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Old 04-09-2013, 04:19   #1
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Lithium BMS critical for...?

No really. With all the expense and distraction of complex BMS systems, I found myself wondering.

Are lithium batteries really so frail that their life would be radically cut if they were simply operated without a BMS, using limited charging voltage and amperage? Plus, perhaps, a low voltage cut-off (like the cheap Battery Buddy/Brain) to ensure they weren't pulled down below their lower limit?

Or, is the only really critical purpose of the BMS really to prevent a thermal incident? If a cell fails and shorts out, in theory it could overheat and start a thermal incident. But that could be prevented by a simple thermal sensor, no need for complicated electronics to prevent that.

So...is the BMS really just a horribly complex way to get the last 10 or 20% of the battery's useful life out of it? Or really, just to prevent runaways, which could be managed as simple thermal management? (Cutting power when something overheats, something like the battery post or top, not just the cables and connectors.)
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Old 04-09-2013, 04:29   #2
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Re: Lithium BMS critical for...?

I'm our batteries BMS, i take heed of both volts and amps and am warned audibly by our battery monitor and Logger. Works for us.
Cheers Frank
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Old 04-09-2013, 07:57   #3
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Re: Lithium BMS critical for...?

Quote:
Originally Posted by hellosailor View Post
No really. With all the expense and distraction of complex BMS systems, I found myself wondering.

Are lithium batteries really so frail that their life would be radically cut if they were simply operated without a BMS, using limited charging voltage and amperage? Plus, perhaps, a low voltage cut-off (like the cheap Battery Buddy/Brain) to ensure they weren't pulled down below their lower limit?

Or, is the only really critical purpose of the BMS really to prevent a thermal incident? If a cell fails and shorts out, in theory it could overheat and start a thermal incident. But that could be prevented by a simple thermal sensor, no need for complicated electronics to prevent that.

So...is the BMS really just a horribly complex way to get the last 10 or 20% of the battery's useful life out of it? Or really, just to prevent runaways, which could be managed as simple thermal management? (Cutting power when something overheats, something like the battery post or top, not just the cables and connectors.)
Now your getting it. I run a BMS mainly for monitoring purposes. The only thing it can control, other than alarms, is the alternator on the main engine, and this is because of the need to run the engine at times when the bank may be full. (also have a cellog and LVC).
Plus my BMS costs less than $200, so it's not like they have to be terribly expensive and complex. (House Power BMS).
Thermal runaway is not likely in these cells, I've shorted a 700 amp cell and, besides the bulging the only evidence of excessive heat or melting of the PVC case was right around the terminals. You'd have to work at getting into a runaway situation, although it is possible as it is in LA.
Are you becoming a "Fanboy" now? Just Kidding.
Dwain
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Old 04-09-2013, 08:43   #4
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Re: Lithium BMS critical for...?

Depends whether you view BMS as a battery Monitoring System or a Battery Management System.

LiFerrous cells, once initially balanced and maintained in < .25C charge environment will quite happily exist without any cell monitoring. Thermal runaway doesnt happen , so even overcharging will not cause any catastrophic issues. Ensuring that the charge voltages never exceed 14.6 handles HVC issues . LVC is a nice to have in so much as it protects your $ investment. ( and is cheaply and easily added) .

However for a few bucks , a simple cell monitor can be added, giving you all that extra security if you would like it

But active BMS as in management such as in-process cell balancing etc is definitely not needed in typical boat environments

dave
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Old 04-09-2013, 12:47   #5
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Re: Lithium BMS critical for...?

"definitely not needed in typical boat environments "
And yet, some suppliers are so vehement about it. Which, to answer dwain's question, is why I'm no fanboy. I'd just be rather pleased to find that under all the FUD, there's a pony in that pile.
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