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Split charge, solar and 240v hookup


Chris Davies

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Hi, quick question for you electrical guru's.

I have a leisure battery that is charging through a split charge relay, the battery just runs a 40l compressor type fridge, a few lights and USB points but I'm finding that if I'm parked up for more than a few days I'm running out of juice so I would like to add a solar panel to keep things topped up. Occasionally we are on a campsite with EHU so I thought about adding a small smart battery charger as well.

My question is, would this be ok? Split charge and a solar panel and a 240v battery charger all connected to the leisure battery? Obviously the split charge and battery charger would never be on at the same time but would the solar panel 'argue' with the 240v battery charger when I have EHU? 

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You need to add a solar regulator with a panel, this reduces the voltage from the panel to ~14v and should turn off the supply to the battery if it's over a certain voltage (ie being charged by another source)

Or you can get one of the fancy ctek control boxes, but I've always got on perfectly well with a £15 solar regulator. 

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We've got a very similar setup on our camper ambulance and we have never bothered with a mains hookup - a single 100W panel and cheapy cheap eBay controller works pretty well, because we're short on battery space (~80AH battery) we doubled up and put a 2nd panel and controller which basically keeps us going almost indefinitely even drinking a lot of beer in the sunshine and not starting the engine for a week. If I had the space I'd have shoved the largest leisure battery I could find in instead.

Bear in mind with a decent split charge if you start the engine it will recharge the leisure battery at a hell of a rate - ours having a P38 alternator will happily chuck 60A into the battery for a fair old while if it's been run down, you can hear the alternator whining under load :lol:

I did the opposite to @Eightpot's advice - I wired the solar controllers a long way from the battery and ran separate (fused) cables to each so that they don't fight each other trying to charge the battery, the small drop in the cables evens out their voltage measurements

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6 minutes ago, FridgeFreezer said:

we doubled up and put a 2nd panel and controller ............ I wired the solar controllers a long way from the battery and ran separate (fused) cables to each so that they don't fight each other trying to charge the battery, the small drop in the cables evens out their voltage measurements

That's interesting. We only use the battery, no electric hookup but on Autumn trips with a heater and fridge to run, the 100W panel doesn't supply enough juice in the daylight available. I have been thinking about adding a second solar panel but to connect in parallel and so into my Ctek controller (don't start!), I'd need to be sure that the voltage is the same. For some reason that I can't remember, picking a suitable panel isn't quite as easy as it sounds. It might be just the physical dimensions and my roof configuration.

So the second (cheapy cheapy) controller is an idea but just how much volt drop have you needed to achieve to match the two panels at the battery terminals? It can';t be much but you obviously think it's necessary.

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Here's the very cheap controllers:

dual_controllers.jpg

 

As you can see I also set one to cut off 0.1v before the other (13.5v / 13.6v) so as the battery charges up they're not cycling on & off.

It's not so much a specific voltage drop as the fact that they are both pushing current down their own ~3m run of cable to the battery so they both "see" battery voltage plus the drop in their cable run (due to current) rather than the current from one bringing up the "local" voltage of the other. Putting them closer to the battery might also work but then I wouldn't be able to see them :lol:

I couldn't see much sensible detail from the likes of CTEK on connecting multiple panels, especially given the effects one shaded panel can have on the other in certain setups - the twin controllers mean the two systems are as separate as they can be, one panel could be totally dead (shaded) and the other would still charge at full whack.

 

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Thanks @FridgeFreezer, that's got me thinking. I can see the controllers on eBay (amongst many others 😱) in the £25 range, although I imagine there will be some import costs also.

I agree about two controllers handling two panels across partial sun & shade better than one. Using just the Ctek, two panels would have to appear as one, as it can only follow one I/V curve as part of the MPPT. They would need to be identical in terms of output voltage at least.

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1 hour ago, Bowie69 said:

@FridgeFreezer, any reason why you didn't just parallel link the solar panels into one controller?

As alluded to above - because solar panels do weird things when they're shaded (even by a single leaf falling on them) and depending on how you join them you risk one shaded panel affecting the performance of the other, or at the very least the solar controller doesn't know it's got two panels attached (it only has one input, so it "sees" one bigger panel behaving poorly) and will jigger up its MPPT algorithm.

With two separate controllers you're always getting the best out of both - and the controllers being dirt cheap means there's not much penalty, maybe £25 and two extra supply wires.

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Thanks for the reply's, the solar panel would be wired through a charge controller, I think what I had forgotten (or never knew in the first place) is that the controller will shut off  if it detects a higher voltage, so whether that is from the alternator or from a 240v charger it should disconnect the solar panel if I'm understanding correctly?

@FridgeFreezer are those voltage settings on your controllers what they will try to deliver to the battery or is the voltage at which they will disconnect?

 

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1 hour ago, Chris Davies said:

Thanks for the reply's, the solar panel would be wired through a charge controller, I think what I had forgotten (or never knew in the first place) is that the controller will shut off  if it detects a higher voltage, so whether that is from the alternator or from a 240v charger it should disconnect the solar panel if I'm understanding correctly?

@FridgeFreezer are those voltage settings on your controllers what they will try to deliver to the battery or is the voltage at which they will disconnect?

You're correct - there's a target voltage for the controller to charge to ("PV Off") above which they just stop charging. When charging they (in theory) use MPPT to get the most watts out of the panel, so the actual output voltage may vary a bit.

The other two parameters (Load Off / Load On) are for the switched output (last pair of terminals on the right) which will disconnect any loads attached when the voltage drops below "Load Off", until it goes back above "Load On", but as you can see I don't really use those.

We've been running with these panels & controllers for at least 5 years and a lot of trips now, between the panels and the split charge we've never had an issue (I did accidentally disconnect the split charge recently when replacing the battery, which did cause a little faff) - our "worst case" use case for the system is LeMans where the truck is sat for ~5 days without moving in sunshine getting heated up and with the fridge working quite hard to keep up with the rate of beer consumption :lol: but it does cope. We're hoping to to go back next year just to make sure ;)

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2 hours ago, FridgeFreezer said:

solar panels do weird things when they're shaded

They alter their output depending on the amount of light (obviously) and this can be seen on a plot of Current v Voltage which is none linear. I think an MPPT controller alters its load in response to the voltage it sees from the panel, so that the current is adjusted and so it follows the curve to maximise the Watts.

It can therefore only really follow one curve and not two.

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I run an aux battery for lights, charging etc, but not fridge, that's gas

For shower, tent lighting and other power I have two seperate batterie. Both with charge controllers. One 27amp hour, for the toilet lighting and shower. One 7amp hour for the tent lighting and children's entertainment systems. Both batteries run small solar panels, that match the trickle charge rate for the individual battery.

When I fianlly move over to a compressor fridge, I have a similar set up ready

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13 minutes ago, Peaklander said:

They alter their output depending on the amount of light (obviously) and this can be seen on a plot of Current v Voltage which is none linear. I think an MPPT controller alters its load in response to the voltage it sees from the panel, so that the current is adjusted and so it follows the curve to maximise the Watts.

It can therefore only really follow one curve and not two.

MPPT in theory is constantly altering the load on the panel to hit the maximum watts (it watches volts and current and good ones also cycle through a wider range every so often to check they're not in a false peak - called perturb and observe sometimes), but as you say it can only really do that reliably for the response curve of one panel.

Solar panels that are shaded can go high impedance - even a tiny bit of shade on one cell of the panel can be enough to make it behave like a very dead battery - so a single leaf, stray shadow, or large bird dropping can really knock a lot off the panel's performance. Dust & dirt also drops it too. Dave at eevblog did a video recently about his rooftop solar panels where the shadow of the guy wires for his radio aerial were noticeably affecting his solar output - if you can put up with his voice it's an interesting video :lol:

 

 

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1 hour ago, FridgeFreezer said:

Solar panels that are shaded can go high impedance - even a tiny bit of shade on one cell of the panel can be enough to make it behave like a very dead battery - so a single leaf, stray shadow, or large bird dropping can really knock a lot off the panel's performance.

I'm going to be really, really annoying and come back to you on this one again, sorry, maybe I should pretend I am someone else 🥸

If one panel gets one cell covered and goes high impedance (not disputing this at all, known fact), then it wouldn't affect a parallel connected panel, would it? Do you have different sized panels?

I did a bit of reading and that suggested the only reason to run more than one controller would be that the controller current capacity was lower than the connected panels could provide, but happy to be proved wrong.

 

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@Bowie69 - the panel doesn't simply go high-impedance, unless the panels are identical in every way (including the light hitting them) one's performance curve will be different to the other's, think of it like building a vehicle where you bolt a TDi directly to a V8 and then try to work out what's the best gear to be in - you've got two different power/torque curves strapped to each other and the answer you get out is going to fall somewhere between the two (and away from what's optimum for either).

If solar controllers were expensive I'd agree you can make do with one, but the cheap ones are hovering around the £15 mark so it hardly seems worth it.

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On 11/1/2021 at 11:55 AM, Chris Davies said:

Thanks for the reply's, the solar panel would be wired through a charge controller, I think what I had forgotten (or never knew in the first place) is that the controller will shut off  if it detects a higher voltage, so whether that is from the alternator or from a 240v charger it should disconnect the solar panel if I'm understanding correctly?

@FridgeFreezer are those voltage settings on your controllers what they will try to deliver to the battery or is the voltage at which they will disconnect?

 

I've got a Steca charge controller which controls the two battery setup off a solar panel, and which can "split -charge" the batteries, and deliver 90% of the incoming charge from the panel to one battery & 10% to another at the same time. It also has an output (load) facility so I've wired the fridge to that. In practice what happens is the solar input maintains the two batteries state of charge, and powers the fridge whilst sufficient current is incoming. Then when the solar input drops, the controller switches over to power the fridge from your second (aux) battery (protecting your starter battery), and if the voltage on the aux battery drops below a certain level (the point where it would damage the battery) the controller shuts the fridge off.  An additional plug-in deivce enables lots more functions but the basic unit is pretty mucch fit and forget.  Here: https://www.steca.com/index.php?Steca-Solarix-2020-x2-en

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