Electronics > Projects, Designs, and Technical Stuff
Balance charging lithium cells
digsys:
Totally concur with Siwa.. on his comments, that's why ours is a plug-in maintenance option, NOT embedded in the pack. It is only ever used IF the BMS says
it needs it. Race batteries do get stressed a heck of a lot more than any other use scenario. To complete an event, we may go well past "ideal" operation.
Also, NEVER leave ANY active device (capable) on a battery pack. It can be very dangerous, should it fail, and transport options are FAR more restricted than
a pack of raw lithiums. We've seen the after effects of "shunt" failures.
Super caps are another ball game. I have been working on a different implementation of super-cap usage the last couple years. Hope to have results in a few
months. It will do away with the very wasteful balancing problem.
OM222O:
so you mean once the batteries are balanced initially there is no need for balancing them ever again?
then why do balance chargers even exist as opposed to a CC-CV charger with different cut off voltages for different packs?
digsys:
There are many answers to THAT question. I have to run very soon, so I'll do a couple -
We receive "hand matched" cells from manufacturers, who are our sponsors. ALL production processes have tolerances, and there is NO WAY they can afford
to batch match every single cell (we're talking about millions of cells). They have no idea what pack sizes are being made anyway. So they ship them to
equipment / vehicle manufactures. They ALSO do not have the time to "hand match" every cell ! so a simple / cheap balancer is installed.
In our case, we STILL spend the time to run 3x FULL EXTRA balance cycles on our already "specially" balanced batch, and always find 1-2 out.
BUT, once they are matched well, AND (big AND), the cells are run within limits, they rarely ever need it again. At the very least not under 1-2 yrs, and then
it may be minor. Outside of "proper" operation conditions, which people often PUSH, all bets are off. On top of that, people are often "re purposing" cells etc.
In short, it is a cheap / easy way to have them start life as best as possible. more later
Siwastaja:
--- Quote from: OM222O on April 26, 2019, 06:13:57 pm ---I haven't done any actual testing with batteries, but when it comes to capacitors (especially super capacitors) they need to be balanced every single time as they have different capacities and discharge and discharge at different rates and become unbalanced very quickly.
--- End quote ---
Supercaps and li-ion cells are completely different beasts. Don't assume any similarity.
Siwastaja:
--- Quote from: OM222O on April 27, 2019, 12:13:13 am ---so you mean once the batteries are balanced initially there is no need for balancing them ever again?
--- End quote ---
No one said that. This being said, sometimes this is the case.
I'd guesstimate that most of the commercial multi-cell li-ion packs do not use either balancing or even cell-level monitoring. At least considerable number; including well known brands, without any problems.
--- Quote ---then why do balance chargers even exist
--- End quote ---
Sometimes the leakage differences are large enough to shorten the product lifetime. My previous example number set isn't such a case; it would do fine without balancing. But assume 10 times worse leakage difference; this is still possible in the real world. Now, if the battery unbalances at about 1% per year, it starts to be a real problem at about 5 years - so balancing helps. But even then, the "payback for redistributive balancing" would still be 30000 years!
So, lithium ion cells tend to be almost good enough to require no in-system balancing whatsoever. This creates the awkward "it depends" situation, necessitating real system level understanding.
In reality, sometimes balancing is needed, sometimes not. When balancing is used, it does so little work that it's important to optimize for:
* Reliability
* Low cost
instead of "efficiency", or a fancy feature set. I have seen so many utterly failed li-ion balancer products that started from a complex list of feature requirements, even though in reality, they are irrelevant.
So when the balancer is needed, it needs to reliably sit there, and do it's minuscule yet important job when needed, and only when needed - and correctly.
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