Dumb MPPT question

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I have a couple of small solar panels gathering dust. They have an OC voltage of around 20V. I would like to use them to charge some NiCads, maybe 3 or 4 in series.

I have read about people using PWM and changing the duty cycle up or down, checking the current and voltage and iterating. Here's the dumb bit: Do I measure the instantaneous current and voltage, and havin knowledge of the duty cycle, calculate the mean power? Or is it more complicated (or possibly less complicated) than that. I'm not looking to squeeze the last microjoule out of the solar panel, but would like to use bits and piece I have on hand, e.g. ATTinyxx, FETs etc. I don't have inductors to hand.

Four legs good, two legs bad, three legs stable.

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It's hard to make a switch mode PSU without any coil, and my guess is that 20V pulses is to high (unless it drop a lot , so we need watt of solar compared to bat. size).

 

If you don't care about efficiency go for a linear voltage regulator that give 1.3-1.4 V pr cell.

 

NiCads are more robust that other rechargeable cells but have some memory effect (my first cellphone discharged in the charger before it started charging)     

Last Edited: Sun. May 17, 2020 - 10:03 AM
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Battery charging involves maximizing current, not power.  A properly sized PWM buck converter with current sensing can maximize that into any battery having voltage less than the panels.

 

But if a step-up conversion is needed than a search for maximum power might be useful.

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Lithium Ion excepted; doesn't charging a battery inherently find the right operating point?

 

I have a small panel; rated 13.4V 350mA, there's no electronics, you just connect to a {car} battery and wait for the sun to come out.

It's rubbish though; in the UK it doesn't work it's only small 2 panels each 4" x 6.5". Perhaps it works somewhere tropical.

 

Last Edited: Sun. May 17, 2020 - 10:38 AM
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PWM buck converter 

 

How would you make that without a coil ?

 

 

If you have a slow PWM perhaps the coil from an small relay could be used I guess OP has that. (but it depends of the power 1W 5W 10W ?) 

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N.Winterbottom wrote:
Perhaps it works somewhere tropical.
or more temperate

Simplify Small Solar Systems* with Hysteretic Controller | Analog Devices

[3/4 page, rule of tens]

These relationships were derived for Milpitas, California to give 4 days’ run time on unassisted battery power, with the panel oriented for maximum winter insolation. ...

 

"Dare to be naïve." - Buckminster Fuller

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N.Winterbottom wrote:

I have a small panel; rated 13.4V 350mA, there's no electronics, you just connect to a {car} battery and wait for the sun to come out.

A sloar panel approximates to a current source i believe.

So if the maximum current you can ever get out of the solar panel is less than the maximum safe trickle charge current (assuming the battery type can be safely trickle charged), no electronics needed, you can just connect the panel to the battery.

Same with nicd, you can quite happily connect the panel directly to a pack of cells, as long as the max current out of the panel is small enough. You can connect it directly to a single cell if you want. Just not very efficient. I presume by adding some electronics in between you can produce a much more efficient setup, particularly if charging a small number of nicd from a nominal 20V panel, but my limited knowledge runs out at this point!

 

 

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John_A_Brown wrote:
They have an OC voltage of around 20V
And the short-circuit current at full insolation?

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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If you want to trickle charge, that might barely load down the solar (good)...

 

Since trickle charging never ends, it is a pretty low current, like 20ma, 50ma, about C/20.. so efficiency is not needed, unless that is near the full capacity limit of the cells.

 

Make a constant (low) current trickle Nicd charger out of an adj regulator. 

You can use a resistor or pot to set the charge current (such as 100ma)...note it will be "off" by the operating current running the regulator (rather small, look it up Iquiescent).

The minimum Vdrop is 1.25V plus the regulator dropout, you can use a regular 5V reg, if a 5V min drop (plus regulator dropout) is acceptable. 

A low dropout vreg might have 100mv dropout ....Take a look at the LP2950/LP2951   https://www.onsemi.com/pub/Collateral/LP2950-D.PDF

 

The regulator maintains the voltage (1.25V or 5v) between the resistor pins....then simply  I=V/R  V being 1.25 for an adj or 5V for fixed (or 3.3v)   ...some Vregs use 1.235V or other voltages

 

10 ohms gives 125 ma of constant charging, 50 ohms give a trickle of 25 ma

 

The switcher is better suited if you don't want to waste power.  Here you may want to waste it, if you only want to trickle--it really depends on how much excess power capacity you have in the first place.

There are solar switchmode chips with MPPT built-in.  I used this chip a few years ago, without much hassle.  It was for high power use, but you might find some of the explanations useful...how they did MPPT is described

https://media.digikey.com/PDF/Application%20Notes/ST%20Microelectronics%20Application%20Notes/SPV1020%20AN3392.pdf ...when Chinese panel prices fell, the user went belly up!

 

  

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Mon. May 18, 2020 - 02:20 AM
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John, I make and sell simple solar chargers for SLA and LiFePO4 batteries, if you need help with your project pm me or ask specific questions here.

glad to help.  The one I make is based on a t25 using pwm.

 

jim

 

 

 

 

 

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joeymorin wrote:

John_A_Brown wrote:
They have an OC voltage of around 20V
And the short-circuit current at full insolation?

I haven't tested that yet.

 

Four legs good, two legs bad, three legs stable.

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ki0bk wrote:

John, I make and sell simple solar chargers for SLA and LiFePO4 batteries, if you need help with your project pm me or ask specific questions here.

glad to help.  The one I make is based on a t25 using pwm.

 

jim

 

Thanks.

Four legs good, two legs bad, three legs stable.

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ki0bk wrote:

John, I make and sell simple solar chargers for SLA and LiFePO4 batteries, if you need help with your project pm me or ask specific questions here.

glad to help.  The one I make is based on a t25 using pwm.

 

jim

 

 

https://www.youtube.com/watch?v=...

 

Jim, has your website been taken down? The link does not work.

 

Cheers,

 

Ross

 

Ross McKenzie ValuSoft Melbourne Australia

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I inked an agreement with another ham to build and sell my products now, the new website is www.flinthillsradioinc.com (shameless promotion), I needed to do this in order to free up my time to attend to our aging parents needs.

I'm working on some new and improved products to add to the line.

 

Jim

 

 

 

 

 

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www.flinthillsradioinc.com wrote:
Pardon the dust! We'll update this very soon. Check back later please.

That's a shame.

 

As gchapman's link stated: It's perfectly acceptable to dump all the available current straight into your NiCds or lead-acids. In the UK of course there's little danger of overcharging cells. (I must admit - today has been really sunny though)

 

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Lead Acid as well as other chemistry cells have a recommended max charging rate, be it C/10 or C/1 etc. so it's best to match the panel size, Isc or (Current short circuit) to this rate, not too small or not too large, but just right! 

In reality, it does not need to be exact, just don't expect a 200w (11amp Isc) panel to charge your 7a/h battery, likewise, it will take a long time for a 7w(1/3amp) panel to charge a 100a/h battery!

Its sort of like impedance matching for best performance.

Good luck with your solar project.

 

Jim

 

 

 

 

 

Last Edited: Mon. May 18, 2020 - 04:44 PM
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dak664 wrote:
Battery charging involves maximizing current, not power

But MPPT is about getting the most power (energy) out of the solar cells

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Nickel cells nearly yes, lead-acid no (current-limited CC vs CV)

Nickel cells can charge at rate 0.1C (limited current) below freezing; lead-acid float voltage is by temperature and class (AGM, gel, flooded cell)

Below freezing, a Ford F350 '00 diesel (dual 12V SLA) takes a few minutes to get over the initial charge hump (reduced loading on the alternator after engine start); above freezing, approximately one minute (auxiliary idle control has a 'charge protect' mode that sets the engine speed to approximately double that of idle)

Simplify Small Solar Systems* with Hysteretic Controller | Analog Devices

...

Figure 2 shows a complete shunt charge controller for a 12V lead-acid battery using an LTC2965 100V micropower voltage monitor as the controlling element. 

...

[1A PV panel, power nFET, Schottky diode, LTC2965, NTC, 12V 7.2Ah SLA]

Figure 2. Shunt mode hysteretic regulator. Trip points are temperature compensated from 0°C to 50°C.

...

Low Vce-sat NPN also have low equivalent Rsat.

 

"Dare to be naïve." - Buckminster Fuller

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joeymorin wrote:

John_A_Brown wrote:
They have an OC voltage of around 20V
And the short-circuit current at full insolation?

250mA

Just checked it.

Four legs good, two legs bad, three legs stable.

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Now we only need AH and max charging current for the batt.'s