The following is clarification on charging
a sealed lead acid battery (SLA) vs the LiFePO4 battery, and why the BMS can't
protect a LiFePO4 from SLA charging damage.
First, let's touch on the charging characteristics as both
batteries have unique charging processes that take full advantage of their
chemistries thus maximizing lifespan and performance!
- SLA: Typical lead acid batteries have a full charge voltage
of 13.8 volts or 2.3V per cell (6 cells). SLA cells have the unique property of
balancing themselves out. As such, lead acid chargers just need to provide a
voltage higher than battery's voltage to bring the battery to its maximum state
of charge (13.8V).
- SLA chargers: Most lead acid smart chargers provide 3
stages of charging: bulk, absorption, and float-- each with their own
respective function. (Keep the float stage in mind, I’ll get back to this in a
bit)
(Float charging keeps a lead acid battery at 100% state of
charge as they have high self-discharge rates)
Voltages are unique to each type of battery as their
chemistries have differing energy densities.
- LiFePO4: LiFePO4's voltage sits at 14.6V or 3.65V per cell
(4 cells). An SLA charger will read 13.8V from this battery when discharged and
think it is fully charged when in it is not, leading to disappointing capacity.
Some chargers may even bypass the bulk and absorption stages and keep the
battery float charging which basically means it won’t even charge since the
charger thinks the battery is 100%! Keep in mind, some SLA smart chargers
perform equalization of the battery's cells which will damage the LiFePO4's
cells as well!
- LiFePO4 chargers: Chargers for LiFePO4 perform 2 stages
called "Constant Current/Constant Voltage" (CC/CV). A preset current
charges the battery up to a preset voltage then the current lowers as soon as
it hits that preset voltage up until it reaches a 100% state of charge (14.6V).
The BMS can't protect a LiFePO4 battery from an SLA charger because keeping a
LiFePO4 battery at 100% through float charging isn't necessarily harmful in the
short-term, but in the long term it will speed up aging of the cells as their
activity is constantly stimulated due to the extra power coming from the SLA
charger. This extra stimulation leads to polarization (buildup of
material where the electrodes make contact with the electrolyte) and
electrolyte decay.
It's absolutely necessary to use the correct charging
algorithm (CC/CV ; 14.4V) as SLA smart chargers WILL NEVER fully
charge a LiFePO4 battery or charge it ADEQUATELY enough to
provide maximum lifespan and efficiency. Too much charging voltage for too long
and you risk breaking down LiFePO4's internal electrolyte, too much charging
current for too long and you shorten the lifespan of the cells.