It seems the Overkill BMS eliminates the need for a shunt

[krfish]

I've graduated from buying pre-built batteries, such as the Ampertime Battery, to building my own. Love it - should have done it from the very start. But now I am realizing that the Overkill BMS literally tells me everything.

I am going to get rid of the lead acid battery in my RV and build a battery for it. It seems like the need for the shunt goes away if I use the Overkill BMS like I did for my off-grid system.

Is this assumption I am making correct?

 

[Ampster]

Is this assumption I am making correct?

Yes, I get all I need from the BMS on my small pack. On a larger pack I like the Coulomb Counters that report kWhs because those are the units that my solar inverters and power company use.

 

[DThames]

I've graduated from buying pre-built batteries, such as the Ampertime Battery, to building my own. Love it - should have done it from the very start. But now I am realizing that the Overkill BMS literally tells me everything.

I am going to get rid of the lead acid battery in my RV and build a battery for it. It seems like the need for the shunt goes away if I use the Overkill BMS like I did for my off-grid system.

Is this assumption I am making correct?

Yes, you are correct. I have 5 or 6 of them in various roles.....love it.

 

[DThames]

A great feature is you can turn off the charging, the output, or the entire battery from the Bluetooth app. Great to turn off before messing with the wiring.

 

[Ampster]

Great to turn off before messing with the wiring.

There is also an optional switch setting that can be used with two wires and a remote switch. I am experimenting with that on a roto tiller conversion. It might eliminate needing a contactor.

 

[DThames]

There is also an optional switch setting that can be used with two wires and a remote switch. I am experimenting with that on a roto tiller conversion. It might eliminate needing a contactor.

The contactor just to turn the battery off, so avoid drain, or is it needed for some other requirement?

 

[Ampster]

The contactor just to turn the battery off, so avoid drain, or is it needed for some other requirement?

Just to turn on and off the motor. There is a surge, which is why I did not want to use a DC switch. Also I did not want to run high Amp cables to the handle bars. I do have a method for quick disconnect if the FETs fail closed.

 

[DThames]

Just to turn on and off the motor. There is a surge, which is why I did not want to use a DC switch. Also I did not want to run high Amp cables to the handle bars. I do have a method for quick disconnect if the FETs fail closed.

Emergency disconnect, no. Between the BMS and the inverters, one should provide low voltage disconnect but some physical switch would be an improvement. On your motor, do you have a snubber to prevent back EMF from the motor? Direct battery power to the motor or is there a PWM controller like on a golf cart?

 

[Zil]

These Things might be available on the BMS. But educate yourself, a BMS is not supposed to be the control switch, it is not that accurate to measure the battery state of charge, It is meant to be the last ditch emergency disconnect to protect the battery from terrible unforeseen disaster. The system, charging and load, should be designed and built to never get the battery near where the BMS trips. If you want the SOC the second best is using a shunt based battery monitor. the BMS to monitor SOC is someplace like fourth or fifth on my list. Maybe better than a voltmeter?
Clarifier, a small battery may not be worth spending the few dollars the monitor may cost. Your call.
The first and best? Belmar SG200.

 

[DThames]

These Things might be available on the BMS. But educate yourself, a BMS is not supposed to be the control switch, it is not that accurate to measure the battery state of charge, It is meant to be the last ditch emergency disconnect to protect the battery from terrible unforeseen disaster. The system, charging and load, should be designed and built to never get the battery near where the BMS trips. If you want the SOC the second best is using a shunt based battery monitor. the BMS to monitor SOC is someplace like fourth or fifth on my list. Maybe better than a voltmeter?
Clarifier, a small battery may not be worth spending the few dollars the monitor may cost. Your call.
The first and best? Belmar SG200.

Not sure why you think the BMS (referenced in the original post statement) is less than okay to determine state of charge? I am misunderstanding your statement?

 

[HRTKD]

I would be OK with skipping the shunt on a single battery system. With two or more batteries, the shunt summarizes the state of charge nicely.

 

[Zil]

No you are not misunderstanding me. The BMS is simply not accurate for tracking SOC. May be fine on a small battery, but my bank needs better tracking.

 

[Ampster]

On your motor, do you have a snubber to prevent back EMF from the motor? Direct battery power to the motor or is there a PWM controller like on a golf cart?

No, it is a brushed DC motor that draws 20 Amps at 24 volts on the bench with no load. I don;t know how much under load, but if it is a lot I will go back to the contactor since I already have the contactor.

 

[DThames]

No you are not misunderstanding me. The BMS is simply not accurate for tracking SOC. May be fine on a small battery, but my bank needs better tracking.

Can you state your experience and findings that support the idea that the Coulomb meter in that BMS is not accurate?

 

[Lt.Dan]

Can you state your experience and findings that support the idea that the Coulomb meter in that BMS is not accurate?

Get a real columb meter or shunt and you'll see the difference. I did with my setup. I find the shunt far superior

 

[DThames]

No, it is a brushed DC motor that draws 20 Amps at 24 volts on the bench with no load. I don;t know how much under load, but if it is a lot I will go back to the contactor since I already have the contactor.

The motor brush noise can cause voltage spikes that might risk the BMS FETs if you don't have some type of snubber to dampen those spikes. Direct switching of the motor can also cause spikes. I wouldn't want those spikes across the BMS FETs, using them as a switch without some extra circuit to help dampen those spikes.

 

[DThames]

Get a real columb meter or shunt and you'll see the difference. I did with my setup. I find the shunt far superior

I have both that BMS and a separate coulomb meter on the same battery. I have not recorded any data to look and see what any differences are. I might do that and report back.

 

[TacomaJoe]

I have the AiLi shunt and it and the JBD track pretty darn close. I like that I can read the JBD from the house 40 feet from the trailer even with the battery sitting in a metal box.

 

[Samsonite801]

I would be OK with skipping the shunt on a single battery system. With two or more batteries, the shunt summarizes the state of charge nicely.

Veeeeeery true, veeerrry true...

 

[time2roll]

I would be OK with skipping the shunt on a single battery system. With two or more batteries, the shunt summarizes the state of charge nicely.

I can double the number in my head is close enough for me. Yes they don't discharge perfectly equal either but still close enough. Worst case l have to look at both. Mostly just a curiosity anyway as I have way more power now with no more worries. Never had a shunt before and they seem to have their own periodic issues too.