Active balancer in cold weather?

[Texas-Mark]

I have seen a lot of threads recently about heating batteries, etc. and it made me think of something I don't recall reading and had not really thought about before. What, if any effect does active balancing have on cells if below freezing? Even if the BMS has low temp CO, it won't cut off a stand alone active balancer. Is the current flow between cells too low to worry about? I have a couple of lifpo4 batteries with active balancers that are in my trailer. I have the batteries disconnected for now because I am not using it and don;'t have any heaters out there yet. I do have a manual switch for the active balancers and did turn them off when I disconnected the batteries, but was wondering if it is an issue that people should worry about that may not have a manual switch and the batteries get below freezing, especially for active balancers that are not being controlled by cell voltage. But even those that are, it may be possible for the battery to get below freezing and the balancers are still working.

 

[timselectric]

Shouldn't be a problem.
Because there's no reason to balance until close to full charge. And at least float charging.
Resting cells aren't usually at the balance voltage.

 

[RCinFLA]

You should not charge at cold cell temps. The line is commonly drawn at water freezing point but you should reduce the rate of charge below 10-15 degs C to reduce cell degradation.

Lithium-ion migration through electrolyte at cold temp is very sluggish requiring a high cell overpotential voltage to drive the required lithium-ion migration rate to support the demanded cell current. This damages electrolyte and increases the detrimental conversion of free lithium to lithium metal which permanently reduces cell capacity and grows conductive lithium metal dendrites from negative electrode that can punch through cell separator creating cell shorts. The lithium metal creation only occurs during charging but the electrolyte degradation can occur for charging or discharging at low temperatures.

You get greater battery voltage slump during discharge current and greater voltage bump up during charging current at colder temperatures. This is the greater overpotential voltage to push the lithium-ion migration rate.

You should not balance unless charging since you should not balance below 3.4v cell voltage which pretty much requires charging current be present to get above 3.4v cell voltage.

Balancing below 3.4v can be done but it can actually create greater cell imbalance that causes cell overvoltage trip by BMS when attempting a full charge.

Reason for the only above 3.4v balancing is below this voltage the SoC cell voltage difference is small and any random discharge current on cells due to inverter loading creates an overpotential cell terminal voltage slump greater than the SoC open circuit cell voltage difference.

Cell mis-matching results in cells' overpotential voltage slump under load current being slightly different which is independent of the cells' SoC. This throws off the balancer decision making on which cells need to be balance bled, possibly balancing in wrong direction worsening the SoC balance.

By setting balancing to start above 3.4v, any sporadic inverter load current will almost immediately drop cell voltage below 3.4v and shut off balancing.

There can also be a small time delta between cell voltage sampling times causing differing time dependent load conditions when a given cell voltage is sampled that prejudges the cell voltage samples.

 

[Texas-Mark]

Because there's no reason to balance until close to full charge. And at least float charging.
Resting cells aren't usually at the balance voltage.

Not all active balancers are on/off controlled by cell voltage. That is why there are several threads here on how people are devising ways to turn them on/off by reading the cell voltages. I have also read posts by people that say they just leave them on all the time (like I usually do). I can see a situation where someone charges a battery (maybe with a generator) just before night and then it goes below freezing. The active balance may still be doing it's thing if it is not is not voltage controlled for on/off, or if the cell voltages are still high.

As I noted, I usually just leave mine on all the time (have had zero issues). But when I disconnected the batteries from the trailer, I turned them off more to keep from draining the batteries than being concerned about the cold. However the trailer (and batteries) have been below freezing many nights recently. Had I not turned them off, I wonder what effect it would have had on the cells (if any) since there would be minimal current flow since they are well balanced.

 

[timselectric]

Balancing below 3.4v per cell is pointless, wasteful, and detrimental to Balancing. Fix that first, and you won't have to worry about the cold temperature issue.

But ,I doubt that you have a large enough active balancer to be a problem with the low temperature.
A trickle charge for seconds at a time, isn't going to hurt the cells.

 

[Texas-Mark]

Balancing below 3.4v per cell is pointless, wasteful, and detrimental to Balancing.

I think there are quite a few here that would disagree. As I said, I have had zero issues leaving mine on all the time and my cells are usually almost balanced to .001v - .002v Then again, I don't run my batteries all the way down either.

But ,I doubt that you have a large enough active balancer to be a problem with the low temperature.
A trickle charge for seconds at a time, isn't going to hurt the cells.

This was the question I was asking. How much "charge" is too much when below freezing.

 

[timselectric]

Everyone is entitled to be wrong. lol

I would not worry, under 5a.

 

[time2roll]

Interesting question. I doubt it matters if the cells are basically in balance before the cold storage.

Typical capacitive balancer would need 500 mV differential to get a full 5 amps flowing. At < 10 mV differential the current flow will be proportionately less or zero.