Monthly Archives: February 2016

What is the recommended charge current ?

Now that the Charge Doctor 10A V2 is available, the question of charging at 4A or more has been frequently asked. A higher charge current reduces charging time but does it reduce the lifetime of the battery?
Answer: it depends on the battery’s “1C” value.

Electric unicycles use 56V batteries (16 LiIon cells Vnominal = 3.6V), the correspondence between Wh and “1C” value in Ah is given below (for hoverboards or other electric bicycles with 42V or 36V batteries, recalculate 1C = Wh / voltage):

Battery capacity
“1C” value
Example of wheel
130 Wh 2.2 Ah Solowheel, Airwheel, NineBot
260 Wh 4.5 Ah Firewheel 260, Gotway, Kingsong
520 Wh 9 Ah Firewheel 520, Dolphin
680 Wh 12 Ah Gotway, KingSong

If 1C value is 2Ah, a “1C charge” means charging at 2A.

It is a convenient normalisation to determine wether the charge current is too high or not, and thus its impact on the battery’s lifetime.

  • 2C charge is a fast charge. It is not recommended (it can apply in RC model world but on LiPo batteries, not LiIon).
  • 1C charge is acceptable but may decrease the battery lifetime. A lot of electric unicycles have 130Wh batteries (Solowheel, Airwheel X3 et clones, Ninebot E…) so they are charged at 1C by a 2A standard charger.
  • 0,5C charge is a slow charge and may be a good compromise between charging time and battery lifetime. A 260Wh battery (Firewheel 260) is charged at 0,5C by a 2A standard charger.
  • 0,2C charge is considered a very soft charge.

With a 680Wh battery for example, a 1C charge means the charge current is 12A! So a 4A or 6A charge (by connecting in parallel a charger 4A + charger 2A or 2A 3 chargers) remains a “slow” charge. Even seemingly so high currents should not affect the battery lifetime. Of course, preserving lifetime also means avoiding overload and deep discharge, the subject of a previous post.

Fast charge with the Charge Doctor V2

The Charge Doctor V2 has a option with 2-input connectors to connect in parallel two chargers. Owners of large batteries will have more connection options (see table below) and can mount a cheap 4A charger using two standard and ubiquitous 2A chargers.


Connection Total current
one 2A charger
2 A
two 2A chargers
4 A
one 2A charger + one 4A charger 6 A use thicker wires*
two 4A chargers
8 A use thicker wires*

* for currents above 4A , the charging wires (between the BMS and the wheel’s charge connector) are too thin and must be replaced by thicker wires.

Case study of a Firewheel 260Wh fast charge

The battery is a 260Wh type and is empty (ridden until pedals tilt-up) before data measurements.

One charger connected => voltage = 61.7V ; current = 1.94AImage

Two chargers connected => current = 3.95AImage

The charging curves were sampled using the Charge Doctor’s serial output. The 4A charging graph can be compared to a 2A graph of the same 260Wh battery. Charging from empty to 90% capacity lasts about 1 hour, meaning charge time has been divided by 2!Image

mark capacity/Wh % total capacity voltage /V current /A observation
0 0 0% 67.3 0 one charger connected
A 0 0% 57,4 2,03 start of charge
B 3,9 0% 60,0 3,99 second charger connected
C 152,8 58% 66,3 3,89 end of constant current phase
D 211,0 80% 67,1 1,84 current <2A => one of the chargers has desactivated by itself
E 244,8 93% 67,5 0,70 auto-shutdown by Charge Doctor, cut threshold = 0.7A
F 244,8 93% 67,5 0,81 manual power-on to charge until 100%
G 263 100% 67.5 0,03 charge termination, disconnection
  • Note 1: when connecting the second charger (A-> B), the internal resistance of the battery can be estimated from the voltage jump : R = deltaV / deltaA = (60-57.4) / (3.99-2.03), or R = 1.3 ohms. It’s a rather high resistance typical of LiIon batteries (LiPos have lower internal resistance). This explains the big voltage sag I observe on my Firewheel when accelerating, with the fuel indicator dropping momentarily from 50% to 20%.
    Bigger batteries, eg 520Wh, would probably have much smaller R.
  • Note 2: setting automatic cutoff at 0.7A stops the charge session at 93% capacity, a value consistent with results from a 2A charge. The correspondence between cutoff current and % of charge seems to depends only on battery capacity, not on load current. A ballpark value would be 1A threshold => 90% of charge for a 260Wh battery.
  • Note 3: charging from 93% up to 100% takes as much time as charging from 0% to 93%, so the last drops of charge are usually not really worth it.

Warning: in theory, connecting two chargers in parallel poses no risk to the chargers. At least, on the chargers used so far since I don’t have any negative feedback from the first users. But given the many different charger models, I can not certify 100% that it would work on your specific chargers. So to test it knowingly. Connecting chargers in parallel is a hack well worth trying for frequent riders and/or owners of large batteries> 260Wh.

Warning: Lithium batteries charging is not without risk, all the more with high currents. So do not neglect the usual precautions : do not charge without being nearby, install a fire detector, keep valuables far from the wheel, wait until the battery is cold after a ride before charging… Stick to the precaution rules and don’t let routine induce you into complacency.

Edit 20/03/2016 : add graph Gotway MSuper 850, fast charge with two 1.75A Gotway chargers. Data Thomas T.


Apparently, it’s a 680Wh battery and the 850Wh nameplate value is false !

Charge Doctor V2, auto-shutdown function

For maximum battery life, it is best to avoid overdischarge and overcharge. It is generally admitted that the ideal range should be within 20% to 90% of full charge, to be kept as often as necessary, when possible, especially for happy owners of large capacity batteries like 520Wh of the Firewheel F520, even 680Wh (!) on some Gotway or KingSong wheels.

On electric monocycles or other electric wheelies (bike, hoverboard, bikeboard, gyropods…), overdischarge is avoided by the main controller board by warning and stopping. But all of them are designed to charge to full charge, which is contrary to recommanded practices above. It is nonetheless what happens with all products for obvious marketing reasons since price is in part determined by the nameplate battery capacity, which corresponds to a full charge.

The Charge Doctor V2 auto-shutdown function is precisely intended to address this inconvenience by cutting power when charge current drops under a user adjustable threshold to avoid overcharge. The auto-shutdown can of course be disabled so Charge Doctor V2 would function as any conventionnal (advanced) charge monitor.

Important : a full charge is still required occasionnaly, e.g. 1 every 10 or 20 charging sessions, to ensure the BMS ( (battery management system, the embedded battery controler board) would trigger charge balancing. Balancing, whose importance is most often exagerated, is still necessary on Lithium batteries. So plan some 100% full charges from time to time if you decide to mostly charge to 80% or 90%.

An example of charge session with auto-shutdown is described below to highlight important points of this process. It is the charge of my Firewheel F260 (batteries 16S2P, 260Wh stated capacity), datalogging is by Charge Doctor V2 and graphic display is by KST (see “Graphic Display” post for more details on how to use these tools).

The Firewheel was driven until pedals tilt-up, meaning the battery was empty before charging (voltage starts at 57.7V ie 3.6V, see “How to take care of your unicycle batteries‘). Charger is a 2A version and auto-shutdown threshold has been adjusted to 1.4A. The total datalogging duration is 3h34min30s (bottom-right number on the graph).

Charge graph of Firewheel F260 with auto-shutdown at 1.4A

A : end of the constant current stage, start of constant voltage stage
B : auto-shutdown by Charge Doctor
C : charging restart (by user)
D : charging up to 100% capacity

The charge session begins by the constant-current stage at 2A, lasting about 100 minutes, until A. Then constant voltage stage kicks in : voltage remains at 67.3V and stop increasing since it corresponds to 4.2V, the maximum voltage per cell. Note that, theorically, reaching the constant voltage phase should be sufficient to trigger cell-balancing by the BMS. After point A, current drops rapidly to point B, where Charge Doctor shutdowns current automatically. At C, Charge Doctor is switched on again to resume charging to point D, where the battery is virtually 100% full.

This full charge graph leads to the correspondance table of currrent threshold – % charge below (values are taken directly from the file “log.txt”). For example, a 1.0A threshold will end charging at 90% capacity.

For 4A or 5A fast chargers and/or different batteries, thresholds are naturally different so such graph must be logged initially by the user to determine the characteristics of his personal setup.

threhold (A)
Battery capacity (Wh) % of full charge


1.52 232.3 85%
1.40 235.6 86%
1.20 242.0 89%
1.00 245.7 90%
0.80 250.6 92%
0.60 256.0 94%
0.40 261.5 96%
0.30 264.4 97%
0.20 267.0 98%
0.10 272.7 100%

N.B. with the Firewheel charger, charge current does not drop to zero, even after hours (even after a night according to my tests). This means that the charger does not cut current at end of charge which is really bad for lithium batteries! My other chargers don’t behave like this, fortunately. Anyway, all chargers systematically charge to full capacity, which is always above the 90% recommanded level. So use the Charge Doctor V2 auto-shutdown, when you can.

Charge Doctor V2 – 10A

Charge Doctor V2 is an upgrade of the previous battery charge controller. It retains the original features of the product like a selectable charge display in Wh or Ah, user-recalibration and storage of calibration factors in eeprom, one-button user interface, datalogging by a transmit serial output. Enhancements have been added :

  • maximum current up to 10A
  • wider voltage range
  • bigger display digits
  • end-of-charge auto shutdown with adjustable current threshold

With its wider range of voltages and currents, V2 is suitable for charge monitoring of most light electric vehicules, especially electric monowheels, but also bikes, scooters, hoverboards, gyropods, e-bikeboards…


Charge Doctor V2 features

  • Voltage: 20.0V – 100.0 V
  • Current : 0.00 – 10.00 A
  • Charge : 0.000-1000 Ah with automatic decimation
  • Energy : 0.0-10000 Wh with automatic decimation
  • End-of-charge adjustable current threashold : 0.1-8.0 A
  • Datalogging serial output 9600 bauds TTL
  • In/out with 3-pin GX16 aviation connector (Airwheel & clones, TG, iezWay, Gotway, Firewheel, most e-bikeboard…) or Lemo connector for Ninebot
  • Dimensions : 85x50x20 mm
  • Weight : 65 g


At startup, displays “C-Dr” (top) and firmware version “2.00” (bottom).


Top display shows alternatively voltage (“65.7U”) and current in amps (“1.99A”).

Image Image

Bottom display has 3 display modes :

Short click => mode change
1. Wh
2. Ah : “Ah” alternates with value to differentiate from Wh mode
Auto ; flashing display of auto-shutdown current threshold
( auto-shutdown disabled => current = 0.00A)
Long click
1. & 2. reset Ah and Wh counters
3. Enable/disable auto-shutdown (“0.00A” = disabled).
N.B. auto-shutdown threshold can
only be changed in “adjustment” mode

Important : after auto-shutdown, the internal switch is in OFF state and no current can pass. To enable charging again, switch ON the internal switch using either methods :

  • long click when in auto-shutdown mode (see above)
  • disconnect/reconnect Charge Doctor (internal switch is always ON at startup)


Enter adjustment mode by holding the button when powering on.
Button hold < 5s => mode 1) auto-shutdown current threshold adjustment
Button hold > 5s => mode 2) voltage & current calibration

1) Adjustement of auto-shutdown current threshold

Top led displays actual parameter. With no button click, parameters changes every 2s in a round-robin schedule for all available parameters. Any click hold up the actual parameter 6s before the round-robin cycle restarts.
“Auto” : auto-shutdown mode. Bottom led displays the shutdown threshold ( “1.00A”). When charge current drops below this threshold, charging is stopped by Charge Doctor. Change by 0.1A steps by clicking the button or by holding the button
Important : disabling auto-shutdown (display = “0.00A”) can only be done in normal mode (see above), not in adjustment mode
“Pr1” : parameter 1. Value = 1 => deep standby mode with Led display off at auto-shutdown. To power on, click button or disconnect/reconnect Charge Doctor.
“PR2”, … “PRn” : future extension

If no click for more than 6s, this adjustement mode is terminated and Charge Doctor reverts back to normal mode.

2) Calibration

Voltage and current are calibrated at +-10mV and +-1mA respectively before shipping. Avoid recalibration unless you have accurate instrumentation.

Top led displays actual parameter. With no button click, parameters changes every 2s in a round robin schedule for all available parameters. Any click keeps the actual parameter 6s before the round-robin restarts. To quit this calibration mode and revert back to normal mode, power-off then on Charge Doctor.

“down U” : decrease voltage gain
(about 10mV per click)
“UP U” : increase voltage gain
“down A” : decrease current gain
(about 1mA per click))
“UP A” : increase current gain
“00 A” : change current offset to calibrate 0.00A
ATTENTION : connect no charge (zero current) and hold button 2s to calibrate (short click is ignored to filter out manipulation error)

Data logging

Charge-Doctor transmits data every 10s by serial 9600 bds protocol, TTL level, 8 bit data, 1 bit stop, no parity. Data are in ASCII format with separator = coma. The format is identical to the first version Charge Doctor, see the blog’s article on Graphic Display for steps to download and process data in realtime.
N.B. Charge Doctor does not store data. For data logging, a PC must be connected during all the charge record.

Connection to dongle : the Charge Doctor’s output Tx must be connected to the serial-USB converter dongle’s input Rx :


IMPORTANT : at power-up, Charge Doctor sends a 2 line header containing various information such as the firmware version :

Charge Doctor V2.00-10A by HMC 01/2015
Time(s) Voltage(x0.1V) Current(x10mA) Charge(x0.1Wh)

These two lines will be recorded in file “log.txt” but they are not parsable by the grapher KST so the realtime display will stop : KST is quite pricky as to the data format and any error will stop the graph updates. Data and graph should change in realtime in KST when new data is logged into “log.txt” by datalogger.exe so if KST’s display freezes, then “log.txt” contains errors. To remove errors, quit KST and edit “log.txt” using a text editor.

The ininterpretable header lines can be removed from “log.txt” by two methods :

  • restart datalogger.exe 2 secondes AFTER restarting Charge Doctor so the header gets lost and not logged
  • Click “Stop logging”, UNcheck option “Append to file” then click “Start logging” in datalogger.exe => old file “log.txt” is deleted and replaced by a new blank file for logging, hence, without the uninterpretable header.

Pictures of Charge Doctor V2, versions with GX16 connector (most electric monocyles, bike, hoverboard, bikeboard) and Lemo connector (Ninebot monocycle).
(click picture for high resolution image)