# I'm designing a three-stage battery charger for a sealed lead acid (SLA) battery. How will I decide which charging stage to apply to the SLA battery?

I am designing a three-stage battery charger for an SLA battery.
The three stages of charging I am referring to are: bulk (CC), absorption (CV) and float. (CC is constant current and CV is constant voltage.)

Before the battery is connected to the battery charger, it will prompt the user for inputting the bulk current, absorption voltage and float voltage values specified according to the battery manufacturer. The battery voltage and current measured will be measured the charging process.

This is how the battery charger will work:

• During the bulk stage, the battery will be charged at a constant current equal to the bulk current.
• When the battery voltage is equal to absorption voltage, the battery will be charged with a constant voltage equal to the absorption voltage until the current drops to 5% of the bulk current value.
• When the current drops to 5% of the bulk current value in the absorption stage, float voltage will be applied to the battery indefinitely.

How do I know which charging stage to apply when an SLA battery of unknown state of charge (SOC) is connected to the charger?

Also a side question, I have looked into a couple of SLA battery datasheets and I noticed that they only give information for charging the battery based on constant current or constant voltage or not at all. Why is there no information for charging battery based on three-stage (CC/CV) charging, for example in this or this datasheet? (CC/CV is constant current constant voltage charging.)

• If you can't find the values for three stage charging in the battery datasheets, what makes you think that the average user with a random battery is going to have these values to put into the system?
– vir
Commented Feb 24, 2023 at 21:46
• In that diagram how is the stage 2 description "voltage increasing" consistent with "constant voltage"? Which is it? Commented Feb 25, 2023 at 4:30
• @SimonFitch Its a typo. It should be constant voltage. Commented Feb 25, 2023 at 6:25

”My question is how do I know which charging stage to apply when an SLA battery of unknown SOC is connected to the charger?”

Why do you need to in the first place? Just apply CC-CV.

If CC sets the limit, you are in bulk phase. If CV, you are in absorption phase. If current drops too low in CV phase, you are in float stage and you need to adjust your voltage set point accordingly.