Electronics > Projects, Designs, and Technical Stuff
Balance charging lithium cells
OM222O:
why would this be as efficient as using a braking resistor? I'm not sure I understood what you said correctly, but what I understood is using an op amp and an output stage which forces the voltage to the midpoint between cells (i.e: taking charge from the battery with higher charge and forcing it to the battery with lower charge). this should be more efficient than just wasting it all as heat?
David Hess:
--- Quote from: OM222O on April 26, 2019, 07:51:08 am ---why would this be as efficient as using a braking resistor? I'm not sure I understood what you said correctly, but what I understood is using an op amp and an output stage which forces the voltage to the midpoint between cells (i.e: taking charge from the battery with higher charge and forcing it to the battery with lower charge). this should be more efficient than just wasting it all as heat?
--- End quote ---
They are both doing the same thing. With the operational amplifier, the transistor is being used as a variable resistance and dissipating the same power that the resistor would have to maintain balance. Power is power no matter how it is dissipated.
OM222O:
--- Quote from: David Hess on April 26, 2019, 01:14:57 pm ---
--- Quote from: OM222O on April 26, 2019, 07:51:08 am ---why would this be as efficient as using a braking resistor? I'm not sure I understood what you said correctly, but what I understood is using an op amp and an output stage which forces the voltage to the midpoint between cells (i.e: taking charge from the battery with higher charge and forcing it to the battery with lower charge). this should be more efficient than just wasting it all as heat?
--- End quote ---
They are both doing the same thing. With the operational amplifier, the transistor is being used as a variable resistance and dissipating the same power that the resistor would have to maintain balance. Power is power no matter how it is dissipated.
--- End quote ---
I didn't mean dissipation, I meant using the op amp to transfer charge between the batteries until they are balanced, therefore very little (almost zero) dissipation. Similar to what the ICs that digsys mentioned (check the app note / datasheet on how they balance the batteries).
Siwastaja:
What you describe is called "active" or "redistributive" balancing. While widely discussed on academical (and hobbyist) level, very rarely used in the real world. Simply because: li-ion cells have almost negligible self-discharge rates, and the differences between self-discharges between the cells (which determines the balancing need) are even lower.
Think about this: a typical cell self-discharges around 2%/year. Let's say it's neighbor self-discharges by 2.1%/year; the difference to balance out is 0.1%/year. At, say, 10Ah capacity, this is 10mAh per year. Assuming an example pack of 10 series cells at 10Ah, this would be around 0.37Wh wasted in heat - in a year! Approximating at $0.15/kWh, the cost for this electricity would be around $0.00005, per year.
Now, passive balancing BMS cost per cell is about $0.50, and well designed redistributive is about $2.00 - assuming both types are cost optimized for mass market. (This is based on my own designs of both types, so I have a general idea of what it costs. Sadly, the differences are actually even bigger, since redistributive balancers are expensive niches.)
Saving $0.00005 per year, the payback in a 10s pack (10 * $1.50) would happen in 300000 years. Of course, assuming that everything else, such as quiescent current, stays the same, which is a ridiculous assumption.
In larger packs, the numbers are less ridiculous. Yet, it's highly improbable that an active system ever makes sense, even in large systems. They are used in very specialized conditions. Another chemistry in the future might require the re-evaluation of redistributive balancing; li-ion is just too good.
Of course, if this pack is in any electric vehicle, the extra consumption of carrying a few dozen grams of extra electronics with you makes the bottom line negative.
Factor in any cost for developing the more complex system, and it falls apart completely.
OM222O:
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. the cost of that would add up very quickly in the waste heat (especially given the main goal is charging at insanely high currents. e.g: 5 or 6 amps!). So I would assume batteries would be the same and discharge at different rates (regardless of self discharge). especially the recycled ones that are used in most DIY power wall type applications where the batteries might not even be the same model (e.g: mixing samsung cells with LG ones). you don't want to be throwing out tons of your input power simply because one cell has been charged to its limit.
I agree in a single pack that wouldn't make much sense but if you scale it to a power wall type project, waste energy scales just as much so efficiency is important.
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