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 |
Remark |
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 = 1.94A
Two chargers connected => current = 3.95A
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!
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 !