My 200W, MMPT system could not recover "full charge" in bad conditions.

[TheSteeleFamily]

Originally Posted by rickst29

And you know what they say around here: "It's a dry heat, not so bad".

So is the flame on my stove, but I ain't putting my finger in it.

[scrubjaysnest]

Originally Posted by LoveToCamp

rickst, no, I don't have "Y" connectors. I have one panel on the back shell, and am adding one on the front shell. Due to the distance between, and being on separate shells, I was told I can run wires from each one to the controller, and join them at the controller. Two wires into the + port, and two wires into the - port.

That will work fine; install breakers in each panel. That is how I wired the roof mounted and the portables. Breakers also give you a means to shut off a panel to work on things if need be.

[scrubjaysnest]

Originally Posted by BrucePerens

I think that a theoretical PWM charger that could handle series panels could provide some additional charge in dim conditions. Not as good as an MPPT.

The internal resistance of the battery is unknown at any particular time until we measure it as R = E/I. We can offer the battery any voltage we want, and we don't know if the battery accepts a charge or not until we measure the current. So, we generally regulate charge by regulating the charge current, rather than the charge voltage, until the battery reaches its target voltage at which point we ramp down the current.

PWM's as a general rule will not step the series voltage down to 12 volts. To my knowledge only the MPPT's will do that. Also the recommended ratio input to out voltage is 2:1.

[scrubjaysnest]

Originally Posted by BrucePerens

If you want to figure out how MPPT works, the various IC manufacturers have published data sheets on how to use their chips in an MPPT charger. These tell you everything in tremendous detail. See Linear, OnSemi, TI, and another TI. And those are the ones that came up on my first Google search!

Most of them use electrolytic capacitors, rather than inductors. Electrolytics will store more energy than inductors for the same size. Also, inductors can release their stored energy as a really high voltage, with ringing, which isn't really what we want.

Electrolytic capacitors have limited lifetimes, though. Over time they will develop increased series resistance and will need to be replaced. I have a meter that measures this, perhaps in a few years I'll check out my controller.

Accoriding to the folks over on the solar-electric forums the best MPPT's use transformers so the input is isolated from the output.

[BrucePerens]

It isn't necessary to step the series voltage down to 12 volts. You can charge the battery with any voltage higher than 16 volts and low enough that there won't be arc-over. So, a pulse at your full series voltage would do it, as long as the overall current is limited by the PWM to that specified for the battery, so that the battery temperature doesn't go too high.

Batteries are not charged by voltage. A current is forced through them to charge them, and the battery increases in voltage as it is charged.

We're used to thinking of this as voltage because it's easier for people to understand voltage, and because of our old alternator charging systems which used linear regulation.

[BrucePerens]

Originally Posted by scrubjaysnest

Accoriding to the folks over on the solar-electric forums the best MPPT's use transformers so the input is isolated from the output.

They are talking about their home power systems with grid-tie, where they are connecting to the regional power grid and isolation is important. That doesn't make sense in the context of an RV. First, you should consider what is necessary for a transformer to work. You would need to change the DC from the panel into AC. Transformers require a constantly-changing magnetic field to work, thus AC. If you are going to go to all of that effort, you might as well take the switches you'd need and use them to drive a capacitor ladder, which can produce any voltage without the use of a transformer.

Transformers are best driven by sine waves. They become lossy when driven by square waves which would be fine for a capacitor ladder. But the circuitry to make sine waves is itself lossy. So, you would not get the conversion efficiency that you get with electrolytic capacitors.

Finally, why do we need to isolate the panels from the rest of the system on an RV rather than a home grid-tie system? For safety? In the case of a short circuit in the controller, the full voltage of the panels could be put across the output. Maybe this happens with cheap controllers, good ones (like the Morningstar I'm using) have crowbar circuits, fuses, etc. to prevent this.

All of this is why we have higher efficiencies in our DC-only RV systems than the grid-tie folks can achieve. They have to make 60 Hz sine-wave AC, and they lose around 20% of the power on the way.

Land-based solar panels require a GFI for safety, but they generally run at higher series panel voltages than we can achieve in an RV. I would not mind having a GFI once someone makes an outdoor-mountable one. It should really go at the panels, rather than the controller. Otherwise, you have a long unprotected wire run with no benefit from the GFI.

[scrubjaysnest]

Originally Posted by BrucePerens

They are talking about their home power systems with grid-tie, where they are connecting to the regional power grid and isolation is important. That doesn't make sense in the context of an RV. First, you should consider what is necessary for a transformer to work. You would need to change the DC from the panel into AC. Transformers require a constantly-changing magnetic field to work, thus AC. If you are going to go to all of that effort, you might as well take the switches you'd need and use them to drive a capacitor ladder, which can produce any voltage without the use of a transformer.

Transformers are best driven by sine waves. They become lossy when driven by square waves which would be fine for a capacitor ladder. But the circuitry to make sine waves is itself lossy. So, you would not get the conversion efficiency that you get with electrolytic capacitors.

Finally, why do we need to isolate the panels from the rest of the system on an RV rather than a home grid-tie system? For safety? In the case of a short circuit in the controller, the full voltage of the panels could be put across the output. Maybe this happens with cheap controllers, good ones (like the Morningstar I'm using) have crowbar circuits, fuses, etc. to prevent this.

All of this is why we have higher efficiencies in our DC-only RV systems than the grid-tie folks can achieve. They have to make 60 Hz sine-wave AC, and they lose around 20% of the power on the way.

Land-based solar panels require a GFI for safety, but they generally run at higher series panel voltages than we can achieve in an RV. I would not mind having a GFI once someone makes an outdoor-mountable one. It should really go at the panels, rather than the controller. Otherwise, you have a long unprotected wire run with no benefit from the GFI.

Has nothing to do with grid tie, they are using the same mppt CC's from Moringstar, Blue Sky and Outback as the rest of us.

[LoveToCamp]

I went out for three nights last weekend, with my additional 100w panel. My wiring seems to have worked properly, as I was fully-charged by noon each day. Ran furnace in the mornings, as DW has thin blood.

Very glad I installed a second 100w panel, as it was overcast most of the time, and I know one panel would not have kept up charging my two 12v batteries.

[Padgett]

Good rule of thumb is 1W of solar for every AH of battery. I have 210 AH in 2 GC2s & 2 100W panels. This works for my needs in sunny Florida.

Can say that one 105 AH Grp 31 AGM can start and run including stops an Olds 455 all day long when an unobtanium alternator failed 100 miles into a 1200 mile run.

[BrucePerens]

Originally Posted by scrubjaysnest

Has nothing to do with grid tie, they are using the same mppt CC's from Moringstar, Blue Sky and Outback as the rest of us.

If you look at The Morningstar MPPT Technology Primer you will see a clear statement that all Morningstar MPPT controllers are buck converters. A buck converter doesn't use a transformer to do its work, it has an inductor or (more recently) a capacitor as its power conversion device. You can read about the older style of inductive buck converters at the Wikipedia article on Buck Conerters. But these days we use switched capacitor buck converters instead because they are more efficient, cheaper, lighter, and don't have the problem of inductive heating of their cores and the mechanical buzz created by an oscillating magnetic field. You can read a simple tutorial about switched capacitor converters in this Maxim technical note and an exhaustive technical paper about them in this U.C. Berkeley paper.

With all due respect, I would like to see your friends show me a schematic for a modern commercial DC-input-to-DC-output MPPT controller using a transformer to do its conversion. It just would not be as efficient as another choice. Also, there seem to be very few
DC-input-to-DC-output MPPT controllers advertising any form of input-to-output isolation. So, I think your friends are not quite up to date with the state of the art of power electronics. Not their fault. Electronics has been changing quickly and much of the way we did things when I learned has been replaced.

Now, I'll show you something that might be confusing your friends. This photo (attached) is from a fellow ham who has taken a few MPPT controlers apart, see his blog at http://www.kg4cyx.net/category/solar/
There are two things that look like transformers, don't they? But they are clearly labeled as inductors. They are in a buck converter circuit.

If designed today, they'd be better with capacitors.

It's just a question of efficiency. Sure, you could use transformers for this. And anyone who learned electronics in the 1970s and hasn't kept up might reach for a transformer, given the job, because that's how we used to do things. But more recently we have very many small devices that need extremely good power efficiency, and need to be small, light, and cheap. And thus we have learned a lot about switching power regulation, including kinds that can trade voltage for current and vice versa without transformers.