Inside the Rad battery and BMS

[DickB]

The Rad battery pack is made up of batteries, Battery Management System (BMS) circuit board, key switch, charging port, power connector, and battery meter.

The Rad BMS uses a Sino Wealth SH367008 Li-Ion Battery Management chip. Sino Wealth is a Chinese integrated circuit manufacturer. The spec sheet for the chip is in Chinese, but Google Translate does a good job.

"SH36700X has built-in high-precision voltage detection circuit and delay circuit to monitor voltage, current and temperature to ensure the safety of the pack. At the same time, the series of chips also have Balance function to extend battery life."

The Rad battery pack has 13 groups of 4 cells each, with the 4 cells connected in parallel and the 13 groups connected in series. Because of the parallel connection, each group of 4 cells is essentially one cell with 4 times the capacity of an individual cell. Unless otherwise noted, further use of the term "cell" refers to a group of 4 parallel-connected cells. To the BMS, each group is in essence one cell.

The Rad chip uses a passive switching shunt resistor method for balancing. It does not use active balancing. Balancing is done during charging only.

Shunt resistors are switched in to equalize the voltage of each cell by dissipating the energy from higher-voltage cells down to the lowest cell voltage. Resistors are connected in parallel with each series-connected cell through semiconductor switches. The switches are controlled by the Sino Wealth chip.

The shunt resistors draw only a small amount of current and would be ineffective at high charge current. Balancing must take place at the end of the charge cycle. This is almost certainly after the Rad Charge light has turned green. To balance cells, Rad instructs that the charger be left on for 12 hours, long after the green Charge light. I have observed the charger continue to supply current to the battery pack after the green light illuminates. If an aftermarket charger cuts off current at charge complete, the pack may not be balanced.

The circuit board has semiconductor switches to disconnect the battery under both charge and discharge conditions as a whole pack. There is no provision to connect or disconnect individual cells or groups of cells. The pack is disconnected under charge overvoltage, discharge overvoltage, charge under voltage, discharge under voltage, short circuit, and high or low temperature.

With the key switch in the Off position, the semiconductor switches are off. Turning the key switch on "wakes up" the chip and the discharge switches are turned on presenting voltage to the battery power connector. As long as the key switch is on, the BMS is active and drawing current, albeit small, from the battery. The key switch does not disconnect power directly, but basically tells the chip to do so.

Construction of the battery pack is high quality. The mechanical construction is very good, with a plastic holder for the cells that is very well secured, as is the circuit board. Battery connections are direct to the circuit board with welded straps. Components and connections to the circuit board are all environmentally protected (think humidity) with conformal coating - the blue stuff that you see in the photo.

[sc00ter]

Nice info and thanks for sharing!

[vudude]

Thanks for the peak inside the Rad battery pack. Are the cells 18650? What is the branding on the cells? Thanks

[fin_rad]

Thanks @DickB! Good info!

[DickB]

Thanks for the peak inside the Rad battery pack. Are the cells 18650? What is the branding on the cells? Thanks

The cells are 18650, which denotes only the size of the Li-Ion cell (18mm x 16 mm), much like the designation AA. None of the visible cells were oriented so as to show the manufacturer, but Rad says that they use Samsung 35E cells:
https://bit.ly/3iAetTr

[DickB]

I'm not 100% sure exactly when balancing takes place; more guessing based on observations and Rad instructions. I may intentionally unbalance a cell and take some measurements to learn more, but that will have to wait as I am a bit busy for a couple of weeks.

Here is a good reference on various Li-Ion balancing methods:
https://onlinelibrary.wiley.com/doi/full/10.1002/est2.203

[DickB]

I have seen comments about how the Rad charger is of lesser quality than some aftermarket chargers.  Based on what I see in the Rad battery, I have to question that.

Here is a competitor's battery pack.  The silver rectangle is the BMS. It is taped to the cells, and there are over a dozen wires connecting the BMS to the cells. Compare that to Rad's construction. You can't tell from my photos, but the Rad plastic battery holder is securely screwed to the base. Dense foam pieces are used to isolate and protect the circuit board from the top of the case. Electrical connectors, such as from circuit board to the LED meter, use locking connectors, but are also secured with tape (second photo is Rad).

Which would you say is of higher quality?

[ejonesss]

has anyone built and ran a 14s battery on their stock rad bike?

preferably radwagon 4 or built around the same time so compatibility conditions would be the same?

will the bike run ok or will it show an over voltage error of some sort?

[DickB]

Curious why you would want a higher-voltage battery.

[RJ in Tucson]

Thanks for the detailed explainer Dick. Are you affiliated with Rad or is this just your inner nerd unable to help himself?
I've built a few 48V cell packs successfully from recycled and load tested laptop batteries for a Sun EZ recumbent I converted back in 2015. Your observations are spot on and much appreciated.

[DickB]

I am not affiliated with Rad. I am an engineer and mostly just like to know how things work, so yes, I am a nerd.  I do have an interest in maximizing battery life, as I ride every day and put on well over 4000 miles a year. And understanding more precisely what the State of Charge (SoC) is at any time. So knowing more about the BMS is helpful. I have also seen a lot of speculation on how the Rad BMS works, some of which is not correct, so I wanted to find out for myself more about it.

[fin_rad]

-- I ride every day and put on well over 4000 miles a year.

Offtopic, but it would be more than interesting to know how your Rad bike has survived doing such high mileage? Perhaps on some other topic here. I've done around 1500 miles on one year so far, all working still great.

[DickB]

I only have 400 miles on my Rad. I just got it a few weeks ago. Previous miles on a mountain bike.

[hbanquer]

This is an EXCELLENT article you have written. From one Nerd to another!

[Jman]

As suspected, the BMS is the likely culprit behind people's dead batteries when they leave them for extended periods of time(if the key switch is left in the on position).