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Last seen: 9 years 4 months ago
Joined: Monday, June 14, 2010 - 19:36
Points: 14

Hi All,
I've been trying to understand how the Bms systems work with chargers. I've seen a couple of Bms systems that monitor cell voltage and shunt charger power into resistors when voltage is >3.7v This seems like a waste of power, especially with limited resource like solar. I also looked at Thunderstruck system which turns off charger if voltage goes high in any cell.
It seems like a better system would monitor each cell and if voltage went high the cell balancer would SHUT OFF power only to that particular cell and keep charging the rest
till they all shut off in thier own time. A simple voltage controlled switch and relay would accomplish this wouldn't it?
Am I missing something or this the basics of a BMS system?


Last seen: 6 months 1 day ago
Joined: Saturday, July 7, 2007 - 23:55
Points: 1686

don't forget the embodied energy required to make a product.

to take a cell out of circuit during charging for the purpose of balancing, you will need a switch per cell.
that switch has to be able to break full pack voltage, and conduct full traction discharge current.

that switch is not small or cheap (whether mechanical or silicon).

from my own experience, the balancing resistive shunt at 3.7v while charging at cell average of 3.6vpc gives the most bang for the buck.

for a good pack, after the initial balancing charge, stay in balance well enough the shunts do very little.
in effect very little energy is wasted while balancing.
because the circuits are rather simple, there embodied energy from design and manufacture is low.

another approach is to use "flying capacitors", where a capacitor is switched between two neighbouring cells.
that requires a switch per cell rated to balancing current and the voltage of two cells (so acceptably small) and a capacitor per cell.

the embodied energy (and cost $) is higher due to the greater complexity but it allows around half of the energy otherwise wasted to balancing to be reclaimed.

as i said before the lithium cells don't really need much in the way of balancing (in energy terms) so it still doesn't pan out.
for other chemistries that do need significant balancing, the benefit is more meaningful.


Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km

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