What are the constraints of MPPT boost controllers (golf cart)

[Porkpie]

Hi folks, I am an intimidated newbie with electronics and solar.

I have bought an old 36V golf cart with new lead acid batteries that I would like to put a solar roof on.
Naturally I am trying to do this on the cheap, with discount panels and a low cost controller.

What I am seeing for panels seem to top out with a VMP of 37V or maybe 41.xV.

Is this enough for a regular PWM or MPPT controller, or do I need a boost controller?

If I need a boost, do they work with slight excesses of volts, such as these panels would produce, or do they need lower volts, like a 24V panel?

How do you size a controller for the batteries?
These panels are in the 4-450W range, and they all seem to have an Imp around 10-11A, which makes me think a 10 amp controller is too small.

This is a shame, as 10A MPPT boost controllers are super affordable. Am I thinking about the controller rating correctly?

I have been considering a slightly smaller panel, like 330-350, because that would drop the amperage down to 9.2A. But if I am thinking about the controller amperage wrong then there is no point in that.

Your thoughts are greatly appreciated,

Ruben.

 

[sunshine_eggo]

Hi folks, I am an intimidated newbie with electronics and solar.

I have bought an old 36V golf cart with new lead acid batteries that I would like to put a solar roof on.
Naturally I am trying to do this on the cheap, with discount panels and a low cost controller.

What I am seeing for panels seem to top out with a VMP of 37V or maybe 41.xV.

Is this enough for a regular PWM or MPPT controller, or do I need a boost controller?

No. 36V lead acid needs to be charged at about 43.2V.+

If I need a boost, do they work with slight excesses of volts, such as these panels would produce, or do they need lower volts, like a 24V panel?

There are very few boost controller options, and they tend to be lower power - 10-20A max

Boost MPPTs

It comes up from time to time if there's a way to use lower voltage PV to charge higher voltage batteries: Here are a few options: https://www.amazon.com/SUNYIMA-Controller-Electric-Charging-Regulator/dp/B0B7WWY5S3...

diysolarforum.com

How do you size a controller for the batteries?
These panels are in the 4-450W range, and they all seem to have an Imp around 10-11A, which makes me think a 10 amp controller is too small.

Array power / battery voltage:

450W / 36V = 12.5A at low state of charge.
450W / 43.2V = 10.4A at peak charge voltage.

Of course, neither condition would allow you to charge due to low panel voltage.

If you want to charge based on a desired current:

36V * charge current = minimum array power.

The MPPT selected will need to be able to output the desired current.

This is a shame, as 10A MPPT boost controllers are super affordable. Am I thinking about the controller rating correctly?

Controllers are most commonly rated by OUTPUT current. They may have additional limitations on the input.

I have been considering a slightly smaller panel, like 330-350, because that would drop the amperage down to 9.2A. But if I am thinking about the controller amperage wrong then there is no point in that.

Again, 10-11A vs. 9.2A on the input isn't necessarily relevant relative to the MPPT output.

IMHO, get a Victron 100/50 MPPT.

Get 60 cell panels with a Vmp around 30V.

Place two in series to form a string and use as many strings as you need to get the desired charge current.

You don't want to charge lead acid too fast. You typically want to charge at 10% of capacity rating. If 150Ah batteries, you want to charge at 15A, though higher may be allowable - depending on battery manufacturer. Trojans allow up to 13% on their FLA and some GEL/AGM batteries can take 20-30%.

 

[Porkpie]

Sunshine_eggo, thank you very much for that.

Thanks for the links to your previous post about boost controllers. It seems I am in a bit of a weird vintage spot, in that there are affordable 20A boosts for 24V and for 48V, but not many for 36V.

There are very few boost controller options, and they tend to be lower power - 10-20A max

Array power / battery voltage:

450W / 36V = 12.5A at low state of charge.
450W / 43.2V = 10.4A at peak charge voltage.

Of course, neither condition would allow you to charge due to low panel voltage.

This part confuses me, for two reasons:

450W / 36V = 12.5A at low state of charge.
450W / 43.2V = 10.4A at peak charge voltage.

These look like they would fit within a 20A controller.

Where does the low panel voltage come from? Just from clouds, and horizontal panel mounting angle?

Get 60 cell panels with a Vmp around 30V.

Place two in series to form a string and use as many strings as you need to get the desired charge current.

I googled 60 cell panels, and it sounds like these are the sort of large panels I am seeing available, in the 400-450W range. Just one panel will overhang the golf cart, so two won't fit.

You don't want to charge lead acid too fast. You typically want to charge at 10% of capacity rating. If 150Ah batteries, you want to charge at 15A, though higher may be allowable - depending on battery manufacturer. Trojans allow up to 13% on their FLA and some GEL/AGM batteries can take 20-30%.

This is a farm cart, so there are not great distances or many heavy loads involved. More just carrying tools to the next task location. So, the constant trickle charge will be a big help, hassle-wise.

More googling...

This controller says it will handle 15A Solar Charge Controller, Boost MPPT 24‑85V
Maybe I could find this one at less than ten units per order GCSOAR 20A 12V 24V 36V 48V Buck-Boost MPPT

So....

If the physical size limits the panel options so that I do require a boost controller, does the voltage still have to be lower than the battery pack, in order to boost UP? That first 15A controller makes it sound like no, but I don't know how many things I am failing to understand here.

Thanks again,

Ruben

 

[sunshine_eggo]

This part confuses me, for two reasons:

These look like they would fit within a 20A controller.

Where does the low panel voltage come from? Just from clouds, and horizontal panel mounting angle?

That's the battery voltage. MPPT are rated based on OUTPUT. They may have input limitations as well.

If the physical size limits the panel options so that I do require a boost controller, does the voltage still have to be lower than the battery pack, in order to boost UP? That first 15A controller makes it sound like no, but I don't know how many things I am failing to understand here.

I would expect it doesn't matter, but you should confirm whatever unit you select behaves as desired.