Forums

Charge controller - power diode question

Started by Unknown December 26, 2012
Could some power electronics guru please 
help ?
I am trying to put together a charge 
controller for a battery pack/solar 
panel array. The maximum solar panel 
output current is around 20.0A, and 
the maximum battery voltage is 12V.
So, in the charge controller, I was 
planning to use the power diode 10A02, 
whose IFmax is 10.0A and the BV is 
100.0V. To deal with the 20.0A maximum 
solar panel output current, I was planning 
to have 3 diodes in parallel to tackle
the current, and the diode's 100.0V   
BV would block reverse battery discharge.
How does this look ? Also, if I have a 
simple shunt type charge controller, 
what would be a good power transistor
to use at these current/voltages ?
Thanks in advance for all your hints
and suggestions.
On 26/12/2012 4:20 PM, dakupoto@gmail.com wrote:
> Could some power electronics guru please > help ? > I am trying to put together a charge > controller for a battery pack/solar > panel array. The maximum solar panel > output current is around 20.0A, and > the maximum battery voltage is 12V. > So, in the charge controller, I was > planning to use the power diode 10A02, > whose IFmax is 10.0A and the BV is > 100.0V. To deal with the 20.0A maximum > solar panel output current, I was planning > to have 3 diodes in parallel to tackle > the current, and the diode's 100.0V > BV would block reverse battery discharge. > How does this look ? Also, if I have a > simple shunt type charge controller, > what would be a good power transistor > to use at these current/voltages ? > Thanks in advance for all your hints > and suggestions. >
Why not use a schottky diode pair (to220 package) such as MBR2545 for lower forward resistance and since the diodes are in the same package they will share the load better. Also for similar reasons a power mosfet might be a better choice to handle switching high current more efficiently.
On 12/26/2012 12:20 AM, dakupoto@gmail.com wrote:

There have been ample discussions about the folly
of solar power when you have any alternatives.
I'll not repeat that here.  I like to help
people with their hobbies.
I've helped engineer more than one solar system
for mountain-top use in the boonies.

Your statements are worrisome on many levels...
> Could some power electronics guru please > help ? > I am trying to put together a charge > controller for a battery pack/solar > panel array. The maximum solar panel > output current is around 20.0A, and > the maximum battery voltage is 12V.
Depending on what batteries you're using and the tradeoffs you've made for life vs maximum stored energy, your battery voltage ranges from 10V to nearly 15V or so.
> So, in the charge controller, I was > planning to use the power diode 10A02, > whose IFmax is 10.0A and the BV is > 100.0V.
To deal with the 20.0A maximum
> solar panel output current, I was planning > to have 3 diodes in parallel to tackle > the current, and the diode's 100.0V > BV would block reverse battery discharge. > How does this look ?
Sounds logical on the surface, but there's a troll under that bridge. Most diodes have a negative temperature coefficient. If there's any imbalance, one diode takes more current and warms up which makes it hog more current which makes it warm up. Pretty soon, you have one diode in the circuit and the other two just sitting there. From then on, it's a matter of time until the silicon melts and shorts or the plastic melts and the device comes apart open. According to the spec, that diode at 10 amps is gonna be 150 C above ambient. That's too hot, even if you could make three share equally. Get something with a bolt hole and put it on a heat sink. You're better off with one 40A diode. And a 20V diode will likely be less lossy and cheaper than a 100V one. And I don't mean a lower voltage one selected out of the same bucket. I mean a diode designed for minimum forward voltage often has a lower reverse voltage. It doesn't have to be a fast diode. But there's a more basic question. Many solar panels have built-in diodes. Are you sure yours don't? That diode has a forward voltage spec of 1V. You've got 15V at the battery and you're wasting another volt or 20 watts. And it's more complex than that. Look at the family of curves for the panel. They're not a straight line. And the 20A is at noon in the desert. At 3PM when the panel isn't pointed directly at the sun and there's more atmosphere in the way and there's some haze and some bird poop on the panel, you're gonna be on another curve of that graph. That diode might be the difference between your charge current going to zero at 3PM instead of 3:30 PM. Those lost amp-hours add up. Also, if I have a
> simple shunt type charge controller, > what would be a good power transistor > to use at these current/voltages ?
Shunt controller sounds simple, but again, the devil is in the details. We built one system that used 4 150W resistors and 4 audio power transistors. A comparator switched the transistors on at 13.7V and off at 13.6V. That worked because The panel was 20A and the batteries were 800Amp-Hour. They didn't have any trouble with that current and voltage. And the thing rarely turned on except on the longest days of the year. If you had a single car battery and a 20A panel, that wouldn't be a good idea. A later version used an oscillator and a crude 300Hz PWM to switch the transistors. For a less robust battery system, you'd want a fast switcher and much better charge management. And today, you'd be able to afford fast switching transistors at that current. For starters, take any 300W plus buck converter design that can run on 10-15V. Put 600W of resistors with values that can draw at least 300W from the output of the converter. Control the converter output voltage to give you the shunt load current you require. Since you're likely to use multiple resistors in parallel. It might be easier to use more buck converters of lower current each. Depends on what you can find in the cheapo bin of the electronic surplus store. We built a MPPT controller and a sun tracker. Worked neat and gained some additional usable power. Problem was that it was deemed unworkable. It's hard to track the sun when the panel is covered with ice and the roads to the site are closed eight months out of the year. For remote stuff that just has to work, it's often better to add another panel than to try to eek out a few more percent on the one you have. Reliability trumps efficiency every time.
> Thanks in advance for all your hints > and suggestions.
The hardest part of any project is writing the spec. Decide exactly what you expect to happen under any and all conditions of insolation and load current and battery charge level. Map that all out and decide what to implement. Then figger out HOW to implement...repeat the cycle until it looks like what you want. Then start ordering parts. If you're on the grid and expect to generate power, you're likely to give up a that point and go play a round of golf. If you're indulging an expensive hobby, you can have great fun with solar power. It's a lot cheaper than golf...and less risky than a mistress.
On Wednesday, December 26, 2012 6:44:53 AM UTC-5, mike wrote:
> On 12/26/2012 12:20 AM, dakupoto@gmail.com wrote: > > > > There have been ample discussions about the folly > > of solar power when you have any alternatives. > > I'll not repeat that here. I like to help > > people with their hobbies. > > I've helped engineer more than one solar system > > for mountain-top use in the boonies. > > > > Your statements are worrisome on many levels... > > > Could some power electronics guru please > > > help ? > > > I am trying to put together a charge > > > controller for a battery pack/solar > > > panel array. The maximum solar panel > > > output current is around 20.0A, and > > > the maximum battery voltage is 12V. > > Depending on what batteries you're using > > and the tradeoffs you've made for life vs > > maximum stored energy, > > your battery voltage ranges from 10V to > > nearly 15V or so. > > > So, in the charge controller, I was > > > planning to use the power diode 10A02, > > > whose IFmax is 10.0A and the BV is > > > 100.0V. > > To deal with the 20.0A maximum > > > solar panel output current, I was planning > > > to have 3 diodes in parallel to tackle > > > the current, and the diode's 100.0V > > > BV would block reverse battery discharge. > > > How does this look ? > > > > Sounds logical on the surface, but there's > > a troll under that bridge. > > Most diodes have a negative temperature coefficient. > > If there's any imbalance, one diode takes more > > current and warms up which makes it hog more current > > which makes it warm up. Pretty soon, you have one diode > > in the circuit and the other two just sitting there. > > From then on, it's a matter of time until the silicon melts > > and shorts or the plastic melts and the device comes apart open. > > According to the spec, that diode at 10 amps is gonna > > be 150 C above ambient. That's too hot, even if you > > could make three share equally. Get something with a bolt hole > > and put it on a heat sink. > > You're better off with one 40A diode. > > And a 20V diode will likely be less lossy and cheaper > > than a 100V one. And I don't mean a lower voltage one > > selected out of the same bucket. I mean a diode designed > > for minimum forward voltage often has a lower reverse voltage. > > It doesn't have to be a fast diode. > > > > But there's a more basic question. Many solar panels have built-in > > diodes. Are you sure yours don't? > > > > That diode has a forward voltage spec of 1V. > > You've got 15V at the battery and you're wasting another volt > > or 20 watts. > > > > And it's more complex than that. > > Look at the family of curves for the panel. > > They're not a straight line. And the 20A is at noon in the desert. > > At 3PM when the panel isn't pointed directly at the sun and there's > > more atmosphere in the way and there's some haze and some bird poop > > on the panel, you're gonna be on another curve of that graph. > > That diode might be the difference between your charge current going to > > zero at 3PM instead of 3:30 PM. Those lost amp-hours add up. > > Also, if I have a > > > simple shunt type charge controller, > > > what would be a good power transistor > > > to use at these current/voltages ? > > > > Shunt controller sounds simple, but again, the devil is in the > > details. > > We built one system that used 4 150W resistors and 4 audio power > > transistors. A comparator switched the transistors on at 13.7V and off > > at 13.6V. > > That worked because > > The panel was 20A and the batteries were 800Amp-Hour. They didn't have > > any trouble with that current and voltage. And the thing rarely > > turned on except on the longest days of the year. > > > > If you had a single car battery and a 20A panel, that wouldn't be a good > > idea. > > > > A later version used an oscillator and a crude 300Hz PWM to switch the > > transistors. > > For a less robust battery system, you'd want a fast switcher and much > > better charge management. And today, you'd be able to afford fast > > switching transistors at that current. > > > > For starters, take any 300W plus buck converter design that can run on > > 10-15V. Put 600W of resistors with values that can draw at least > > 300W from the output of the converter. > > Control the converter output voltage to give you the shunt load current > > you require. Since you're likely to use multiple resistors in parallel. > > It might be easier to use more buck converters of lower current each. > > Depends on what you can find in the cheapo bin of the electronic surplus > > store. > > > > We built a MPPT controller and a sun tracker. Worked neat and gained > > some additional usable power. Problem was that it was deemed unworkable. > > It's hard to track the sun when the panel is covered with ice > > and the roads to the site are closed eight months out of the year. > > > > For remote stuff that just has to work, it's often better to add another > > panel than to try to eek out a few more percent on the one you have. > > Reliability trumps efficiency every time. > > > Thanks in advance for all your hints > > > and suggestions. > > > > The hardest part of any project is writing the spec. > > Decide exactly what you expect to happen under any and all > > conditions of insolation and load current and battery charge level. > > Map that all out and decide what to implement. Then figger out HOW > > to implement...repeat the cycle until it looks like what you want. > > Then start ordering parts. > > > > If you're on the grid and expect to generate power, you're likely to > > give up a that point and go play a round of golf. > > > > If you're indulging an expensive hobby, you can have great fun > > with solar power. It's a lot cheaper than golf...and less risky > > than a mistress.
Thank you very much for your detailed and insightful explanation. I really loved your last paragraph.
On Wed, 26 Dec 2012 03:44:53 -0800, mike <ham789@netzero.net> wrote:

>To deal with the 20.0A maximum >> solar panel output current, I was planning >> to have 3 diodes in parallel to tackle >> the current, and the diode's 100.0V >> BV would block reverse battery discharge. >> How does this look ? > >Sounds logical on the surface, but there's >a troll under that bridge. >Most diodes have a negative temperature coefficient. >If there's any imbalance, one diode takes more >current and warms up which makes it hog more current >which makes it warm up. Pretty soon, you have one diode >in the circuit and the other two just sitting there. > From then on, it's a matter of time until the silicon melts >and shorts or the plastic melts and the device comes apart open.
The usual method to avoid this is to put individual series resistors in front of each diode, thus, sharing the current more evenly. Of course, this dissipates some power. However, since there are usually quite a long distance between the panels and the batteries (and diodes) at such low voltage as 12 V (instead of 24 V or 48 V), there are still going to be some voltage drop in the cable with cross section A, before to current goes into three 10 A diodes. Why not use three separate insulated conductors with A/3 cross section from the panels, each connected separately to each diode ? The total power loss in the wiring will be the same, but the separate wiring will now act as small series resistors to distribute the current more evenly.
On Wed, 26 Dec 2012 03:44:53 -0800, mike <ham789@netzero.net> wrote:

>On 12/26/2012 12:20 AM, dakupoto@gmail.com wrote: > >There have been ample discussions about the folly >of solar power when you have any alternatives. >I'll not repeat that here. I like to help >people with their hobbies. >I've helped engineer more than one solar system >for mountain-top use in the boonies. > >Your statements are worrisome on many levels... >> Could some power electronics guru please >> help ? >> I am trying to put together a charge >> controller for a battery pack/solar >> panel array. The maximum solar panel >> output current is around 20.0A, and >> the maximum battery voltage is 12V. >Depending on what batteries you're using >and the tradeoffs you've made for life vs >maximum stored energy, >your battery voltage ranges from 10V to >nearly 15V or so. >> So, in the charge controller, I was >> planning to use the power diode 10A02, >> whose IFmax is 10.0A and the BV is >> 100.0V. >To deal with the 20.0A maximum >> solar panel output current, I was planning >> to have 3 diodes in parallel to tackle >> the current, and the diode's 100.0V >> BV would block reverse battery discharge. >> How does this look ? > >Sounds logical on the surface, but there's >a troll under that bridge. >Most diodes have a negative temperature coefficient. >If there's any imbalance, one diode takes more >current and warms up which makes it hog more current >which makes it warm up. Pretty soon, you have one diode >in the circuit and the other two just sitting there. > From then on, it's a matter of time until the silicon melts >and shorts or the plastic melts and the device comes apart open. >According to the spec, that diode at 10 amps is gonna >be 150 C above ambient. That's too hot, even if you >could make three share equally. Get something with a bolt hole >and put it on a heat sink.
If you look at actual diode curves, diode TC is negative at low currents and positive at higher currents. That's because the ohmic component of VF dominates at high current, and it has a positive TC. It's generally safe to parallel identical diodes on the same heat sink.
>You're better off with one 40A diode. >And a 20V diode will likely be less lossy and cheaper >than a 100V one. And I don't mean a lower voltage one >selected out of the same bucket. I mean a diode designed >for minimum forward voltage often has a lower reverse voltage. >It doesn't have to be a fast diode. > >But there's a more basic question. Many solar panels have built-in >diodes. Are you sure yours don't? > >That diode has a forward voltage spec of 1V. >You've got 15V at the battery and you're wasting another volt >or 20 watts. > >And it's more complex than that. >Look at the family of curves for the panel. >They're not a straight line. And the 20A is at noon in the desert. >At 3PM when the panel isn't pointed directly at the sun and there's >more atmosphere in the way and there's some haze and some bird poop >on the panel, you're gonna be on another curve of that graph. >That diode might be the difference between your charge current going to >zero at 3PM instead of 3:30 PM. Those lost amp-hours add up. > Also, if I have a >> simple shunt type charge controller, >> what would be a good power transistor >> to use at these current/voltages ? > >Shunt controller sounds simple, but again, the devil is in the >details. >We built one system that used 4 150W resistors and 4 audio power >transistors. A comparator switched the transistors on at 13.7V and off >at 13.6V. >That worked because >The panel was 20A and the batteries were 800Amp-Hour. They didn't have >any trouble with that current and voltage. And the thing rarely >turned on except on the longest days of the year. > >If you had a single car battery and a 20A panel, that wouldn't be a good >idea. > >A later version used an oscillator and a crude 300Hz PWM to switch the >transistors. >For a less robust battery system, you'd want a fast switcher and much >better charge management. And today, you'd be able to afford fast >switching transistors at that current. > >For starters, take any 300W plus buck converter design that can run on >10-15V. Put 600W of resistors with values that can draw at least >300W from the output of the converter. >Control the converter output voltage to give you the shunt load current >you require. Since you're likely to use multiple resistors in parallel. >It might be easier to use more buck converters of lower current each. >Depends on what you can find in the cheapo bin of the electronic surplus >store.
Couldn't you do a series switcher with no inductor and no catch diode? Just connect the solar array to the battery through a mosfet or SSR, on/off, from a comparator with maybe a little timing, like a clocked d-flop in the path. It's simple and power dissipation would be very low.
On 12/26/2012 5:44 AM, mike wrote:

> But there's a more basic question. Many solar panels have built-in > diodes. Are you sure yours don't?
Acutually, the solar panels I have in front of me have the diodes in _parallel_ with the panel output. The purpose, AIUI, is to provide a path for the current of shaded panels when wired in series. In that case, the provided diodes will not prevent current back into the battery.
On Wed, 26 Dec 2012 13:32:58 -0600, John S <Sophi.2@invalid.org> wrote:

>On 12/26/2012 5:44 AM, mike wrote: > >> But there's a more basic question. Many solar panels have built-in >> diodes. Are you sure yours don't? > >Acutually, the solar panels I have in front of me have the diodes in >_parallel_ with the panel output. > >The purpose, AIUI, is to provide a path for the current of shaded panels >when wired in series. > >In that case, the provided diodes will not prevent current back into the >battery.
They prevent current from an illuminated panel from being loaded by a non-illuminated panel. IOW, electrons only get to spit in one direction from all elements. That being INTO the charging circuit input.
On 12/26/2012 11:30 AM, John Larkin wrote:
> On Wed, 26 Dec 2012 03:44:53 -0800, mike<ham789@netzero.net> wrote: > >> On 12/26/2012 12:20 AM, dakupoto@gmail.com wrote: >> >> There have been ample discussions about the folly >> of solar power when you have any alternatives. >> I'll not repeat that here. I like to help >> people with their hobbies. >> I've helped engineer more than one solar system >> for mountain-top use in the boonies. >> >> Your statements are worrisome on many levels... >>> Could some power electronics guru please >>> help ? >>> I am trying to put together a charge >>> controller for a battery pack/solar >>> panel array. The maximum solar panel >>> output current is around 20.0A, and >>> the maximum battery voltage is 12V. >> Depending on what batteries you're using >> and the tradeoffs you've made for life vs >> maximum stored energy, >> your battery voltage ranges from 10V to >> nearly 15V or so. >>> So, in the charge controller, I was >>> planning to use the power diode 10A02, >>> whose IFmax is 10.0A and the BV is >>> 100.0V. >> To deal with the 20.0A maximum >>> solar panel output current, I was planning >>> to have 3 diodes in parallel to tackle >>> the current, and the diode's 100.0V >>> BV would block reverse battery discharge. >>> How does this look ? >> >> Sounds logical on the surface, but there's >> a troll under that bridge. >> Most diodes have a negative temperature coefficient. >> If there's any imbalance, one diode takes more >> current and warms up which makes it hog more current >> which makes it warm up. Pretty soon, you have one diode >> in the circuit and the other two just sitting there. >> From then on, it's a matter of time until the silicon melts >> and shorts or the plastic melts and the device comes apart open. >> According to the spec, that diode at 10 amps is gonna >> be 150 C above ambient. That's too hot, even if you >> could make three share equally. Get something with a bolt hole >> and put it on a heat sink. > > > If you look at actual diode curves, diode TC is negative at low > currents and positive at higher currents. That's because the ohmic > component of VF dominates at high current, and it has a positive TC. > It's generally safe to parallel identical diodes on the same heat > sink. > > > > > >> You're better off with one 40A diode. >> And a 20V diode will likely be less lossy and cheaper >> than a 100V one. And I don't mean a lower voltage one >> selected out of the same bucket. I mean a diode designed >> for minimum forward voltage often has a lower reverse voltage. >> It doesn't have to be a fast diode. >> >> But there's a more basic question. Many solar panels have built-in >> diodes. Are you sure yours don't? >> >> That diode has a forward voltage spec of 1V. >> You've got 15V at the battery and you're wasting another volt >> or 20 watts. >> >> And it's more complex than that. >> Look at the family of curves for the panel. >> They're not a straight line. And the 20A is at noon in the desert. >> At 3PM when the panel isn't pointed directly at the sun and there's >> more atmosphere in the way and there's some haze and some bird poop >> on the panel, you're gonna be on another curve of that graph. >> That diode might be the difference between your charge current going to >> zero at 3PM instead of 3:30 PM. Those lost amp-hours add up. >> Also, if I have a >>> simple shunt type charge controller, >>> what would be a good power transistor >>> to use at these current/voltages ? >> >> Shunt controller sounds simple, but again, the devil is in the >> details. >> We built one system that used 4 150W resistors and 4 audio power >> transistors. A comparator switched the transistors on at 13.7V and off >> at 13.6V. >> That worked because >> The panel was 20A and the batteries were 800Amp-Hour. They didn't have >> any trouble with that current and voltage. And the thing rarely >> turned on except on the longest days of the year. >> >> If you had a single car battery and a 20A panel, that wouldn't be a good >> idea. >> >> A later version used an oscillator and a crude 300Hz PWM to switch the >> transistors. >> For a less robust battery system, you'd want a fast switcher and much >> better charge management. And today, you'd be able to afford fast >> switching transistors at that current. >> >> For starters, take any 300W plus buck converter design that can run on >> 10-15V. Put 600W of resistors with values that can draw at least >> 300W from the output of the converter. >> Control the converter output voltage to give you the shunt load current >> you require. Since you're likely to use multiple resistors in parallel. >> It might be easier to use more buck converters of lower current each. >> Depends on what you can find in the cheapo bin of the electronic surplus >> store. > > Couldn't you do a series switcher with no inductor and no catch diode? > Just connect the solar array to the battery through a mosfet or SSR, > on/off, from a comparator with maybe a little timing, like a clocked > d-flop in the path. It's simple and power dissipation would be very > low. > >
I didn't want to complicate the discussion. What you suggest probably works fine. In our case, there was also a wind generator that had some control mechanisms in the head. It didn't like to be disconnected when the wind was blowing hard. And the solar had a crude controller. The two controllers wanted to fight. And the pieces were a hundred miles away and a mile straight up. Was easier to just shunt the thing into a resistor to regulate it. It can be a minor consideration, but series control is loss when you need the juice most. Shunt control is loss when you've got excess you can't use.
On Dec 26, 2:32=A0pm, John S <Soph...@invalid.org> wrote:
> On 12/26/2012 5:44 AM, mike wrote: > > > But there's a more basic question. =A0Many solar panels have built-in > > diodes. =A0Are you sure yours don't? > > Acutually, the solar panels I have in front of me have the diodes in > _parallel_ with the panel output. > > The purpose, AIUI, is to provide a path for the current of shaded panels > when wired in series. > > In that case, the provided diodes will not prevent current back into the > battery.
Hmm, well first I know squat about solar panels. But that seems wasteful. Doesn't it cost a bit more than one solar panel 'photovoltage' to overcome the diode drop of the panel in the shade? Or is there a series stack of 'PV's with one diode across the whole lot? George H.