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Battery damage when cells in series?

cupoftea

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Hi
We have 3 series rechargeable lithiums which supply a constant 2A.
We don’t want the expense of a balancing circuit. We hope the cells will stay balanced.
But would you agree, we must have a diode across each cell? To not do so would otherwise mean danger?
Also, the out-of-balance condition is more likely to happen when the cells are near fully discharged, would you agree?
So actually, would you agree that we also need a voltage detector across each cell, so that we can stop the battery charging when any cell gets up to 4V? If any cell gets down to 2V5 while the battery is discharging, then we don’t need to worry about that, because the diode across the cell will stop that cell getting too far charged in the wrong polarity?..so nothing to worry about there?
Also, would you agree, balancing circuits are a waste of time because if cells get so far out of balance that they need a balancing circuit, then they are usually soon-to-die anyway, and will soon get so imbalanced that even the balancing circuit cannot rebalance them?
 

KlausST

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Hi,

A diode across each cell?
What is it good for?
I mean, it does nothing until the voltage becomes slightly negative ... but if the voltage is near zero ... or -even worse - negative, tyen the cell is most probably killed yet.

Klaus
 

BradtheRad

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Do you mean an led across each cell? As a monitor? With 2A constant load it's feasible to spare a few mA constantly through an led.
Or two led's.

One would go dim when cell voltage drops below a threshold.

One could trigger a shunt circuit to divert current when the cell becomes overcharged.
 

cupoftea

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A diode across each cell?
What is it good for?
Thanks,...its for safety's sake. To stop it going even more negative.
I doubt anyone recomends battery cell balancers?....when a cell goes rogue, it usually quickly gets worse and worse till even the balancer can't handle it.
 

FvM

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Also, the out-of-balance condition is more likely to happen when the cells are near fully discharged, would you agree?
Lithium-ion cells must be never deep discharged. Otherwise they may catch fire. Therefore they are not used in any commercial product without low voltage cut-off. IMHO Voltage balancing is also required for reliable applications.
 

cupoftea

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Thanks, the question then , if i may, AYK, becomes, what power of balancer do you select for say 2Ahr 3.7V Li cells?
 

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