BU-303: Confusion with Voltages
BU-303: Confusion with Voltages
A battery is an electrochemical device that produces a voltage potential when placing metals of different affinities into an acid solution (electrolyte). The open circuit voltage (OCV) that develops as part of an electrochemical reaction varies with the metals and electrolyte used.
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Applying a charge or discharge places the battery into the closed circuit voltage (CCV) condition. Charging raises the voltage and discharging lowers it, simulating a rubber band effect. The voltage behavior under a load and charge is governed by the current flow and the internal battery resistance. A low resistance produces low fluctuation under load or charge; a high resistance causes the voltage to swing excessively. Charging and discharging agitates the battery; full voltage stabilization takes up to 24 hours. Temperature also plays a role; a cold temperature lowers the voltage and heat raises it.
Manufacturers rate a battery by assigning a nominal voltage, and with a few exceptions, these voltages follow an agreed convention. Here are the nominal voltages of the most common batteries in brief.
Lead Acid
The nominal voltage of lead acid is 2 volts per cell, however when measuring the open circuit voltage, the OCV of a charged and rested battery should be 2.1V/cell. Keeping lead acid much below 2.1V/cell will cause the buildup of sulfation. While on float charge, lead acid measures about 2.25V/cell, higher during normal charge.
Nickel-based
In consumer applications, NiCd and NiMH are rated at 1.20V/cell; industrial, aviation and military batteries adhere to the original 1.25V. There is no difference between the 1.20V and 1.25V cell; the marking is simply preference.
Lithium-ion
The nominal voltage of lithium-ion is 3.60V/cell. Some cell manufacturers mark their Li-ion as 3.70V/cell or higher. This offers a marketing advantage because the higher voltage boosts the watt-hours on paper (voltage multiplied by current equals watts). The 3.70V/cell rating also creates unfamiliar references of 11.1V and 14.8V when connecting three and four cells in series rather than the more familiar 10.80V and 14.40V respectively. Equipment manufacturers adhere to the nominal cell voltage of 3.60V for most Li-ion systems as a power source.
How did this higher voltage creep in? The nominal voltage is a function of anode and cathode materials, as well as impedance. Voltage calculations include measuring the mid-way point from a full-charge of 4.20V/cell to the 3.0V/cell cutoff with a 0.5C load. For Li-cobalt the mid-way point is about 3.60V. The same scan done on Li-manganese with a lower internal resistance gives an average voltage of about 3.70V. It should be noted that the higher voltage is often set arbitrarily and does not affect the operation of portable devices or the setting of the chargers. But there are exceptions.
Some Li-ion batteries with LCO architecture feature a surface coating and electrolyte additives that increase the nominal cell voltage and permit higher charge voltages. To get the full capacity, the charge cut-off voltage for these batteries must be set accordingly. Figure 1 shows typical voltage settings.
End-of-charge voltage must be set correctly to achieve the capacity gain.
Battery users want to know if Li-ion cells with higher charge voltages compromise longevity and safety. There is limited information available but what is known is that, yes, these batteries have a shorter cycle life than a regular Li-ion; the calendar life can also be less. Since these batteries are mostly used in consumer products, the longevity can be harmonized with obsolescence, making a shorter battery life acceptable. The benefit is longer a runtime because of the gained Wh (Ah x V). All cells must meet regulatory standards and are safe.
The phosphate-based lithium-ion has a nominal cell voltage of 3.20V and 3.30V; lithium-titanate is 2.40V. This voltage difference makes these chemistries incompatible with regular Li-ion in terms of cell count and charging algorithm.
How should i charge 3 lithium ion batteries?
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See also:Can I use a regular battery instead of a AGM battery?
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i'm aware that i need to use a 3S balance board, and a constant current converter to charge them
You need more than just a current source. The charger must limit the voltage to 4.20V per cell (12.6V total), and (if you want to get more than 80% charge into the battery) then continuously reduce the current at that voltage until it reaches ~1/10 the set charge rate, then shut off. If you are using a basic CC/CV (Constant Current / Constant Voltage) converter then it probably won't shut off, so set the 'float' voltage to 4.15V per cell.
To prevent over-discharge you should have a cutoff circuit that disconnects the load when the battery reaches 3.0V per cell (9V total). If the battery gets very low (<3V/cell) it should be charged at a lower rate until the voltage reaches ~3.7V per cell (11.1V total).
is it safe to draw power from the batteries while charging?
Yes, but the battery won't charge if the load current is higher than the charging current. Assuming you have 3.7A at >12.6V available and the load only takes 1A, there is 2.7A free for charging. However the charge current must not exceed the battery's rating, which might be eg. 1.5A. So you could set the charge current to 2.5A, but if the load is turned off then the battery would get 2.5A which is too much. Therefore the charger has to monitor battery current separately from the load, or charge at 1.5A which will only get 0.5A into the battery while the load is on.
and where do i connect the load to?
To the battery through the 'balance' board.
Alternatively you could power the load from the power supply when it is on, and charge the battery at the same time with its output isolated from the load. This has the advantage that the battery can be charged at full rate even when the load is on, but the circuit is more complex.
If the load can work at slightly higher than 12.6V then a simple diode switch-over circuit might suffice. Details are important though. With the charger and load sharing a common ground the charger must monitor current in the battery positive lead, not negative, otherwise load current would upset the charger circuit.
Li-ion batteries can explode if they are overcharged, so don't charge unattended until you are sure that the circuit is working properly.
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