Shunt or battery monitor usage

[gerrys]

I looked through many of the designs Will presents. It seems like few of them include a battery monitor or shunt. Then I searched the site and aside from a listing in the links to purchase items, I can't find descriptions of when to use battery monitors and shunts, and how they work. Yet I see many mentions of them in the forums and videos. I have a 12V FLA system on my camper, but i just base the SOC assessment on battery voltage. So... What is a shunt? How do shunts work? How are they wired? How do shunts differ from a battery monitor? When must they be used? Why can I not find shunts or battery monitors in most of Will's example system designs?

 

[Wjm1964]

Shunts are a bit like a fuel gauge, it measures the energy going into and out of the battery. I have the Victron smart shunt but I use lifepo4 battery. You can't really rely on voltage with lifepo4 but for LA yes. The shunt connects from the main negative output of the battery to the load side.

 

[sunshine_eggo]

I looked through many of the designs Will presents. It seems like few of them include a battery monitor or shunt. Then I searched the site and aside from a listing in the links to purchase items, I can't find descriptions of when to use battery monitors and shunts, and how they work. Yet I see many mentions of them in the forums and videos. I have a 12V FLA system on my camper, but i just base everything on SOC. So... What is a shunt? How do shunts work? How are they wired? How do shunts differ from a battery monitor? When must they be used?

What reports SoC?

If you're using SoC based on voltage, it is a very poor indicator of charge in an active system. It also varies with load. You could have a fully charged battery under a heavy load reporting only 12.1V - about 50% based on voltage though it's actually nearly fully charged.

FLA voltages are only somewhat representative of SoC after 10+ hours of sitting with no charge or discharge of any kind. Better if you can wait 24 hours. This is absurdly impractical in active systems.

A shunt is a very precise resistor with a calibrated voltage drop. A 500A shunt may have a 50mV voltage drop. This is linear, so a 100A current flowing through the shunt would be ±10mV drop depending on direction of flow.

The battery monitor is programmed with the battery information. It then counts the net current in and out of the battery and compares that to the rated capacity. If a 100Ah battery has experienced a 10A discharge for 1 hour, it is at 10Ah/100Ah = 90% SoC.

Bottom line is if you want accurate SoC, you use a battery monitor.

If you want REALLY accurate, the Victron line can also factor in Peukert and temperature-based efficiencies.

 

[gerrys]

Shunts are a bit like a fuel gauge, it measures the energy going into and out of the battery. I have the Victron smart shunt but I use lifepo4 battery. You can't really rely on voltage with lifepo4 but for LA yes. The shunt connects from the main negative output of the battery to the load side.

If the shunt is between the battery negative and the load, then I see how it monitors how much energy is used. But how does the shunt monitor how much energy flows into the battery?

 

[WoodySolarVan]

My experience: in my van I use battery monitors with shunts only on the solar controllers/ DC chargers to battery so I see exactly how many amps are going in. I don't use ground based shunts elsewhere as the vehicle is nothing but a big negative bus and I typically only run positive wiring and run a ground at the appliance from the nearest vehicle metal access point. I found that negative use shunts even at the battery ground simply weren't accurate enough Exception is the inverter.


Battery monitors that use Hall sensors to measure voltage and amps out off positive I use for determining amps under load. Drawback to this method is if close to battery it still picks up some of the charging amps as a Hall sensor is bi directional.


That's how I now do it for the vehicular application....

 

[gerrys]

The battery monitor is programmed with the battery information. It then counts the net current in and out of the battery and compares that to the rated capacity. If a 100Ah battery has experienced a 10A discharge for 1 hour, it is at 10Ah/100Ah = 90% SoC.

Bottom line is if you want accurate SoC, you use a battery monitor.

The rated capacity (and estimated capacity for that matter) of the battery is never the actual capacity. So if the entered data for battery capacity (or charging efficiency) is off by a typical 1%, then every time the shunt records a full charge/discharge cycle, the shunt/battery monitor would "think" the battery was full at 1% less/more than reality. This would be cumulative though? Which means each charge cycle the error would increase. Is there some sort of smart or AI type function built in that learns the real full capacity of the battery and charging efficiency? How does the monitor/shunt account for the error in every cycle?

 

[gerrys]

The rated capacity (and estimated capacity for that matter) of the battery is never the actual capacity. So if the entered data for battery capacity (or charging efficiency) is off by a typical 1%, then every time the shunt records a full charge/discharge cycle, the shunt/battery monitor would "think" the battery was full at 1% less/more than reality. This would be cumulative though. Which means each charge cycle the error would increase. Is there some sort of smart or AI type function built in that learns the real full capacity of the battery and charging efficiency? How does the monitor/shunt account for the error in every cycle?

I think i just answered my own question: The answer is that the shunt doesn't control battery recharge, the charge controller does. So even if the estimate by the shunt is off by 1%, the charger will still charge the battery to full capacity. If the battery is brought to full state of charge any cumulative error is erased. So the error is not cumulative over repetitive cycles as I presumed, wrongly. However, the inaccuracy would be cumulative over time, if the charge controller does not have a chance to fully charge the battery to the fully charged point. The cumulative portion of the error is only reset every time the battery is fully charged.


Thank you everyone for your answers.

 

[sunshine_eggo]

I think i just answered my own question: The answer is that the shunt doesn't control battery recharge, the charge controller does. So even if the estimate by the shunt is off by 1%, the charger will still charge the battery to full capacity. If the battery is brought to full state of charge any cumulative error is erased. So the error is not cumulative over repetitive cycles as I presumed, wrongly. However, the inaccuracy would be cumulative over time, if the charge controller does not have a chance to fully charge the battery to the fully charged point. The cumulative portion of the error is only reset every time the battery is fully charged.


Thank you everyone for your answers.

Yep.

The shunt is also programmed with the fully charged trigger, e.g., charge to 28.8V, hold until the current drops to 0.04C, re-sync to 100%.

Victron recommends batteries be fully charged twice a month to maintain shunt accuracy.

 

[time2roll]

I never had trouble using a voltmeter with lead-acid. At this level I would prefer to expand actual capacity rather than have slightly better indication of charge level. Those that have the meter always seem to like them.

 

[gerrys]

This all leaves the question. Why doesn't Will show a battery monitor/shunt in any of his designs?

 

[sunshine_eggo]

This all leaves the question. Why doesn't Will show a battery monitor/shunt in any of his designs?

Only those that haven't watched many of his videos or reviewed any of his designs, OR don't understand what they're seeing would say this.

I just spot checked three of the designs on his website, and all three specify a battery monitor.

Other reasons not to use Battery Monitor:
1) Many choose to utilize the SoC reported by a smart BMS as a battery monitor. This is reasonably accurate.
2) The server rack batteries communicate SoC and other information back to compatible inverters.

Neither are an option with FLA.

 

[gerrys]

Only those that haven't watched many of his videos or reviewed any of his designs, OR don't understand what they're seeing would say this.

Only those that didn't read the original post or didn't understand what they were reading OR haven't reviewed several of his designs would say that. Because the original post clearly says that there are many mentions of battery monitors and shunts in his videos and in Will's 4 designs below a battery monitor or shunt is not an obvious part of the design.

A shunt or battery monitor is not shown in pictures or the summary of costs or suggested parts, that I can see, for the systems below (obviously not including the generic list of affiliate purchase links included below each design... refrigerators, crimpers, strippers, entry glands... etc) . But then again, maybe I just haven't reviewed ANY of his designs. Perhaps these don't count as any of his designs?

Van Dweller Solar Power Blue Prints

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

RV Solar Power Blue Prints

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

RV Solar Power Blue Prints

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

Large RV Solar Power Blueprints

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

 

[sunshine_eggo]

Recommend you scroll down on each one of those pages and pay attention. Each has a battery monitor on it.

In case you continue to miss it, look for this:



That appears on each of the pages you linked.

 

[gerrys]

Recommend you scroll down on each one of those pages and pay attention. Each has a battery monitor on it.

In case you continue to miss it, look for this:

View attachment 120080

That appears on each of the pages you linked.

I'm glad you agree with my point that the battery monitors are only shown in the purchase links in the designs I shared, and not shown in the pictures or blueprints, or cost summaries, parts summary lists ...etc.

 

[sunshine_eggo]

Since you seem to like changing the narrative:

Why can I not find shunts or battery monitors in most of Will's example system designs?

You can... on pretty much every one.

Like all other components, it's under the "what I recommend" heading.

 

[gerrys]

Since you seem to like changing the narrative:

You can... on pretty much every one.

Like all other components, it's under the "what I recommend" heading.

Again, it's so nice you agree with me, that in the designs I shared, the battery monitor is only mentioned in the section where it shows where to purchase things. But for some reason it is not included in the example system pictures or recommended components. I don't know why you keep repeating what I say. That the battery monitors are all shown in the lower section.

 

[time2roll]

The battery monitor shunt goes on the battery negative terminal. Every design is the same unless there is an extraordinary consideration.
No need to belabor the shunt explanation for every build. Want a monitor, add a monitor.

 

[gerrys]

The battery monitor shunt goes on the battery negative terminal. Every design is the same unless there is an extraordinary consideration.
No need to belabor the shunt explanation for every build. Want a monitor, add a monitor.

This is a great explanation. While the shunts are mentioned in so many videos and listed in most builds, there was little explanation about when, why or where to use them. Mostly just talk about which brands are good and their accuracy. Thus why I legitimately asked in the beginner's corner.

 

[sunshine_eggo]

Again, it's so nice you agree with me, that in the designs I shared, the battery monitor is only mentioned in the section where it shows where to purchase things. But for some reason it is not included in the example system pictures or recommended components. I don't know why you keep repeating what I say. That the battery monitors are all shown in the lower section.

Because you think the upper section or the "Major Components of this system mounted to a wooden board" is the "design."

The entire page must be considered. You can't look at that page and draw a line saying, "only this part is the design."

This is a great explanation. While the shunts are mentioned in so many videos and listed in most builds,

Wait... "most builds"... I though you said none...

there was little explanation about when, why or where to use them.

On each page you linked:

"This device will tell you how much power you have left before your batteries go to zero. Simply attaches at the main negative lead of your battery bank."

 

[gerrys]

Time2Roll gave a great explanation. This is the beginner's corner. There are a lot of items listed at the bottom that are not part of the design shown at the top. A beginner trying to piece together a system asked a legitimate and honest question about why the shunt listed at the bottom don't appear in the build videos or pictures for these simpler designs. Yet the only response was a harsh and rude judgement of whether the beginner 'watched videos, reviewed any designs, or understands what he was reading'. The beginner did all of those a lot. Yet for some reason the expert wanted to just attack. The beginner kept repeating that his confusion is why the items were listed on the bottom purchase area but don't seem to be in the top item list and pictures. A very legitimate question for a beginner who doesn't know which items are required and which aren't. The expert will certainly want to reply to this obviously sincere statement of sadness over the rude responses received with more attacks.