No BMS, just balancing?

RyanM923

10 mW
Joined
Jul 27, 2009
Messages
28
In my application, I will have a charging source (alternator) that has a voltage regulator to charge at a maximum of XX.X volts, so over-voltage isn't an issue (barring alternator failure). As far as discharging goes, 3.2V cells would never go under 2.5V, and 3.7V cells would never see less than 3V. Now, with that being said, assuming the cells being used can safely handle 2C charge and 10C discharge...is there really any need to have an expensive BMS that monitors the charging and discharging, if my application stays well within the spec'd C ratings and where the voltage is controlled on the upper end while also never being close to dangerous low voltage levels? I figure a type of cell balancing system that uses the excess capacity at the top of the charging cycle to then balance the cells as the battery drops to it's natural resting voltage, would be enough to keep everything in check.

Am I just over-simplifying this?
 
I am also considering removal of the BMS from my Elettra 2500 scooter.
To safeguard against low voltage I will install this cell checker tuned at 2.9V; I will put 3 of them to cover all 20 cells and fit them in the dashboard so that I have constant control of the battery pack; plus they blink and make a loud noise when cut-off limit is reached:
http://eclipsebikes.com/cell-checker-alarm-lipoliionlimnlifepo4-wires-p-1093.html?osCsid=fcc9141440c999645218c10016c1564e
As far as extra voltage protection is concerned, I will rely on an external balancer that I will connect only during battery charge together with a 20s charger that will stop at 72V.
I have a 50 Amp fuse which will fire way below the 2c discharge limit of the battery.
So, at the end of the day, there is really no point of having an onboard BMS, it is just an extra point of failure.

http://luca-tosolini.eu/WiKi/Elettra/elettra.html
 
eTrike said:
Application specific... you might be over simplifying. Lots of folks use no BMS and just monitor and balance manually as needed. Barring cell failure it should be fine, but there is only one type of battery I would use for -10C +2C operation.
What have you found to be the best cells for that application?
 
eTrike said:
Is there even a reliable source for these anymore? All I seem to see are links to places that don't exist anymore, or horror stories about fake/reject cells. Scares the hell out of me when it comes to buying a large quantity.

I'm debating on getting some Xalt 37ah UHP cells that are rated for 15C constant and 25C burst.
 
To oversimplify even more,, you ALWAYS need a bms. The question is whether the bms is a circuit board, or you holding a voltmeter or other devices.

For some situations, a bms can be awkward. For example, if you need high amps and can't tolerate a bms shutting you off when you need them.

Permanently installing voltage monitors can be a problem. Because of the display, they draw more than a bms, and that draw can wreck your battery if left unattended long enough, or in the short term it can essentially cause your battery to be more out of balance than you can stand, permanently. The type that runs on less than full pack voltage will draw that power from just one or two cells, making it mandatory that every charge be a balance charge.

One strategy I have liked when I am forced to discharge a naked ( no bms) pack to 100%, is just watching the voltage of the cell I know is the weakest one in the pack. If I know for a fact cell 11 is the lowest capacity one in the pack, then all I need to know is the voltage of cell 11.

In the end, you can't simplify it. You have to analyze the system, and then determine if human bms is better, or easier, or cheaper, or whatever than a bms.

But if you have a bms,, do check on it. Blind trust is not rewarded if a bms is defective, or other malfunctions occur.
 
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