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.
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.
Nominal cell voltage | Typical end-of-discharge | Max charge voltage | Notes |
3.6V | 2.8–3.0V | 4.2V | Classic nominal voltage of cobalt-based Li-ion battery |
3.7V | 2.8–3.0V | 4.2V | Marketing advantage. Achieved by low internal resistance |
3.8V | 2.8–3.0V | 4.35V | Surface coating and electrolyte additives. Charger must have correct full-charge voltage for added capacity |
3.85V | 2.8–3.0V | 4.4V | Surface coating and electrolyte additives. Charger must have correct full-charge voltage for added capacity |
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.
Comments
My question is about Float Voltage:
I have qty-6 12 volt lead acid deep cycle batteries in series for a 30 amp UPS. The UPS keeps them charged and then goes into a float state. All 6 batteries were purchased at the same time and all are the same part number. When charged and sitting at float, one battery voltage measures 14.7 while all the others measure 13.2 to 13.3 volts.
Question: Is this normal and / or OK? And, why would this be the case any why?
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I found this page when trying to find an answer to the following: I have (the remains of) a fairly new RP-NPF970 DV battery pack. This was returned by a student after they had dropped (?) it smashing the case and breaking some of the internal contacts. Re-making the contacts between the six F-166958 (LS LR18655K FM1588 10117361) cells as apparant would seem to give a pack of 1.2 v (I do not know the charge status before disruction) despite the casing reading 7.4v. Until the casing has been recontructed it is too fragile to put on charge, but because of the high cost of the pack I am reluctant to recycle it at this stage of its life. I am not sure how 6 x 3 volt cells in paralel would give an expected voltage of 7.4v unless there is a hole in my maths. If anyone has any ideas about this or is able to fill the possible hole in my knowledge I would be most gratful.