Using a CC Power Supply in Series with a Battery to Limit Output Current MPPT?

[Farsider]

Hi all, hopefully some of the clever people here can put me straight on this as have had a good dig around the google verse and forum and cant find this scenario discussed.

With my limited experience and and small scale experiment this appears to be viable but sure I'm missing something.

So here is the situation. Lets assume we have a multiple 12V car batteries with "effectively" unlimited current output potential. We want to hook it up as an emergency "fake" solar input to an MPPT solar charge controller.  It well understood that this isn't a good idea as the MPPT will see the high current capability as effectively a dead short and the tracker may do all sorts of odd things. 

So we could get a beefy DC-DC converter with CC/CV and use that to limit current. But that is a fairly expensive solution as soon as we go much over ~36V DC-DC threshold that most cheap Power supplies can do.

So it go me thinking.  If current in a series circuit is limited to the smallest current source and you wanted say a ~100V CC DC Supply.  Could you have 7x12V Batteries in series with an 8th connected also in series via a 12-36V Boost converter.  Then lets say the PSU is set in CC mode to 3A @16V.  Does this mean we now have a 100V 3A supply overall.  Is the PSU now handling 300W(100*3) or is it just doing 48W(16*3). Does it depend if it high or low point in the circuit.  Or is that the crux of why it cant work, the small PSU would effectively see the full 300W and 84V(7*12V) across its input and basically blow up.

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POC Bench experiment to prove very basic concept was 3x9V batteries in parallel as a ~9V/6A supply in series with a PSU to power a ~35V LED.  Was able to set the current limit for the whole circuit via the PSU in this setup but overall voltages/current were also within limits of the supply itself.

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So please be kind and educate me where my logic is wrong with the above as sure I must be missing something as otherwise this would be a solution or idea that cropping up everything to Current limit batteries without high voltage DC-DC power supplies.

[MrAl]

Hi all, hopefully some of the clever people hear can put me straight on this as have had a good dig around the google verse and forum and cant find this scenario discussed.

With my limited experience and and small scale experiment this appears to be viable but sure I'm missing something.

So here is the situation. Lets assume we have a multiple 12V car batteries with "effectively" unlimited current output potential. We want to hook it up as an emergency "fake" solar input to an MPPT solar charge controller.  It well understood that this isn't a good idea as the MPPT will see the high current capability as effectively a dead short and the tracker may do all sorts of odd things. 

So we could get a beefy DC-DC converter with CC/CV and use that to limit current. But that is a fairly expensive solution as soon as we go much over ~36V DC-DC threshold that most cheap Power supplies can do.

So it go me thinking.  If current in a series circuit is limited to the smallest current source and you wanted say a ~100V CC DC Supply.  Could you have 7x12V Batteries in series with an 8th connected also in series via a 12-36V Boost converter.  Then lets say the PSU is set in CC mode to 3A @16V.  Does this mean we now have a 100V 3A supply overall.  Is the PSU now handling 300W(100*3) or is it just doing 48W(16*3). Does it depend if it high or low point in the circuit.  Or is that the crux of why it cant work, the small PSU would effectively see the full 300W and 84V(7*12V) across its input and basically blow up.

---

POC Bench experiment to prove very basic concept was 3x9V batteries in parallel as a ~9V/6A supply in series with a PSU to power a ~35V LED.  Was able to set the current limit for the whole circuit via the PSU in this setup but overall voltages/current were also within limits of the supply itself.

---

So please be kind and educate me where my logic is wrong with the above as sure I must be missing something as otherwise this would be a solution or idea that cropping up everything to Current limit batteries without high voltage DC-DC power supplies.

I don't think you can do that, but I may not understand what you want to do exactly.  Maybe draw up a diagram that makes it unmistakably clear.

You might be able to use a power resistor in series with the batteries, but you would end up wasting power, depending on how much you needed to limit the current.

[Berni]

Do NOT combine batteries with lab PSUs. It is really easy to blow stuff up doing that.

If the aim is to simulate a solar panel for testing purposes, just add a power resistor in series with the output of a bench PSU. Wont perfectly simulate a solar panel, but it will give the effect of voltage doping as more current is drawn from it, some point being the max power point. The "ilumination" of the solar panel is mostly the value of that resistor, so you could also use a bank of batteries as your power source and then have a switchable bank of resistors to simulate different amounts of solar illumination.

However just to make a MPPT controller work you don't really need simulate a solar panel at all. Just hook up a voltage source directly to the solar input and the MPPT controller will continue to rise the power draw up to 100% power and stay there. It is the same as illuminating your solar panels using 1000 suns, making the panel able to maintain voltage even if you draw a lot of current.

[Farsider]

Attached a quick diagram to aid understanding.  My understanding without some sort of current limiter the short circuit protection would trip as MPTT would raise current over the example 22A limits.

And yes could use a resistor or other load but that just wasted heat.  In fact already have tested that setup with an array of 60W incandescent light bulbs which works.

Also before anyone  worries this is thought experiment which may make it as far as an isolated lab/bench setup but isn't anything that looking to put anywhere near a true grid connection.   

Just have an old off grid solar Inverter (that if it dies it dies) and various old batteries etc so it more the realms of possibility in a real disaster scenario.

[Terry Bites]

Battery+power supply= fire+expense.

Most adjustable regulators can act as a current limiter.

See attached basic concept and boosted version for high currents (>1A)

An LM317 will give up the will to live if the input goes above 40V. I'd keep to 36V or less.
For higher voltages try LM317HV (60V) or TL783 (125V). The 2N3906 can be any other small pnp. eg BC557
Any Darlington that can handle the full current will work. The TIP142 will go as high as 100V and 10A, though not at the same time of course.
Think about what the transistors wil experience with a shorted load. A thermal trip would not be a bad idea.
For more current, parallel up the boost stage transistors and add ballast resistors as shown.
All devcies including the regualtor need adequate heatsinking or it will all go very badly! https://www.allaboutcircuits.com/tools/heat-sink-calculator/

[Farsider]

Appreciate the tips and possible ICs if wanted to build a power supply but trying to understand the fundamental reason why

Battery+power supply= fire+expense.

what is the risk, run away condition that happens in this case. As if follow the core basic principles of voltage in series and currents in series then it logically works so there must be a reason it doesnt.

As I say this is more of understanding the why it don't work.

Can fully see why if you using a AC lab bench PSU you have isolation to GND to consider, or if you trying to include charging of the batteries as well as discharge them then yes lots of risks of common GND etc.  But in the theoretical example struggling to understand what i'm missing.

Also my low voltage setup seems to concur with the theory. And cant find any odd differently voltages or currents at play.

[Terry Bites]

1. Potentially putting more volts across the psu output terminals than its max or set point and killing the regulation loop.
2. Frying the PSU pass elements with over V or reverse V
3. Frying the PSUs current sense circuit and sense resistor.
4. Dont try and second guess your PSU designer.
5. Dont tempt fate and Mr Murphy.
6. Why risk your PSU on a whim?

The up side to using your psu as a limiter- well, there isnt one.....
 

[Farsider]

So is the fundamental principle around what is zero volts/gnd point in the circuit.

What I mean is there lots of example of isolated PSUs in series to make high voltage supplies. Or split rails by using say 2*12v supplies in series to effectively make a +/- 12v suplly by using the center point.

This is basically the fundamental principle I think I'm failing to understand. In the example the PSU itself is within its normal output limits. I'm assuming that isn't the case and hence it will die but why?

[MrAl]

Attached a quick diagram to aid understanding.  My understanding without some sort of current limiter the short circuit protection would trip as MPTT would raise current over the example 22A limits.

And yes could use a resistor or other load but that just wasted heat.  In fact already have tested that setup with an array of 60W incandescent light bulbs which works.

Also before anyone  worries this is thought experiment which may make it as far as an isolated lab/bench setup but isn't anything that looking to put anywhere near a true grid connection.   

Just have an old off grid solar Inverter (that if it dies it dies) and various old batteries etc so it more the realms of possibility in a real disaster scenario.

Hi,

Not sure if that is a good idea.  The power supply may see a reverse polarity battery which would cause it to blow the internal reverse protection diode which would mean you'd have to replace the diode just to get it to work again.

You can still use a series power resistor just to test because you can limit the current to a lower value.  This assumes you just want to test not to run indefinitely.  The maximum power transfer theorem states that the max power occurs when the generator impedance matches the load impedance, so to test you would need a resistance that matches the load resistance.  That's unless you wanted to test at lower power, and then you would still see a max power point but it would be lower than maximum power that you would get with the right size series resistor.

This is sounding like a bit of a tall order.  It is starting to sound like a switching power converter would be needed in order to lower the available output power.  It would have to have a smooth current cutback so the MPPT doesn't go nuts trying to figure out what the MPP is.  This sounds like either a special switching converter or a converter with an output series resistor, not necessarily large enough to waste a lot of power.

The way you do this in Spice is with a current source and diode, but in the real world that's not going to cut it unless maybe you could string a lot of diodes of the right current rating in series.  We could look into this and other options if you can't find any simpler way to do it.

[Berni]

Attached a quick diagram to aid understanding.  My understanding without some sort of current limiter the short circuit protection would trip as MPTT would raise current over the example 22A limits.

And yes could use a resistor or other load but that just wasted heat.  In fact already have tested that setup with an array of 60W incandescent light bulbs which works.

Also before anyone  worries this is thought experiment which may make it as far as an isolated lab/bench setup but isn't anything that looking to put anywhere near a true grid connection.   

Just have an old off grid solar Inverter (that if it dies it dies) and various old batteries etc so it more the realms of possibility in a real disaster scenario.

If your MPPT controller blows up due to willingly drawing more current than it is capable of.. well then you have a shitty incorrectly designed MPPT controller that belongs in the garbage anyway (and so you should not be trusting your batteries to such a poorly designed device)

The MPPT is in control of the current and the designers of it know how much current it is capable of handling. Heck since MPPTs always tend to have a microcontroller inside it could even be made to reduce the current if it is overheating, letting it safely run at the maximum possible power even if the user installed it in some poorly chosen spot with no air flow.

If it is for backup power, just hook it up directly and let the MPPT control how much power it wants to take. As long as your battery bank is within the MPPTs input voltage operating range it is fine.

Do NOT wire up power supplies as a current limiting like that. You only get current limiting in the exact correct conditions where the load would be drawing roughly the right current anyway. But as soon as the load becomes too low impedance the power source will just force current trough the regulated PSU. At low currents you just get non regulated current, at high currents you blow up the PSU. However since batteries are capable of a LOT of current, if you short the output the batteries will send their full wrath into the PSU. Not only will this blow up the PSU, but it will also involve a lot of smoke, molten metal, some sparks, potentially a fire and a PSU that needs a lot more than just a new diode on the output. Not even joking. This does set power supplies ON FIRE

[MrAl]

Attached a quick diagram to aid understanding.  My understanding without some sort of current limiter the short circuit protection would trip as MPTT would raise current over the example 22A limits.

And yes could use a resistor or other load but that just wasted heat.  In fact already have tested that setup with an array of 60W incandescent light bulbs which works.

Also before anyone  worries this is thought experiment which may make it as far as an isolated lab/bench setup but isn't anything that looking to put anywhere near a true grid connection.   

Just have an old off grid solar Inverter (that if it dies it dies) and various old batteries etc so it more the realms of possibility in a real disaster scenario.

If your MPPT controller blows up due to willingly drawing more current than it is capable of.. well then you have a shitty incorrectly designed MPPT controller that belongs in the garbage anyway (and so you should not be trusting your batteries to such a poorly designed device)

The MPPT is in control of the current and the designers of it know how much current it is capable of handling. Heck since MPPTs always tend to have a microcontroller inside it could even be made to reduce the current if it is overheating, letting it safely run at the maximum possible power even if the user installed it in some poorly chosen spot with no air flow.

If it is for backup power, just hook it up directly and let the MPPT control how much power it wants to take. As long as your battery bank is within the MPPTs input voltage operating range it is fine.

Do NOT wire up power supplies as a current limiting like that. You only get current limiting in the exact correct conditions where the load would be drawing roughly the right current anyway. But as soon as the load becomes too low impedance the power source will just force current trough the regulated PSU. At low currents you just get non regulated current, at high currents you blow up the PSU. However since batteries are capable of a LOT of current, if you short the output the batteries will send their full wrath into the PSU. Not only will this blow up the PSU, but it will also involve a lot of smoke, molten metal, some sparks, potentially a fire and a PSU that needs a lot more than just a new diode on the output. Not even joking. This does set power supplies ON FIRE

That's an interesting take on max power point tracking devices.
I think a lot of them are designed to work with certain solar arrays that have some sort of current limit themselves, because all solar panels will have a maximum current rating unlike a lead acid battery. That could be why they do not incorporate a max current themselves.

Maybe because they use a linear method rather than a switching method.  If they use a linear method they cannot limit the current they would have to shut down or limit it to a very small amount in some cases.  If they use a switching method then I think they could do just about anything.  I never checked into the method my store-bought MPPT device uses.  I know the ones I had designed years ago had current limit, but they were switching type MPPT circuits and were very expensive.  They had to work with giant solar arrays used in major solar projects.