- Labs project
July 10, 2017
There are hundreds of balancers. But there's none for big batteries, none for 20 cells at a single blow, none, which equalizes to the last Millivolt. The Balancecharger 20s is doing all this and on top it balances actively, without waste and with a simple cell phone charger! The basic idea was a circuit, which switches the phone charger from one series connected cell to the next by means of relais. The length of the USB-cable limits the current, an Atmel 8bit Microcontroller determines the weakest of all cells by measuring the voltage across the cell. Controller and the Charger ground potential are floating, but "fixed" when connected to one cell. The controller supplies individual charge to the each cell by adapting the charging time. At the beginning, each cell will be "Voltmeasured" and according to its voltage, the following charging time will be longer ar shorter. Cells of the same voltage get the same charging time, which can be as long as 120 seconds. Hence the "fullest" batteries will be almost no charged and the "emptiest" will be charged the longest time during a run over all cells. When a cell voltage exceeds 3.5 volts (LiFePo4) or 4.0 volts (Lipo), the voltage monitoring is intensified, so that there will be no overcharge. The type of cell (LFP/Lipo) can be chosen in the first 5 seconds after start: if no key is pressed, the device assumes LFP, if pressed, it automatically sets the parameters for charging Lipo (or NMC, NCA and so on). You don't have to connect 20 cells, it works with any number between 1 and 20. And there's also no need for strict series connections, you can, for instance, also connect one 3s, one 4s and one 6s to the terminal and leave a pin space between the connectors. The program determines the "missing" cells and skips these for the next runs. A P-Mosfet switches the cell to the charger shortly after the relay was activated and the initial voltage was measured. So it is possible to switch off a relay without breaking the current. This leads to a long relay life and makes a 20+20ms time period possible for brake and make. Although the charging current is measured and it would be possible to control the charging current by the Mosfet, it is recomended to use a "USB charger doctor" or similar to initially know the maximum current. If higher than 2 amps, it would be a good idea to switch another USB extension cord in the line. The code is prepared to serially send out voltage and current of each charged cell, but the board is not. This feature worked well in a 16s balancecharger, but when using it in the 20s balcharger, you will have to protect the port against shorts, because the serial out uses the same pin as the button. Please keep in mind that you always charge just one cell. If you charge a 20s battery with a current of 2amps per cell, this equals to an overall current of 100mA only. Not much but high enough to balance a "precharged" 100Ah battery of a small vehicle over night.
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