Hi Michael,
Ned got it right. The typical BMS systems monitor for over or under voltage at the invidual cell. I use EV-Power BMS modules (same functionality as the Mini-BMS modules) with my 160Ah Li cells and my Elcon charger. The charger is wired into the modules and will shut off completely if any cell registers 4.2V. I'm not using the modules to monitor for undervoltage, but the module monitor circuit will open if any cell drops below 2.5V. I could use that signal to reduce throttle or shut down the motor altogether, but I'm not operating my boat down to those voltages (100% discharge).
For balancing, my modules are considered "passive", once the cell voltage passes 3.65V, the module will shunt up to 0.8A of charging current, slowing down the charging of that cell while the other cells catch up. Now that the cells ahve had a chance to balance through a number of charge cycles, they all reach 3.65V within a couple of minutes of each other. The modules continue to shunt until the Elcon charging profile reaches 60.8V for the bank (average 3.8V/cell).
There are "active" BMS that state that they will use current fromm one cell to charge another. If you thiink about it, this seems unlikely unless you're in the middle of a charging cycle. During a charging cycle, an active BMS could re-direct current that is pointed at a "high" cell towards a "low" cell. The passive BMS only dissapates (shunts) as minimal heat some of the current aimed at the high cell. While not as efficient, the actual losses from shunting are fairly low and only happen when the battery bank is more than 95% full. An active BMS is more efficient, but not by much considering the extra expense and complexity.
Thinking about an active BMS when dealing with resting voltage is different. Put in 12V terms, let's say that you have 4 batteries, with one battery at 12.8V, two batteries at 12.65V and one at 12.4V. You might expect that you could use the 12.8V battery to charge the 12.4V battery. But a 12V battery doesn't really accept any current unless the voltage is above 13.5V or so. So now the active BMS needs to boost the 12.8 to 13.5 or more to get any current into the 12.4V battery. Chances are that the BMS will waste more energy converting the voltage than the few amps that head to the low battery. In most cases, the high battery will drop much more than the low battery will come up. We can see that this would be more common and not very efficient. I would not want a BMS that did this at any time that the battery bank was not charging.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Ned Farinholt <nedfarinholt@...> wrote:
>
> Michael,
> It may indeed be possible to build such a BMS but that is not the way most of them work. The device generally referred to as a BMS for Lithium batteries monitors the cell voltage on each cell in the serial string, either using analog or digital electronics looking for an overvoltage or undervoltage condition. An output of the BMS can be connected to the charger itself, a relay on the AC side of the charger, and/or to an alarm. If any of the cells exceeds a predetermined voltage the BMS shuts down the charger and/or rings an alarm. It can also balance the pack by shunting some of the current around the higher voltage cells. While the pack is under load, the BMS monitors each cell for an undervoltage condition and can shut the system down, reduce the throttle, and/or sound an alarm.
> Available BMS systems for lithium batteries can be more or less sophisticated, from a mini-bms which performs the above functions for a few hundred dollars to very complex systems with computer outputs, CAN bus, etc.
> I have a mini-bms on my 16 cell 160 Ah traction battery and an Elcon charger. I have tested it extensively and use it on my boat. It is very satisfactory.
> Best wishes,
> Ned
>
> On Apr 15, 2012, at 4:38 PM, Michael Mccomb wrote:
>
> >
> > Hello all,
> >
> > A new question. I am interested in a 48v 200ah LiFePO4 battery (bank) and BMS but I am very fuzzy on exactly how the BMS functions. I know that there is both a maximum charge rate and a maximum discharge rate that should not be exceeded with the battery pack. I assume that the BMS regulates and prevents such problematic usage of the battery pack.
> >
> > Can someone tell me if there is a required min or max to the voltage and amperage fed to a BMS for charging purposes. I assume that the BMS can regulate discharge and charge at the same time and that in effect it is only the net difference between the two functions that can adversely impact the battery.
> >
> > I'd love to be able to feed the BMS 48v (or 52v or whatever) and 50amps and then have the BMS completely manage the pack. Does anyone think this completely feasible?
> >
> >
>
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