The difference between active equilibrium and passive equilibrium

Nami

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Battery management system BMS in addition to preventing battery pack overcharging and overdischarge,
Good balancing is required to maintain the consistency of the battery pack.
At present, almost all BMS in the market have the balancing function, which is mainly divided into passive balancing and active balancing.

☆ Active equalization (lossless equalization)
Active equalization equalizes the whole set of voltage by transferring the high energy of a single cell to a battery with low energy of a single cell in a way of energy transfer, with little energy loss involved in the transfer process.
1.Start Equilibrium Conditions
Batteries start to balance whenever the pressure difference is greater than the set value, whether they are charging, discharging or resting, so as long as there is an active balance with the pressure difference, it should work 24 hours a day until the pressure difference is less than the set range.
2.Balanced current
Since active balancing is the way of energy transfer and does not heat up, all balanced currents can be large without affecting heat loss. Generally, active balanced currents can be 1~2A more common.
Since active equalization is not limited by charging time, has a longer equalization time, and has a larger equalization current, it is suitable for use in large capacity battery packs.
☆Passive Equilibrium (Lossy Equilibrium)
Passive equalization generally discharges batteries with higher voltage by means of resistance discharge, releasing energy in the form of heat, balancing the whole set of voltage, and gaining more charging time for other batteries.
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1.Start Equilibrium Conditions
Passive equalization can start discharge equalization only when the battery is close to full voltage, so the start-up equalization time of passive equalization is relatively short, from full battery to full-up, and generally several hours equalization time depending on the charger.
2.Balanced current
Because passive equalization is a resistance-consuming equalization, the energy consumed is released in the form of heat, so charging BMS during the equalization process will heat up, resulting in a balanced current that cannot be set too high, otherwise the battery pack temperature will be too high. Generally, the passive equalization current is from 35mA to 200mA, the larger the equalization current, the more severe the fever.
If the equalization current is small, the efficiency of the power balance in large capacity battery packs with large power differences is low. It takes a long time to reach the balance, and there is a scratching sensation in the application. Passive equalization circuit is simple, low cost and suitable for low capacity battery packs.

☆Advantages of active equalization method:
1) The active equalization circuit has high equalization efficiency.
2) Balance during charging, discharging and static process.
3) The balance current is large and the balance speed is fast.
 

toms

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The fewer components involved, the greater the inherent reliability the system has. With a properly set up BMS, small resistive balancing maintains cell balance easily and reliably.

Complex balancing systems are a bandaid for poorly designed systems, and have proven to be unreliable.
 

RCinFLA

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The more balancing current available, the better. With small variations in cells and interconnecting resistance, along with relatively flat discharge curve of LFP cells, you should avoid balancing below 3.4v. When moderate inverter load currents amplify small resistances differences between cell voltage sense points, it can overtake actual SOC voltage variation, causing balancing in wrong direction.

Since each BMS sense wire shares connection and measurement between two cells, make sure you consistently make sense wire connections so you have no more than one series bus bar connection between adjacent sense wire pairs. BMS's momentarily shut down balancing current when they make cell voltage measurement, but BMS has no control over random inverter loading currents.

Restricting balancing to above 3.4v cell voltage has two benefits. There is more cell voltage variance as cells approach full charge, so it is easier to determine which cells actually need balancing. If inverter load pulls moderate current load when cell is above 3.4v, the cell will almost immediately drop below 3.4v which shuts down balancing and more importantly balancing direct decisions, which may be corrupted by random inverter load currents through inter-cell resistance variations effecting the voltage sense readings by BMS.

Downside of balancing only above 3.4v is achieving balance before highest SOC cell hits over-voltage shutdown limit. This is where having greater balancing current greatly helps.
 

toms

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Downside of balancing only above 3.4v is achieving balance before highest SOC cell hits over-voltage shutdown limit. This is where having greater balancing current greatly helps.

I disagree, especially where the balancing is resistive and housed along with the rest of the BMS. High current = high heat.

A well set up system will taper charge current during balancing, and i’ve set up lots of 400 - 600ah (48V) systems that easily maintain balance with 0.5A balance resistors.

If your system allows a charge controller to let a cell reach its high voltage cutoff - it is a failure of a design. It should reduce current when the first cell reaches balancing voltage, then pause charging but continue balancing when the first cell reaches top balancing voltage.

If you insist on using a cheap BMS - then sure, be ready with the bandaid balancing methods.
 

RCinFLA

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A well set up system will taper charge current during balancing, and i’ve set up lots of 400 - 600ah (48V) systems that easily maintain balance with 0.5A balance resistors.
Only equipment that tapers charge current during balancing is when there is communications link between BMS and inverter/charger.

There are not very many BMS's with passive balancers that will provide 0.5A of balancing current.
 

toms

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Only equipment that tapers charge current during balancing is when there is communications link between BMS and inverter/charger.

Yes, and if you don’t have this then you are going to need bandaids. The very first design decision when using LiFePO4 should be to ensure your BMS has control of your inverter and charger.
 

ocular

Solar Enthusiast
Joined
Aug 16, 2020
Messages
43
Battery management system BMS in addition to preventing battery pack overcharging and overdischarge,
Good balancing is required to maintain the consistency of the battery pack.
At present, almost all BMS in the market have the balancing function, which is mainly divided into passive balancing and active balancing.

☆ Active equalization (lossless equalization)
Active equalization equalizes the whole set of voltage by transferring the high energy of a single cell to a battery with low energy of a single cell in a way of energy transfer, with little energy loss involved in the transfer process.
1.Start Equilibrium Conditions
Batteries start to balance whenever the pressure difference is greater than the set value, whether they are charging, discharging or resting, so as long as there is an active balance with the pressure difference, it should work 24 hours a day until the pressure difference is less than the set range.

Interesting discussion and some great wisdom shared. Not sure for active balancing the Start Equilibrium Conditions should only be just the pressure difference between cells (delta V) for LiFePO4 sytems. I notice that in some smart active balancers this delta V to start balancing can be set as well as a setting to start balancing above a certain cell voltage. So if we are to use the pressure difference to initiate and allow the active balancer to balance 24/7 across the whole voltage curve, what is the correct set value for the pressure difference (delta V) above which balancing should occur - >5mV, >10mV, >20mV, >50mV - ?
If the set value is too low then the balancer will be sensitive enough to unbalance (aggressively) in the flat part of the curve. How should we determine the correct set value for our system?
 
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