ElectroClassic EV
Classic Cars Reborn into the Electric Future

Balancing Act

Having a well-balanced pack has a direct bearing on how much range and longevity is possible from your batteries. Out of the box, all cells should theoretically have the same factory charge, but that is usually the exception. The end user should always insure a good initial state of balance before putting the pack into service, or cells will charge and discharge unevenly, hobbled by the highest and lowest cell in the pack. During charging, if one cell reaches its ceiling before the others, it triggers a high voltage alert that shuts down the charger. This prevents damage to any of the cells due to overcharging, but also robs the lowest cells from the chance to come up to full capacity. While driving the car, those lowest charged cells will empty first, triggering a low voltage cutoff that will slow or even stop the vehicle to prevent cell damage, limiting your range even though there may be plenty of juice in the rest of the pack.

Connecting all cells in parallel is called static balancing, but there are also dynamic methods, such as bottom balancing. This involves discharging all cells to the same floor level, so they begin their next charge cycle equally at the bottom of their capacity. Top balancing requires bringing all cells to their maximum level, so they start their next discharge cycle equally at their fullest capacity. CleanPowerAuto recommends top balancing as the only method to use with their MiniBMS.

One way to top balance is charging the pack until the fullest cell trips the alarm, then individually bringing the rest of the cells to the same level with an auxiliary charger. An alternative and easier method is to siphon a bit of juice from the fullest cells one-by-one as they trip the alarm. That’s done by placing a load across the cell’s terminals without removing it from the pack, and allowing it to discharge for a few minutes. Then a new charge cycle is started until the next fullest cell trips the alarm. This is repeated until the last cell comes up to capacity. Easy is good. This is the scheme I will use.

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Here is a garden-variety load resistor. It creates resistance by passing current through a predetermined length of wire that is coiled and encased in ceramic. Depending on the type of wire and length, precise resistance values can be offered, measured in ohms. The lower the ohm rating of the resistor, the more current will flow through it, and the quicker it will discharge the cell. But the higher the current, the more heat the resistor will generate. That’s why it’s important to choose a load resistor that can handle the wattage, or it will burn up. For example: If the nominal voltage of each lithium cell is 3.2 volts, and I want to discharge at no less than 1 amp, I can deduce the resistance needed to drain individual cells using Ohm’s Law. Here comes the math. Since volts divided by amps equals ohms, then 3.2 volts divided by 1 amp = 3.2 ohms. Watts equals amps multiplied by volts, so the minimum power rating required for a 3.2 ohm resistor at 1 amp is 3.2 watts.

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The load resistor that comes closest to this requirement at Radio Shack is rated 1 ohm at 10 watts. Again, I’ll use Ohm’s Law to calculate the expected current and the wattage rating needed. A 1 ohm resistor at 3.2 volts would pass 3.2 amps, which equates to 10.2 watts. Unfortunately, that exceeds the 10 watt rating of the resistor, so I’ll put two of them in series for a total of 2 ohms. This will double the resistance and halve the current to 1.6 amps, for a safer 5.1 watts.

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The device used to drain the cells is nothing simpler than this: two 1 ohm load resistors tied together in series, and crimped to a couple clamps that will bite onto the battery terminal bolts. These resistors are designed to do nothing more than convert electricity to heat, so I made sure they’re suspended in midair when the clamps are in place, allowing them to get as hot as they like without damaging anything.

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Now the fun begins. The first step is locating the fullest cell that’s shutting down the charger. I’ll begin a charge cycle and take notes on which cells go into shunt mode first. The moment a high voltage alarm is tripped, the module displaying only a red LED will indicate the culprit cell. The green LED on that module only abates from 5 seconds to a minute, so I’ll need to watch the pack closely. But to my surprise, an HVC (high voltage cutoff) event did not happen, although the charger shut itself down automatically. I’ve since learned from CleanPowerAuto that this is a happy thing. The MiniBMS is designed to shut down the charger only during the initial CC (constant current) phase of the charge cycle, when the current is at max limit, and the danger of exceeding cell voltage is highest. During the next CV (constant voltage) phase, the charger itself limits the voltage, preventing the cells from being overcharged. Because the charger shut down without an HVC alarm, it means the pack is yet closer to coming into balance.

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Even without an HVC event, it’s still possible to discharge some of the fullest cells a bit to level the pack. This shot shows the drain clamped onto cell #26 in the front pack. The instruction sheet recommends discharging cells at 1 amp for no more than 10 minutes, but since the drain will pass about 1.6 amps, I’ll need to adjust that time downward. After about 7 minutes, I removed the clamps, rebooted the charger, and began another charge cycle. I noted which cells were fullest by how soon and how brightly the red shunt LED lit up on each module. After the charger shut itself down, I drained a few of the fullest cells before starting the next charge cycle. As I worked my way down the through the ranks, I also noticed more cells reaching shunt mode that had not done before. That means that more cells were gradually coming up to full charge. It was getting late, so I packed it in for the night.

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After the pack rested for a couple days, I started another charge cycle to finish locating and draining the cells that were filling up before the others. But a curious thing happened – none of them went to shunt mode, and after a short time the charger turned itself off. An email exchange with CleanPowerAuto assured me that this was the desired result, indicating that the pack was finally well-balanced, and none of the cells were charging beyond their limit. After all, if the charger was delivering more power than the batteries were designed to accept, they would all go into shunt mode at the end of the cycle, and would all be overflowing. As it is, they are now equally full to the brim, but not over.

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A couple of checks confirmed the pack was charging properly. Simply reading the voltage across each cell with a digital multimeter showed a consistent 3.46 to 3.48 volts per cell, indicating a good balance. Another measurement of the overall pack voltage during the CC charging phase showed an average of 122 volts, and during the CV phase showed a shade over 126 volts. This reflects the correct 501v charging curve programmed into the Elcon at the factory.

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All cells should now stay balanced until the end of their life, when they gradually begin to lose the ability to hold a charge after about 2000 cycles. At that point, another pack balance could squeeze an extra 1000 cycles from the cells, but with limited top range. That equates to over eight years of service!

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3 Responses to “Balancing Act”

  1. Great Job Mark.
    Dave Kaufmann, Longtail electric bike

  2. Very interesting. I am curious to learn about how you set “LVC” which controls the acceleration so that the batteries are protected from over-discharging.

    • Hi Masa,

      The LVC is preset by CleanPowerAuto – the manufacturer of the MiniBMS. I have not investigated whether it is possible to adjust the LVC floor, but I do not think it’s possible to adjust the shunting or HVC values either.

      Nice eBOBBER project. I like the idea of converting motorcycles. Good work!

      Mark


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