lithium batteries

Plucka

Is a 12 volt lithium battery made up of 4 x 3.2 volt units or 4 x 3.7 volt units to show a charge of 13.8 volts and a float of 14.4 volts?

mcgivor

The 3.2V nominal would be of different chemistry to the 3.7V nominal so the series charge voltage would differ accordingly. Normally the charging would be constant current until close to fully charged voltage is reached then it becomes constant voltage, a BMS would be used to limit charging current and or provide cell ballancing. This constant current / voltage  may seem complicated but it's actually not, it means full current until 

Lifepo4 has a nominal voltage of 3.2V per cell with maximum of 3.6V which coincides with 14.4V for a 4 series topography, the 3.7V  nominal cells would likely be LIPO which typically would be arranged 3 series for a 12V battery with a maximum of 4.2V per cell or 12.6V for the string.

It is of extreme importance to limit the charging current when at the maximum limitations, either with termination, or by the preferred method with a BMS. For off grid applications it is common to program the charge controller to not venture into the upper voltages in order to improve the cycle expectancy, the same applies to the lower termination voltage, keeping within the 20 to 90% state of charge is typical. 

Some operators rely solely on the charge controller along with inverter low battery cut out to achieve this, however having the added protection of a BMS is something I believe is important because it protects against not just the voltage but many other parameters. It must be noted that most inverters are designed for lead acid batteries so have LBCO well below the requirements of lLifepo4 which would destroy the cells.

The pre made packs such as drop in replacement type will most likely have an on board BMS along with specified charging voltages such as 14.4V for a 12V nominal lifepo4, this doesn't mean the terminal voltage couldn't be set lower to prevent long periods at maximum but it would be prudent to check with the manufacturer.

All the above is in general terms mainly concerning lifepo4, where my experience is, LiPo may be very similar however I think it's best to focus on a single chemistry to avoid confusion.

Plucka

So the answer is 3.2 volts x 4 is the normal 12 volt lithium battery size.

mcgivor

No, each chemistry within the lithium umbrella has different characteristics, LiFePo4 is one and it's nominal voltage is 3.2V per cell. There are advantages along with compromises along with different cell voltages for each type, LiFePo4 whilst not having the highest energy density are one of the safest, LiPo on the other hand has higher energy density but much more dangerous. Understanding the chemistry being used is critical to its use, this link
https://batteryuniversity.com/learn/article/types_of_lithium_ion explains in detail each type.