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.
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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 = 1.94AImage
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Two chargers connected => current = 3.95AImage
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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.

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

8 thoughts on “What is the recommended charge current ?

  1. RotoGluOn

    Hi,

    All the pictures/images on your site are not working.

    I own a MSUPER v2 HS 850 Wh and I’d like to know whether it’s interesting or not for me to get the dual input CD V2 ?

    How can I be sure I have a 850 Wh and not a 680 Wh like Thomas T. ?

    I really want to know because I paid for the extended battery capacity and I’d be angry if I don’t have it…

    Thanks for your help.

    I’ll order as soon as you answer my few questions.

    Regards.

    Clément B.

    Reply
    1. hobby16 Post author

      Oh yes, for such a big battery, the CD with double input is a must (when you have tasted fast charging, you’ll see, you will find it hard to do without). It’s the best service/price of the year you can get

      In a ballpark, count about 12Wh per km for a 65kg rider, tyre pressure = 3 bar (so about 70km for a 850Wh battery).
      But with the CD, it’s quite easy and more accurate to know the real capacity of your battery.
      Ride until tilt-up (near empty or empty battery). Then charge until full and read the Wh counter on the CD. You can even log the charge data and display it on your PC then looking at the charge curve, other users on the forum (or me) can tell you immediately if your battery is right or not.

      Things to remember:
      – ride until empty, otherwise, the total Wh you have injected would be lower than the nameplate capacity
      – reset the counter at the beginning of the charge to start from 0Wh
      – CD comes with a default autocutoff at (about) 90%, so just disable if you want to charge to 100% for your measurement.
      If any more question, please let me know.

    1. hobby16 Post author

      The Monster is with 20S battery and GX16-4pins connector, so it will work with the CD without problem

  2. Philippe Hoyet

    Bonjour,
    Pour une Dualtron Ultra chaud V2, on est bon si je vous prends le CD V2 à 3 broches ? Je crois que oui mais c’fest juste pour confirmer.

    Cdlt.

    Reply
    1. hobby16 Post author

      oui, je confirme. Pour toutes les trotts Dualtron, c’est bien le CD V2 avec prises GX16-3 (3 broches), à 1 entrée, 2 entrées (duo) ou même 3 entrées (triplex).

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