts of heat.

 

It is standard practice to over size the array compared to the controller by up to 20%. In some special cases its even higher.

 

If its giving off more heat then it's a crappy controller.

 

Using a bigger controller with a smaller array will lower the efficiency rating.

Edited October 8, 2013 by Justme

[KJT]

We have a similar set up as you. 4 x 100 watt panels running through a Tracer 30amp MPPT controller.

 

I was going to go for the 40amp version which is considerably more expensive. The engineer at the suppliers said the 30 amp one would be fine as the amount of time the panels are delivering their maximum output would, in practice, not be that often. The Tracer apparently can cope well with a reasonable amount of overload, on those occassions I understand you will be wasting the extra output which would be utilised by the 40 amp one.

 

We were running that system all this summer and even during the very hot spell it coped very well. I can only speak for the Tracer though. Other makes might be less tolerant.

 

Ken

[IainW]

if you can afford it, get a bigger ampage controller to give you some 'head room' if you decide to get more panels, or get a direct hit from a solar flare :-)

[Detling]

Been pleased with my Tracer so far (6 months) good value for money

[mutleee]

Thank you all for getting back to me with your views.

 

I see both NB Ellissiana and Detling have Tracer controllers and are happy with them. Is that the same as the EP Solar Tracer MPPT Controllers that are available here? http://www.sunstore.co.uk/EP-Solar-Tracer-MPPT-Charge-Controller-12v-24v-40A.html.

 

If so I think at that price (£165 for a 40a MPPT) I may take the plunge. The other ones I was looking at (MorningStar, Outback etc) are over £400!. So I was assuming that one at £165 must be a bit cheap n cheerful.

 

At least then if I wanted to add another panel at a later date it would cope.

 

That controller has a maximum PV Open Circuit Voltage of 100VDC, as each of my 100w panels have a Max Open Circuit Voltage of 21.84v, should I wire them all up in series (21.84 x 4 = 87.36v) or is it more efficient as 2 lots of 2 x 100w in series. then parallel the 2 together?

[Supertramp]

I would link them in series, and keep the voltage high, ( = lower current - smaller cables, less loss). This will also allow the controller to do it's best for you on cloudy days.

[KJT]

Thank you all for getting back to me with your views.

 

I see both NB Ellissiana and Detling have Tracer controllers and are happy with them. Is that the same as the EP Solar Tracer MPPT Controllers that are available here? http://www.sunstore.co.uk/EP-Solar-Tracer-MPPT-Charge-Controller-12v-24v-40A.html.

 

If so I think at that price (£165 for a 40a MPPT) I may take the plunge. The other ones I was looking at (MorningStar, Outback etc) are over £400!. So I was assuming that one at £165 must be a bit cheap n cheerful.

 

At least then if I wanted to add another panel at a later date it would cope.

 

 

 

That's the one! £165 seems a good price. I have seen them advertised for £240.

 

I think it's worth paying the extra £35 for the remote display to allow you to see what the panels are actually doing.

 

Ken

[smileypete]

Bimble Solar do good prices on Tracer MPPTs.

 

With panels in parallel the cables will lose a little bit of power, in series the MPPT will likely be a bit less efficient, so it might not make much difference - that said series is simpler to wire up.

 

Bear in mind with the 40A 100V Tracer it won't handle five 18V panels in series, they'd all have to go in parallel. There is a 40A 150V Tracer around, but usually costs a bit more.

 

The 30A Tracer has the same size case and heatsink as the 40A one, so I'd say it's more over engineered cooling-wise than the latter.

 

cheers, Pete.

~smpt~

Edited October 8, 2013 by smileypete

[Justme]

 

That controller has a maximum PV Open Circuit Voltage of 100VDC, as each of my 100w panels have a Max Open Circuit Voltage of 21.84v, should I wire them all up in series (21.84 x 4 = 87.36v)

 

 

You also need to know the temperature coefficient factor. The 21.84v will be rated at say +20c.

 

For every 1c lower the voltage will rise by a fraction of a %.

 

So when it hits -20c (was not that long ago when it did) you could be looking at a much higher voltage.

[by'eck]

 

 

It wont be running at its limit for very long each day. Even in summer the peak will only be for a short time.

 

The heat dissipated should only be equal to the efficiency losses.

 

So a 400w array with a 90-95% controller should only be giving off 20 to 40 watts of heat.

 

It is standard practice to over size the array compared to the controller by up to 20%. In some special cases its even higher.

 

If its giving off more heat then it's a crappy controller.

 

Using a bigger controller with a smaller array will lower the efficiency rating.

 

Well my BlueSky 20 amp controller (one of the major US manufacturers) used to get almost too hot to touch supplied by 2 x 130 watt panels whilst supplying up to a max of 18 amps @ 14.7 volts - slightly more than spec, although this was in the Caribbean . I wouldn't have wanted anything smaller. In the UK we have less sun but lower panel temperatures which offsets the lower output to some degree.

 

Mppt controllers do by design dissipate heat requiring large heat sinks and fan cooling on larger models compared with much smaller heat sink requirement for equivalent pwm controller. I suspect the high efficiencies you and the manufacturers mention are conversion efficiencies and not overall. My Blue Sky one quoted a conversion efficiency of 96% for example. There was a little more than 5.2 watts of heat emanating from it though

 

In any case its simply not good policy generally to under-rate equipment on the premise that it might not be taken beyond its limits.

Edited October 9, 2013 by by'eck

[Ex- Member]

We have an Outback 60 and we're about to add another panel @ 240 watts to our already 500 watt array so a possible 61 amps However the odds of the panels ever achieving that are lottery odds.

 

We've had some really good sunshine and our best recorded amps was 37amps which isn't bad but It probably wasn't 37 amps for long.

 

I have to say though with regards to temperature the Outback has hardly been luke warm even when in the 30 amps range. Will be interesting to see how it fairs with the additional panel.

 

The new panel we'll hopefully erecting later today, It's going to have a high tilt to glean more winter sun, I'll post more later if we get it fitted today.

[DaveP]

I have a pair of 250w panels and the 40A Tracer controller. I'm happy with the setup. One feature the more expensive controllers seem to have, that the Tracer lacks, is the ability to divert excess solar power into a dump load - which I'd love to be able to run into the calorifier.

[Justme]

 

Well my BlueSky 20 amp controller (one of the major US manufacturers) used to get almost too hot to touch supplied by 2 x 130 watt panels whilst supplying up to a max of 18 amps @ 14.7 volts - slightly more than spec, although this was in the Caribbean . I wouldn't have wanted anything smaller. In the UK we have less sun but lower panel temperatures which offsets the lower output to some degree.

 

Mppt controllers do by design dissipate heat requiring large heat sinks and fan cooling on larger models compared with much smaller heat sink requirement for equivalent pwm controller. I suspect the high efficiencies you and the manufacturers mention are conversion efficiencies and not overall. My Blue Sky one quoted a conversion efficiency of 96% for example. There was a little more than 5.2 watts of heat emanating from it though

 

In any case its simply not good policy generally to under-rate equipment on the premise that it might not be taken beyond its limits.

 

It wont / cant be taken beyond its limits. It simply does not happen. The peak amps is controlled by the controller & it simply does not harvest the excess current from the panels. The panels do not force the controller to take all the available power.

Virtually every on grid PV install in the country will have an inverter that is rated at less than the size of the array it is connected too. Remember that an ongrid system is not limited by a battery's state of charge so can run at full out put for as long as there is power available. The inverter manufacturers recommend it this way.

 

If what your saying was true then as the battery charged the controller needs to "burn off" or dump the excess. That is not how they work.

 

I agree that cheaper controllers might not be rated for 100% duty cycle at the peak output but quality makes like Outback, Struder ect are rated for continuous use when hot.

[by'eck]

 

It wont / cant be taken beyond its limits. It simply does not happen. The peak amps is controlled by the controller & it simply does not harvest the excess current from the panels. The panels do not force the controller to take all the available power.

Virtually every on grid PV install in the country will have an inverter that is rated at less than the size of the array it is connected too. Remember that an ongrid system is not limited by a battery's state of charge so can run at full out put for as long as there is power available. The inverter manufacturers recommend it this way.

 

If what your saying was true then as the battery charged the controller needs to "burn off" or dump the excess. That is not how they work.

 

I agree that cheaper controllers might not be rated for 100% duty cycle at the peak output but quality makes like Outback, Struder ect are rated for continuous use when hot.

 

I agree it makes sense not harvesting unneeded current from panels and that a controller should be able to supply its max rated current indefinitely.

 

I find it difficult to accept though that you choose a set of pv panels and couple them to an expensive mppt controller that's underrated to cope with their max rated output such that under ideal light conditions (however rare this may be in the UK) you will never be able to draw full power from them. This apparently to avoid incurring (minute) extra inefficiencies.

 

Is this just UK policy or have I misunderstood, since you mention underrated inverters as well on grid systems?

Edited October 10, 2013 by by'eck

[Justme]

 

I agree it makes sense not harvesting unneeded current from panels and that a controller should be able to supply its max rated current indefinitely.

 

I find it difficult to accept though that you choose a set of pv panels and couple them to an expensive mppt controller that's underrated to cope with their max rated output such that under ideal light conditions (however rare this may be in the UK) you will never be able to draw full power from them. This apparently to avoid incurring (minute) extra inefficiencies.

 

Is this just UK policy or have I misunderstood, since you mention underrated inverters as well on grid systems?

 

 

The main idea is that you calc your power needs & for how long you want the PV to be able to provide them. Let me use my set up as an example.

 

Our daily needs averaged over a year are a tad over 3kWh (thats 242ah @ 12.4v) per day.

 

To provide 100% of that in summer I would only need an array of about 375 watts. (plus some for autonomous days)

 

To provide that in deepest winter I would need nearer 10,000 or more watts.

 

So what I wanted to do was to find the sweet spot between both those extremes.

 

In summer my 1.2kWp array can & does make up to 8kWh per day into a flat battery Even though the controller will cap the current at 80amps. (without that cap it could hit 10kWp) So instead of a neat curve from dawn till dusk you ended up with a sugar loaf (steep rise, flat top steep drop) curve.

 

That also means that I can harvest my 3kWh either side of the summer & heading into the winter.

 

So fitting a 100amp controller would have been over kill. I could have dropped even further to a 60 amp but then I would loose the ability to recharge quickly after 3 to 5 days of bad weather.

 

With an on grid system most domestic installs are limited to 3.68kWp of exported power. If you had just 3.68kWp of array then you would only hit that limit in ideal conditions. To max the energy they can make / export you could fit a 5kWp array yet still connect it to a 3.68kWp capped inverter. You will have full power available from mid morning till late afternoon. Yes you would have got more in total with a bigger inverter but the rules do not allow you to do this without substantial costs.

 

In addition to all of that the inverter makers have different models that have different efficiencies at different points in the power curve. The higher in the power curve the better the efficiency normally. Using the makers calculator you can find the right combination of panels and inverter to maximise the total yearly gain whilst still keeping within the regs for domestic installs.