Shunt or battery monitor usage

[RCinFLA]

A good battery monitor measures both the current verses time and battery voltage.

Only thing the battery voltage is used for is to reset amp-secs counter when battery voltage reaches full charge voltage and secondary check for totally depleted battery voltage drop out.

A shunt is just the low value resistor used to measure battery current. The accuracy of shunt resistance is very important. A shunt can get quite hot at higher current, so its design needs to keep its resistance constant over temperature extremes. A good quality shunt is made of alloyed metals to compensate resistance for temperature. Shunts have small separate voltage sense terminals to avoid the high current terminal connections from impacting voltage read across shunt resistance.

Smart BMS's include shunt resistors and battery monitoring firmware, although the chip shunt resistors are not the best quality. The advantage of BMS battery monitor is it can monitor individual cells' voltage where a separate battery monitor can only monitor total battery voltage.

The voltage measurement across the shunt resistance used to compute current also has its own accuracy. All this accumulates errors in the amp-second tally over time. This is why the 'full' reference needs to be periodically reset with a full battery charge to allow the counter to reset and re-reference the 100% state of charge of battery. This clears out accumulated errors in the amp-secs counter.

 

[Mattb4]

... Yet for some reason the expert wanted to just attack. ...

Forums have all kinds of folks that post comments. It is important to take anyone's posting for what it is worth. There is a handy feature on most Forums to choose to ignore certain posters if you find there posts to be a distraction.

I am not an expert, though I did sleep at a Holiday Inn Express last night.

 

[gerrys]

Forums have all kinds of folks that post comments. It is important to take anyone's posting for what it is worth. There is a handy feature on most Forums to choose to ignore certain posters if you find there posts to be a distraction.

I am not an expert, though I did sleep at a Holiday Inn Express last night.

Sooo funny. Hahaha. Holiday Inn Express. Omg, did you get the awesome breakfast?

Thank you for the response. I appreciate it a lot. It's unfortunate because I can see the expert is actually very knowledgeable.

 

[gerrys]

...Smart BMS's include shunt resistors and battery monitoring firmware, although the chip shunt resistors are not the best quality. The advantage of BMS battery monitor is it can monitor individual cells' voltage where a separate battery monitor can only monitor total battery voltage. ...

This is very interesting. I would expect a good brand smart BMS to come with a good shunt. Do any good brand batteries come with a BMS that has a good chip shunt resistor? Or do any good batteries come with a BMS shunt that is not chip based but higher quality individual component? I see that some SCC units can receive data from a separate shunt (Victron?). It seems like it would be very useful/easier to have all data in one spot, like the Bluetooth program from the battery BMS.

 

[RCinFLA]

This is very interesting. I would expect a good brand smart BMS to come with a good shunt. Do any good brand batteries come with a BMS that has a good chip shunt resistor? Or do any good batteries come with a BMS shunt that is not chip based but higher quality individual component? I see that some SCC units can receive data from a separate shunt (Victron?). It seems like it would be very useful/easier to have all data in one spot, like the Bluetooth program from the battery BMS.

There are some BMS's that have external shunts. They are rare but are the best.

Self-contained 12v batteries with internal BMS are usually the cheapest BMS possible. All BMS's have current detection to control battery overcurrent but just controlling overcurrent requires a lot less accuracy than a battery monitor. Some really cheap BMS's just use voltage drop across their MOSFET's to trigger overcurrent protection. MOSFET resistance varies a lot with temperature. Likely a self-contained 12v battery with internal BMS that has BT RF link to report individual cell voltage and battery current would be a better quality BMS.

 

[gerrys]

There are some BMS's that have external shunts. They are rare but are the best.

Self-contained 12v batteries with internal BMS are usually the cheapest BMS possible. All BMS's have current detection to control battery...

Great info. Thanks