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Advice Needed: MPPT design guidelines, reference design

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rgbphil

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Hi,

I've recently been asked to help out in an artwork installation using solar powered LEDs.
We want to have a set of self enclosed (no interwiring) units consisting of a battery, solar cell or array and a single 350ma (1W) LED to operate for 6-8 hours at full brightness per night and a contingency for 2-3 nights capacity on rainy days.

I'm capable of specifying the battery size, cell size and MPPT controller working out the maH needed. However I'd like to design a MPPT to reduce cost as off the shelf solutions meeting the specs are a little too capable and expensive.

Maintenance and battery replacement are to be as low as possible....I'm looking at SLA or gel batteries. If anyone has other advice....please advise.

From what I gather a MPPT will track the PV voltage and current and the battery voltage and current, then using a switch mode DC-DC converter adjust the battery charge voltage to maximise the charge current. In essence its a variable DC-DC converter. As we need to roll in a fixed ON time, I'm expecting to use a small micro with a fast PWM to the DC-DC converter.

Can anyone provide a link or advice on designing MPPT controllers. Perhaps a reference design or theory link? Any specifically designed chips for this application?

Thanks
Phil
 
solar-guppy.com/forum/download/mppt_thesis_Hannes_knopf.pdf

or just Google 'HANNES KNOPF'
 
Last edited:
Thanks for the link, it's a good reference.

Forgive me if I've only skimmed the material, 177 pages is quite a lot. I'm open to corrections if I've got the assumptions below wrong. I'm less a build from first principals person and more a select from ready made solutions and bolt them together with an eye on the first principals.

Simplifying...You have a buck/boost DC-DC converter controlled by the PWM from a micro that monitors the V and I of the cell array. The micro uses an algorithm to adjust this PWM, looking at the V/I to maximise the power output of the panel, whilst keeping the output voltage to the battery constant.

I had originally thought of having the micro looking at the charge current to the battery, and the algorithm seeking back and forth to maximise this charge current (with some overvoltage/current limits set), varying the battery voltage as needed. Is this equivalent? Or will it end in an unstable system?

Phil
 
Hi,

I've recently been asked to help out in an artwork installation using solar powered LEDs.
We want to have a set of self enclosed (no interwiring) units consisting of a battery, solar cell or array and a single 350ma (1W) LED to operate for 6-8 hours at full brightness per night and a contingency for 2-3 nights capacity on rainy days.

I'm capable of specifying the battery size, cell size and MPPT controller working out the maH needed. However I'd like to design a MPPT to reduce cost as off the shelf solutions meeting the specs are a little too capable and expensive.

Maintenance and battery replacement are to be as low as possible....I'm looking at SLA or gel batteries. If anyone has other advice....please advise.

From what I gather a MPPT will track the PV voltage and current and the battery voltage and current, then using a switch mode DC-DC converter adjust the battery charge voltage to maximise the charge current. In essence its a variable DC-DC converter. As we need to roll in a fixed ON time, I'm expecting to use a small micro with a fast PWM to the DC-DC converter.

Can anyone provide a link or advice on designing MPPT controllers. Perhaps a reference design or theory link? Any specifically designed chips for this application?

Thanks
Phil
first off ... forget the SLA and probably the Gel as well. They are notoriously short lived.

6 hours x 3 nights x 0.35 amps = 6AHr but 3-4 is probably fine.

if you are serious about low maintenance use something like this: Custom LFP 26650 3.2v 6600mAh Module Linked by Tabs (CU-MM133 PID# 5284) (1.98)

a 6V 1A solar panel should be good enough to fully charge the system on a bad day.

you would then need a buck boost LED driver.

or this Custom LFP 26650 6.4v 3300mAh w/o PCB (CU-MM145 PID# 5372) (1.98) with a 12V 0.5A panel and a normal buck driver.

on the MPPT end you do not "adjust" a battery voltage. simply monitor the charge current and adjust the drive PWM for max current until the battery requirements dictate that you supply less.
 
Hi Ubergeek63,
Thanks for the link on the LiFePO4 battery. I initially rejected Li batteries due to their limited cycle life....but this chemistry would last 2-3 years, which is fine for this app.

I thought that gel or SLA batteries were the best choice for solar because of their long life. Can you elaborate on this please? I'm happy to be wrong....it would make things cheaper.

Thanks for the confirmation on monitoring things at the battery end of the MPPT. I have seen several complex MPPT designs with all sorts of multiple input on the solar end which I wanted to avoid.

Phil
 
Maximizing battery charge current is an acceptable method to implement MPPT controller. Looking at PV current gives more direct info for knowing what to do then looking at battery charge current. It's not a bad method.

LiFePO4 are good but intolerant of overcharging. They should have every cell voltage monitored and a way to correct for divergence of state of charge on individual cells that occurs over multiple cycles.
 
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