Solar Charger Voltage difference

[sjansky]

I have two 130 watt panels and a MorningStar Pro 30M charger. This charger has the usually battery connection for charging battery and additional connections for "Battery Sense" wires. The battery sense voltage can be as much as 0.6 to 0.8 volt lower than the charge line connections voltage - greater difference when charger is running at a higher rate of charge. MorningStar indicates this is supposed to help prevent under charging batteries. When I disconnect solar panels from charger the two voltages are the same regardless where messaged. My set up was wired prior to my ownership with the usual 10 gauge wires in a protective sleeve.

I am thinking about getting an MPPT charger as prices have started to come down - none of the MPPT chargers I have researched have an additional "Battery sense" connection. My question is will the MPPT charger undercharge the battery because it uses the same wires to charge as to sense voltage? Short of rewiring my charger to battery with 6 or 8 AWG wire any other suggestions?

[ScottBrownstein]

Your maximum current will be about 11 or 12 amps for which the voltage drop would be about .4 volts for a 20 foot run. Remote sense on your current unit takes care of that. Frankly MPPT is not a magic bullet and for a system of your size it probably doesn't make sense anyway.

[sjansky]

Your explanation of a voltage drop for the size of wire seems to be the opposite of what I am seeing. The battery sense wires are 16 AWG, per Morningstar's recommendation as these wires carry little or no current. The charge wires are 10 AWG and show higher voltage when the wires are carrying a higher charge current to the batteries. When solar panels are disconnected the battery sense and charge terminals on the controller show same voltage. The same wires show higher voltage when carrying current - confusing.
I am using a Fluke multimeter for voltage measurements.

[ScottBrownstein]

Well, I guess it all depends on how you take it. If you pump 12 amps from the converter, the #10 will cost you about .4 volts. So...the reading that you get at the converter itself will be .4 volts higher that what you get on the batteries (remote sense leads.) However, the controller will be bumping the output voltage by that amount so the battery will get the right voltage but there will still be a .4 volt difference when the current is flowing.

[WolfWhistle]

I must be missing something and I can be quite dense. Why would you not expect to see higher voltage when charging? Since, voltage drop % stays the same, the higher the charging voltage the higher the voltage will be but at the same %. I also don't see why the voltage drop on the sense wires are a concern.

The SCC is simply compensating for the voltage drop to the batteries and adding more tenths based on that loss. It should be also compensating for temperature if you have a temperature probe.

I am not sure if you have a monitor or not, but I wouldn't be without a good one.

As far as your question, I don't know if generically speaking an MPPT will compensate, or if you have to compensate by setting Absorb and Float voltages "artificially" higher. I would think they would.

MPPT has it place, such as when using 60 or 80 cell panels (higher volts), or when used on-grid.
Bogart Engineering puts it this way: "We have compared at least one commonly used MPPT charger with the SC-2030 and found that under very ordinary conditions the SC-2030 delivered more charge to the batteries. We measured this when the ambient temperature was 70 F degrees in full sun, and when the proper panels matched to the batteries were being used and when charging over 13.0 volts (the most common charging range with lead acid batteries.)

The SC-2030 is a "PWM" (Pulse width modulated) type that is simpler and less costly than a "MPPT" (Maximum Power Point Tracking) type charger. As said, the SC-2030 can give even better performance under some common situations. MPPT technology can give some advantage when temperatures are low, and it is necessary for good power transfer when panel voltage is much higher than the battery system voltage. With the SC-2030 (or other PWM charger), you may be able to get more total performance at the same cost by purchasing another properly matched solar panel instead of a more expensive MPPT solar charger.

A common mistake for evaluating MPPT performance is to compare their (lower) solar input current with (higher) output battery current, and thinking this additional current is solely due to the MPPT charger. This is incorrect, and will give an exaggerated impression of its advantage. A comparison must be done by changing to the PWM controller and then comparing battery currents."

HandyBob (https://handybobsolar.wordpress.com/) agrees with this publication and I trust him, since he doesn't have a quota to fill or a hidden agenda.
WW

[ScottBrownstein]

All totally true. The OP was concerned with replacing his PWM controller with an MPPT that does not have remote sense! In that case the voltage at the batteries would be lower due to the drop in the charging wires.

I too am a really big fan of the Bogart stuff. Well designed, well executed! MPPT is necessary for grid tied systems where you want to series the panels to drop the current and raise the voltage.

[Skyliner]

Many solar systems are underwired meaning that their is excessive voltage drop due to too thin gauge wire and/or too long runs for the gauge wire used.

Most solar installers feel 3% drop in voltage is acceptable. I don't. At 13.8volt, 3% drop is about .4 volts and I feel that is too much.

My solar system is designed for a 1% drop in voltage and that mean ditching 10 gauge wires in favor of thicker wires and deliberately keeping runs short.

[ScottBrownstein]

You are right on balance, however, a controller with remote sense eliminates the voltage drop entirely. Most solar panels provide the controller with way more voltage than the batteries can take. Remote sense with 300 watts of solar will never use more than 15 amps of charge current, its really not necessary to over wire the system, just let the controller do its thing and Bogart's SC 2030 does it to perfection when paired with a TriMetric.

[Skyliner]

I disagree but it is understandable for one to not want to admit their system not ideal and designed to extract maximum efficiency from the solar panels.

I'm a die hard boondock camper and rely on my solar to conserve gasoline that is better used on the ATVs then the backup generator.

In perfect conditions there may be excess voltage that one can afford to lose due to power transmission losses (resistance). However in overcast and early morning or evening conditions, that .4 volt drop can make all of the difference. My panels will put out 20.1 volts when it is very sunny outside and sun is overhead. However, these are ideal conditions which occur only several hours a day. The 20.1 volts is nice because it gives me greater amperage via the MPPT controller to top off my 696 amp-hours of battery capacity at 14.8 volts

My controller will push voltage to batteries up to 14.8 volts which is almost ideal (prefer 15 volts). I can get that voltage to my batteries when the sun isn't as bright and every tenth of a volt counts especially in less then ideal conditions.

I started doing solar installs and solar repairs/upgrades this year and have have come across many solar systems that didn't do much other than occupy space. The common reason was too thin gauge wire resulting in voltage loss. I don't have any websites or fancy credentials to stand behind but if you go to Handy Bob's website, he pretty much states the same as I do regarding wire size.
'
Read HandyBob's comments here about voltage: https://handybobsolar.wordpress.com/...ging-puzzle-2/

[pmsherman]

I don't have solar panels on my RV (yet) but have a lot of experience with them as part owner of an off-grid solar/wind powered remote observatory.

For a single solar panel, as pointed out, there's often very little difference between a PWM and MPPT controller. Once you get into multiple panels, however, an MPPT controller allows additional options. MPPT controllers allow panels to be connected in series instead of parallel. Series connections provide power from the panels at higher voltage and lower current, which significantly cuts voltage drops in the wiring from the panels to the controller. If you cut the current in half, you cut wire losses to 1/4 of what they would be at the higher current.

Solar panels also don't produce full voltage until the sun strikes them at a certain angle. Series wired panels can produce a charging voltage for a little longer time each day than parallel wired panels. Series wired panels do have one downside though, if any of the panels in the series is shaded, there's very little output from the entire set of panels.

This is a case where multiple factors need to be balanced - wiring size, costs, potential shade issues and other factors. There's not single solution that's right for everyone.

Phil

[WolfWhistle]

S, how long wire run from panels to the Prostar 30? Gauge? How far to batteries? By keeping the SCC closer to the batteries, the voltage compensation feature is less needed. And, if you have your heart set on MPPT (I can't imagine why, but who am I) can you bump up the Absorb / Float voltages up a skosh in setup?

Responding to #9. And the last one.

Tony, Handy Bob cautions his readers that spending money on cabling that goes beyond the manufacturers recommendations (-3% in my case) is a waste of resources and money, while pointing out many, if not most are indeed undersized. He got on me for going with #4/0 - inverter to batteries - when #2/0 would have worked ... if marginally.

I read about 21 volts from my panels no matter the ampere available /demand as long as its enough sun for them to be charging. So, like SB so correctly stated (#2), "controller with remote sense eliminates the voltage drop entirely" .. with higher voltage available in abundance and as needed ... as long as it isn't beyond the SCCs design limit ... 3% in the case the SC-2030.

Pmsherman, X2. I think for a house were shade can be avoided and nothing is moving, MPPT and series panels makes perfect sense especially considering the small wire size and longer wire runs. But, RVs move and shadows fall and the run is often not so long as to consider larger wire sizes too costly. Most people don't want to crawl onto an RV roof to tilt panels for a short stay; I would add a panel rather than do that. Many folks don't realize how affected panel output is when just a small (in close) shadow falls on one cell. If it falls on a parallel panel only that panel is affected, but if in series the entire string's output could drop to near nothing.
WW

[ScottBrownstein]

This always descends to more copper!!! Frankly, we all have to look at the specifics of our units to determine the best course of action. Most Georgetowns with rear power bays suffer from one basic limitation. The converter is almost 22 feet from the batteries and is wired with #6 or #8. Forgetting solar, my 70 amp PD converter (with no remote sense) loses .3 to .4 volts when charging, which lengthens genny run times. It seems that FR has not only wired the converter with small leads but has run both the positive and the negative all the way to the batteries, rather than resorting to a chassis ground for at least the negative, which effectively cuts the run length in half. For those who point out that there are issues with chassis ground, let's not forget that the whole battery bank (in my case 4 deep cycles) connects its negative ONLY through a chassis ground! The best fix here is to ground the negative and up the positive to something a little bigger, which will help all around.

My 300 watts of solar (parallel) comes down from the roof into the rear bay with standard #10 PV wire. The voltage drop from the panels to the controller is far less important since this voltage is unregulated and is generally far higher than the voltage needed by the controller to maximize output to the batteries (even for 15 volt equalization!) They connect to the SC2030 in the rear bay and piggy back to the battery bank using the converter connections, as does my 1000 watt xantrex inverter. The main bus from the converter is used by the inverter (converter not running); by the SC2030 with remote sense and by the converter when connected to shore power.

Now the converter wants to put out 70 amps and does not have remote sense. The 12 to 15 amps or so from the panels has a much lower v-drop, since it tops out at maybe 20 amps and the 2030 will up the voltage to compensate for the battery bus drop.

Copper is not a magic bullet. Unfortunately most RVs are designed to be connected to shore power all the time. Boondocking is another thing and you will have to adjust the design to accommodate it best. Using MPPT for 300 watts is pretty out there, especially if you already have a Trimetric.

[sjansky]

My system when I purchased the rig was a single 75 Watt panel and two T105. I purchased two additional 130 Watt Kyocera panels and the ProStar 30 charger. I removed the 75 watt panel and old charger to trickle charge my truck batteries when in the garage.

All wiring is 10 AWG - length from panels to charger is about 5 feet and length from charger to batteries is 12 feet. I live in Colorado but do not boondock in the winter - temperatures in Spring and Fall can be in the thirties. My panels are mounting flat. I do not currently have an inverter so just running lights, pump, and furnace on 12 volt. I have converted most of the daily use lights to LED. I had to replace the T-105s earlier this year with a couple of T-145. The T-105 lasted ten years

Would the Bogart charger/meter make a significant difference in charging and/or battery like compared to the ProStar 30?

[ScottBrownstein]

Originally Posted by sjansky

...Would the Bogart charger/meter make a significant difference in charging and/or battery like compared to the ProStar 30?

Not really. You have a pretty good unit. Besides, the Bogart controller only works in conjunction the their TriMetric battery monitor which mounts inside the RV and gives you a really good view of your state of charge, your charging sources like converter, engine, solar, etc. It would cost you almost $300 to switch and i don't see the value. You should be pretty good since the ProStar has remote battery sense as well.

[WolfWhistle]

I agree with SB but many people have the Trimetric TM-2025 or the newer TM-2030 to monitor what ever they have. Not a thing wrong with the PS30.

What is your system not doing that you would like it to do better? Could it be you are not getting those great batteries fully charged? But, if the first lasted 10 years, you are doing something right
WW

[sjansky]

I do not believe that the batteries are getting a full charge - the charger seems to go to absorb and float very quickly. I probably just need to get a good hydrometer to check for sure. I am thinking about getting a temp sense probe and put in fridge to "fool" the charger into adding more voltage to batteries. If Handy Bob is right and you need higher voltage and more amps to batteries - I really think the T-145 are under charged. The settings on ProStar 30 is 14.4 for bulk - it does an equalize charge every 25 days to 14.9 for an hour and 15.1 for two hours if battery voltage goes to 11.7.

[ScottBrownstein]

The Bogart SC2030 has a profile for Trojan batteries that does an equalize at 16.2 volts. This is changeable and settable manually, of course. Trojan generally advocates significantly higher voltages than some of the other vendors. Don't know how the rest of the stuff in the system will react to a dc voltage of 16.2, but a solar controller easily has that much voltage at its disposal.

The problem with converters like Progressive Dynamics is that their power supply sections usually top out at 14 volts or so and can't manage much of an equalize.

[WolfWhistle]

Originally Posted by sjansky

I do not believe that the batteries are getting a full charge - the charger seems to go to absorb and float very quickly. I probably just need to get a good hydrometer to check for sure. I am thinking about getting a temp sense probe and put in fridge to "fool" the charger into adding more voltage to batteries. If Handy Bob is right and you need higher voltage and more amps to batteries - I really think the T-145 are under charged. The settings on ProStar 30 is 14.4 for bulk - it does an equalize charge every 25 days to 14.9 for an hour and 15.1 for two hours if battery voltage goes to 11.7.

I like the idea of fooling your SCC.

I have my Absorb point (the voltage where Bulk ends) set to 14.8 volts; Float is 13.2v right now. I don't have Trojans but I have not needed to Equalize in the year I have had these Duracells, but I am close. Higher charging voltages (nothing wrong with 14.6v to 14.7v just takes longer) seems to be the key. My SCC considers full charge (and resets to 100%) when 14.8 is reached the same time 2% of the Ah that is set (460*.02=9.2A). Using the Magnum (set to 80% of its 100A) I have to lower the reset voltage on the SCC to 14.7v to get a reset to 100%. I can also raise the % to 2.5 or 3 and accomplish the same thing. At the resting voltage (and SG) for 50% battery, the Magnum can get these 460 Ah of battery through Bulk... 1.5 hours of Absorb to Float (if I am using float) in about 3 hours. The Trimetric is showing 102 to 105% by that time. I love having a good charger. These are so programmable that frankly, I am still learning how it all works.
WW

[ScottBrownstein]

Looks like if you get a sensor and put it in the freezer, you will get between .125 and .250 more than you have at 25 degrees C. Might work, sort of the kind of thing that I might try before my wife would throw it out!