Battery balancing

[DickB]

The Standard Rad battery is made up of 52 cells, with groups of 4 cells connected in parallel (to provide more current) and 13 groups connected in series (to provide more voltage). 

Cells are in balance when all cells are at the same State Of Charge (SOC) and voltage, and out of balance when at a different SOC and voltage.  The cells connected in parallel are always in balance, because they are all at the same voltage.  The series-connected groups can get out of balance, with one or more groups having more or less SOC than the others.

It is desirable to have all cells at the same SOC to get maximum performance – run time – out of the battery as a whole.

The Standard Rad battery uses passive cell balancing. Passive balancing is performed only during the charge cycle, and when the charger is in Constant Voltage (CV) mode. It does so by bypassing charge current away from cells that are already at the desired SOC and into cells that are at a lesser SOC.

Looking at the attached figures, each group of 4 cells in parallel has a shunt resistor and a switch connected in parallel with the cells. When the switch is off, all charge current flows to and through the cells. When the switch is on, up to about 40 mA (0.04A) of current flows through the shunt resistors, and not to the cells.

When a depleted battery is first connected to the charger, the charger operates in Constant Current (CC) mode. About 2A of current flows to and through the groups of cells – about 0.5A to each cell – and none through the shunt resistors, because the shunt switches are off. In Figure 1, note that Cell Group 2 is at a lower SOC than the other cell groups.

As the cells are charged, their SOC and voltage increases. After (typically) 2-3 hours, the combined voltage of all cells approaches the charger float voltage of 54.6V, and the charge current decreases dramatically.  Cell Group 2 has been charged along with the other groups, but remains at a lower SOC and voltage than the other cell groups. The Battery Management System (BMS) detects this voltage imbalance, and turns on the shunt resistor switches for all cell groups that have reached full SOC, bypassing current around the cells. It leaves the switch off for any cell group that is at a lower SOC, so that the current continues to flow into those cells. This stops the fully-charged cell groups from getting more charge current, and only the lesser-charged cell groups continue to get current. The BMS is balancing the cells. Because the balancing current is much smaller than the initial charge current, it can take several hours to bring the unbalanced cells up to the desired SOC.

The BMS will enable balancing any time the charger is in CV mode (green light) and any of the cell groups is at a lower SOC and voltage than the others. Balancing is enabled based on relative cell voltages, not an absolute voltage. There is nothing special about a 12 hour charge, other than 12 hours pretty much guarantees that the BMS will have had adequate time to balance. Once all cells are in balance, all the switches are turned on, and all low charge current bypasses the cells.

[JimInPT]

Brilliant!  Well-written and illustrated so that even us dumb mechanical engineers can understand.

Thanks and Happy New Year!

[Eric7]

Thank you.  Nice writeup.  Thanks so much.

[Ddaybc]

Yes, thank you. Very well explained.

[john in Idaho]

Excellent description.  So........ if the charger is left hooked up, not just for 12 hours but forgot about for a few days, does any harm result?  I have a countdown timer that i can set for up to 12 hours then it automatically shuts off.  But how long I set it for is just an educated(?) guess.

[JimInPT]

I have a countdown timer that i can set for up to 12 hours then it automatically shuts off.  But how long I set it for is just an educated(?) guess.

Shortly after I got my MiniST, I added a countdown timer to prevent forgetting about it being plugged in (probably can't hurt over a couple of days, but even Rad doesn't recommend leaving it connected indefinitely) and set it to 7 hours.  I almost never discharge below 2 bars (20-80% is the optimum range for battery longevity) and 7 hours will bring it to 100% every time no matter how much is discharged, then shuts down.  I asked a Rad tech about my 7-hour setting during a support call and they're cool with that.

I'm awaiting one of Peanutbutter's charging regulators so I can set it to 80% charge-to-shutoff for normal use but will occasionally take it back up to 100% for recommended rebalancing now and then or for expected long rides.

[Veggyhed]

As for timers I use a Kasa smart plug and a WeMo Insight smart plug to charge my batteries.

Sent from my Pixel 3a using Tapatalk

[DickB]

Excellent description.  So........ if the charger is left hooked up, not just for 12 hours but forgot about for a few days, does any harm result?  I have a countdown timer that i can set for up to 12 hours then it automatically shuts off.  But how long I set it for is just an educated(?) guess.

https://www.radowners.com/index.php?topic=1502.msg7519#msg7519

[GasBlaster]

Only the weakest cells get charged.

Balancing makes it so there are no more weak cells, and all cells get charged.

[John Rose]

This makes perfect sense.
Once a group of 4 is charged ...

... it's "NO CURRENT FOR YOU!"

But - is there some feature for dealing with a failure in one of the cells in a group of four?

[DickB]

But - is there some feature for dealing with a failure in one of the cells in a group of four?

No.

[John Rose]

Having just received my RadExpand 5, I noticed that there is no mention of "battery balancing" in the Owners Manual at all.

In the manual for the RadMini ST2 (2020?), they recommend it for new batteries and after long-term storage, and then go on to describe the procedure.

I wonder if the newer batteries have been balanced at the factory, or do they have a newer BMS  that deals with it automagically?

[handlebar]

Rad's Battery Charging Guide, dated October, 2023, says,

"You do not need to balance your battery unless your Owner's Manual or Rad Power Bikes Product Support recommends it. Batteries from Rad Power Bikes stay well balanced under normal use conditions. Follow the use, charging, and storage instructions in the Owner's Manual, and your battery will provide you with excellent performance. "

They used to say you should routinely cycle the battery three times of 12 hours on the charger. Now they say not to do it unless you call Product Support to complain about your battery. As long as cells don't all have the same rate of self-discharge, balancing will be necessary. I think the new policy reflects a belief that most users leave their chargers on enough to keep up with balancing.

I plug my Kill-A-Watt meter into the wall and my timer into the meter. Normal charger wattage with a green light is about 1.7. If the green light is on and the charger is drawing 2.0 or more, I assume it's balancing and leave it on. Normally, I don't need to run a charger overtime. The time I let a battery sit several months, it needed hours of balancing, according to my watt meter.