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Two Solar controllers, bad idea ?


Lesd

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On another recent thread someone said having two seperate solar controllers connected to your batteries was a bad idea, there wasn't much further explanation as to why this is so. Can anyone expand on this please ?

 

I have two panels (18V) with a Stecca PWM controller and am considering adding another couple of larger panels (36V ones).

Bimble Solar said the best way to connect this up would be to add a second controller for the new panels and run this in parralel to the existing set up but this clashes with the assertion above so I'm not sure how to approach this. Any help appreciated.

 

Cheers

Les

 

 

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I imagine it's a bit like paralleling alternators. When the batteries are low, both will work at the maximum output they can manage with the sun available.

When the batteries get reasonably well charged, one controller will shut down due to the high voltage it perceives.

If I'm right the only downside would be that you lose the power from the panels whose controller is the more "sensitive" and shuts off first.

There might also be an issue if your new controller is the more effective mppt type, rather than another pwm. I don't imagine this would again be any more than an issue of lost efficiency.

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As said above one controller could easily cancel out the other one, most likely the more efficient MMPT would make the PWM think the batteries are charged.

You could use one panel and the PWM just for the starter battery (assuming you have some form of split charger fitted) then connect then put the rest through one MMPT controller to the leisure batteries, or see if you can sell the PWM on here, get some money back.

K

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I also reckon that all it will do is bring up the bulk phase more quickly and then one will "cut out" before the other, leaving that one to "finish off"

 

Perhaps what was being thought was that charging at too high a rate would give a false / premature end point as the battery voltage would be artificially high for the next stage of charging ? That might still be the case, depending on the bank size and the total charge current, ( probably unlikely) but at least that point is reached quicker even if it is a bit out. On less sunny days and days when you have a higher load on though, I would think it would be a bonus...

 

Nick

Edited by Nickhlx
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I also reckon that all it will do is bring up the bulk phase more quickly and then one will "cut out" before the other, leaving that one to "finish off"

I think (and it is just the way I see it might happen) is that the bigger solar array with the more efficient MMPT will stop the PWM ever going into bulk charge mode, the PWM will never see a reading at the batteries low enough for it to deem it necessary to go into bulk mode so the MMPT will do al the work and the PWM will just float around doing nothing.

K

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I think (and it is just the way I see it might happen) is that the bigger solar array with the more efficient MMPT will stop the PWM ever going into bulk charge mode, the PWM will never see a reading at the batteries low enough for it to deem it necessary to go into bulk mode so the MMPT will do al the work and the PWM will just float around doing nothing.

K

 

 

Yes - I think that would be the case but it probably depends on power of the arrays and the bank sizes too

 

Nick

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as said above if you mix controllers the PWM won't be doing much, if you are getting another controller i suggest you have like for like, either 2 PWM or 2 MPPT, we have a large array and have 2 MPPT controllers that work well together.

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If you have a PWM and a MPPT controller onto the same battery bank the PWM will kid the MPPT that the batteries are full. This is because the PWM will output narrow pulses of upto 18V onto the batteries as a way of regulation, the batteries will smooth out these pulses but the raised voltage will cause the MPPT to think the batteries are full. Below 90% charge there should be no problem. Two PWM ot two MPPT will work in harmony but one will still think the battery is full before the other and we all know which one 'S*D' will chose don't we.

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If the battery is nearly fully charged, it won't be taking much current anyway, so it doesn't matter if one controller shuts down. That is what my twin Victrons do. With a low battery they both churn out 50 amps, and with a full one they either share the load equally or one has a cuppa and leaves the other to it.

 

But I do believe it makes sense to use identical controllers.

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I have 2 sets of 2 x 230Watt panels. Each set goes to a 30A MPPT controller. So each controller is connected to 460Watts of solar.

I connected a clamp meter on the output of each controller on a very warm bright day, and each controller was giving off about 25Ah each......making a total supply to the batts of 50Ah

 

If I start the boat engine, the alternator raises the batteries to 14.4V and the clamp meter shows almost nothing from each controller...as their is no demand.

 

I suspect if I add more batteries, I would be able to save more power...and am losing a lot of power at the moment because I cant store it. I'm installing a calorifier with an immersion heater....should be interesting to see if solar could drive that...?

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I have 2 sets of 2 x 230Watt panels. Each set goes to a 30A MPPT controller. So each controller is connected to 460Watts of solar.

I connected a clamp meter on the output of each controller on a very warm bright day, and each controller was giving off about 25Ah each......making a total supply to the batts of 50Ah

 

If I start the boat engine, the alternator raises the batteries to 14.4V and the clamp meter shows almost nothing from each controller...as their is no demand.

 

I suspect if I add more batteries, I would be able to save more power...and am losing a lot of power at the moment because I cant store it. I'm installing a calorifier with an immersion heater....should be interesting to see if solar could drive that...?

Yes dean the solar will handle the immersion heater when the batteries are full, this is what we do to use up the extra solar on a sunny day.

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Thank you all for the replies, its starting to make sense now, I can see why Bimbles suggestion may not be the best way forward.

Sounds like Im going to have to spent some ££ on the existing set up, so following on from Bottle's point the other possible way to connect this up may be to series wire the two existing 18v/130w pannels to give a 36v output and put this in parallel with the two new 36v/165W panels and connect all into a new MPPT controller. The main issue being the cost of that single controller as it will need to handle around 50amps. Just for completion the battery bank is 4 x TrojanT105's (house bank) and two cheapie 110ah wet cells (start and BT) all connected via smart bank.

 

Any thoughts on this alternative and recommendations for the controller ?

 

Cheers

Les

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Maybe get a 30A Tracer for the new panels then try it along side the Steca with the old panels. If you're not happy with then later on get another 20A Tracer MPPT to use with the old panels.

 

Also one of those DC clamp ammeters can be very handy for telling what each controller is putting into the batts.

 

cheers, Pete.

~smpt~

 

 

 

 

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Thank you all for the replies, its starting to make sense now, I can see why Bimbles suggestion may not be the best way forward.

Sounds like Im going to have to spent some ££ on the existing set up, so following on from Bottle's point the other possible way to connect this up may be to series wire the two existing 18v/130w pannels to give a 36v output and put this in parallel with the two new 36v/165W panels and connect all into a new MPPT controller. The main issue being the cost of that single controller as it will need to handle around 50amps. Just for completion the battery bank is 4 x TrojanT105's (house bank) and two cheapie 110ah wet cells (start and BT) all connected via smart bank.

 

Any thoughts on this alternative and recommendations for the controller ?

 

Cheers

Les

 

But Bimble usually suggest using identical controllers. Did they really suggest pairing a Steca with an MPPT?

Yes dean the solar will handle the immersion heater when the batteries are full, this is what we do to use up the extra solar on a sunny day.

 

How do you achieve this? Do you manually switch on power to the immersion heater, or do you have some clever circuitry to do it for you?

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immersion heater has a switch which we turn on manually, usually 20 minutes at a time to give the batteries time to refill.

 

Well, that's certainly the simplest way to do it - and the cheapest. The downside is that you have to be around to do it. I'd like to find a simple circuit that would click the immersion on and off depending on battery state - but it's hard to know what that is while it's being charged.

 

Perhaps something that switches on for a pre-determined time - perhaps twenty minutes - when the amps flowing into the battery from the solar panels drop below five, though I am not sure how to measure that. I'd be using my inverter to power a 1kW element. If I could get two or three twenty minute "heats" during a day it would make a difference.

 

I have two 220W Yinglis connected to a 40A Tracer (all from Bimble), which in turn charges two 255Ah AGMs, which in their turn feed a Rich Electric low frequency 3,000W inverter.

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Well, that's certainly the simplest way to do it - and the cheapest. The downside is that you have to be around to do it. I'd like to find a simple circuit that would click the immersion on and off depending on battery state - but it's hard to know what that is while it's being charged.

 

Perhaps something that switches on for a pre-determined time - perhaps twenty minutes - when the amps flowing into the battery from the solar panels drop below five, though I am not sure how to measure that. I'd be using my inverter to power a 1kW element. If I could get two or three twenty minute "heats" during a day it would make a difference.

 

I have two 220W Yinglis connected to a 40A Tracer (all from Bimble), which in turn charges two 255Ah AGMs, which in their turn feed a Rich Electric low frequency 3,000W inverter.

I use a Voltage Sensitive Relay(set at 14.5 Volts)switching the immersion feed,as the batteries deplete ,the Relay switches out allowing the Voltage to rise again.

 

CT

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But Bimble usually suggest using identical controllers. Did they really suggest pairing a Steca with an MPPT?

 

 

Yes indeed, I just checked my email and I did include that detail. Ive just emailed them back asking for clarification.

Maybe get a 30A Tracer for the new panels then try it along side the Steca with the old panels. If you're not happy with then later on get another 20A Tracer MPPT to use with the old panels.

 

Also one of those DC clamp ammeters can be very handy for telling what each controller is putting into the batts.

 

cheers, Pete.

~smpt~

 

 

 

 

 

Thanks Pete

Do you think I could wire it as I described above if I wanted just the single controller ?

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What might help is finding out or timing how long the Steca maintains it's absorption charge voltage before dropping back to the float voltage (without any large loads pulling the voltage down and triggering a new charge cycle).

 

cheers, Pete.

~smpt~

Edited by smileypete
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Have just ordered 2 x 165W panels and the 30A Tracer MPPT controller. I'll wire these two in series and then connect the controller in parallel with the existing set up.

I'll see how it goes once its all installed and report back.

Thanks for the advice everyone.

Les

Edited by Lesd
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Sounds good, the 30A Tracer MPPT has the same box as the 40A so might run a bit cooler which could help reliability. Charge voltage at 14.6V for flooded batts is a shade below the 14.8V Trojans prefer, but I doubt it'd be a big problem.

 

To make the screw connections at the controller more secure it could be worth considering putting ferrules on the ends of the wires, eg:

 

http://www.ebay.co.uk/itm/Insulated-Cord-End-Terminal-ALL-SIZES-German-Ferrule-Terminals-Bootlace-/111322858890

 

Even if squished on with pliers (which I would NOT normally recommend for crimping ohmy.png), they'd still be much better than nothing.

 

For the cable between panels and controller 4mm2 should be perfectly adequate, tho if 6mm2 is only a little more why not go for that. Between controller and batts 10mm2 should do fine if it's reasonably close. Also don't forget the appropriate fuse (30A I guess) in the positive near the batt end. Plug 'n' play.... smile.png

 

cheers, Pete.

~smpt~

Edited by smileypete
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