MPPT controller - voltage sensing without separate wires?

If you have 1/2 volt drop from controller to battery then the sense wire could make sense. Larger cable also makes sense.

My calculated drop is .064v, certainly not .500v

As mentioned before, most people who install charge controllers in a RV are cognoscente of line drops and and know that a 1% drop is equivalent to losing 1% of PV power. The cable will be beefy. There will be minimal drop and no big deal in lost charge times.

As far as additional loads, well that's why I said 'in most cases'. They are usually short lived. Once you're in absorption, does it really matter if battery voltage is 100 mV lower when a heavy load turns on?

The point is that you might get to absorption maybe half a volt early if you have too much drop in your cabling, and stay half a volt low if your controller is supplying a lot of current after the transition. Does that matter? Yes.

Perhaps you feel compelled to add sense because the big boys do it? You got a 300W max unit? I question if it's cost effective for lower power units to add a sense circuit.

The new unit can handle almost 2000w with a 48v battery bank. The old unit could handle about 1000w. I think battery sense is a useful addition, and doesn't add that much extra cost.

My offer still stands to test your unit. Don't want to keep it, just offering to test and give you fair publicity if warranted.

No thanks.

I realize there's no ground sense line in my 7345 mod. It's not needed. About 80% of the line drop comes from the positive cable. I have a 23 ft MH, and my converter to battery cable is 23 ft long. It snakes all over the place. The ground on the other hand connects to chassis (a good sized frame) through a short cable. It's good. That's how many RV's are built.

Happy that works for you.

As mentioned before, most people who install charge controllers in a RV are cognoscente of line drops and and know that a 1% drop is equivalent to losing 1% of PV power. The cable will be beefy. There will be minimal drop and no big deal in lost charge times.

As far as additional loads, well that's why I said 'in most cases'. They are usually short lived. Once you're in absorption, does it really matter if battery voltage is 100 mV lower when a heavy load turns on?

Perhaps you feel compelled to add sense because the big boys do it? You got a 300W max unit? I question if it's cost effective for lower power units to add a sense circuit.

My offer still stands to test your unit. Don't want to keep it, just offering to test and give you fair publicity if warranted.

I realize there's no ground sense line in my 7345 mod. It's not needed. About 80% of the line drop comes from the positive cable. I have a 23 ft MH, and my converter to battery cable is 23 ft long. It snakes all over the place. The ground on the other hand connects to chassis (a good sized frame) through a short cable. It's good. That's how many RV's are built.

lorelec wrote:

I disagree. The onset of absorption may occur too early if the battery voltage is not being measured correctly at higher currents. As well, if you have significant loads that the controller is supplying current to in absorb and float modes, the controller may be undercharging the batteries during those times. Believe me, I wouldn't go to the trouble of adding the feature if there wasn't any benefit of doing so. Also, you're missing the negative sense wire in your schematic a few posts back, so you're not compensating for the voltage drop across the negative battery wire in that design.

Nice! How does it do on say a 50% cloudy day with strong sunshine between clouds? My Vmpp will quickly jump from 16 or 16.5 in the sun down to 14 Vmpp when a cloud comes over. If the controller is set at 16v I get essentially no output until it passes. If set to 14v quickly, I get max power. In some shade conditions the Unisolars can produce almost 50% power into my AGMs compared to full sun.

What is the quickest user selectable sampling interval available?

Jim

I've not measured the actual time it takes to track a 16v -> 14v change in MPP, but a 2v change would be tracked quickly...I would imagine less than a second, based on observing the change in pulse width on a scope.

The shortest user-adjustable interval is 10 minutes.

Thinking about sense lines for small PV systems (as in RV's) some more made me realize they're not needed. There are exceptions, but the only time the charge controller outputs max amps is during boost. You don't need sense lines at this time as the system is current limited by the maximum power point. It doesn't matter how much line drop there is. The system is under current control: I ~ Pmpp/Vbat. At the onset of absorption, it can be an issue. But current quickly tapers, making line drops small and hence not needing sense lines.

I disagree. The onset of absorption may occur too early if the battery voltage is not being measured correctly at higher currents. As well, if you have significant loads that the controller is supplying current to in absorb and float modes, the controller may be undercharging the batteries during those times. Believe me, I wouldn't go to the trouble of adding the feature if there wasn't any benefit of doing so. Also, you're missing the negative sense wire in your schematic a few posts back, so you're not compensating for the voltage drop across the negative battery wire in that design.

Salvo wrote:
Thinking about sense lines for small PV systems (as in RV's) some more made me realize they're not needed. There are exceptions, but the only time the charge controller outputs max amps is during boost. You don't need sense lines at this time as the system is current limited by the maximum power point. It doesn't matter how much line drop there is. The system is under current control: I ~ Pmpp/Vbat. At the onset of absorption, it can be an issue. But current quickly tapers, making line drops small and hence not needing sense lines.

Sal

Yes bulk and float has little need. Just high current side of absorption.

If absorption is timer based then the timer may start prematurely. Adjusting the time or voltage setting could be just as effective.

double post

Thinking about sense lines for small PV systems (as in RV's) some more made me realize they're not needed. There are exceptions, but the only time the charge controller outputs max amps is during boost. You don't need sense lines at this time as the system is current limited by the maximum power point. It doesn't matter how much line drop there is. The system is under current control: I ~ Pmpp/Vbat. At the onset of absorption, it can be an issue. But current quickly tapers, making line drops small and hence not needing sense lines.

Sal

Nice! How does it do on say a 50% cloudy day with strong sunshine between clouds? My Vmpp will quickly jump from 16 or 16.5 in the sun down to 14 Vmpp when a cloud comes over. If the controller is set at 16v I get essentially no output until it passes. If set to 14v quickly, I get max power. In some shade conditions the Unisolars can produce almost 50% power into my AGMs compared to full sun.

What is the quickest user selectable sampling interval available?

Jim

Nice! Can I limit the voltage sweep range by entering min and max Vmpp (hot panels in shade, cool panels in edge effect) if I know them? Or is it so fast now and the quickest interval between scans large enough that those few milliseconds don't really matter? Think edge case like solar roof panels charging batteries while driving past buildings and under bridges.

There's really no need to limit the voltage range of the sweep. The "down time" during the sweep is very small, and even if it was longer, there's virtually no energy lost during that time. The amount of energy lost during brief shading is also negligible (if the whole panel is shaded, you're not getting much from it anyway). The sweep occurs only once during MPPT - at the start - unless the user chooses the option of having the sweep occur at an interval. The dynamic tracking is very good, so there's generally no need to do recurring sweeps. But the option is there for those that want to play with it.

Almot wrote:

HiTech wrote:

Measuring Voc requires a disconnect of the PVs from charging and is generally a sign of a crude MPPT strategy. Who still uses this?

I think Salvo meant that controller "can" measure battery voltage via power cables accurately when there is no charging current. Which is correct. The problem is that there is always some current.

Well - since users complained on "inconvenience" of accurate voltage sensing with separate cables, those who don't care about under-voltage, should be happy now :) ... If the battery is sitting next to controller, there is not much error, but then, there is not much inconvenience with extra sensing wires either. Whatever...

Correction to my previous post - it has to be ~6ft from battery to controller, to get 1% error at 30A. 12ft total run.

There is an ultra crude form of MPPT that really is not tracking at all but a SWAG, that is based on fully disconnecting panels from the load, measuring Voc of the array, and hoping that the aggregate system Vmpp is .80 of the aggregate Voc, and just fudging it down 4 percent more and using that voltage. It does not track power point at all. It tracks Voc, and makes SWAG at Vmpp. And it interrupts solar power generation to do it. Most of the time it is too low (for the many panels in the .80-.85 Vmpp of Voc range), and for panels like my Unisolar it is too high, potentially yielding less output than PWM would.

If a controller were close to the price of a PWM controller plus a buck converter, I could see using such a crude control strategy. But for anything high end it's yield is too low. It's basically never going to find the max power point, and if you are lucky it might be closeish for your specific panels.

You really need to to a sweep through all to local maximums to find the system max power point.

Jim