Monthly Archives: August 2015

How to take care of your unicycle’s battery

Most unicycles use 16 LiIon battery cell packs. There are a few variants, InMotion battery has 18 cells, Pinwheel uses Lipo batteries, first generation Solowheels have LiFePo batteries, so the thresholds listed below are to be adapted but the operating principle remains unchanged.

All batteries have a BMS board (battery management system) to manage the cells, especially to balance the voltages of 16 cells and to automatically shut off charging current to prevent overcharge. A large majority of BMS also features overdischarge protection by cutting off power (without warning !!!), a horribly faulty design responsible for countless accidents, sometimes serious, see this blog’s BMS many articles.

Note that according to Shane Chen himself, Solowheel’s founder and popularizer of the electric unicycle concept, his BMS never cuts off power during a ride (vital requirement for an unicycle!). If the battery has a problem, the BMS just reports it to the mainboard thanks to a dedicated signal wire, the only reasonnable thing to do.

Battery capacities

To double the capacity of a 16S1P battery pack, an additional cell is welded in parallel to each existing cell to make a 16S2P (16 series or 2 parallel cells). Smallexis made such mod for his Airwheel Q3 battery, find it with the “search” button). To triple the capacity, add two cells (16S3P), to quadruple, add 3 cells (16S4P eg on the Firewheel 680Wh). The most common variant is two parallel packs, each with its own BMS, eg two packs 340Wh in parallel to have a capacity of 680Wh (Gotways).

Important thresholds

Here are some important values for a ??LiIon pack. They are useful to know to charge & discharge properly, to make a quick diagnosis in case of the wheel’s power supply problem and to monitor battery (inevitable) aging.
Values are for cell and pack (1 pack = 16 cells in series, values ??measured by the Charge Doctor )

Note V cell V pack Threshold
1 3.0 48 Quick deterioration
2 3.2 51 Minimum, avoid going below
3 3.45 55 Stop limit on most wheels
4 3.6 58 Storage voltage
5 3.7 60 For calculating the nominal value Wh
6 4.0 64 Open circuit after full load
7 4.1 66 Balance triggering by BMS
8 4.2 67.2 Constant voltage charge threshold

(1) the battery may not be destroyed by such a low voltage, it only “takes a hit”. This is a deep discharge state. Avoid at all costs !
(2) this limit is normally never reached because the wheel’s mainboard emits low voltage warnings (buzzer and/or pedals tilt-up) and forces the rider to descend before. If you have this value or lower, then your package has a problem, usually some defective cells.
(4) indicative value, may vary. Use this value to get 50% charge state for storage (a Charge Doctor counting capacity from voltage and current would be much more accurate). The goal is not to store the battery neither full (high voltages tend to stress the cell’s electrolytes separation membrane), nor empty to avoid self-discharge from deeply discharging the battery (see (1)).
(6) after the load, open circuit (no load) voltage tends to sag, especially if the battery is old.
(8) constant voltage charging zone: the charge current gradually decreases. When it is below a few tens of mA, the load is automatically switched off by the BMS. It can also be manually turned off by the user when the red LED on the charger switches to green, which corresponds to a charging current less than 0.25A. At this point, the battery is almost 100%, keeping on charging is unnecessary because only a few Wh would be added.
(8) The voltage at the charging socket (read by the Doctor load) may be 68V due to the BMS’s voltage drop, especially the reverse voltage protection diode.

Charge profile example (Firewheel 260 Wh)

Preserve the pack

  1. It is preferable to not charge more than 90% (even 80%), nor to discharge to less than 10% of the nominal capacity in order of the reduce stress to the battery. It’s not always possible with low capacity batteries (eg 130Wh) so if you have a beefy battery, just do it.
    A 100% charge or discharge to 0% results in a more stress to the electrolyte separation membrane and the electrodes respectively and reduces the number of usefull charge / discharge cycles. A LiIon battery does NOT like overcharge.
    Most of the times, it’s OK to stop charging when the current drops below 1A on a 2-ampere chargers (see graph). For discharge, the wheels stop early enough by raising the pedals so that we do not have to worry about a deep discharge.
  2. If the wheel is not used for a long time, eg more than a year, the battery should be voltage monitored and if necessary recharged to 50% to compensate for self discharge. However, LiIon battery’s self-discharge is very small (compared to NiCd or NiMh batteries) so checking it every year is enough.
  3. Log at least once the battery charging curve (using the Charge Doctor) and store it for later comparison in case of problems or to find the battery aging conditions after a few months or years of use. The reference curve gives a lot of important information, including how to determine the duration for charging to 90% (read the voltage at the beginning of charge, place the point on the curven use the x-axis to determine the time it takes for current fall below 1A, and use this duration to end the charge).