BMS or Shunt

Pete and Liz

Pete and Liz

Joined
Mar 13, 2021
Posts
80
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Location
East Yorkshire, UK
Funster No
79,726
MH
Hymer B698
Exp
2018
A question for the electrical minded the collective.
I am just finishing off the electrical (12V) upgrade to our new to us van. Having installed the new solar ,solar controller, B2B and a fab Fogstar 300Ah battery I was thinking why do I need the Shunt and its associated LCD screen when the Fogstar BMS bluetooth app appears to give me what I need for monitoring the usage Watts, current and the charging?
So am I over looking something?
 
You possibly don't would be my first thought, if the Fogstar app shows amps in/out and state of charge as capacity in ah and % then there is not much more a stand alone shunt will tell you.
 
Personally, I would go with a shunt, as far as I know the monitor within the BMS is typically a hall effect monitor and tends to be less accurate particularly where small currents are concerned.
I cannot comment specifically on the Fogstar, but certainly with the KS Energy batteries the bms monitor is fine as a guide over a few days but over longer periods it can become way out, the only way to correct it again is a full discharge / recharge.
 
Dave101 said:
Personally, I would go with a shunt, as far as I know the monitor within the BMS is typically a hall effect monitor and tends to be less accurate particularly where small currents are concerned.
I cannot comment specifically on the Fogstar, but certainly with the KS Energy batteries the bms monitor is fine as a guide over a few days but over longer periods it can become way out, the only way to correct it again is a full discharge / recharge.
Click to expand...
I suppose if they started with just the Fogstar BMS they could work out if it was accurate or not, we have a Buttner/Dometic shunt based battery monitor and I laways thought it was lazy counting the amps back in, I would come back from a sunny day and work out in my head how many amps we should have gained but the computer never agreed, it would go to 85%-90% and I could see the solar cutting back and then it would jump to 100%, we now have a Victron shunt as well and that is more accurate and confirmed my suspicions re the Buttner.
 
I would go with the shunt will be more accurate and will give you more information. The smart Shunt is good with the BMV712 you have the display but that is not really necessary but also get a programable relay which the smart shunt doesn't have which can be very useful.

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I have a Fogstar 280ah lithium. Coming over the France a couple of weeks ago I did 4 nights off grid, using the battery with inverter for kettle etc. and charging during the day while driving. Final night of the 4 went to bed with I think 40% remaining and in the morning zero, flat. After contacting Fogstar their advice is that their BMS is primarily a tool to protect the battery, its function as a device to monitor the charge is secondary and not accuate. In my case they say that a number of partial discharges and charges without the battery getting to 100% can cause the BMS to incorrectly display charge status. They advised me that if I want to know accurately what the state of charge is then I need to get a smart shunt like a Victron.

Their words, not mine.

I’ll be fitting one when I get home as having a measure of something which is inaccurate is as good as a chocolate fire guard. 😄

For the technically minded is the Victron the best to get or are their others to be considered?
 
The SOC monitor in the BMS is a good basic monitor that will be ok for most people. However, if you have an inverter with high current demands it can cause the SOC monitor to get out of sync pretty quickly.

My suggestion would be, try it and see. If it goes out of sync too quickly for your use case then add in the victron shunt later. No point spending on a shunt if you don't actually need it?

I am a bit of a "why spend it if you don't need it" type of a person these days.
 
I've got a FogStar and a Victron shunt. I'm not convinced either is more accurate. I already had the shunt, so I left it in the setup. It provides temperature (not that it matters with a FogStar and its heater) and voltage to my Victron chargers. It means they can adjust for the voltage drop in the cable run to the battery.
 
I'm using a Victron BMV712 which along with my Victron MPPT's and my JK BMS are hooked up to a Raspberry Pi for viewing my system both in the van and remotely via the Victron VRM.
I can also view my fresh and grey tanks levels as well as my LPG tank level on the Raspberry Pi screen and the VRM.

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Well thanks for the advice , I think I will go with the Shunt. I have a Renogy one but no bluetooth and I can't be bothered to cut any holes in the new van so I will swap that for Victron as its Bluetooth.

As for the culinary advice from Two on Tour hooking his whole van up to a Raspberry Pi , I did try the same thing with a rhubarb crumble but it just gave poor results and kept going off. :ROFLMAO:
 
We have a lot of experience with a number of BMS‘s (JK, JBD, Daly, Overkill, Smart123 etc) I think Fogstar use a JBD with their own app (or at least branded).
My take on it is that for the most part a LiFePO4 BMS provides vastly superior info compared to an old lead-acid voltage-based meter. It‘s usually OK for most people. JK and JBD are quite good in terms of calculating SOC, probably Overkill could be the best. Daly are horrific.
For the most part the BMS is within a few % and is a handy guide for SOC.
However …..
Every now and then the BMS’s get it quite wrong. This happens especially if you’re not getting up to 100% SOC for a few days, such as if you’re at a campsite (static) without hookup and solar not enough to get full SOC. In these conditions the BMS can go out significantly. Especially with a high-current inverter.

In general shunts are more accurate.
In particular the Victron shunts can be very accurate. I say “can be” because if you use incorrect settings they can also introduce progressively worse error over a few days when not reaching full SOC. If you have bad settings your BMS could be more accurate than the shunt. The Smartshunt defaults are for lead acid, so need to be changed.

One of the nice things about the Victron Smartshunt is that it ties in with the rest of the Victron stuff, particularly the Cerbo GX. In my case my Cerbo is connected to my fresh water tank and my LPG tank, so from anywhere in the world I can see what my tank levels are, see what my battery SOC is, and also my starter battery (which is also connected to my Smartshunt). I also have temperature probes and Ruuvi tags. I have some alarms set up (for example to warn me when starter battery goes a bit low, of the inside of the van is approaching freezing).

So my vote is for a shunt and to set it up properly.
 
Andy MacD said:
I have a Fogstar 280ah lithium. Coming over the France a couple of weeks ago I did 4 nights off grid, using the battery with inverter for kettle etc. and charging during the day while driving. Final night of the 4 went to bed with I think 40% remaining and in the morning zero, flat. After contacting Fogstar their advice is that their BMS is primarily a tool to protect the battery, its function as a device to monitor the charge is secondary and not accuate. In my case they say that a number of partial discharges and charges without the battery getting to 100% can cause the BMS to incorrectly display charge status. They advised me that if I want to know accurately what the state of charge is then I need to get a smart shunt like a Victron.

Their words, not mine.

I’ll be fitting one when I get home as having a measure of something which is inaccurate is as good as a chocolate fire guard. 😄

For the technically minded is the Victron the best to get or are their others to be considered?
Click to expand...
When the fogstar app displayed zero did the battery still work?
 
jezport said:
When the fogstar app displayed zero did the battery still work?
Click to expand...
No, 10.4 volt at the terminals. Fogstar said the BMS did as it was supposed to and shut the battery down. It was the 40% capacity reading the night before which was wrong and there was no way for me to know at the time. In other words going on the BMS reading is no reassurance that you have power in the battery. I’m not reluctant to let it below 50% just in case.

On another point I’m not confident that the solar is actually charging the battery. The solar controller has flashing lights but as to whether it’s charging? I’ll test properly when I ge home in a couple of weeks.

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Bms or shunt they are only there as guide and BMS to shut systems down if there's a problem I have both If I really want to no the exact voltage in the Battery I use my amp meter EASY job done
 
BOWHOUSE52 said:
shunt they are only there as guide
Click to expand...
A good shunt like the Victron SmartShunt or the Victron BMV-712 are much more than a "guide" and I would trust as much as a good professional meter.
A lithium battery needs more than just voltage to determine its state of charge.
 
Hi yes I could not agree more if you are teck minded by all the means go for it I myself have done just that by fitting 2x 100 amp Renogy Lithium Batteries 1 x 2000xRenogy inverter 410 watt of solar power and a b2b charger which by the way I was advised to buy but after 10 months of quite heavy use in the UK and abroad in Europe not had to use it No EHU in that time and system seems to be running fine I am not teck minded and as long as my inverter tells me what's coming in and what is going out it suit me All these manufacturers throw all these figures at us and we get brainwashed into thinking we must have an I don't really think we do And of course you get the lads who buy because it's a new toy for the boys As I say I have all these different things but was just trying to say easy is simple
 

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