What's the point of having a coulometer when you have a BMS

[ramstein]

Hi,
What's the point of having a coulometer if the BMS already gives you all the informations you need?
I mean, my JBD displays the amps when charging, and when discharging too, it also gives the battery's voltage (I am using a lifepo4). These informations are all you need, or am I missing something ?
Thanks

 

[timselectric]

Generally, what the BMS reports is not very accurate.
A shunt is the most accurate option.

 

[Danke]

Generally, what the BMS reports is not very accurate.
A shunt is the most accurate option.

As a shunt adds some resistance, I was thinking of trying a Hall Sensor. Is something like this very accurate?

https://www.amazon.com/gp/product/B0BGWTSD5D/ref=ox_sc_saved_title_2?smid=AYY4I0D4PZJ0T&psc=1

 

[Browneye]

I like the display for a quick glance. A meter also counts in and out so it is much more accurate than the bms. Which only reports SOC based on cell voltage. (EDIT: Also counts coulombs but fails back to SOC by voltage depending on use. More on this later in the thread.)

A meter you zero out at full - 100% - and then it keeps track. Bms doesn’t do that. (EDIT: BMS does this automatically - when the battery gets to the full voltage setting then it resets and reports 100% SOC.)

For the $45 this one was a no-brainer. Works extremely well, and also has a very nice BT app for smartphone.

Bms is very good for individual cell health, and its log of activity and any errors.

 

[ramstein]

Thanks guys for your replies.
I am surprised about the BMS not displaying accurate values. Since it is plugged right on the battery, controling each cells, I thought it was the best device to get an idea of a battery's health.
I measured the current while charging with a DCDC 20A, I got 20.8A. While the BMS shows 21A. This is pretty accurate?
I also made some test with device I know the power of and the BMS shows a pretty right value too.
I ordered a shunt/coulo though, but I was wondering if it was really usefull since the bluetooth BMS shows eveything I need.

 

[Browneye]

My overkill/jbd doesn’t count, it displays SOC based on cell voltage in its settings. It will jump from 50% to 100% when voltage comes up during charging. The two devices are a nice compliment to each other.
Remember, that flat charge/discharge curve of LFP makes it very difficult to estimate state of charge, And charging or discharging also affects voltage. Voltage measure is only accurate after resting - no current in or out.

 

[timselectric]

As a shunt adds some resistance, I was thinking of trying a Hall Sensor. Is something like this very accurate?

https://www.amazon.com/gp/product/B0BGWTSD5D/ref=ox_sc_saved_title_2?smid=AYY4I0D4PZJ0T&psc=1

It can be, if good quality.
Through research, I found that you just can't beat a Victron shunt. So that's where I chose to spend my money.

 

[TomC4306]

My overkill/jbd doesn’t count, it displays SOC based on cell voltage in its settings. It will jump from 50% to 100% when voltage comes up during charging. The two devices are a nice compliment to each other.
Remember, that flat charge/discharge curve of LFP makes it very difficult to estimate state of charge, And charging or discharging also affects voltage. Voltage measure is only accurate after resting - no current in or out.

MY 2 Overkill/JBD don't work like yours.
The shunt in MY BMS' do a fine job of displaying SOC. Mine read gradual changes while charging all the way to 100%. They generally reach hundred before the batteries are finished absorbing energy but they sit there for that last 8 to 10 minutes of absorption until the current falls to zero. This time is the "reset" of the BMS. Then they read gradual discharge all the way to my 0%.
Smoothly. No jumps or hiccups.
None of the smooth gradual behavior would be possible if it was using voltage.
Perhaps you and I have different model numbers.

 

[timselectric]

My JK's count energy in and out. But, accuracy is not a BMS's strong point. It's close, but the shunt is always spot on.

 

[TomC4306]

My JK's count energy in and out. But, accuracy is not a BMS's strong point. It's close, but the shunt is always spot on.

Agreed. But for me SOC is always a moving Target. My system is never still. So the difference between 28% and 24% on the discharge at any given point in time or the difference between 76% and 80% on the charge is irrelevant to me.

 

[timselectric]

BMS's mathematically calculate SOC. Based on full charge (3.65v per cell) a full discharge (2.5v per cell). But, nobody runs their system this way. So, the accuracy degrades constantly.
The victron shunt resets every time you reach your preset 100%. So, it's more consistently accurate.

 

[TomC4306]

BMS's mathematically calculate SOC. Based on full charge (3.65v per cell) a full discharge (2.5v per cell). But, nobody runs their system this way. So, the accuracy degrades constantly.
The victron shunt resets every time you reach your preset 100%. So, it's more consistently accurate.

Actually that top voltage and bottom voltage are adjustable in JBD. Mine are 3.500 and 3.125..
Mine hit 0% at 50 volts.

 

[timselectric]

Actually that top voltage and bottom voltage are adjustable in JBD. Mine are 3.500 and 3.125..
Mine hit 0% at 50 volts.

I assume that also lowers the reported capacity.
I know that my BMS reported capacity is a moving target.

 

[EastTexCowboy]

It can be, if good quality.
Through research, I found that you just can't beat a Victron shunt. So that's where I chose to spend my money.

What size Victron shunt are you running?

 

[Ampster]

As a shunt adds some resistance, I was thinking of trying a Hall Sensor. Is something like this very accurate?

My Orion JR2 uses a Hall sensor and is very accurate. Less expensive BMSs just use voltage to estimate SOC. Unfortunately, it is useless where you often need it in the middle of the range. Voltage works to tell you when you are full or empty.

 

[timselectric]

What size Victron shunt are you running?

1,000

I expect to see over 700a charging in full sun.
When my system is complete.

 

[ramstein]

Thank you for all your messages this is very informative.

I was wondering, while waiting for my shunt, should I also plug my 1500W inverter on its negative or inverters should only be plugged directly to the battery?
Thanks

 

[Ampster]

I was wondering, while waiting for my shunt, should I also plug my 1500W inverter on its negative or inverters should only be plugged directly to the battery?

If you want to measure current from the battery to the inverter or vice versa you have to place the shunt on the circuit between the battery and the inverter.

 

[timselectric]

That

 

[ramstein]

ok thanks I was not sure if it was possible, I will plug it on the shunt negative then

 

[Ampster]

I will plug it on the shunt negative then

Plug? It is better to connect those cables with bolts.

 

[EastTexCowboy]

1,000

I expect to see over 700a charging in full sun.
When my system is complete.

That's a lot of those little electron thingies.

On semi-related but not really subject - when I add my second bank of 30kWh batteries I'm thinking I need to upgrade my bus bars to the 3/8" 600a versions. I think the current ones are 5/16" 250a. I'm not increasing the charging amps - just more batteries - but I keep thinking I'm a little light on those bus bars.

 

[Ampster]

I think the current ones are 5/16" 250a. I'm not increasing the charging amps - just more batteries - but I keep thinking I'm a little light on those bus bars.

I have 42kWh in batteries but never charge at more than 180 Amps. My discharge rate is even lower. 250 Amps at 50 volts is 12.500 Watts.

 

[timselectric]

That's a lot of those little electron thingies.

On semi-related but not really subject - when I add my second bank of 30kWh batteries I'm thinking I need to upgrade my bus bars to the 3/8" 600a versions. I think the current ones are 5/16" 250a. I'm not increasing the charging amps - just more batteries - but I keep thinking I'm a little light on those bus bars.

You only need to be rated for expected max current.
Capacity is only extending runtime.

 

[pollenface]

I have neither. If the batteries get into float I'm happy.