Your Charge Controller "Charges" the line between your Battery.

Hi. So, there is a persistent misconception that the line voltage from your charge controller to your battery IS the batteries voltage, and if it is off, there is something wrong with the system.

This is inaccurate.

The charge controller "charges" the line from your "charger" to your battery.

In the "First" Images 1 & 2, you see the Batteries BMS shows a Voltage of 13.6, and the Charge Controller shows the Voltage of 14.7
In these images the System is on, and charging normally, the charger is receiving energy from the solar panels and then sending it to the battery by charging the line from the controller to the battery. (The BMS is showing the actual voltage of the battery.)

In the "Second" Images 1 & 2, you see the BMS AND the Charge Controller read the same exact voltage of 13.4/13.5v This IS the Batteries Voltage. I unplugged the Solar Panels from the Charge Controller so there was no current, and you can clearly see, the Charge Controller immediately dropped the reported voltages down from 14.7 to 13.5 BECAUSE: THERE WAS NO CHARGE ON THE LINE. 13.5 IS the Batteries Voltage.

In the "Third" Images 1 & 2, I reconnected the Solar Panels and the system is back in normal operation. And again, the Charge Controller shows the voltage as 14.7, while the Batteries BMS shows the Voltage of 13.5 - Again, the Charge Controller "Charges" the line from the Charger to the Battery.

The wires from my Charge Controller are 2 feet long and 8 AWG - 8 AWG is rated to handle 50 Amps.

There is no issue with the system, equipment or devices, what you are seeing is common with PWM, Lead Acid, MPPT, LIFEP04, or any of the configurations and combinations you can imaging for a Solar System receiving a lot of sun, charging the line and putting amps into the battery.

This is normal. Anyone saying that the "Charge Controller" line voltage is the Batteries Voltage is incorrect.

I am in the process of doing just that, I will use my Lead Acid Batteries and PWM that I have replaced as I have upgraded the components on my system, I will buy and use heavy gauge wires, I will use MPPT with Lead Acid, PWM with LIFEP04 etc, and make a video so people can see, with any configuration, with enough solar, your charge controller charges the line and it is NOT your batteries voltage - and that is normal and how electricity works.

Please make sure to look at the images.

It only 'charges' it if your SCC voltage is higher than the battery, otherwise your battery 'charges' it the other way. That's how current works - it flows from high to low, and there has to be a differentiation for that to happen.

The only place you'll get accurate battery voltage is on it's own terminals with no other loads or charge, and after allowing it to reach its resting voltage.

Michael77 said:

This is normal. Anyone saying that the "Charge Controller" line voltage is the Batteries Voltage is incorrect.

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Obviously you have not seen the voltage when the battery is charged and the charger is at rest.

Have you validated those readings with a multimeter? Without that you are trying to reach a conclusion that is inconsistent with science. I have seen small differences between my BMS, my inverter/charger and my multimeter but nothing that large that was not diagnosed. When I have seen larger discrepancies, it has often been a loose connection or something that caused resistance in the path from one of the devices. The miultimeter readings helped me diagnose the actual issue.

Browneye said:

It only 'charges' it if your SCC voltage is higher than the battery, otherwise your battery 'charges' it the other way. That's how current works - it flows from high to low, and there has to be a differentiation for that to happen.

The only place you'll get accurate battery voltage is on it's own terminals with no other loads or charge, and after allowing it to reach it's resting voltage.

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Thank you. Please tell that to half the other people posting on the posts about this. This misconception and misinformation is so bad it actually is causing people (like me when I was researching) to have wrong information. I have been told so many times on this forum by many different sources that the Charge Controller Voltage reading IS the Batteries Voltage and if they are off by even the slightest, there is "something wrong with the system..." I feel like I am literally fighting a never ending army of zombies... lol

Mattb4 said:

Obviously you have not seen the voltage when the battery is charged and the charger is at rest.

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I do not understand what you are trying to say, are you saying that once the battery is full and the charger is at rest? I have seen when the battery is fully charged and the charger is at rest. The Batteries BMS shuts off charging, and drops from its set charge parameter of 14.2v and slowly drops down to 13.33 Volts which is its rest voltage. Once the battery is used and the voltage drops below the set parameter of ~13.32 it turns charging back on and starts charging again. That is the normal operation for a modern LIFEP04 Battery System.

Ampster said:

Have you validated those readings with a multimeter? Without that you are trying to reach a conclusion that is inconsistent with science. I have seen small differences between my BMS, my inverter/charger and my multimeter but nothing that large that was not diagnosed. When I have seen larger discrepancies, it has often been a loose connection or something that caused resistance in the path from one of the devices. The miultimeter readings helped me diagnose the actual issue.

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Yes, Many many times, I have validated this with a Multimeter. The system is working normal. There are no loose connections, there are no errors or problems with the system - will make video soon. Thank you.

Ideally your BMS should not be turning off charging. Your charge controller should be handling that.

Mattb4 said:

Ideally your BMS should not be turning off charging. Your charge controller should be handling that.

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Dear Sir, I hate to break it to you, but you are completely inaccurate. Incorrect. The Battery BMS Manages the Battery.

Michael77 said:

Dear Sir, I hate to break it to you, but you are completely inaccurate. Incorrect. The Battery BMS Manages the Battery.

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You need to be promoted t

Michael77 said:

Dear Sir, I hate to break it to you, but you are completely inaccurate. Incorrect. The Battery BMS Manages the Battery.

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The "Solar Charge Controller" controls the charging!

Michael77 said:

Dear Sir, I hate to break it to you, but you are completely inaccurate. Incorrect. The Battery BMS Manages the Battery.

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to quote from Wikipedia

A battery management system is any electronic system that manages a rechargeable battery, such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.

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That is not a battery charger.

Unless you're using shit wire, have garbage connections or components, the statement you're refuting is essentially true. Notable differences between charger and battery voltages indicate excessive resistance.

First, Before you get all nutz with experimentation, have you confirmed both the BMS and MPPT readings with a separate voltmeter?

Second,

Using Ohm's Law:

(Vc - Vb) = I * Rw

Vc = Voltage at charger
Vb = voltage at battery
I = current delivered by charger
Rw = Resistance of wire.

This is an idealized calculation because both the charger and battery influence the values, but these are relatively small effects.

4' of 8awg wire is 0.003Ω.

Using your #1 example.

I = 27.6A
Vb = 13.6V
R = 0.003Ω

(Vc - 13.6) = 27.6V * .003Ω
Vc = 27.6V * .003Ω + 13.6V = 13.68V

Yes, connection and internal resistance will make this number a little higher, but there is NO WAY yo will measure 14.7V @ the charger unless something is horrifyingly wrong.

Using your numbers to calculate resistance:

(14.7 - 13.6) = 27.6V * RΩ

RΩ = (14.7V - 13.6V) / 27.6V = 0.040Ω - about 10X what the wire contributes.

You either have a measurement error or excessive resistance.

It's not Ohm's suggestion. It's Ohm's Law.

sunshine_eggo said:

Unless you're using shit wire, have garbage connections or components, the statement you're refuting is essentially true. Notable differences between charger and battery voltages indicate excessive resistance.

First, Before you get all nutz with experimentation, have you confirmed both the BMS and MPPT readings with a separate voltmeter?

Second,

Using Ohm's Law:

(Vc - Vb) = I * Rw

Vc = Voltage at charger
Vb = voltage at battery
I = current delivered by charger
Rw = Resistance of wire.

This is an idealized calculation because both the charger and battery influence the values, but these are relatively small effects.

4' of 8awg wire is 0.03Ω.

Using your #1 example.

I = 27.6A
Vb = 13.6V
R = 0.003Ω

(Vc - 13.6) = 27.6V * .003Ω
Vc = 27.6V * .003Ω + 13.6V = 13.68V

Yes, connection and internal resistance will make this number a little higher, but there is NO WAY yo will measure 14.7V @ the charger unless something is horrifyingly wrong.

Using your numbers to calculate resistance:

(14.7 - 13.6) = 27.6V * RΩ

RΩ = (14.7V - 13.6V) / 27.6V = 0.040Ω - about 10X what the wire contributes.

You either have a measurement error or excessive resistance.

It's not Ohm's suggestion. It's Ohm's Law.

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Hi thank you for that. I hate to say it, but you are not correct. The resistance is AT THE BATTERY. The battery is resisting the current flowing into it, and you need to increase the voltage to increase the "current" flowing into the battery. If you do not understand this simple fact; all I can say (in a kind, friendly, helpful gesture type of way) is get your head out of the class books and set up and measure your own system, with a large enough solar bank that your amps into your battery maxed out or close to max. You will see. Real world and theoretical ideology (that may or may not take into account many many factors) are two totally different things.

(You literally have to have higher voltage delta from the charger to the battery, or no current will flow - the higher the resistance from the battery, the larger the voltage delta has to be to facilitate any increase in current. That is what you call English. lol)

Michael77 said:

Hi thank you for that. I hate to say it, but you are not correct. The resistance is AT THE BATTERY.

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Nope. It's everywhere. I qualified it:

Yes, connection and internal resistance will make this number a little higher, but there is NO WAY yo will measure 14.7V @ the charger unless something is horrifyingly wrong.

Michael77 said:

The battery is resisting the current flowing into it, and you need to increase the voltage to increase the "current" flowing into the battery.

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By the amount dictated EXACTLY by Ohm's law: V = I * R.

Michael77 said:

If you do not understand this simple fact; all I can say (in a kind, friendly, helpful gesture type of way) is get your head out of the class books and set up and measure your own system, with a large enough solar bank that your amps into your battery maxed out or close to max. You will see. Real world and theoretical ideology (that may or may not take into account many many factors) are two totally different things.

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Done and done. Real world follows theoretical ideology. The problem is gathering all the real world data. Getting accurate measurements for the internal resistance of the battery, all the connections in between, any resistance in the charger itself, etc. If you have all that data, it will exactly follow V = I * R.

Internal resistances are typically substantially lower than the wiring resistance. I would never expect to get the theoretical value - maybe 2-3X even, but > 10X. Something's wrong.

Michael77 said:

(You literally have to have higher voltage delta from the charger to the battery, or no current will flow - the higher the resistance from the battery, the larger the voltage delta has to be to facilitate any increase in current. That is what you call English. lol)

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Right. In English it's still V = I * R.

The interesting thing I noticed from looking at the charge controller pictures was the solar voltage was 15.1 volts and the battery voltage was 14.7 volts. I thought MPPT controllers needed a larger delta between solar voltage and battery voltage? I have an AIO inverter and my solar voltage is over 200 volts and my batteries are 54 volts.

Ampster said:

The interesting thing I noticed from looking at the charge controller pictures was the solar voltage was 15.1 volts and the battery voltage was 14.7 volts. I thought MPPT controllers needed a larger delta between solar voltage and battery voltage? I have an AIO inverter and my solar voltage is over 200 volts and my batteries are 54 volts.

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Not sure. I did take off the load (mean, I turned off the BMS so that the battery stopped charging, simple toggle on and off...), just to test and I saw the solar voltage was 19.v so maybe its due to voltage drop over the whole system, from the panels, 4 panels using standard parallel connector to 10 AWG into the Charge Controller and 2 panels paralleled w/standard parallel cable into 10 AWG wire into the Charge Controller, and then 8 AWG to the battery... V droop, or just a dip due to the high amps being absorbed by the battery...

Michael77 said:

Dear Sir, I hate to break it to you, but you are completely inaccurate. Incorrect. The Battery BMS Manages the Battery.

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It is generally accepted that your charging termination parameters are set at the charger level so that the BMS does not need to get involved. Using the BMS to do this is considered bad practice. The BMS shut down of current in or out of a lifepo4 bank is considered emergency use or application.

The battery voltage displayed on the SCC is with the charge current added on top of the system or battery voltage level. It is common for it to read a little higher at the SCC. Again, the only way to get a true volts reading on a battery is after resting with no load or charge current. At that point, your SCC and battery should pretty well read the same. With charging current it is going to read higher at both the SCC as well as the actual battery, and any meters you employ. At least that has been my experience.

Same for loading a system - an inverter will show a slightly lower battery voltage that the actual battery - but the battery also reads lower than at rest, called load sag.

Once your system is built and you are monitoring loads and charging, and what everything displays, you'll have a lot better feel for how your system is performing.

Michael77 said:

The resistance is AT THE BATTERY. The battery is resisting the current flowing into it, and you need to increase the voltage to increase the "current" flowing into the battery.

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Yes, batteries have internal resistance.

Michael77 said:

If you do not understand this simple fact; all I can say (in a kind, friendly, helpful gesture type of way) is get your head out of the class books and set up and measure your own system, with a large enough solar bank that your amps into your battery maxed out or close to max. You will see. Real world and theoretical ideology (that may or may not take into account many many factors) are two totally different things.

(You literally have to have higher voltage delta from the charger to the battery, or no current will flow - the higher the resistance from the battery, the larger the voltage delta has to be to facilitate any increase in current. That is what you call English. lol)

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Love the false sense of superiority, it comes off well.



Remember the battery is a chemical process and to get energy into the battery, the voltage at the battery terminals must increase? Now this increased voltage is still present at the battery terminals and inside the battery, thus is battery voltage. You still run the risk of damaging the cells if you exceed the spec sheet limits.

Do you have familiarity with car batteries or lead acid? Your car charges the battery at roughly 14 volts. After you shut off the engine (stop the alternator), the battery voltage will be something like 12.6-13.0 volts depending on how long you wait.

Both the 12.6 volts and 14 volts are "battery voltage"

If you car battery is mostly dead and you throw huge current at it and get voltage up to 17 volts you will damage/degrade it even if the SOC is low.

Browneye said:

It is generally accepted that your charging termination parameters are set at the charger level so that the BMS does not need to get involved. Using the BMS to do this is considered bad practice. The BMS shut down of current in or out of a lifepo4 bank is considered emergency use or application.

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With respect, and only about this point I quoted above; I beg to differ. I had a NON PROGRAMABLE Charge Controller designed for Lithium Iron Phosphate and recommended by Will on several of his videos (Renogy Wanderer 30 Amp, He got an affiliate commission too...) that I used for years, and it worked great! It charged my battery and when my Battery was full the BMS in the Battery disconnected the Charge.

I respect you, because you are one of the only people who is coherent and IMO on the same page as me, but I can only surmise that there are a lot of old folks in here that are stuck in their ways of doing things and have not played with new, modern equipment.

That is the only reason, I can imagine you would make a statement like that. If you every used any charger that wasn't connected to an advanced charge controller, you would know, with years of experience; that wall chargers, non-programmable PWM Chargers and even old "Lead Acid" Battery Chargers are technically all designed to work with "Drop In LIFEP04 Batteries" (Well, its the other way around really; Drop In LIFEP04 Batteries are designed to be compatible with as much of the old 12v hardware as possible, and the BMS is designed to be robust and protect the battery.

If I could only ask, pleas be mindful of this, I did my research over the years and I saw this forum come up a lot when searching for information, and honestly, I never joined or used this forum because of things in the threads, like some of the issues I am personally experiencing, where people, dare I call them Trolls, just spit out (what I consider to be) nonsensical, or downright completely incorrect information, though I have seen some really good, concise posts, I have also seen tones of people muddling up the heart of the information that is being investigated.

What are you going to say to the person that only has a wall charger or non-programmable PWM charger, you are doing it wrong? He is not doing it like you, that is for sure. But he is also not doing it wrong, and his best practice works really well, AND exactly like it was designed to.

Do you see my point on this simple little matter? You are implying an "incorrect" usage of hardware that is properly being used, times that by the telephone game, and you get a situation where people have said to me just within the last hour on this forum that: You can't use your BMS to cut off charging or it will crash your whole system, and you have to manually rest the battery, charge controller, etc. That is completely and totally inaccurate information, and it really has to be called out and stopped, or the core value of the forum will be lost...

Anyway, as for the rest of the points in your reply; YES! Yes, I Agree - now tell that to half the other people in this forum that are telling everyone otherwise and sending them spinning in the wrong direction... GG

Michael77 said:

I have been told so many times on this forum by many different sources that the Charge Controller Voltage reading IS the Batteries Voltage and if they are off by even the slightest, there is "something wrong with the system..."

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This IS correct. The ONLY difference you should have between the OUTPUT terminals of the charge controller and the BATTERY terminals would be the voltage drop (resistance) of the cable connecting the two. If that drop (resistance) is too high, there IS something wrong with your system.

I suggest you go take a basic electronics class before telling people here with countless electrical degrees and experience, that they are wrong every other post.