Custom built solar charger

[seandepagnier]

Originally Posted by catakate

Sorry mate but you are totally wrong. THE ONLY WAY to determine the state of charge is measuring the density of the electrolyth, not possible with today's sealed bbatteries, what I'm asking for is most common recommendation by manufacturers. Please let me know what reads the charging specs of batteries you have used.

Yes and state of charge is irrelevant. You only want to maximize the number of amps flowing into the battery.

[daddle]

Originally Posted by boat_alexandra

Yes and state of charge is irrelevant. You only want to maximize the number of amps flowing into the battery.

I agree. In my opinion, no device is more stupid that a Smart Charger. It is frustrating to see the idiot device, which cannot really know the charge state, cut back the current when I know the battery is a long long way from charged. What a piece of marketing BS. A simple switch or knob would be fine to vary the maximum voltage between "Float" and "Get 'er Done Now Because Clouds are Coming".

[noelex 77]

Originally Posted by catakate

Sorry mate but you are totally wrong. THE ONLY WAY to determine the state of charge is measuring the density of the electrolyth, not possible with today's sealed bbatteries, what I'm asking for is most common recommendation by manufacturers. Please let me know what reads the charging specs of batteries you have used.

If you can get a solar controler to measure the electrolyte density you will have a unique product
Most good solar regulators can be manually forced on to an absorption or float cycle. The trick is to develop an automatic system that works well.

I have used batteries with recommendations for absorption voltages from 14.7 to 14.2v
Float voltages vary less with different battery types but sometimes like to tweak these from the manufacturers recommendations ( for example lowering the float voltage during periods when the boat is not used and raising in winter when its desirable to push 100% into the batteries when possible)
A range of 13.9 to 13.5 would cover all requirements.

Absorption time is very dependent on the system a range of 0.5-3 hours would be helpful

Don't forget the provision for an equalisation cycle needed by some batteries.

[sailinglegend]

Originally Posted by catakate

Sorry mate but you are totally wrong. THE ONLY WAY to determine the state of charge is measuring the density of the electrolyth, not possible with today's sealed bbatteries......

Sorry catakate YOU are wrong.

Measuring SG of a battery is a difficult and time consuming procedure. SG readings are often used to measure the state of charge of an open flooded battery, but the SG falls very slightly as it ages. So what is the SG at a 100% SOC, and how accurately can it be measured? If the cells in the battery are not equal then an average of the 6 cells has to be made, or the battery equalized – or both; temperature compensation also has to be applied. The SG must only be taken when the cells have been topped up with distilled water and after the batteries have been charged, this stirs up the electrolyte to give a true SG reading for each cell. The major problem is the SG reading must only be taken when the charged battery has been allowed to rest with no charge or discharge for several hours. This is not very practical on a typical cruising boat so the potential for errors can give poor results.


One way to accurately measure the state of charge is when the current going into the battery at the absorption voltage is less than 1% of the known current battery capacity. With any charger you have to "force" it back into boost mode to get this absorption voltage. The easiest and in my opinion the best way is with a SmatGuage battery monitor that intelligently measures and interprets the battery voltage 1000 times a second. I also have a BEP Battery Monitor which gets more and more inaccurate as the batteries age.

[catakate]

Originally Posted by boat_alexandra

For optimal power transfer you would need to adjust the duty cycle to compensate so you need to be able to react fairly quick.
I doubt most commercial ones are designed for boats in this way.

A digital device to do this in real time would cost more then many boats

Originally Posted by sailinglegend

This is not very practical on a typical cruising boat so the potential for errors can give poor results.

Thanks for your data with the mentioning that this is published by every manufacturer based on determined state of charged. I'm only asking what those recommmendations are in average.

[Lake-Effect]

Thanks catakate for posting the schematic. This is an interesting thread.

Assuming there's some loads still present (instruments, radios, etc) when the solar panels are providing charge, the charger circuit should ideally be aware of this, so that the current from the charge controller equals the sum of the desired charge current (depending on battery state) and the load current.

[seandepagnier]

Originally Posted by catakate

A digital device to do this in real time would cost more then many boats

I was using a microprocessor that cost $4 for 1, or under $2 in quantity. It was one of the PWM avr's. Now there are much better cheaper ones since this was 4 years ago. It had a PLL built in to give it 64mhz for pwm.. so 0.1% duty cycle increments at 64khz switching frequency. Then you need two mosfets for sync rectification (you posted a schematic which used a diode in place of one of the mosfets which is less efficient) and an inductor and filter caps on input and output.. although they can be ceramics these days and very cheap. I also used gate driver chips for the mosfets since the microprocessor's io pins had difficulty. Should be possible to get 95% efficient here.. critical if you want to actually gain from mppt. Faster switching frequency allows for smaller cheaper parts, but uses more power to drive the mosfets. Also, higher voltage allows for more efficiency in many ways, but mosfets will have higher on resistance, so it's not as much an improvement as wiring.

It probably costs under $10 for all the parts, pcb and to fabricate this.. in quantities of 1000 or more, and could handle up to 20 amps. For more amps you need bigger power parts (inductor caps and mosfets) but the microprocessor would be the same. It would also be nice to have a switching supply to drop the battery voltage down to 5v for the logic.. I used a linear regulator.

Then there is the issue of the microprocessor and switching action consuming more power than it saves in high-cloud states. To really do things well you need to adjust switching frequency as well as duty cycle to get that last 2-3% overall.. and switch to a simple dc mode (one mosfet is always on the other off) in the cases where that is best.. this is pretty simple and can all be done in software.

I implemented fixed-point math support directly into the avr-gcc backend specifically to use for the microprocessor logic for this application.. I got this bit done, but then got distracted and never finished perfecting the mppt. Then I found I have excessive solar power as it is so it doesn't really matter. In some ways getting an extra 20 watts of solar panels is simpler and effectively gives the same resulting power while adding redundancy.. and hardware that isn't going to break if it gets wet.

If you really want a good regulator you will actually have separate inputs for each solar panel. This way the controller can run each one at its peak power. It may be best just to use separate controllers in this case, but it would be good to syncronize them somehow so they don't confuse each other as the battery changes states. This is really important if they are at different angles relative to the sun, or if the panels are different voltages.

[catakate]

Originally Posted by Lake-Effect

Assuming there's some loads still present (instruments, radios, etc) when the solar panels are providing charge, the charger circuit should ideally be aware of this, so that the current from the charge controller equals the sum of the desired charge current (depending on battery state) and the load current.

You're welcome but I'm not sure what you mean. The charger should limit the current available for other loads and preserve enough current for charging the battery? If at somme point you need transmit power from your radio (25W) and the fridge kicks in (75W) that would exceed the output of an 100W panel. It should be capable to draw current from the battery when needed.

[catakate]

boat_alexandra thanks for the interesting reading. I know of a few tweaks doable and any feed-back is much appreciated. I'll pass the info regarding the controller to those more knowledgeable then me

[Lake-Effect]

Originally Posted by catakate

...I'm not sure what you mean. The charger should limit the current available for other loads and preserve enough current for charging the battery?

No, I only meant to say that when there's a battery load while solar-charging, the charge controller must supply the load current PLUS the current required for charging... or else you don't have optimal charging.

Of course if you have 2 battery banks and can draw on just one bank while charging the other, this requirement is avoided.

[hellosailor]

"THE ONLY WAY to determine the state of charge is measuring the density of the electrolyth"
In the real world, it is also impossible to measure the density of the electrolyte. The real world facts are that the density of the electrolyte will vary depending on the location of the measurement (above or below the plates, inside the plates) and how ell the electrolyte has circulated and blended within the battery. In practice, you will need to circulate the electrolyte, blend it, and let it stand down so any bubbles can come out, and only then can you get the real picture of the density. Take the big turkey baster, swirl it around, you've got a good start but it still needs to stand down and now of course, you've got that acid to mess with, some of it always gets on something.

So as long as you're going to provide for all that, sure, electrolyte density is great. In the real world? That's all part of why these big dumb electronics companies find other ways to measure what is happening inside a battery. To do it instantaneously, and accurately enough to accomplish their needs.

Similarly, you speak of using the MPPT controller as part of a three-stage charging process. That's dead, obsolete. I've watched a brand-name MPPT charger, which uses a microprocessor and look-up table and is programmed for the specific battery type and capacity, and it actually changes voltage every few minutes so that it provides maximum amperage (from the available power) and it doesn't go to a set voltage during the bulk charge at all. It provides a certain lead, perhaps i.e. 1/2 volt or one volt higher than whatever the battery is at, and as the battery voltage comes up, it changes and stays slightly above the battery voltage, putting all the power into amperage--not an arbitrary voltage.

These big dumb companies, the ones that make the specialized large scale integrated circuits that are doing real MPPT controlling, and the smaller ones building devices around them, have PHD level EEs who have spent decades improving charging schemes by working full time on them. They won't discuss a lot of what they do because they consider it proprietary.

But the bottom line? Is that all of the "common" knowledge about conventional charging schemes has been obsoleted, and trying to build your own systems may be effective, especially cost-effective, but it is unlikely to compete with what modern products are doing on the market today.

As Robert Heinlein said "Any sufficiently advanced technology will be indistinguishable from magic."

MPPT charging systems today? There's a lot more happening than what meets the eye. And there are eval kits from the maker chip makers, which will give you a good product quickly without having to reinvent the wheel, if a commercial product doesn't suit you. (Again, assuming you can access them.)

[Lake-Effect]

Originally Posted by hellosailor

I've watched a brand-name MPPT charger, which uses a microprocessor and look-up table and is programmed for the specific battery type and capacity, and it actually changes voltage every few minutes so that it provides maximum amperage (from the available power) and it doesn't go to a set voltage during the bulk charge at all. It provides a certain lead, perhaps i.e. 1/2 volt or one volt higher than whatever the battery is at, and as the battery voltage comes up, it changes and stays slightly above the battery voltage, putting all the power into amperage--not an arbitrary voltage.

Yes. Good shore-power chargers do that too. I don't think anyone would disagree with this approach for the bulk charge.

But the bottom line? Is that all of the "common" knowledge about conventional charging schemes has been obsoleted, and trying to build your own systems may be effective, especially cost-effective, but it is unlikely to compete with what modern products are doing on the market today.

I dunno about the common knowledge being 'obsoleted'. Battery manufacturers haven't radically changed their charging recommendations. It's true that charger manufacturers, including solar charge controllers, have gotten smarter about achieving the optimum charge process.

Like most technology, battery chargers and charge controllers have reached a level of complexity that is hard to duplicate as a DIY project. But not impossible, if you're motivated enough. (great learning exercise, would make a good project for an EE student)

I confess I don't know all the manufacturers in the charger/MPPT field.To all you solar-charging cruisers - is there a single gold-standard MPPT that most everyone endorses and would use if cost was no object?

[hellosailor]

Lake, i don't think there's a gold standard although there are a couple of brand names that seem respected. There are also, IIRC, three separate ways to test and set the MPPT value being used so there are also different logics being applied to determine how to set the MPPT, how often, etc. and as usual each has different pros and cons. I don't think any one has shaken out as "best" overall yet.

And then of course, we're on the cusp of lithium batteries and doing everything differently for them. Including updating the fire extinguisher types.<G>

[Paul Elliott]

Originally Posted by catakate

And here's the block schema

Can you discuss the voltage feedback (regulation or clamp) portion of this schematic (13.3V Zener, BD139 transistor, 1N4007 diode, BC547 transistor)? It looks like it kicks in at about 14.5V, but the exact voltage relies on base-emitter voltage and diode drop matching, and will be subject to temperature variations. Is this some sort of overvoltage protection? If so, perhaps the absolute threshold voltage isn't critical. If it is part of the charging regulation I would be concerned about the setpoint stability over temperature and component variation.

[goboatingnow]

I don't quite understand how the circuit provided can be regarded as MPPT anything. It does no actual tracking of the Vmp. Its basically a regulator that keeps the input voltage fixed. That's fine but its in no way mppt or pseudo mppt.

Then we have this bit from hello sailor

These big dumb companies, the ones that make the specialized large scale integrated circuits that are doing real MPPT controlling, and the smaller ones building devices around them, have PHD level EEs who have spent decades improving charging schemes by working full time on them. They won't discuss a lot of what they do because they consider it proprietary.

But the bottom line? Is that all of the "common" knowledge about conventional charging schemes has been obsoleted, and trying to build your own systems may be effective, especially cost-effective, but it is unlikely to compete with what modern products are doing on the market today.

As Robert Heinlein said "Any sufficiently advanced technology will be indistinguishable from magic."

MPPT charging systems today? There's a lot more happening than what meets the eye. And there are eval kits from the maker chip makers, which will give you a good product quickly without having to reinvent the wheel, if a commercial product doesn't suit you. (Again, assuming you can access them.)

Which is quite frankly nonsense. Firstly most mppt techniques are commonly available in research papers ,and trade tech like EDN/ieee papers over the years.

To suggest that " conventional " charging schemes have been obsoleted is nonsense. The LA battery is the same and is charged the same as it always was, IUI or IUU

Mppt has absolutly nothing to do with charging batteries.

Most EES don't discuss proprietary implementations because often companies don't want you to see how simple they implement an idea.

Mppt, LA charging , very little is not available , techniques , circuit implementation, 1st and 2nd order mppt algorithms are all out there to be read about, there's no mystery.

As Robert Heinlein said "Any sufficiently advanced technology will be indistinguishable from magic

That quote has its limits , it only applies to those that don't bother understanding what they are looking at.

Commercial mppt regulator /chargers are not particularly complex devices. There is no mystery. ( well nothing a scope wouldn't reveal )
Dave