Soc. bms or smartshunt?

[Davidgabriel]

I have a Fogstar 24v 270 ah battery connected by can bus to the cerbo gx. I also have a smartshunt connected. On the cerbo I use the bms as the measure for soc. I have a choice. Bms, automatic or smartshunt. I ticked bms. When I look on the Victron connect app. The smartshunt always shows a higher [not huge] Soc than the Fogstar bms. Which do you think is most accurate please?

[blackrose]

I'm just guessing Smartshunt would likely be more accurate, but only because my Fogstar BMS SoC is fairly inaccurate.

 

But that might depend on how your system and Smartshunt is set up.

 

I tend to use voltage rather than SoC because the SoC on my Victron battery monitor drifts and isn't very accurate either. I know the voltage curve for LiFePo4 is very flat, but you can still see what's going on and it doesn't drift! The only thing is you need to check voltage with no charge to or discharge from the batteries.

 

Edited July 11 by blackrose

[Tony Brooks]

Not an answer to your question, but the talk of a Frogstar BMS says that you have lithium batteries, and as they do not sulphate like LAs do the need to keep them as fully changed as possible for as long as possible does not apply, so the state of charge. As the general opinion OS that you try to avoid discharging below roughly 20% I don't see that a 5% error in reported battery capacity is of any concern whatsoever. You should still have enough capacity left to continue modest use before the BMS shuts down. 

[Davidgabriel]

Yes. It’s no big deal really. And it’s only a few percent. With general use they’re kept between 60 and 90 percent in any case

[Russ T]

It will depend on how you have them set up. If both are tailored for your specific installation the Victron will likely be more accurate. The gap will widen the older the BMS is.

 

The fogstar needs a few cycles to sync, and the victron will need syncing occasionally too.

Edited July 11 by Russ T

[PeterF]

1. Best SmartShunt settings, Peukert coefficient 1.05, charge efficiency 99%, charged voltage 0.2-0.4V below absorption voltage, 4% tail current, charged detection time 3 mins.

2. Wiring per illustrative diagram below.

If points 1 and 2 are correct then I would always trust the SmartShunt.

 

The only time the SmartShunt can get things wrong are

1. If the battery is left unused and uncharged for a while, the SmartShunt does not allow for self consumption from the BMS.

2. If you have a heated battery and use the heaters, the SmartShunt will see current going into the battery as charging.

[Tony Brooks]

6 minutes ago, PeterF said:

The only time the SmartShunt can get things wrong are

1. If the battery is left unused and uncharged for a while, the SmartShunt does not allow for self consumption from the BMS.

2. If you have a heated battery and use the heaters, the SmartShunt will see current going into the battery as charging.

 

Because LFP batteries are so much more tolerant of being left under charged and can discharge to a much lower level that LAs, then these two points would lead me to come to the opposite conclusion, simply for a simpler life.

[GUMPY]

Providing it's set up correctly then Smartshunt  every time.

On older Fogstar if low currents (under 1 amp) were involved then I have seen discrepancies of  40% and above😱

[Davidgabriel]

It’s about a year old. Would the bms be more accurate if I were using dvcc on the Victron 

[GUMPY]

21 minutes ago, Davidgabriel said:

It’s about a year old. Would the bms be more accurate if I were using dvcc on the Victron 

A year old and it should be fairly accurate there was a software change in the BMS not long before that. Mine is about 2 years old.

[PeterF]

43 minutes ago, Davidgabriel said:

It’s about a year old. Would the bms be more accurate if I were using dvcc on the Victron 

DVCC will have no impact on the battery BMS accuracy, DVCC communication is all one way, the battery sends info to the rest of the system, it does not receive.

 

One point to think about on DVCC with a third party battery is if you should enable SVS (share voltage sense) or not. As you have a SmartShunt you can enable SVS but you MUST choose the SmartShunt as the source of the voltage. Do not choose the BMS because if the BMS disconnects then the reported voltage to the Cerbo GX may not be the battery terminal and system voltage, it could be the voltage over the disconnected cells. With some third party BMSs, DVCC often disables SVS.

[Davidgabriel]

On the Fogstar instructions it says svs on and sts on when connected by can and dvcc on. It’s all very confusing for an old man😄

Edited July 11 by Davidgabriel

[PeterF]

16 hours ago, Davidgabriel said:

On the Fogstar instructions it says svs on and sts on when connected by can and dvcc on. It’s all very confusing for an old man😄

If Fogstar confirm that the voltage reported by the BMS is the terminal voltage then you can use SVS from the BMS. Many BMSs report the voltage of the cells so on a disconnect DVCC see a voltage that it can not control and can result in overvoltage on the system.

[Davidgabriel]

Ok. I get that. Question for Fogstar then?

what I would really like to do is get everything on DVCC including the Wakespeed ws500. But the ws500 doesn’t accept info from the Fogstar. I was hoping the Cerbo would act as a bridge as it does with the mppts and the multi plus. But it doesn’t. 

[cheesegas]

On 11/07/2026 at 14:44, Russ T said:

The fogstar needs a few cycles to sync, and the victron will need syncing occasionally too.

There's no 'learning' as such over multiple cycles with any of the BMS units fitted to Fogstar batteries - they don't need multiple cycles to sync or calibrate.

 

The only time the BMS calibrates is when the under voltage discharge disconnect or over voltage charge disconnect is triggered. The instructions for the BMS state that for the first time it's used, both of these disconnects need to be triggered.

 

The BMS's SoC appears to be done with a combination of cell voltage thresholds and amp-hour counting. Unless the over voltage disconnect is triggered, it doesn't sync at 100% charge like Victron monitors.

[Russ T]

3 minutes ago, cheesegas said:

There's no 'learning' as such over multiple cycles with any of the BMS units fitted to Fogstar batteries - they don't need multiple cycles to sync or calibrate.

 

The only time the BMS calibrates is when the under voltage discharge disconnect or over voltage charge disconnect is triggered. The instructions for the BMS state that for the first time it's used, both of these disconnects need to be triggered.

 

The BMS's SoC appears to be done with a combination of cell voltage thresholds and amp-hour counting. Unless the over voltage disconnect is triggered, it doesn't sync at 100% charge like Victron monitors.

Fair enough. Thanks for setting me straight 

[IanD]

4 minutes ago, cheesegas said:

There's no 'learning' as such over multiple cycles with any of the BMS units fitted to Fogstar batteries - they don't need multiple cycles to sync or calibrate.

 

The only time the BMS calibrates is when the under voltage discharge disconnect or over voltage charge disconnect is triggered. The instructions for the BMS state that for the first time it's used, both of these disconnects need to be triggered.

 

The BMS's SoC appears to be done with a combination of cell voltage thresholds and amp-hour counting. Unless the over voltage disconnect is triggered, it doesn't sync at 100% charge like Victron monitors.

 

Does that mean that the SoC error gradually increases over time unless you have an OV disconnect? (which should never happen)

 

That certainly happens with a lot of other batteries with built-in BMS, unless they do what Victron/REC do and reset SoC to 100% when you do a cell balance cycle -- and you do one, obviously.

 

Doesn't the Fogstar do this?

On 12/07/2026 at 14:04, Davidgabriel said:

Ok. I get that. Question for Fogstar then?

what I would really like to do is get everything on DVCC including the Wakespeed ws500. But the ws500 doesn’t accept info from the Fogstar. I was hoping the Cerbo would act as a bridge as it does with the mppts and the multi plus. But it doesn’t. 

The Cerbo should be able to control the WS500, and presumably can read data from the Fogstar?

[cheesegas]

Just now, IanD said:

Does that mean that the SoC error gradually increases over time unless you have an OV disconnect? (which should never happen)

 

That certainly happens with a lot of other batteries with built-in BMS, unless they do what Victron/REC do and reset SoC to 100% when you do a cell balance cycle -- and you do one, obviously.

 

Doesn't the Fogstar do this?

From what I understand, it uses the initial under voltage/over voltage disconnect cycle to set the battery capacity, it never resets the SoC to 100% based on amp-hour counting alone. It seems to use a number of configureable cell voltage thresholds (every 10%) in combination with amp-hour counting and possibly current to work out the SoC, I'm not entirely sure how this is done though.

 

The balancers on the non-Pro Fogstars are passive and appear to be independent of the SoC entirely - they activate over a configurable voltage threshold, and also on a certain cell voltage delta. It's impossible to force a balance cycle. Not sure about the Fogstars with an active balancer though.

 

Either way, the early JBD units were very bad at SoC tracking but they've done something to the firmware in the past couple of years which now means it's much more accurate. For example, even over winter where the battery doesn't hit 100% for a couple of months, my Fogstar's BMS is never more than 4-5% different to the Victron.

[Momac]

On 11/07/2026 at 13:29, Davidgabriel said:

I have a Fogstar 24v 270 ah battery connected by can bus to the cerbo gx. I also have a smartshunt connected. On the cerbo I use the bms as the measure for soc. I have a choice. Bms, automatic or smartshunt. I ticked bms. When I look on the Victron connect app. The smartshunt always shows a higher [not huge] Soc than the Fogstar bms. Which do you think is most accurate please?

I expect any battery monitor can only be an estimate. This is because we do not consume  a constant current . eg a fridge cuts in and out which will influence the remaining time .  More likely  the Ah capacity off the battery may not be exactly as per the label on the battery and it is possibly the Frogstar built in monitor is factory set at something slightly different to 270Ah. So I would say the Frogstar is likely to be the more accurate figure. Try settings for the Victron smart shunt to say 275Ah and see if this makes the Victron readings closer to the Frogstar.

Or remove the now redundant victron smart shunt and refit  it on the engine start battery,  or sell it .

[cheesegas]

1 minute ago, Momac said:

I expect any battery monitor can only be an estimate. This is because we do not consume  a constant current . eg a fridge cuts in and out which will influence the remaining time .  More likely  the Ah capacity off the battery may not be exactly as per the label on the battery and it is possibly the Frogstar built in monitor is factory set at something slightly different to 270Ah. So I would say the Frogstar is likely to be the more accurate figure. Try settings for the Victron smart shunt to say 275Ah and see if this makes the Victron readings closer to the Frogstar.

Or remove the now redundant victron smart shunt and refit  it on the engine start battery,  or sell it .

As I mentioned above, the Fogstar's BMS 'learns' the battery's capacity when the under voltage cutoff is triggered, followed by the over voltage cutoff. This is often slightly more than the claimed capacity. From the factory, I can confirm that the BMS is set at exactly what's on the label.

 

The discharge current not being constant doesn't affect the stage of charge %, only the time to go reading.

[Momac]

14 minutes ago, cheesegas said:

As I mentioned above, the Fogstar's BMS 'learns' the battery's capacity when the under voltage cutoff is triggered, followed by the over voltage cutoff. This is often slightly more than the claimed capacity. From the factory, I can confirm that the BMS is set at exactly what's on the label.

So if the Frogstar BMS learns its own capacity  the Victron shunt settings might be adjusted to a slightly higher Ah capacity in an attempt to equalise the  percentage SOC reported by the frogstar and victron shunts ?

 

[PeterF]

46 minutes ago, IanD said:

The Cerbo should be able to control the WS500, and presumably can read data from the Fogstar?

The problem with some 3rd party batteries is they can not run at the 250kpbs speed of the VE CAN bus which the WS500 runs at and only run at 500kbps so need to connect to the BMS CAN bus on older Cerbo models or the second CAN bus set to 500kpbs on newer Cerbos. connects to. The WS500 only connects to VE CAN bus at 250kpbs and reported experience is that the cerbo does not run the WS500 as externally managed with a 500kbps battery on the second CAN bus. If you have a Victron Lynx BMS (either NG or older version) running VE CAN at 250kbps then the WS500 works fully under DVCC as I can attest.

[IanD]

53 minutes ago, cheesegas said:

From what I understand, it uses the initial under voltage/over voltage disconnect cycle to set the battery capacity, it never resets the SoC to 100% based on amp-hour counting alone. It seems to use a number of configureable cell voltage thresholds (every 10%) in combination with amp-hour counting and possibly current to work out the SoC, I'm not entirely sure how this is done though.

 

The balancers on the non-Pro Fogstars are passive and appear to be independent of the SoC entirely - they activate over a configurable voltage threshold, and also on a certain cell voltage delta. It's impossible to force a balance cycle. Not sure about the Fogstars with an active balancer though.

 

Either way, the early JBD units were very bad at SoC tracking but they've done something to the firmware in the past couple of years which now means it's much more accurate. For example, even over winter where the battery doesn't hit 100% for a couple of months, my Fogstar's BMS is never more than 4-5% different to the Victron.

On mine (REC-BMS) the SoC drifts down by a pretty consistent 1% per week, which actually works very well -- it starts at 99% in float (54V), then after about a month it's dropped to 95% which is the threshold where the BMS starts a "charge to 100%" cycle by raising the CVL from 54V to 57V. After hitting 57V and balancing the cells (usually a couple of hours) SoC is reset to 100% and CVL back to 54V (float).

 

I don't know if this behavior is intentional, but it works very well... 🙂 

Edited July 15 by IanD

[cheesegas]

1 minute ago, Momac said:

So if the Frogstar BMS learns its own capacity  the Victron shunt settings might be adjusted to a slightly higher Ah capacity in an attempt to equalise the  percentage SOC reported by the frogstar and victron shunts ?

 

You could do, although that isn't the main cause of the error between the Victron shunt and BMS SoC. It's usually only 1-5aH more than the claimed amount.

 

The cause of the difference is that they use different methods to calculate the SoC - Victron is amp-hour counting only apart from when it syncs at 100%, and the Fogstar used a combination of cell voltage, current and amp-hour counting.

 

The SoC error between the newer Fogstars as of a couple of years ago and a properly set up Victron shunt is so small it's not worth worrying about. Again, mine drifts only 5% or so even in winter when it's not taken up t0 100% for months.

[Davidgabriel]

To be fair the difference is fairly minimal. But consistent. The smart shunt always shows slightly higher Soc