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Old 10-10-2016, 04:53 PM   #1
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Question Solar Charger Voltage difference

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?
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Old 10-10-2016, 05:42 PM   #2
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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.
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Old 10-10-2016, 06:27 PM   #3
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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.
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Old 10-10-2016, 07:22 PM   #4
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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.
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Old 10-11-2016, 04:47 PM   #5
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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.
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Old 10-11-2016, 05:08 PM   #6
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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.
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Old 10-11-2016, 06:34 PM   #7
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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.
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Old 10-11-2016, 07:20 PM   #8
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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.
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Old 10-11-2016, 09:40 PM   #9
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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/
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Old 10-12-2016, 08:02 AM   #10
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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
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