Step up (boost) MPPT charge controllers?

[yahastu]

I want to charge a 52V 12ah lipo battery directly using 2x 36 cell, 120 watt max solar panels in the most efficient way possible.

My current plan is to wire the two solar panels in series to produce one effective 72 cell, 240 watt panel. The peak output voltage is 34 volts at 14 amps. The charge controller therefore needs to step up the 34v input to 52v output.

My understanding is that MPPT charge controllers are regarded as the most efficient method -- more efficient than shunt or PWM based controllers. However, I understand that usually one is using an MPPT controller to reduce the input voltage to a lower voltage battery...and I'm wondering if MPPT controllers are still the most efficient type of charge controller given that I need to boost the voltage, not reduce it.

According to the discussion here, they seem to be advising against it:
https://www.solarpaneltalk.com/foru...ms/10413-can-mppt-controllers-step-up-voltage

 

[anorlunda]

Most likely, series is the answer. However a better answer is to follow the instructions that come with the MPPT controller. Not all MPPT controllers are alike. The manufacturers usually put their recommendations in the manuals.

 

[yahastu]

Most likely, series is the answer. However a better answer is to follow the instructions that come with the MPPT controller. Not all MPPT controllers are alike. The manufacturers usually put their recommendations in the manuals.

I'm not sure you understood my question. I am asking about which type of charge controller would be most efficient, so I don't have an instruction manual to refer to...in fact the main reason I am asking is because I can only find 1 manufacturer of an MPPT controller that meets my required input/output voltage ranges (MPT-7210A), and it has some known quality issues that cause it to reset frequently...so if I'm not restricted to MPPT controllers, perhaps I can find some higher quality products..

I just found this interesting article:
https://www.renvu.com/Learn/What-does-an-MPPT-charge-controller-do
It basically says that the advantage of MPPT over PWM is that MPPT can increase the current when it decreases the voltage to the battery level, thereby delivering the full wattage of the solar panel...whereas a PWM controller can only regulate output voltage.

However, in my case the solar array would be producing 34 volts at 14 amps, so if you have a PWM controller that regulates the output to 52V then you have a maximum of 52V * 14 amps = 728 watts, which is greater than the 240 watt generation capacity. Obviously that's not possible...I'm not sure about the limitations of PWM controllers, but I'm guessing this means that either the PWM controller is incapable of stepping up the voltage to 52 V (meaning I can't even use it), or it means that the current will be naturally reduced by the available power of the panel...which would mean that it delivers full power output, and is a cheaper and better choice than using an MPPT controller. I'm not sure which is the case..

 

[essenmein]

If the MPPT algo is implemented the same between both a boost and buck type DC-DC there should be no difference in efficiency (assuming neither are running into voltage limits).

Then DC-DCs are power converters, that is if you have an input of 34V and 14A, and you boost it to 52V then the current you have available at 52V is (34*14)/52=9.1A (ignoring DC-DC efficiency for simplicity), ie output power cannot be higher than input power!

Then a side note, PWM is something that is used in most DC-DC converters, ie PWM description alone gives no indication on what that PWM is doing, it could be boosting, it could be a buck, it could be forward or sinusoidal inverter etc etc. When you refer to "PWM solar controllers" they are just choppers which use the battery as a sort of filter, if that chopper is running at 50% duty then the average current is also reduced by half, full current only flows if PWM duty is 100%.

 

[fredtserl42]

I want to charge a 52V 12ah lipo battery directly using 2x 36 cell, 120 watt max solar panels in the most efficient way possible.
...

Did you ever solve this? I am tempted to use 2 12v panels and use 2 "step up"'s to then put the output 48v's in series to get 96v. This would then go into the MPPT, my output needs to charge a 72v pack (86v bulk charge)

 

[berkeman]

Did you ever solve this? I am tempted to use 2 12v panels and use 2 "step up"'s to then put the output 48v's in series to get 96v. This would then go into the MPPT, my output needs to charge a 72v pack (86v bulk charge)

Welcome to PF.

The MPPT module needs direct access to the solar panels, so it can operate them at their maximum power point (output voltage and current). You can't put a boost circuit between the panels and the MPPT module.

Which MPPT module are you planning on using?

 

[DaveE]

Did you ever solve this? I am tempted to use 2 12v panels and use 2 "step up"'s to then put the output 48v's in series to get 96v. This would then go into the MPPT, my output needs to charge a 72v pack (86v bulk charge)

First, I have no idea what you can buy to do this. I'm out of touch with the marketplace. But I do know the theoretical solution.

What you want is a DC-DC power supply that will take a 24V nominal input and output a much higher voltage, something like 70-90V. No, it doesn't have to be done in factors of two, or with multiple "step-ups". This can be one DC-DC converter. That converter can also do the MPPT of the PV input by smart control of the output load. For example, I would control it as a current source at the output at whatever voltage your battery generates as it's charged, it doesn't have to be a fixed value. Then I would control the battery charging current to maximize the power flow (accommodating the other limits needed for good battery charging). This would essentially be the same as maximization of the charging current.

Stringing together a bunch of building blocks that don't really do what you want will be very kludgy, inefficient, and confusing. MPPT only makes sense if you already have a pretty efficient system to control, otherwise you will be better off focusing on not wasting the power you get whether it's "maximum" or not. PVs are still pretty efficient when operated near the MPP.

So, one well designed system can do the whole job; MPPT and battery charger. However, it's not a simple DIY thing if you don't have SMPS design experience. Still, maybe you can buy it, IDK.