Any BMS that offers functionality based on energy levels?

[Jeremiah]

So since voltage isn't the best way to indicate SOC with LiFePO4 does anyone know of a BMS which allows protection at x% energy levels (i.e. watt hours)?
I've been considering doing a PIC controlled implementation myself but I'd much rather get something off the shelf if it's not too expensive.
I don't understand, for example, why my ANT BMS which apparently knows the energy level of the battery only allows me to define cut off levels, low and high, based on voltage (which is not precise enough to hit the cutoffs I want, ie 15% on the bottom and 15% on the top).
Does anyone know of an affordable solution to base BMS operations on energy level instead of simply on voltage?
This seems like a holdover from Lead Acid tech...

 

[snoobler]

A hold over from lead-acid tech? Since when to lead-acid batteries have a BMS?

It's because the typical low-end BMS isn't meant to do this. Your other devices are. The BMS is meant to protect the cells. Some higher-end BMS possibly have this function.

The user is to determine how to manage their SYSTEM between the BMS limits according to their use.

Do you want your BMS cutting the battery out of the circuit when you hit 85% charge? Do you want your BMS cutting the battery out when you hit 15% SoC?

The charger should be programmed to charge to whatever target you seek to limit the upper end.

The load should be programmed to disconnect at whatever target you seek to limit it.

Let's pick 3.1V as a lower limit. During your normal daily use, it's not a factor, but what if you use very heavy loads for a brief period of time. Your battery may be at 50% SoC, but heavy loads could pull the voltage lower than this threshold and cut off when you're well above your intended discharge floor? Do you want to trip your BMS off just because you use higher loads? Yes, I'm making your point for you, but I'm illustrating the hazard of having the BMS manage it. You want your BMS to be neither seen nor heard... until it does its job of protecting the cells.

The flatness of the discharge curve makes this a very difficult proposition. It's more about you discovering how your overall system, components and use.

A Victron BMV device has a "discharge floor" value that can be programmed to operate a relay thus providing a means of disconnecting the load. When used with other victron hardware, it can signal the charger stop charging and the inverter stop inverting.

 

[Steve_S]

Ummm, with Lead Acid the Real Way to determine state of charge & health is to check the specific Acid Gravity level for each cell within a battery & within the bank overall.

With Lithium Chemistries, you obviously have no means to do that. Voltage & Internal Resistance is the only two things you can pull from Lithium and then do eh match around it. This is the same mechanism used in high-end EV's, Grid, Residential & Off-Grid energy storage systems.

 

[Bob B]

I haven't seen any BMS that makes decisions based on SOC .... Plus, you will tick off the BMS police by even thinking about using a BMS this way.

 

[Dzl]

I believe the SBMS0 lets you take actions and set limits based on State-of-Charge. I'm not sure if this can or should be in place of voltage based limits, or only in addition to voltage based limits.

I'm still unclear how BMS' like the Ant BMS and the JBD BMS actually measure state of charge, and how accurate it is, I would not trust SOC based limits without understanding how accurate these BMSes are at calculating SOC.

 

[Jeremiah]

A hold over from lead-acid tech? Since when to lead-acid batteries have a BMS?

Yes, of course, but why did they put this in? I'm only speculating what the programmers were thinking. By "holdover from lead acid" I mean the way many folks think of lead acid.

Ummm, with Lead Acid the Real Way to determine state of charge & health is to check the specific Acid Gravity level for each cell within a battery & within the bank overall.

Yeah that's ideal, but not with closed systems. Folks and tech often rely on voltage levels with Lead Acid, AGM, etc. to get a ballpark idea of remaining capacity and it's pretty close.

 

[Jeremiah]

I believe the SBMS0 lets you take actions and set limits based on State-of-Charge. I'm not sure if this can or should be in place of voltage based limits, or only in addition to voltage based limits.

I'm still unclear how BMS' like the Ant BMS and the JBD BMS actually measure state of charge, and how accurate it is, I would not trust SOC based limits without understanding how accurate these BMSes are at calculating SOC.

Isn't this one that can't do 48v systems though? I may be confusing it with another model.

 

[Dzl]

Isn't this one that can't do 48v systems though? I may be confusing it with another model.

You are correct, no 48v with the SBMS0:
I was not intending to make a recommendation, I was only responding to this:

I haven't seen any BMS that makes decisions based on SOC

There may be others that allow this functionality and have a 48v model, but I am not aware of them. I would look into some of the higher end BMS', 123smartBMS, TinyBMS, for starters.

 

[Cal]

I haven't seen any BMS that makes decisions based on SOC .... Plus, you will tick off the BMS police by even thinking about using a BMS this way.

Guess you haven’t seen Chargery.

However, a woefully incorrect SOC makes their decision making totally incorrect.

 

[Bob B]

Guess you haven’t seen Chargery.

However, a woefully incorrect SOC makes their decision making totally incorrect.

What actions does the Chargery take based on SOC?

 

[curiouscarbon]

Victron BMV-702 has State of Charge based relay. It functions as BMS. You can configure SOC % to disconnect below AND SOC % to connect above.

 

[Cal]

What actions does the Chargery take based on SOC?

Come on man, isn't it about time to read the manual?

Chargery manual

 

[JoeHam]

Victron BMV-702 has State of Charge based relay. It functions as BMS. You can configure SOC % to disconnect below AND SOC % to connect above.

View attachment 24610
View attachment 24611

I believe it’s the same with the Schneider battery monitor in their ecosystem.

 

[Bob B]

Come on man, isn't it about time to read the manual?

Chargery manual

I have read the manual and the charge and load decision are all based on cell voltage or temperature ... So, tell me what I missed.

 

[Cal]

Did you read page 6?

 

[Bob B]

Did you read page 6?

Ahhh, I had forgotten about. I think I set that value to zero on mine which I think disables it. I hope it does since the SOC accuracy is so poor.