how important is balancing?

Topics concerning OEM and open source BMSes
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muehlpower
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how important is balancing?

Post by muehlpower »

If I understand correctly the function of a BMS is to make sure that every cell of the battery is healthy. It must ensure that overcharging, undervoltage or overheating is avoided. It determines the possible charge/discharge current and the SOC. But when and how balancing? During charging? With a full or empty battery? When the car is driving or after a rest period when parking. Are cells with high voltage discharged through a resistor until their voltage reaches the lowest level?

What happens if I only monitor the maximum and minimum cell voltage and limit the charger or motor if necessary and control cooling and heating?
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Re: how important is balancing?

Post by arber333 »

muehlpower wrote: Sun Oct 30, 2022 9:42 pm If I understand correctly the function of a BMS is to make sure that every cell of the battery is healthy. It must ensure that overcharging, undervoltage or overheating is avoided. It determines the possible charge/discharge current and the SOC. But when and how balancing? During charging? With a full or empty battery? When the car is driving or after a rest period when parking. Are cells with high voltage discharged through a resistor until their voltage reaches the lowest level?

What happens if I only monitor the maximum and minimum cell voltage and limit the charger or motor if necessary and control cooling and heating?
True! With Kokam cells balancing wouldnt even matter below 3.9V They are such low resistance. After 3.9V they become softer and you can balance them noticably. However i also noticed some harmonic effect when balancing long string 96S. After 5 cells the 6th one would see higher voltage state throughout balancing. Maybe was just my BMS but i decided i dont want to balance them for some time and i just charged them lower to 4.05V per cell.
It worked! Even now in Mazda outside the cells are connected without balancing. And i charge them to 4.05 with discharge down to 3.4V. I only observe their voltage. Works!

In Pug i use active balancing 1A which is ON all the time. I have to avoid leaving the car at low SOC because those balancers are very efficient and as they would balance at low SOC they would also UNbalance at higher SOC. Which would be dangerous when charging at higher power unattended.
I am now trying to build VCU that would keep balancers OFF for the most of the SOC range and only turn them ON for the last 10% after cells would go higher than 3.9V.
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Re: how important is balancing?

Post by johu »

All OEM BMSes use balancing currents below 100mA, the older Leafs just 15mA. The Leaf only balances when "on" i.e. charging or driving. It seems to remember which cells need charge taken out and always keeps the respective resistors on, no matter the cell voltage. The high level strategy is top balancing, i.e. a Leaf pack will look unbalanced when empty and perfectly balanced when full.
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Re: how important is balancing?

Post by MattsAwesomeStuff »

For a few years, the popular meta was kind of to just... not balance.

With E-Bikes, it was, hey, check your cells once a season and manually top balance them. Good enough.

With vehicles, it was still true, because it takes one glitch for a BMS to ruin your whole battery pack. That was more common than actual cell imbalances. That was in the days before OEM BMSs.

You could never sell a vehicle without a BMS, but for a DIYer, I'd say it's not essential because you have context and alertness and occasional maintenance.

The thing is, if you start with a pack that's capacity-matched, and you're state of charge matched... energy is going in and out at the same rate. So the amount of drift is microscopic. Keep an eye on it, and you're fine.

If the amount of drift isn't microscopic, a BMS isn't going to save it. That cell is already done, the BMS can't keep up.

Not pushing your cells to their absolute limits, leaving a few percent above and below, is going to save you as well, because now you can allow a few percent of imbalance without causing problems.

The bigger concern than imbalance, is, if your charger fails and overvoltage happens. So, that'd be something good to trigger a breaker or something. I.E. Johannes nearly burned his car to the ground, and that's even with an overcharge trigger.
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Re: how important is balancing?

Post by catphish »

As you say, a BMS provides several functions.The most important function is to ensure that no cell is charged above its maximum voltage, or discharged below its minimum voltage. It's essential to have some mechanism to ensure this as overcharging an individual cell can be dangerous. If your BMS provides this function then you don't *need* to balance.

If you didn't balance, you would up with less capacity over time, because the most charged cell will reach its maximum voltage sooner, and the least charged cell will reach its minimum voltage sooner. You will be wasting battery capacity.

For this reason, it makes sense to do some kind of balancing. The most common (or at least the simplest) approach is to wait until the pack is near full charge, and then discharge the highest voltage cell through a resistor until it's in line with the others. Repeat for each cell until they're all the same. This could be done manually, but if you've already got a BMS monitoring the cell voltages, it makes sense to automate this.
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Re: how important is balancing?

Post by tom91 »

One thing to keep in mind is the wired connections and what the balancing vs measurement timing is. You can quite quickly end up with all sorts of issues, mainly voltage dips or fluctuations because of the current flowing due to balancing.
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Re: how important is balancing?

Post by johu »

Yes that is quite terrible when extending the leads of the Nissan BMS...
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