Need some guidance for small project series-parallel 18V 1.25A charger

[SunEagle]

It does not have to meet NFPA requirements. 10 AWG with high temperature insulation like Teflon in free air can take 130 amps for short periods of time. In its current package I can see some applications on the load side of a charge controller with battery. As it is now completely useless for AC.

It was designed for DC.. When you use a 14.8v 6.0 mah battery and a big honken DC motor to run your prop you can easily pull 100 amps. I have a smaller motor and run it on 11.4v 3.3 mah battery and can pull close to 60 amps. I also use a UT203 DC clamp on meter for both my RC planes and Solar panels.

[Sunking]

It was designed for DC..

Yes I understand that. Bu twith up to 60 volt input gives it application on a off grid battery system. You just could not use the full 130 amp potential it has.

[FloridaSun]

It does not have to meet NFPA requirements. 10 AWG with high temperature insulation like Teflon in free air can take 130 amps for short periods of time. In its current package I can see some applications on the load side of a charge controller with battery. As it is now completely useless for AC.

yup, DC only. The 10AWG is fine for my application of continuous use under 30A. Even if I get two more panels I'd only be sending a bit over 20amp @ 24volt at most. Some of those lookalike brands of nearly the same design may be cheaper but use 12 or 14AWG, not so good.

[Sunking]

Voltage drop would be a concern.

[SunEagle]

Yes I understand that. Bu twith up to 60 volt input gives it application on a off grid battery system. You just could not use the full 130 amp potential it has.

Yeah. I agree that the 130 amp may be the rating but not good to actually expose it to that level. Just like I would not use my CAT III 600V meter on anything more than a little over 500 volts. Not worth putting it through that test.

[inetdog]

Will this be a problem for the MPPT since having unequal strings in parallel will make tracking the MPP difficult at best? This is the part that had me scratching my head trying to find a rationale for one solution over another.... 1) One MMPT and both panels in parallel, or 2) One PWM for each panel.

It all depends on what you mean by unequal.
If they are different numbers of panels and as a result the Vmp of the two strings differs by more than 10%, or the Vmp of one string is above the Voc of the other, then you really need two inputs.
If they are the same number and specs, but one of them is partially shaded when the other is not, then the activation of the bypass diodes in the shaded panels will drop the voltage of that string and can cause the >10% mismatch I warned about. What you will potentially lose is the whole output of the lower voltage string during that time.
Otherwise the Vmp of a panel or string will not vary much, if at all as a result of the amount of light hitting it, so two similar enough unshaded strings can go to one MPPT input. The Imp will vary, but that does not cause a problem when putting them in parallel. And since they will not both be generating max power at the same time you can use more total panel with the same single CC and get more power through it over the course of the day.

Since you are talking specifically about very short (one panel) strings, you really just need to apply the voltage comparisons to the two panels if they are not identical.

[sgking2]

puzzling .....

It all depends on what you mean by unequal.

The 2 panels are identical, each having the same (5) strings of 18Volts with a blocking diode at the end of each of the 5 strings before they are all tied together and fused before going to the MPPT input. Each panel of (5) strings will see between 18 and 20Voc in direct light, closer to 18 as the cells warm up and 20 when cool. Once connected to the MPPT, of course they drop to about 15 as the controller tries to boost the current with the excess voltage to deliver a 14.2 bulk rate charge.

What I don't understand is what will happen when the 2nd panel of (5) strings with their own blocking diodes is connected in parallel to the 1st at the input of the MPPT. With one panel being East and the other West, clearly they will not be delivering equal power. Will this screw up the tracking and voltage/current selection of the lit panel by the MPPT or will it disregard the 2nd panel because it is lesser power? For a short time both panels will be lit equally and I am sure everything will work ok, but what about before and after that?

Are the blocking diodes enough to effectively isolate each of the 10 strings (2 panels) from each other that I do not need a 2nd charge controller?

Last question - Hypothetically, if you did have 2 charge controllers going to the same battery, would it be a problem for their outputs being tied to eachother at the batt? What if they were not identical controllers or one was MPPT and the other PWM?
Thanks a million guys!

[inetdog]

What I don't understand is what will happen when the 2nd panel of (5) strings with their own blocking diodes is connected in parallel to the 1st at the input of the MPPT. With one panel being East and the other West, clearly they will not be delivering equal power. Will this screw up the tracking and voltage/current selection of the lit panel by the MPPT or will it disregard the 2nd panel because it is lesser power? For a short time both panels will be lit equally and I am sure everything will work ok, but what about before and after that?
Last question - Hypothetically, if you did have 2 charge controllers going to the same battery, would it be a problem for their outputs being tied to eachother at the batt? What if they were not identical controllers or one was MPPT and the other PWM?
Thanks a million guys!

Here is the beautiful part:

Since the panels within each series string will see the same unshaded light, just not striking them at the same angle, they will both produce exactly the same open circuit voltage, Voc (unless one set is a lot warmer than the other, and that will usually only happen early in the morning when the unlit ones are not producing any power anyway.) In addition, although the two strings will deliver vastly different amounts of current (and therefore power) the two strings will each deliver their maximum power at the same Vmp voltage value.
The result is that for all practical purposes, the MPPT operating point for the higher power string is also the best voltage operating point for the lower power string. Since they are in parallel, they will see the same voltage.

If you have two identical generators and spin one faster than the other they will deliver different amounts of power, and at different voltages. Solar cells do not work that way.

Last, but not least, as long as the two charge controllers together will not supply more power than the batteries can take when they are running flat out, and the voltage settings on the two controllers are set to compatible levels, they will work just fine. Anybody with really large battery banks does that regularly, as do those who charge from generators or wind in addition to solar.
What you cannot do is run two CCs from the same panel array whether they are connected to the same battery bank or not. With two PWM controllers, you might get away with it, but if at least one of them is MPPT it will not work right.

[sgking2]

Awesome! Thanks, that was exactly the information I was looking for.

It might seem like overkill, but I was thinking of (3) battery monitors, one from each panel so see how much they are contributing individually and one between the battery and controller to see total current in/out and voltage or DOD status. If I can get those battery monitors for $30 or so I don't mind having 3. Do you see anything wrong with that setup/use?

[Naptown]

Awesome! Thanks, that was exactly the information I was looking for.

It might seem like overkill, but I was thinking of (3) battery monitors, one from each panel so see how much they are contributing individually and one between the battery and controller to see total current in/out and voltage or DOD status. If I can get those battery monitors for $30 or so I don't mind having 3. Do you see anything wrong with that setup/use?

Just make sure the monitors don't consume too much power particularly with a very low power solar source.

[sgking2]

Just ordered (3) Watt's Up meters for $40/ea. Not a bad deal. Will let you know how it turns out after I add the 2nd panel. I think they are about 2Wh/day so that shouldn't present a problem (12v x 7mA).

[sgking2]

Watt's Up Meter

Hey Guys,
My Watt's Up Meter is in circuit but I would like your advice. I want it to automatically reset everyday so you can see the accumulated totals (for that day). The easiest way to do this is to power it by the panels so when they drop below 4V (dark) the meter shuts down (resets) and does not come back on until dawn when the voltage climbs from 0 to 18V again and crosses that 4V threshold. However, since the meter doesn't seem to like to see it slowly climb into range, it comes in with a messed up screen and I have to again manually disconnect and reconnect it to clear and properly 'reboot' the device.

Is there a way to use a 5.1 Zener diode or even a voltage regulator that would not provide any voltage to the meter until it crossed 5.1 or so volts so it comes on instantly instead of seeing this gradual climb? Or maybe even a cap to act as a buffer so it doesnt come in shaky (nonlinear).

I know a simple momentary pushbutton clear solves this, but I would like it to be cycled daily automatically using the power swing to faciliate it - through the dark (<4V) to reset.
Thanks.

[FloridaSun]

Hey Guys,
My Watt's Up Meter is in circuit but I would like your advice. I want it to automatically reset everyday so you can see the accumulated totals (for that day). The easiest way to do this is to power it by the panels so when they drop below 4V (dark) the meter shuts down (resets) and does not come back on until dawn when the voltage climbs from 0 to 18V again and crosses that 4V threshold. However, since the meter doesn't seem to like to see it slowly climb into range, it comes in with a messed up screen and I have to again manually disconnect and reconnect it to clear and properly 'reboot' the device.

Is there a way to use a 5.1 Zener diode or even a voltage regulator that would not provide any voltage to the meter until it crossed 5.1 or so volts so it comes on instantly instead of seeing this gradual climb? Or maybe even a cap to act as a buffer so it doesnt come in shaky (nonlinear).

I know a simple momentary pushbutton clear solves this, but I would like it to be cycled daily automatically using the power swing to faciliate it - through the dark (<4V) to reset.
Thanks.

Why clear it automatically, daily, when your interest is in the data and it needs to be recorded daily anyway? I record evening or early morning, break the circuit from battery, turn it back on, all is well.
Not sure why you're getting 'shaky' results. Mine works perfectly whether I reset it daily or not. Could be what you are seeing is 'shaky' output from your panels? The CC is not supplying the minimum 4volts to power the instrument and the panels will not without decent sun. You're not going to be getting any charge capacity at that rate anyway so.... will it stay on after the panel puts out steady voltage? Using it on an individual panel would be an interesting check for home made panels but really seems the daily charge into battery more relevant to your operating system, having one wired between charge controller and battery.

[inetdog]

Why clear it automatically, daily, when your interest is in the data and it needs to be recorded daily anyway? I record evening or early morning, break the circuit from battery, turn it back on, all is well.
Not sure why you're getting 'shaky' results. Mine works perfectly whether I reset it daily or not. Could be what you are seeing is 'shaky' output from your panels? The CC is not supplying the minimum 4volts to power the instrument and the panels will not without decent sun. You're not going to be getting any charge capacity at that rate anyway so.... will it stay on after the panel puts out steady voltage? Using it on an individual panel would be an interesting check for home made panels but really seems the daily charge into battery more relevant to your operating system, having one wired between charge controller and battery.

Some DC powered electronics, particularly if it includes any kind of CPU, will require a clean reset of the electronics at power up. This is often provided by a voltage detector which starts the CPU in a known state, reading programming from ROM, when the CPU detects a power up from an off state. One way to do this is to hold the RESET line of the CPU asserted until a DC voltage has risen above a preset level. If the voltage threshold for this circuit is low enough that some other part of the electronics (memory, I/O, etc.) is not reliable at that voltage, the CPU will end up in an unexpected program state and will likely crash part way through the startup sequence. A simple Schmitt-Trigger circuit with a higher threshold and hysteresis would provide a clean startup in this situation, but if you have to work entirely from the outside of the device, then latching circuit of the above type which controls power to the Watt's Up would be one way to go. I cannot think of a passive circuit using zeners or other components that would do what you want.

PS: Depending on the voltage thresholds involved, some units of the same product may have very different behaviors when faced with a slowly rising (or rising and falling!) DC input voltage. YMMV.

[sgking2]

Why clear it automatically, daily, when your interest is in the data and it needs to be recorded daily anyway?

If it was easy enough I just figured it would eliminate the need for a reset and not consume power through night cycles (albeit tiny amounts). I would take the readings near dusk when it was still under power.

Not sure why you're getting 'shaky' results. Mine works perfectly whether I reset it daily or not. Could be what you are seeing is 'shaky' output from your panels?

Maybe I miscommunicated, when the panels are driving more than 6V, the power is stable enough and the meter works as intended. It just seems to like an on or off state and not a gradual climb to that 4.8v threshold. When it climbs gradually from 0 to 18 it locks up and stays locked up even when the power it sufficient (all three do), but a quick cycle of the power and they are all fine. Its that slow transition that seems to do it.

Using it on an individual panel would be an interesting check for home made panels but really seems the daily charge into battery more relevant to your operating system, having one wired between charge controller and battery.

That was the main reason for this experiment. These are 2 homemade custom panels, one facing SE and the other SW and I was just curious what each was contributing over the course of the day. Ultimately once I am clear what they harness, I will just move the meter between the CC and the battery as you suggested. At that point I will just add the momentary reset switch and leave it powered 24/7.