To shunt or not to shunt...

[KneeOh]

Greets all, I'm a noob to this solar system.

I recently built a solar system that lives on a car trailer, so I can camp out with a laptop, video projector and a fridge full of beers

Several friends helped out. Then someone told me I need to put a SHUNT onto my system; I replied please dont swear at me buddy

Do I really need one, and if so what Amps?

Tanks in Advance and Happy Festivus for the restofus!

 

[Dzl]

Then someone told me I need to put a SHUNT onto my system

Do I really need one,

Not strictly needed, but you may want one. A shunt is used to measure current in/out of the battery, which can then be used to measure state of charge, consumption, etc. A poor and partial analogy is that a battery monitor is like a gas gauge, its pretty damn useful to know how much is left in the tank (and what is going in/out), but its not strictly necessary. Many would consider some type of battery monitor to be a required piece of a decent system design though.

A shunt based battery monitor is the best option with LiFePO4, but with lead acid batteries, I think a simple voltage based battery monitor works for many people (I personally know next to nothing about lead acid).

Why did the person you talked to tell you to use a shunt?

and if so what Amps?

More than your max potential current in/out. I believe a rule of thumb is that you want your max continuous current to be very roughly 2/3 of the shunts rating, but I may be misremembering the specifics.

 

[sunshine_eggo]

The answer is ALWAYS.

2000W/12V/.8 = 208A

Will recommends https://www.mobile-solarpower.com/battery-monitors.html

The "budget" one has a 350A version.

Please confirm the following:

1. 12V system
2. PWM controllers
3. 300W panels, one per PWM controller

 

[KneeOh]

Why did the person you talked to tell you to use a shunt?

It was like a mystery, they said search it out and see. I read a little bit, because I am not so cluey with this stuff I thought I would check here.

Please confirm the following:

1. 12V system
2. PWM controllers
3. 300W panels, one per PWM controller

yessiree!

I actually have a meter installed a mate gave me but no shunt I believe. See attached just above a temp gauge (which only monitors temp in toolbox unit housed in. If a shunt required or helpful, I imagine they available without a monitor/meter, saving doubling up?

 

[RCinFLA]

If you want reliable columb counting measurement of battery consumption and charging you should use a quality external shunt.

Most BMS's have multiple shunt chip resistors distributed in parallel along the string of parallel MOSFET's. Because they are physically distributed, the PCB trace conductor becomes part of the shunt resistance. They are cheap chip resistors, rated at 1 or 2 watts each, and subject to extra repeated heating and cooling stress cycles from the nearby MOSFET's.

It is easy for a BMS to calibrate out their chip shunt resistors' tolerance but do not accurately compensate for shunt resistance change due to temp variations, even with internal BMS temp measurement.

A high quality external shunt resistor is made from metal alloys that have a near zero resistance temp coefficient, and mechancally structured to survive temp cycling. Some of the cheap Chinese shunt resistors are not very good quality and do not have well controlled temp coefficient. Shunts should have a solid mechanical support mount structure to avoid flexing stress on the resistance element which can effect the value of the shunt.

 

[12VoltInstalls]

with lead acid batteries, I think a simple voltage based battery monitor works for many people

It’s a reliable indicator if you get familiar with your system but it’s not necessarily an accurate indicator. Specific gravity is accurate.

 

[sunshine_eggo]

It was like a mystery, they said search it out and see. I read a little bit, because I am not so cluey with this stuff I thought I would check here.

That gauge is junk and operates purely on voltage - a terrible way to determine state of charge.

A shunt measures the actual usage and calculates % capacity remaining based on the capacity of the battery.

yessiree!

Then you have a configuration problem.

Your 300W panels are crippled.

When using a PWM controller, you MUST use panels with the same nominal voltage as the batteries. Your 300W panels are 60 cell panels, which have a nominal voltage of somewhere around 20V.

A PWM works by shorting the panels directly to the battery thus pulling the panel down to battery voltage. As a result, your 32.1Vmp panels are being forced to operate in the 13-14.4V range. This means that your 300W panels will NEVER put out more than 14.4 * 9.35A = 134.6W. To reiterate, your 300W panels will NEVER produce more than ~140W of power due to your use of PWM controllers.

The only solution is to replace both PWM with one or two MPPT controllers that allow the panels to operate at or near their Vmp. You will need a total of 600W/12V = 50A.

 

[Zil]

A shunt is part of a battery monitoring system. I use and recommend this one; https://marinehowto.com/installing-a-battery-monitor/
Here is one that is state of art. https://marinehowto.com/balmar-sg200-self-learning-battery-monitor/

 

[PerryB67]

As far as I'm concerned is isn't worth a tinkers damn if it doesn't keep history. We don't sit and watch the meters endlessly. The AiLi and Renogy's are only meters (I see Will finally came around and now calls the Aili a meter). Neither the AiLi or Renogy (or many other meters) would have kept our 21v intermittent charge from our WFCO in history, because they have none.

Enjoy,

Perry

 

[sunshine_eggo]

As far as I'm concerned is isn't worth a tinkers damn if it doesn't keep history. We don't sit and watch the meters endlessly. The AiLi and Renogy's are only meters (I see Will finally came around and now calls the Aili a meter). Neither the AiLi or Renogy (or many other meters) would have kept our 21v intermittent charge from our WFCO in history, because they have none.

How did you discover it?

 

[PerryB67]

How did you discover it?

Last January something was causing our AGM batteries to get too low a voltage overnight. Our Victron BMV-712 monitor's history showed us our batteries had received 21v at some point in time. I reset the monitor and the next day it showed another 21v charge. It was intermittent and we were at electrical sites before the failure. I only saw the 21v max charge history when hooked to a power pole. The AGM's were failing after only two years, we camp often in cold weather, so I don't want or need lithium, so a set of Soneil 260 ah SiO2's were purchased.

I searched the DYI Forum, RV Life forum, Good Sam forum and other sites to find we were not the first to have a intermittently failing WFCO charger. At the same time I purchased the 260 ah SiO2 batteries I replaced the dumb GoPower charge controller with a Victron 100/30 SCC. I also learned how to pull the fuses to disconnect the WFCO charger portion. The SiO2 batteries charge to 90%, in similar time as lithiums would with our 170 watts on the roof, and we have no problems keeping them full. They charge much, much quicker than our AGM's charged.

Since disconnecting the WFCO I've never seen over 14.8 (in bulk) in the 712's history. Do not confuse the 712 with the cheap Aili meter (I refuse to call it a monitor) or the Renogy meter (again, not a monitor). Without history how many sets of batteries would have been ruined?

I rarely use the round monitor meter/interface that had to be cut into our cabinet, except to reset the history. I almost exclusively use the bluetooth app. If purchasing a Victron battery monitor today I would purchase the SmartShunt version.

Enjoy,

Perry

 

[JoeHam]

@KneeOh I wouldn’t worry about adding a shunt.

When your beer gets warm you will know your battery is dead. ?

A shunt would help you know when the battery is getting low so you can turn off non-critical loads like the laptop and projector to keep the beer at a critical drinking temperature but it’s clearly not necessary.

If the beer does get warm expect some heated discussion during the festivus airing of grievances.

 

[PerryB67]

@KneeOh
If the beer does get warm expect some heated discussion during the festivus airing of grievances.

I have Festivus with my son tomorrow!

We learned a long time ago no drinking is allowed during Festivus.

Wishing everyone a Happy Holidays!

Perry

 

[A.Justice]

I have the Renogy shunt. It does work, but it's not worth what I paid. Those $20 generic Amazon shunts do just as good a job, if not better.

 

[curiouscarbon]

https://www.amazon.com/gp/aw/d/B07FGFFHC6

"AiLi Voltmeter Ammeter Voltage Current Meter Ammeter 100V 350A Caravan RV Motorhome 999 AH" 45 usd right now

this one seems to work ok for me

 

[Supervstech]

That gauge is junk and operates purely on voltage - a terrible way to determine state of charge.

A shunt measures the actual usage and calculates % capacity remaining based on the capacity of the battery.



Then you have a configuration problem.

Your 300W panels are crippled.

When using a PWM controller, you MUST use panels with the same nominal voltage as the batteries. Your 300W panels are 60 cell panels, which have a nominal voltage of somewhere around 20V.

A PWM works by shorting the panels directly to the battery thus pulling the panel down to battery voltage. As a result, your 32.1Vmp panels are being forced to operate in the 13-14.4V range. This means that your 300W panels will NEVER put out more than 14.4 * 9.35A = 134.6W. To reiterate, your 300W panels will NEVER produce more than ~140W of power due to your use of PWM controllers.

The only solution is to replace both PWM with one or two MPPT controllers that allow the panels to operate at or near their Vmp. You will need a total of 600W/12V = 50A.

I sincerely hope OP read this.

For his build, I would ABSOLUTELY change his two pwm cc’s to a single 60A MPPT…

 

[JoeHam]

I agree.

If money is tight I would rather see it spent on a single MPPT instead of a shunt.

If not, then get both and Merry Christmas to yourself !

 

[curiouscarbon]

happy holidays & merry christmas! ?
good luck with the solar stuff!

 

[Baughb]

I dropped the change on the Victron, bluetooth shunt. The battery's bms/bluetooth information was a bit wonky so the Victron is more "truthful". I also plan on expanding the portable system and I wanted separate (s.o.c.) monitoring points in the system.

 

[time2roll]

I spent my shunt money on more battery. Shunt does not add capacity.
As long as my voltmeter shows top voltage at some point I know I am good for a few more days.