# Charging 6V flooded lead acid battery in different stages?

Joined Mar 29, 2019
17
Hey, I have 6V flooded lead acid battery and its 235 Ah.

I am designing a MPPT controller with a buck converter. I was told that, output voltage regulation is only in float or any stage after Bulk stage. In the Bulk stage, the output voltage is clamped to the battery voltage as long as Vin > Vout. So we are controlling the current in this case and only the current. Since the voltage is clamped to the battery voltage in bulk stage, why it is not clamped to the battery voltage in float stage or any stage after? Meaning why do we need a voltage regulation in float stage since the output will be clamped to the battery voltage?

let's say I have a 12V power supply and I directly connected to a 6V battery that is in bulk stage, does this power supply voltage will be clamped to 6V and current coming is determined by battery resistance? if yes, then why it is not clamped to it after bulk stage?

I really appreciate if someone can help me understand this

#### crutschow

Joined Mar 14, 2008
26,789
In the initial bulk charge, you limit the current to what the battery can tolerate without damage (current-mode charge).
This current is typically no more than 5-10% of the Ah rating.
When the battery voltage reaches the top-off voltage, the voltage is then limited to that value, causing the charge current to drop (voltage-mode charge).
When the current drops to a low level, indicating the battery has been topped off and is fully charged, the voltage is dropped to the float or maintenance level (also voltage-mode charge).

A diagram of this 3-step charge technique from BatteryUniversity.com is shown below:

If you don't limit the voltage at the end of the Stage 1 bulk charge, the voltage will continue to rise at the high charge current, and you will overcharge and damage the battery.

Make sense?

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