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27
June 2020
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Solar Stik® Inc.
24VDC Li Expander Pak 1300 Operator Manual
If the BMS has disconnected the battery terminals from service due to low SOC, it will continuously
scan the terminals to see when charging voltage is being applied so it can reconnect the battery into
service. This “sense” circuit is very active when the battery is first disconnected at the terminals due
to low SOC. If the discharged condition persists and the cells are not recharged in a timely fashion,
the cell voltages continue to deteriorate.
Self-recovery from an Overdischarged State
It is possible for a battery to self-recover from an overdischarged condition.
If the battery is discharged slowly (low current) to the point at which the protection circuits engage,
there is only a minimal chance cells can reconnect on their own.
However, if the battery cells are discharged at a high rate, the chemical reaction inside the cells will
often struggle to “keep up” with the demand, therefore, the voltage will drop and the protection
circuit will disconnect the cells from service.
When cells are allowed to rest from a rapid discharge, the cells will continue to chemically react,
causing voltage in the cells to rise. When the cell voltage recovers back above 2.9 V, the protection
circuit will enable the output of the battery.
If either of these conditions occur, the battery should immediately be put into charge mode.
BMS Operation in an Overdischarged State
LiFePO
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battery cells, as discussed earlier, will drop in voltage very quickly when they have reached
low SOC. The BMS will disconnect the terminals to prevent further discharge by an external
component, but the BMS itself requires power to operate. THE BMS WILL CONTINUE TO USE
POWER FROM THE CELLS TO PERFORM ITS PRIMARY FUNCTIONS, even if the battery has
disconnected itself from service. The cells will continue to discharge internally in support of the BMS
functions, and the voltage can fall very rapidly to the point of non-recovery. If the battery protection
circuit is engaged due to low SOC, there is precious little energy left in the battery, so timely
recharging is extremely important.
As time passes and the cells continue to lose voltage, the BMS will slow down its functions to
preserve as much energy in the cells as is possible. One of these functions is to search for charging
voltage at the battery terminals. Once the battery terminals are disconnected from service, the BMS
uses a sense circuit to pulse the terminals, sensing for voltage. When the presence of higher voltage
(greater that 23.2 V) is sensed at the terminals, it will begin allowing charging current into the cells.
The longer the battery has been in the discharged state, the greater the interval between the pulses;
thus, the longer the charge voltage must be applied in order for the BMS to sense the voltage. In
extreme cases where the cells have remained at the overdischarged state for an extended period,
the BMS will pulse the terminals only sporadically; therefore, the Li Expander Pak may need to
remain connected to a charging source for multiple days in order for the BMS to allow enough
power back in through the terminals to effect a recovery.