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Old 15-03-2013, 01:50   #1
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Custom built solar charger

Between brand name expensive chargers and cheap chinese crap, I have a third option: custom build.

To give a little background here: back when your "toys" were not available beyond the wall in Berlin, we had to build at home many things, from TV antennas, to electronic car ignition, to ham radios.. This breed of talents is still going strong with passionate offsprings.

I found a team on a local electronics forum that put together a "pseudo MPPT" controller designed for 36 cells panels that work around 17 V. After two years in use they are very satisfied with the results. For those of you
electronically minded I'll attach the block schema.

"Pseudo" means is not a true MPPT as it would be pretty much useless, as many knowledgeable users also said here. What it does, analogically and in real-time, is to monitor the performance of the panel and adjust the current drawn, keeping the panel at optimum voltage thus giving the best possible output with over 90% efficiency. To do this digitally would be way too expensive and not feasible.

They use this to feed a normal automotive battery charger, which is working great for home usage, but not so good for a boat. Therefor I tried to resurrect end continue the project, with a very positive response.

So I appeal to your experience in what is the best Lead-Acid deep-cycle charging profile. What I know this far and please correct me if I'm wrong, is:
1st stage - boost charge - 14.4V adjusted with temperature, 3mV/ Celsius degree over 25 degrees?
2nd stage - absorption carge - 14.7V limiting the current at 1/10C
3rd stage - absorption charge - 13.6V to keep the battery from discharging

Thanks in advance for all your input!
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Old 15-03-2013, 02:23   #2
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Re: Custom built solar charger

I love electronic projects its great to build something, but it is getting harder to build something better and cheaper than the commercial products.

I few general comments:
I am not sure way you are describing it as "pseudo" MPPT. There are lots of pseudo MPPT regulators that just measure Voc then drop down a set amount and assume this is the Vmp. It sounds like your regulator is tracking Vmp (as it should do).
90% efficiency is not very good. The 10% loss will wipe out all the gains from MPPT, so I hope "over 90%", is well over most of the time.

The charge algorithm you have proposed looks reasonable, but the voltages only suit a flooded lead acid battery. If feasible it is much better to make them adjustable (preferably infinitely)
Normally the 1st and 2nd stage are the same voltage.
Limiting the second stage urgent to c/10 is a complication that is generally not needed on a boat system.
The second stage needs some means of terminating and dropping back to the third stage voltage. Normally this is simple timer ( but only counting the battery is at the second voltage) this is best to be an adjustable time. Around 2 hours is most commonly needed. termination by battery return is much more precise, but requires an extra shunt and few regulators do this.
The third stage is normally called the "float voltage" or "float stage"
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Old 15-03-2013, 02:48   #3
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Re: Custom built solar charger

Yeah, third stage was a typo...can't edit any more

The calculated efficiency is around 94%, from what I understand could be a little higher with a well designed circuit board. It's also my understanding that any electronic component consumes some current, over 90% is an acceptable standard.

There is practically no MPPT gain is this specific case, the 17V is too low for a true MPPT to harvest extra current. This has been explained why and how in many threads, will not go there in this one.

The charging will be digitally controlled by a micro-controller, availability/price plus programming effort will determine if algorithms for other chemistry/technology will be implemented. As much as I would like, I can't afford LiFePO4 batteries in the near future, that can be programmed when needed.
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Old 15-03-2013, 02:55   #4
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Re: Custom built solar charger

And here's the block schema
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Old 15-03-2013, 03:23   #5
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Re: Custom built solar charger

Quote:
Originally Posted by noelex 77 View Post
Limiting the second stage urgent to c/10 is a complication that is generally not needed on a boat system.
I'm not an expert but if this a requirement to properly charge a battery, this will be the same regardless of the place of installation

Quote:
Originally Posted by noelex 77 View Post
The second stage needs some means of terminating and dropping back to the third stage voltage. Normally this is simple timer ( but only counting the battery is at the second voltage) this is best to be an adjustable time. Around 2 hours is most commonly needed.
You mean this can be done by monitoring the current drawn and once a threshold is reached, just start a timer for the second stage?

Quote:
Originally Posted by noelex 77 View Post
termination by battery return is much more precise, but requires an extra shunt and few regulators do this.
Can you please explain the characteristics of this battery return?
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Old 15-03-2013, 04:35   #6
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Re: Custom built solar charger

Quote:
Originally Posted by catakate View Post
I'm not an expert but if this a requirement to properly charge a battery, this will be the same regardless of the place of installation
Yes batteries don't change when they are fitted to yacht , but the circumstances the battery is in means its not worth limiting the charge current to C/10 because:

1.This is very unlikely to reached by just solar anyway. For example panels that are capable of a genuine 40A are unlikely to teamed with a battery capacity less than 400AHrs
2. The times when it could be a problem is when the alternator is running as well, so you need a sophisticated system that also monitors the other charge sources.
3.c/10 is a bit conservative for some battery types.


Quote:
Originally Posted by catakate View Post
You mean this can be done by monitoring the current drawn and once a threshold is reached, just start a timer for the second stage?
The timer runs when the absorption voltage is reached. It stops when the voltage drops below this level ( say due to cloud or a new load). So only the total time at the absorption voltage is measured. Not just the time from when the absorption voltage was first reached.
When this reaches the absorption time (say 2hours) the charging is dropped back to float.
Most controllers work in this way.

Quote:
Originally Posted by catakate View Post
Can you please explain the characteristics of this battery return?
This is a better way than using a set absorption time. When the current in to the battery drops below a level ( usually 1-2%, so 4-8A for a 400AHr battery) and the voltage is at the the absorption voltage, the controller recognises that the battery is near fully charged and drops back to float
It means measuring the current into the battery. The current out of the controller is not the same as typically there will be some loads on the system.
It needs a shunt near the battery, with communication back to the controller.

Only the most sophisticated controlers will do this.( Some will use the controllers output current, but this is of little use). The %, voltage, (and often a short time delay) are ideally all adjustable.
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Old 15-03-2013, 04:47   #7
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Re: Custom built solar charger

Quote:
Originally Posted by catakate View Post
There is practically no MPPT gain is this specific case, the 17V is too low for a true MPPT to harvest extra current. This has been explained why and how in many threads, will not go there in this one.
.
Why bother,at all, with MPPT and voltage conversion then?

A simple regulator without MPPT is much easier and cheaper to build.
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Old 15-03-2013, 05:24   #8
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Re: Custom built solar charger

Quote:
Originally Posted by noelex 77 View Post
c/10 is a bit conservative for some battery types.
I see, there was a question mark in my sentence

Quote:
Originally Posted by noelex 77 View Post
When the current in to the battery drops below a level ( usually 1-2%, so 4-8A for a 400AHr battery) and the voltage is at the the absorption voltage, the controller recognises that the battery is near fully charged and drops back to float
This would also mean no absorption charge and the battery will be only ~80% charged?

Quote:
Originally Posted by noelex 77 View Post
Only the most sophisticated controlers will do this.
Might be, but I have a $70 automotive charger that does just that. Can't be that expensive but I would really like my batteries as high over 80% as possible.

Quote:
Originally Posted by noelex 77 View Post
Why bother,at all, with MPPT and voltage conversion then?
A simple regulator without MPPT is much easier and cheaper to build.
A simple regulator will not keep the panel at its peak, loosing power. If you let a battery draw as much as it can out of the panel, the voltage will drop, thus the output will drop even further, is that correct? Think the schema above to be expensive?
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Old 15-03-2013, 06:59   #9
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Re: Custom built solar charger

Quote:
Originally Posted by catakate View Post

This would also mean no absorption charge and the battery will be only ~80% charged?
No there will be a normal absorption charge, but the length of charge (the absorption time) will vary depending on the conditions.
My system is set up this way and the absorption time varies from day to day from 15 minutes to a couple of hours.
Quote:
Originally Posted by catakate View Post
Might be, but I have a $70 automotive charger that does just that. Can't be that expensive but I would really like my batteries as high over 80% as possible.
It is easy for a battery charger to do. It is the only charging source and there are no discharge currents when its being used.
Some solar regulators do this (with a backup timer) but its of little use because the regulator only looks at its output. The actual current into the batteries will be a combination of the other charge and discharge sources. The current into the battery (which is what is important) bears little relationship o the output of the regulator.
A more complicated system is needed measuring the total input into the battery. If you can incorporate this into your project its a worthwhile improvement especially for gel and AGM batteries.

Quote:
Originally Posted by catakate View Post
A simple regulator will not keep the panel at its peak, loosing power. If you let a battery draw as much as it can out of the panel, the voltage will drop, thus the output will drop even further, is that correct? Think the schema above to be expensive?
Sorry you have lost me there.
The regulation function of a solar controller is different from the MPPT process. It's worth having an accurate regulation algorithm, but this can be incorporated into a MPPT, or non MPPT regulator/controller.

The considerable added complication of MPPT extracts (if its well designed) a bit more power out of the panels. To do this it needs to have efficient tracking and voltage conversion, otherwise it will extract less power than a non MPPT regulator.

Unfortunately many of the cheaper commercial MPPT regulators do exactly this. Even the best MPPT regulators only extract 5-15% more power on average (than a non MPPT regulator) on a typical yacht installation.

Given the limited space for solar panels on a yacht some people feel this small gain justifies the expense and complication.
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Old 15-03-2013, 07:57   #10
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Re: Custom built solar charger

Quote:
Originally Posted by catakate View Post
....So I appeal to your experience in what is the best Lead-Acid deep-cycle charging profile. What I know this far and please correct me if I'm wrong, is:
1st stage - boost charge - 14.4V adjusted with temperature, 3mV/ Celsius degree over 25 degrees?
2nd stage - absorption carge - 14.7V limiting the current at 1/10C
3rd stage - absorption charge - 13.6V to keep the battery from discharging
Don't both to limit the current - it is only important in the first stage, in the second stage the battery state of charge limits the current anyway.

If you really want to know what good solar controllers do then down load the Morningstar software to programme their controllers @
Morningstar Corporation Tech Support

You'll see a dozen or so things you can adjust, like the time in Floast before it goes back to Boost, or if it will even go into float if the previous day the batteries were heavily discharged.

This is a very misunderstood and complex subject, but we would all welcome a regulator that does the job properly and not drop down to float too early.
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Old 15-03-2013, 08:05   #11
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Re: Custom built solar charger

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This is a very misunderstood and complex subject, but we would all welcome a regulator that does the job properly and not drop down to float too early.
My father, after a lifetime as an aviation electrician told me just that!
That should be the best guide for the guy programming the charge controller, awsome!!
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Old 15-03-2013, 16:19   #12
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Re: Custom built solar charger

Quote:
Originally Posted by noelex 77 View Post
It is easy for a battery charger to do. It is the only charging source and there are no discharge currents when its being used.
Some solar regulators do this (with a backup timer) but its of little use because the regulator only looks at its output. The actual current into the batteries will be a combination of the other charge and discharge sources. The current into the battery (which is what is important) bears little relationship o the output of the regulator.
A more complicated system is needed measuring the total input into the battery. If you can incorporate this into your project its a worthwhile improvement especially for gel and AGM batteries.
I also see this useful in two ways.
One is the ability to count the actual current in and out the battery and second is having a stabilized 13,6V output which would eliminate the need for additional LED drivers. A modular design would allow multiple charging sources to feed a single charging controller module, nothing else touching the battery directly.

Quote:
Originally Posted by noelex 77 View Post
Sorry you have lost me there.
The regulation function of a solar controller is different from the MPPT process. It's worth having an accurate regulation algorithm, but this can be incorporated into a MPPT, or non MPPT regulator/controller.
Good, then we speak the same language Many non-technically minded people consider a MPPT to be the only device capable of having a regulation function. That's the reason for the "pseudo" in the naming. Although I've seen no charger providing a thermal sensor to be fitted on the solar panel, it's practice proved that's worth having for accurately tracking the panel performance. And this one has it

Quote:
Originally Posted by noelex 77 View Post
Unfortunately many of the cheaper commercial MPPT regulators do exactly this. Even the best MPPT regulators only extract 5-15% more power on average (than a non MPPT regulator) on a typical yacht installation.

Given the limited space for solar panels on a yacht some people feel this small gain justifies the expense and complication.
Then we should agree that a proper and complete device that uses every known way of extracting the maximum power from such a system it's worth every trouble. Which leads us to actually being able to build cheaper and better devices then those commercially available

This is the first one built, including the LCD display
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Old 15-03-2013, 23:32   #13
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Re: Custom built solar charger

Quote:
Originally Posted by catakate View Post
I also see this useful in two ways.
One is the ability to count the actual current in and out the battery and second is having a stabilized 13,6V output which would eliminate the need for additional LED drivers.
Yes once you are using battery return amps to correctly terminate the absorption charge you can use that information to feed a battery monitor.
This is how a lot of commercial designs work.
The battery will only be at float voltage 13.6v when its near fully charged and the input exceeds the output. This will only occur during daylight hours so its not much use for led lights that will be used at night.
It is not difficult to build a separate stabilised supply but LEDs are much better driven by a constant current source.
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Old 16-03-2013, 02:15   #14
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Re: Custom built solar charger

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you are using battery return amps to correctly terminate the absorption charge
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.
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Old 16-03-2013, 02:36   #15
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Re: Custom built solar charger

I built one using a microprocessor. It would run for a bit and then vary the duty cycle +- 5%, and measured current. Then it could determine the peak point using a 3 point least squares fit of a parabola to this data, and adjust the new duty cycle.

This would possibly have some issues if you have highly variable battery load as the battery voltage changes causing large changes in current. In these cases the peak power tracker needs to quickly change duty cycle during the load and switch back after. Then there is the case of a boat rolling causing partial shade of panels to occur at the boat frequency. 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.
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