Hi folk's,
I need to build a battery charger, fed from PV, and plan to manage the batteries using a uP
However it seemed wise to build a charger that had inherrent limits and then adjust it down when necessarry as opposed to having the uP in full control.
If this achieves nothing else it would allow me to operate manually if needs be.
It will need to deliver circa 50A on occasion, but should limit at that by default.
It will need to limit the maximum voltage to about 14.8
It should maintain a minimum PV voltage of about 16, possibly as much as 19, I will not actually know until it is running.
Assiming I use a buck converter, which I have, it starts out as a simple voltage source but finding such a high current device wasnt looking promissong, at least not with the budget I have.
I did some reading on paralell devices and also did some testing and found that the cheap V & I limmiting units all used the negative line to measure current. which clearly wasnt going to work keeping several units ballanced because the shunt wasnt in the positive output and there was no practiacal way to ensure that the negative currents are balanced.
However I did find that all the units I looked at used a simmilar control strategy when implimenting current limmiting... some circuit, it isnt importent what, raised a voltage as the current went up. This reference was then fed to the sens pin, enhancing the voltage that is applied by the voltage devider, via a diode. The net result is that until the forward bias of the diode is reached nothing happens and when it is the sens pin voltage rises causing the. buck chip to back off... Neet I thought.
After a bit of fiddling, and a little smoke, I got a simple circuit to work that allowed me to bend the buck board with an analouge, intigrated PWM,ii output from the IP.
I also found that many of these aditional 'limits' could be applied easily as the diode isolated each circuit keeping them simple.
Now I think about it is no different than a summing circuit with an opamp.
Anyway... I decided to try the simple buck boards in paralell. Obviously each needed a current limit as a minimum requirement. I added a current sensor, 5v supply +/_ 2v arround a 2.5v mid point. The sensor output go's via a current limiting resistor to a trimmir which in turn applies voltage to the buck sens pin via a smal diode. The limit is a little soft but in all honesty that just works to eliminate oscillation, or at least it seems to.
I could, and may, amplify the signal a little to get a harder tirn off but it will do for now.
After I had 4 simmilar units all modified with the high side current sensing and limiting circuit I hooked them all in paralell to see if it worked and it initially looked good. The at very low currents the ballance isnt great but still acceptable. At higher currents all the units top out nicely and show no obvious signs of oscillation or feedback, at least not that I am recognizing.
There is no uP in play just the stock voltage devider, athe new current limit and a second inverted voltage sensing circuit on the PV side.
Holdng the converters off untill the panel voltage is over 16, well above battery voltage, seems to prevent any reverse curent and apart from addressing some form of soft start, another little circuit and a diode I thought I was there.... Silly me!
Despite the fact tbat everything looked OK the convertrs are running very hot. Yes they are from afar with dodgy build quality but I bought 15A units anticipating being able to run them at 8 or 9A so that they wern't stresed at all.
Well that isnt working... I have 4 in paralell now and even st at 5A each they are warm enough to want forced cooling despite some chunky heat sinks.
Does anyone know what I am missing and or what to do about it?
In all honesty if I thought it was all stable but with way overstated specs I would simply add more modules but it seems more likely that I am stressing the kit in some way that I have failed to apriciate and if that is the case they will faill at some point however low I set them.
Thoughts, I dont have a clue.
Actually a couple of specific. questions, should I remove the output caps from each module and have a single separate bank, could too much capacitance be a problem. It would certainly have an effect on an AC circuit and as the effect increases with frequency I can see that it would be a problem at some point but I dont know how to work out when.
Also will a buck converter run hotter or cooler as the differential between input and output voltage increases.
Thanks for looking,
regards,
Al
I need to build a battery charger, fed from PV, and plan to manage the batteries using a uP
However it seemed wise to build a charger that had inherrent limits and then adjust it down when necessarry as opposed to having the uP in full control.
If this achieves nothing else it would allow me to operate manually if needs be.
It will need to deliver circa 50A on occasion, but should limit at that by default.
It will need to limit the maximum voltage to about 14.8
It should maintain a minimum PV voltage of about 16, possibly as much as 19, I will not actually know until it is running.
Assiming I use a buck converter, which I have, it starts out as a simple voltage source but finding such a high current device wasnt looking promissong, at least not with the budget I have.
I did some reading on paralell devices and also did some testing and found that the cheap V & I limmiting units all used the negative line to measure current. which clearly wasnt going to work keeping several units ballanced because the shunt wasnt in the positive output and there was no practiacal way to ensure that the negative currents are balanced.
However I did find that all the units I looked at used a simmilar control strategy when implimenting current limmiting... some circuit, it isnt importent what, raised a voltage as the current went up. This reference was then fed to the sens pin, enhancing the voltage that is applied by the voltage devider, via a diode. The net result is that until the forward bias of the diode is reached nothing happens and when it is the sens pin voltage rises causing the. buck chip to back off... Neet I thought.
After a bit of fiddling, and a little smoke, I got a simple circuit to work that allowed me to bend the buck board with an analouge, intigrated PWM,ii output from the IP.
I also found that many of these aditional 'limits' could be applied easily as the diode isolated each circuit keeping them simple.
Now I think about it is no different than a summing circuit with an opamp.
Anyway... I decided to try the simple buck boards in paralell. Obviously each needed a current limit as a minimum requirement. I added a current sensor, 5v supply +/_ 2v arround a 2.5v mid point. The sensor output go's via a current limiting resistor to a trimmir which in turn applies voltage to the buck sens pin via a smal diode. The limit is a little soft but in all honesty that just works to eliminate oscillation, or at least it seems to.
I could, and may, amplify the signal a little to get a harder tirn off but it will do for now.
After I had 4 simmilar units all modified with the high side current sensing and limiting circuit I hooked them all in paralell to see if it worked and it initially looked good. The at very low currents the ballance isnt great but still acceptable. At higher currents all the units top out nicely and show no obvious signs of oscillation or feedback, at least not that I am recognizing.
There is no uP in play just the stock voltage devider, athe new current limit and a second inverted voltage sensing circuit on the PV side.
Holdng the converters off untill the panel voltage is over 16, well above battery voltage, seems to prevent any reverse curent and apart from addressing some form of soft start, another little circuit and a diode I thought I was there.... Silly me!
Despite the fact tbat everything looked OK the convertrs are running very hot. Yes they are from afar with dodgy build quality but I bought 15A units anticipating being able to run them at 8 or 9A so that they wern't stresed at all.
Well that isnt working... I have 4 in paralell now and even st at 5A each they are warm enough to want forced cooling despite some chunky heat sinks.
Does anyone know what I am missing and or what to do about it?
In all honesty if I thought it was all stable but with way overstated specs I would simply add more modules but it seems more likely that I am stressing the kit in some way that I have failed to apriciate and if that is the case they will faill at some point however low I set them.
Thoughts, I dont have a clue.
Actually a couple of specific. questions, should I remove the output caps from each module and have a single separate bank, could too much capacitance be a problem. It would certainly have an effect on an AC circuit and as the effect increases with frequency I can see that it would be a problem at some point but I dont know how to work out when.
Also will a buck converter run hotter or cooler as the differential between input and output voltage increases.
Thanks for looking,
regards,
Al
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