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):
||« 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.
|one 2A charger
|two 2A chargers
|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 !