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Is ISDT enough? What does a full BMS add?

mgnimike

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Aug 30, 2020
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Campbell, CA
I have a Giandel 24v 2000W inverter, an Epever 40A controller, and will build an 8s, 280ah lifepo4 battery.

The controller will properly limit the charge voltage for the whole battery. The inverter will shut off with low voltage. The ISDT will balance the cells. Do I really need a full BMS?

I can imagine that if one cell has a catastrophic failure the controller could keep pumping power and something could explode. Is that a realistic scenario?
 
The controller will properly limit the charge voltage for the whole battery.
The idea behind a BMS is that it provides cell-level voltage protection. Unlike lead acid batteries, lithium cells can diverge drastically near the high and low charge states.

The safe range for LiFePO4 cells for example is 2.5v to 3.65v. If your controller were to cut off charging at 14v for example, if the cells were unbalanced (and they likely will be at that voltage and higher) the cell voltages (equaling 14v in total) can easily be 3.4v, 3.4v, 3.5v, 3.7v. While 3.7v may not destroy your cell, its probably outside where you intend to charge.

Similarly, and perhaps more likely to kill a cell, if your inverter low voltage cutoff is 10v (2.5v per cell), it would not be uncommon for cells to be 2.7v, 2.7v, 2.7v, 1.9v. The 1.9v would possibly be the death nell for that cell (it is certainly doing some damage).

That is why cell level protection is necessary (or a watchful eye and very conservative charge/discharge settings and a rabbits foot).
The ISDT will balance the cells.
ISDT makes half a dozen different chargers, monitors and balancers. From my experience, most have millivolt level balancing which is dwarfed when you are charging and discharging in the tens of amps range.

The ISDT alarm settings are helpful because they can be set to provide an audible alarm when cells get out of range or even significantly unbalanced. Very nice feature that you never hear about.

Did that answer any of your questions?
 
Thank you so much for the very detailed answer.

So, about the need for cell balancing. The model I was looking at for ISDT is a monitor/balancer, but indeed there could be a problem with the amount of current the balancer provides, though some of the BMS do not have good balancing current capabilities. It seems that only the relay based BMS can carry high current from one cell to another.

From experience, how much current is needed for balancing these high amp batteries?
 
It seems that only the relay based BMS can carry high current from one cell to another.

The fact a BMS uses relays is totally unrelated to its balancing current. The relays are carrying the charge/discharge current and the balancer is a separate thing.

From experience, how much current is needed for balancing these high amp batteries?

It depends on the capacity of the cells and how well matched they are.
 
With most BMS’s the ’balance’ function is a non event anyway. Do not rely too much on your BMS cell balancing function - set up your BMS protection settings properly and check your cells physically every 6 months or so and then manual balance as required. Consider this as ‘planned maintenance’ .
 
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