Solar MPPT Question

[Major_Clanger]

I'm a bit of a dunderhead when it comes to electrics and need a bit of advice please.

Currently the boat has an old NASA 10w panel running into a basic MPPT. I'm now adding an additional (removeable) 100w panel and intend buying a new Victron MPPT but don't know which one. I think either the 75/10 or 75/15 - which do you think?
https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-75-10-75-15-100-15-100-20

 

[PaulRainbow]

I'm a bit of a dunderhead when it comes to electrics and need a bit of advice please.

Currently the boat has an old NASA 10w panel running into a basic MPPT. I'm now adding an additional (removeable) 100w panel and intend buying a new Victron MPPT but don't know which one. I think either the 75/10 or 75/15 - which do you think?
https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-75-10-75-15-100-15-100-20

The 75/10 will be fine. There is no point in connecting a 10W panel to a controller, the controller will use most (maybe all) of the output from the panel.

 

[Chiara’s slave]

The 75/10 will be fine. There is no point in connecting a 10W panel to a controller, the controller will use most (maybe all) of the output from the panel.

People forget that. Besides, a 10w panel puts out about 0.8 amps, which is hardly going to fry your battery. Our home solar drives home the point re charging efficiency. Obviously another league in all respects, but when the panels are pretty much maxed out, the inverter is ‘stealing’ over 100w, and making a noise to suit. Overall it’s not significant in 6kw, but of we had just a couple of 100w panels, it’d need to be a fairly good solar day to actually be in power profit.

 

[Baggywrinkle]

I'm a bit of a dunderhead when it comes to electrics and need a bit of advice please.

Currently the boat has an old NASA 10w panel running into a basic MPPT. I'm now adding an additional (removeable) 100w panel and intend buying a new Victron MPPT but don't know which one. I think either the 75/10 or 75/15 - which do you think?
https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-75-10-75-15-100-15-100-20

Go to the Victron MPPT calculator and either search for your panel or use "custom panel settings" to enter the details of the panel you have or are thinking of buying.

Go for a panel with a Vmpp over 18V, the higher the better, as the Victron MPPT will need a voltage of Vbatt (12,8V) + 5V to start - Vmpp over 18V will give a good margin to ensure it never shuts itself down on cloudy days.

On the Victron MPPTs, the first number is the input voltage of the attached panels, this must not be exceeded which is why it is a good idea to use the calculator link above because the voltage rises as the temperature drops, which can cause an overvoltage condition. (V temp. coeff. should be on the spec sheet of the panel) This will not apply with a single 100W panel but is worth remembering if a larger series array is planned.

The second number is the current it can supply to charge your batteries. 100W will provide approx. 100 / 12,8 = 7,8A - so as @PaulRainbow stated, a 75/10 will be fine.

Finally, it is really not worth keeping the 10W panel, mixing it with the 100W panel could actually reduce the power output of the system as they are unlikely to be compatible.

See what happens when mixing different panels in this video ...

 

[mogmog2]

Re 10W panel - as has been said, not worth keeping* and panels have fallen in price massively (I've just bought 2 100W bifacial rigid panels for about £70 - thats nuts! I paid £50 each for 50W rigid ones 4 years ago and though that wasn't bad)

(*or possibly wire it to your starter battery if you have space to leave it somewhere)

 

[noelex]

As others have said, the Victron 75/10 will be fine for a 100W panel. Leave the 10W panel connected to your existing controller.

However, if you think there is a possibility of replacing the 100W panel with a larger option in the future, the 75/15 is worth considering.

This will work just as well as the 75/10 for a 100W panel but give you some headroom to fit a more powerful panel. Solar panels are becoming more efficient and cheaper. If a replacement is needed, many end up choosing a larger panel, and it is a shame when this involves also replacing the solar controller.

There is typically not much price difference between the 10A and 15A version, but this will depend what deals are available in your local area.

The Smartsolar version (with Bluetooth) is worth the cost difference over the slightly cheaper Bluesolar version in both cases.

 

[PaulRainbow]

As others have said, the Victron 75/10 will be fine for a 100W panel. Leave the 10W panel connected to your existing controller.

That's not a good idea. Better to use it without a controller, the controller will have an overhead that will cancel out most (or all) of the yield. It's so small it won't hurt the batteries.

 

[noelex]

That's not a good idea. Better to use it without a controller, the controller will have an overhead that will cancel out most (or all) of the yield. It's so small it won't hurt the batteries.

Paul, I agree if the 10W panel was the only charging device, but the 100W panel will likely keep the battery at 100% much of the time. The additional unregulated at times 0.5A won’t kill the battery but is less than ideal.

While self-consumption of solar controllers can be an issue, this small a panel has likely been teamed with a cheap PWM controller (even though it labeled as MPPT) or at best a very crude and basic MPPT controller. These have very minimal self-consumption. If in doubt this can be easily measured. In addition, even the most basic PWM controller will prevent any discharge from the battery to the solar panel at night, and while this discharge is normally small, it can rise in older panels.

In short, considering the OP already has a solar controller connected to the 10W panel I would leave the system unchanged, but wire the new 100W panel completely independently to a new controller. Just my 2c worth.

 

[PaulRainbow]

Paul, I agree if the 10W panel was the only charging device, but the 100W panel will likely keep the battery at 100% much of the time. The additional unregulated at times 0.5A won’t kill the battery but is less than ideal.

0.5A is the absolute max from a typical 10W panel, it will more likely never see that and it will be for maybe 8 hours in 24. That amounts to about enough to stop a single engine battery from losing it's charge, in the Summer. My engine batteries have a 4A trickle charge from the Multiplus, 24/365 doesn't seem to do them any harm.

While self-consumption of solar controllers can be an issue, this small a panel has likely been teamed with a cheap PWM controller (even though it labeled as MPPT) or at best a very crude and basic MPPT controller. These have very minimal self-consumption. If in doubt this can be easily measured. In addition, even the most basic PWM controller will prevent any discharge from the battery to the solar panel at night, and while this discharge is normally small, it can rise in older panels.

It is nothing to do with discharge back to the panel. The point is, the controller is 100% not necessary and it has an overhead, however small.

In short, considering the OP already has a solar controller connected to the 10W panel I would keep in place, but wire the new 100W panel completely independently to a new controller. Just my 2c worth.

Just because something is already there does not make it correct.

 

[noelex]

It is nothing to do with discharge back to the panel. The point is, the controller is 100% not necessary and it has an overhead, however small.

The discharge from the battery into the panel at night is also an "overhead". This is prevented when a controller is used. A blocking diode can be added to the panel, but this has its own "overhead" (the diode will slightly reduce the solar panels’ output) and requires some electrical knowledge to install.

Especially when using simple controllers, the self-consumption of the controller can be less than the self-discharge that would occur at night if a controller is not fitted. Your assumption that a controller always represents a net "overhead" is not accurate.

 

[PaulRainbow]

The discharge from the battery into the panel at night is also an "overhead". This is prevented when a controller is used. A blocking diode can be added to the panel, but this has its own "overhead" (the diode will slightly reduce the solar panels’ output) and requires some electrical knowledge to install.

Especially when using simple controllers, the self-consumption of the controller can be less than the self-discharge that would occur at night if a controller is not fitted. Your assumption that a controller always represents a net "overhead" is not accurate.

Whatever, you're the World expert, as usual.

 

[Major_Clanger]

Thanks for all your input chaps, appreciated.

I'm going to keep the little 10w panel but, as Paul suggests, running it without a controller. It sits neatly on the lazarette hatch and would probably leave a witness if I junked it.

I'm only planning to use the new panel when not underway and will wire it into the 75-10. It's only a little Contessa so with such a low electrical appetite, I might even be able to sell some power into the Grid!

 

[NBs]

I have an SRNE DC-DC charger with a built-in MPPT controller that has pretty good efficiency. Plus, the LiFePO4 batteries store electricity really well. Here’s a screenshot from this morning—it was a bit cloudy.

 

[RupertW]

Thanks for all your input chaps, appreciated.

I'm going to keep the little 10w panel but, as Paul suggests, running it without a controller. It sits neatly on the lazarette hatch and would probably leave a witness if I junked it.

I'm only planning to use the new panel when not underway and will wire it into the 75-10. It's only a little Contessa so with such a low electrical appetite, I might even be able to sell some power into the Grid!

I think it’s worth checking the current between 10A panel and the battery back both at full sunlight and complete darkness to see if there is any backflow from battery to panel in darkness as the time spent out of full sunlight is much more than in it. Just in case but hopefully the panel has a diode to stop it happening.

 

[noelex]

I have an SRNE DC-DC charger with a built-in MPPT controller that has pretty good efficiency. Plus, the LiFePO4 batteries store electricity really well. Here’s a screenshot from this morning—it was a bit cloudy.

That is a very nice display, but why is solar panel voltage so low? Even on a cloudy morning the Vmp should be 17V+ for a typical "12V panel" unless it is very hot.

The other interesting point is the display suggests the MPPT controller performing the voltage conversion so efficiently that even after also allowing for its self consumption the loss between input and output is less than 1W.

The reality is MPPT controllers, while efficient, have more significant losses than the display is indicating.

Many manufacturers calibrate their controllers in this way to exaggerate the efficiency. On occasions, some will even indicate a higher wattage output than the input. A perpetual energy machine .

 

[noelex]

I think it’s worth checking the current between 10A panel and the battery back both at full sunlight and complete darkness to see if there is any backflow from battery to panel in darkness as the time spent out of full sunlight is much more than in it. Just in case but hopefully the panel has a diode to stop it happening.

This is a good suggestion. If there is a battery monitor on board that can display low currents, it is easy to do. If not, use a multimeter.

The solar panel will almost certainly have some bypass diodes installed, but it is very unlikely to have a blocking diode. Bypass diodes prevent damage to the solar panel from hotspots, but do nothing to prevent discharge from the battery at night.

The solar cells themselves represent a high resistance, so the discharge current from the battery to the solar panel will only be small even when connecting the solar panel directly to the battery without a controller. However, this small current all night can be enough to impact the gain from the panel during the day, so it should be checked. It varies considerably from panel to panel.

If a controller is used, this test is unnecessary as one of the functions of the controller is to disconnect the panels at night so there is no overnight discharge. All controllers, even the cheapest PWM models, will do this.

 

[Major_Clanger]

Quick questions chaps.... The panel I've bought is semi-flexible. Should I mount it on some thin ply or will it be okay just laid down as is? To re-cap, it's only going to be used when not underway, most of the time it'll be stowed under a bunk cushion.

 

[thinwater]

I'm not a fan of removable panels. Typically, the stress of moving them does micro-damage, and they end up lasting no longer than installed panels. Under a bunk cushion is bad; every time you lie down it will be squeezed, causing micro-fractures in the cells (Google it). Just don't. MANY panels are very slowly ruined that way. You will doubtless flex it a little just moving it. Removable panels produce less power, even in good locations, because they are out for many fewer hours. Just find a place to screw/glue/Velcro it down and leave it there. Installation is not complicated. I bet a 50W installed panel would produce as much power, and it's simple to find space for that.

I'd probably scrap the 10 W panel. If it's old, as you say, it's probably 5 W. It's just in the way. Or leave it.

At 10 W MPPT is most likely less efficient than PWM. The self-consumption of MPPT is higher. Morningstar, for example, makes and recommends PWM for 50 W and below.

 

[mogmog2]

Quick questions chaps.... The panel I've bought is semi-flexible. Should I mount it on some thin ply or will it be okay just laid down as is? To re-cap, it's only going to be used when not underway, most of the time it'll be stowed under a bunk cushion.

That might depend on where/how you're going to deploy it. If I were doing it, I'd probably try not to need a piece of plywood and use it a bit, but if it turns out it would help, then go for it. I'd also be looking at knocking up a basic bag/sleeve for it (with or without plywood)

 

[Major_Clanger]

Okay, a fresh wave of confusion has passed over me after reading the install manual for the 75-10..... I had assumed this was going to be really straightforward with the following set-up:

100w solar panel --- Victron 75-10 MPPT --- batteries (2 100ah wired in parallel) using tinned 2.5mm cable (max run length only 3m).

That makes sense to me but Victron are talking about setting-up virtual loads, real DC loads and warning about the battery being destroyed in a short space of time due to repeated partial charging. Do I need to be concerned about any of this or can I wire it as I envisaged? At the moment, all the DC loads are taken from a busbar on the main panel, I don't want to mess about with this and take it all from the MPPT - or am I misunderstanding what Victron are suggesting?

Not sure whether it's relevant but batteries are new lead-acid leisure jobbies and the boat, a Co26, is also fitted with a small Victron mains charger and has a NASA BM-1 monitor. The boat's not power hungry.