Solar - voltage drop and mppt

[stranded]

Using an mppt controller sensed at the battery terminal, is the solar harvest reduced by the same %age as the cable voltage drop or can the mppt compensate to some extent? To avoid ambiguity from my uncertainty about the right terminology, if I have 10% voltage drop in my cable will I lose 10% of my panel’s 5A output? I am aware there are many other variables.

 

[samfieldhouse]

10%??? How long/thin are your cables And what are you trying to calculate? I think your answer is basically "no".

There is always voltage drop in any cable. You can minimise this by using thicker cabling for long runs, or keeping the runs short.
An MPPT cannot compensate for voltage drop.

Are you confident in your understanding of how an MPPT works and how that relates to the ratings of the panel.

For example, my 100w Renogy Panels specify 5.2a at 19v. In practise (looking back at my MPPTs output), last summer my two panels in series were generating 2.9a at 34v which the MPPT converts to 4.2a of charge at 13v. One of the key advantages of MPPT is that they handle much higher voltages which means lower amps, which the MPPT then converts to higher amps at a lower voltage.

Standard MC4 4mm cables between your panels and the MPPT will be just fine. Keep the MPPT close to the battery and use quality 4mm cable and you'll experience minimal drop.

If you want to calculate it accurately you can use an online voltage drop calculator - i found this useful when installing my fridge as its a long way from the battery bank.

 

[VicS]

Using an mppt controller sensed at the battery terminal, is the solar harvest reduced by the same %age as the cable voltage drop or can the mppt compensate to some extent? To avoid ambiguity from my uncertainty about the right terminology, if I have 10% voltage drop in my cable will I lose 10% of my panel’s 5A output? I am aware there are many other variables.

An MPPT controller will control the load on the solar panels at the point at which to power extracted from the panel is at the maximum.

(If the current draw is higher the panel terminal volts will be lower and the power extracted will be lower
If the current draw is lower the terminal volts will be higher but the power extracted will be lower.
All because the panel itself has a significant impedance.)

10% voltage drop is far too high. It should be no more than 3%, preferably no more than 2%. You cables are much to thin.

Make the connections between the controller and the battery as short as possible because voltage drop here will directly affect the battery charging.

 

[rotrax]

We have 2 X 200W rigid panels, EPEver 30A MPPT Controler and I used the reccomended cable/connectors/fuse when installing. controller cable barely 1.5 metres long, another 2.3 to the panel connector.

Minimal voltage drop, performance exceeded expectation.

My tame 'Boat Minder' and his motorcycling friend have just made their monthly inspection. The report was good " AC and DC voltages normal, timer, heaters, dehumidifyer and thermostatic controller working as normal. "

Despite the awful wet weather and dull conditions the battery bank was on float,13.2V.

 

[stranded]

Thanks Sam. I used 10% (and 5A) for simplicity of explanation.

As I have battery sensing, then within reason the mppt will make sure voltage drop won’t mean the battery is getting too low voltage.

The reason for my question is that I am trying to increase the efficiency and robustness of my cabling by moving the paralleling from the panel end to the regulator/battery end of the circuit, allowing me to use thinner wire, and for tidiness brings twin core 4mm2 into the equation. At 3.9% drop for 10m, compared to 2.6% for 6mm2, the 4mm has the advantage for my purpose of being available in tinned twin core, whereas I don’t think the 6mm is. A loss of c.1.3% of my 30A of solar output equates to about 0.39A, or say 2Ahs a day, which I can live with for the convenience.

 

[stranded]

An MPPT controller will control the load on the solar panels at the point at which to power extracted from the panel is at the maximum.

(If the current draw is higher the panel terminal volts will be lower and the power extracted will be lower
If the current draw is lower the terminal volts will be higher but the power extracted will be lower.
All because the panel itself has a significant impedance.)

10% voltage drop is far too high. It should be no more than 3%, preferably no more than 2%. You cables are much to thin.

Make the connections between the controller and the battery as short as possible because voltage drop here will directly affect the battery charging.

Cheers Vic. Mppts are inches from the mastershunt which is about a foot from the battery terminal via 95mm2 wire, and the mppts work on battery terminal voltage by a Victron smart sense, so I don’t think voltage drop there is a concern.

I don’t pretend to have full understanding, but I wanted to make sure that the %age amperage loss will be the same as the %age voltage drop and not need to be factored up or something which would render my “can live with that” calculation wrong.

 

[geem]

A neat way to minimise volt drop is to buy two smaller panels and bolt them together. You can then run these panels in series giving you about 40v output. The volts go up and the amps go down. This vastly reduces volt drop so you can use small cables. Once at the MPPT the magic happens and you get lots of nice amps to your battery. It also help on cloudy days as the 5v difference needed between panel voltage and battery voltage happens easily, so a theoretical improved output in marginal conditions. The only downside is shading. If one panel is shaded the output drops a lot more than a pair of parallel panels

 

[stranded]

A neat way to minimise volt drop is to buy two smaller panels and bolt them together. You can then run these panels in series giving you about 40v output. The volts go up and the amps go down. This vastly reduces volt drop so you can use small cables. Once at the MPPT the magic happens and you get lots of nice amps to your battery. It also help on cloudy days as the 5v difference needed between panel voltage and battery voltage happens easily, so a theoretical improved output in marginal conditions. The only downside is shading. If one panel is shaded the output drops a lot more than a pair of parallel panels

That’s what I tried last year - 1 pair of panels on each pushpit rail, each side in series to its own mppt - but the shading was a problem unless I was very attentive - hauling the boom out of the way etc., so reverted to parallel. Hence my current thinking is to run individual cables from each panel to keep amperage low, only paralleling them at the battery box. I need to rewire anyway because I stupidly did all the paralleling at the panel when I hurriedly installed last year and have an ugly muddle of mc4 single and multi connectors tied to the pushpit rails - this way I will just have 1 pair of single mc4s and one twin core cable per panel above deck, which should be neater and with fewer potential points of failure.

 

[PaulRainbow]

That’s what I tried last year - 1 pair of panels on each pushpit rail, each side in series to its own mppt - but the shading was a problem unless I was very attentive - hauling the boom out of the way etc., so reverted to parallel. Hence my current thinking is to run individual cables from each panel to keep amperage low, only paralleling them at the battery box. I need to rewire anyway because I stupidly did all the paralleling at the panel when I hurriedly installed last year and have an ugly muddle of mc4 single and multi connectors tied to the pushpit rails - this way I will just have 1 pair of single mc4s and one twin core cable per panel above deck, which should be neater and with fewer potential points of failure.

How long is the cabling from the solar panels to the controller ?

4mm cable for a 30w panel is excessive.

 

[PaulRainbow]

Thanks Sam. I used 10% (and 5A) for simplicity of explanation.

As I have battery sensing, then within reason the mppt will make sure voltage drop won’t mean the battery is getting too low voltage.

The reason for my question is that I am trying to increase the efficiency and robustness of my cabling by moving the paralleling from the panel end to the regulator/battery end of the circuit, allowing me to use thinner wire, and for tidiness brings twin core 4mm2 into the equation. At 3.9% drop for 10m, compared to 2.6% for 6mm2, the 4mm has the advantage for my purpose of being available in tinned twin core, whereas I don’t think the 6mm is. A loss of c.1.3% of my 30A of solar output equates to about 0.39A, or say 2Ahs a day, which I can live with for the convenience.

Your sums are wildly off. 10m of 4mm cable for a 30w panel would be a drop of 1% at the very most. 2.5mm cable would be more than adequate.

 

[stranded]

Thanks Paul - might have mixed up my amps and watts somewhere above. 30 odd amps is roughly the total output of all the panels - 6 x 90 or 100watt. Does that make more sense?

 

[PaulRainbow]

Thanks Paul - might have mixed up my amps and watts somewhere above. 30 odd amps is roughly the total output of all the panels - 6 x 90 or 100watt. Does that make more sense?

Can we just clarify exactly what panels you have and how they are connected ?

 

[stranded]

I have 2 pairs of 90 watt Victron panels, one pair on each side of the pushpit, so 180 watts each side. Each pair is connected in parallel to its own Victron smart solar mppt. I then have a pair of 100 watt Victron panels on the foredeck, again connected in parallel to its own Victron MPPT.

Plus I have today taken delivery of a Photonic Universe folding 150 (ie 2 x 75 watt in parallel) panel which I will connect with a mobile coil of 6mm2 via the currently unused mppt charger built into the Mastervolt Mass Combi inverter charger - as the folding panel will only be in use when we are onboard and the Mass Combi anyway switched on, which it appears to need to be for its solar charger to work.

 

[PaulRainbow]

I have 2 pairs of 90 watt Victron panels, one pair on each side of the pushpit, so 180 watts each side. Each pair is connected in parallel to its own Victron smart solar mppt. I then have a pair of 100 watt Victron panels on the foredeck, again connected in parallel to its own Victron MPPT.

So, if all wired in parallel, you only have to consider voltage drop for each panel, between the panel and the controller. Each of those panels is 4a max at about 19v. 2.5mm cable gives about 3% voltage drop over 10m. But you should be able to connect each pair in series and reduce that to 1.5% as long as one of the pair doesn't suffer a lot of shading.

 

[stranded]

Cheers Paul - very helpful as ever. I need this bloody weather to improve so I can actually go sailing instead of dreaming up boaty projects and bothering you on here!

 

[Neeves]

4mm cable for a 30w panel is excessive.

Apart from cost is there any downside to the cables being too big?

If you have a 4mm cable, already with eye terminals, of the correct length but also have some 2.5mm uncut and with no terminals (but you have the terminals loose and the crimps).....and the cable run is not long. If the cable run is long then better to save the 4mm cable to a more appropriate location.

Jonathan

 

[PaulRainbow]

Cheers Paul - very helpful as ever. I need this bloody weather to improve so I can actually go sailing instead of dreaming up boaty projects and bothering you on here!

You're welcome, no bother at all.