I was wrong: common vs separate port BMS

I’ve been following the development and use of LiFePO4 cells for a few years so I am aware of the major concepts. I haven’t been following BMS functionality and made a false assumption.

BMS have several protections that can affect charging (like temperature, overcurrent, overvoltage) and discharging (temperature, undervoltage). Some BMS control charging and discharging channels individually and some of them shut both channels down if there is a problem on one.

Under normal circumstances both channels should be open. If it’s freezing it would be optimal to disallow charging but allow discharging (especially if you are trying to run a battery warmer!). If the battery has been run to unhealthy low levels you would want to disallow further discharge but allow charging.

my error

My error was thinking that common port BMS were all or nothing, and that separate port BMS had seperate controls for charge/discharge. If there is any consolation it’s that this appears to be a common misunderstanding.

getting a clue

First, I read a blog post called Single Port BMS MOSFETs Explained. It contained this explanation:

To allow discharge, but not charge we turn off the charge MOSFET. In doing so, we still have a current path: through the body diode of the charge MOSFET…

I asked a clarifying question:

… those that control both together (early CHINS, for example) lack the body diode?

and he answered:

It could be that earlier BMS implementations completely disconnected on fault, essentially using the MOSFET banks as is typically done when using them as a bi-directions switch. In this case, the gates of both banks are tied together and controlled with a single signal.

So all-or-nothing control is not a function of common port BMS; it’s a function of a type of common port BMS that uses a single signal rather than separate signals to the charge and discharge MOSFETs.

After coming to grips with this new reality I started googling with greater specificity; if you are interested read this post in the DIY Solar Forum’s BMS common port vs seperate port thread, and maybe the dozen posts after it.

The JBD BMS is the one that’s in the 100Ah Rebel I ordered. The Overkill BMS is apparently a rebadged JBD with custom settings and better documentation.


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