Wheelbarrow built-in battery, whether it is used or 18650 lithium polymer, are used in parallel and then the first multi-section series, so when charging is luxurious, and totally different mobile phone charger (phone is a single, continuous charge 100 hours charge to death there will be no big deal).
In s2 and s3, for example, their charger voltage is 67.2 volts, then it is the number of batteries in series it? 67.2 / 4.2 = 16 (not discuss lithium iron phosphate, a majority of the 18650 or lithium polymer 4.2v is off), that s2 battery section 16 is connected in series or section 32, and then every two parallel 16 series. (All of the following to s2, for example, other unicycle brand is the same reason, such as F400 is 50.4 / 4.2 = 12 (or 24), rw is 58.8 / 4.2 = 14 (or 28))


So, s2 of 16 batteries, each section needs to reach the cut-off voltage of 4.2v, 67.2v charger voltage exactly equal, when external pressure is equal to the charging stops.
But this is the ideal state, the reality is sometimes unattainable, because each cell in the production there are subtle differences (termed consistency) resistance, we know that the series resistance of the pile, two pressure resistance pressure drop is large, the voltage across the load resistance is high, a small resistance is low. The battery is the same, due to a slight difference between the internal resistance, so that some 16 batteries on the first full 4.2, while some have not filled, but filled up to 4.2 battery if it continues to punch, there is an explosion (18650) may occur, or the drum and the combustion (lithium polymer), so as wheelbarrow such devices, the design of the battery protection board.
Digression: the best way is to charge the battery model aircraft commonly used balancing charge is to charge each battery separately, are off to 4.2, but the balance of the charge section 16 in series requires 17 lines, in the case where the battery is not removable wheelbarrow around and do the 17 line interface is not realistic. The charger balance charge the high cost, it includes solo and sbu so dead expensive wheelbarrow filled not balanced manner, all with all the common mode protection board.
Then we say what is the role of a protective plate, protective plate is? As the name suggests is to protect the battery, but it is only to protect the battery does not explode does not burn, simply did not protect the life of the battery! Even in the long process of charging will accelerate damage to the battery life!

Here, we have for protection board works, there are two types of protection board, but the basic principle is the detection voltage of each cell and take action when one or several sections to achieve a 4.2v:
  The first is: reduce the overall charging current, the effect is to make to achieve that slow charging sections 4.2, other batteries are charged slowly, until the total voltage reaches the charging voltage of the charger off again, this is actually a protective plate in the sections that overcharge the battery first reaches 4.2, but that by limiting to prevent rapid heating sections explosion or fire, so that such protection board, in the long-term charging of several batteries is that overcharge damage, the first sections of the bad, the performance of the overall life wheelbarrow decline; 
  The second: The first full battery bypass treatment, the majority of current through the bypass circuit protection board to charge the whole series, but at the same time, the bypass circuit will cause the first full discharge behavior of the battery discharge when the voltage is low after bypass disappear, so these batteries also participated in the overall charge of the process, and so on ad infinitum, until all batteries have reached 4.2, the total voltage is equal to the voltage of the charger, the full stop. This type of protection, although no board that several batteries overcharge damage, but in a period for which it carried out several battery charge and discharge operation, add up the total life of the battery has been reduced by some, that your group 16 batteries connected in series, assuming a total of 10 times full charge cycle (from the unicycle completely dead to full), then there is a charge and discharge sections 10 times, in addition to the actual sections charge and discharge times reached 10 + N for example, 30 times or even 50 times or more.


Having said that, we must show a problem, all of the protective plate in the long charging time, are to ensure safety (non-explosive does not burn), but does not guarantee lifetime (overcharge or over the cycle)


That wheelbarrow how should you charge? You should see a green light charger immediately unplug the power! Even where N section 4.2v, additional 16-N section only 4.19v or 4.18v does not matter, stronger than harm life! If you can not see the stare charger, please buy a mechanical timer socket, is the kind of man given a few hours of charge timer socket, in accordance with the requirements of the specification, the red car out of power specified by the manufacturer on the adequacy time (for example, 3 hours), as well as half of the electric car on the set (for example, 1.5 hours), full charge is not right, fear is a lithium battery overcharge and over discharge! Charge is overdone car crash! Not only is the wheelbarrow, ordinary electric bicycle, electric motorcycle is also a reason, do not pay attention and timely charging power, the battery will be broken within one year! Many electric vehicles, but three months on the bad battery, which is rechargeable and the user's bad habits have a great relationship!
Plug in the charger on the kind of laissez-faire filled night, absolutely no good fruit, waiting for the sharp decline in vehicle life now!