Addressing your first question:
“While charging the battery for the first time can I use a constant voltage charger?”
The short answer is:
Yes, but with specific voltage and current monitoring (and/or time).
Yuasa site recommends its batteries to be charged with their chargers. Searching for them, most are multistage charges, with at least bulk, absorption and floating modes, each one with different voltages and current values.
The following figure summarizes such characteristics for 6V and 12V chargers - helping a broader range of readers too.
Please see the information within the highlighted rectangles.
About your second question: “Can the charging voltage be 7.2-7.5V like SLA or should it be lower for a flooded cell?”
As could be seen, the absorption mode would be the “main” charging mode, constant voltage mode set at 7.2V for 6V batteries (or 14.4V for 12V models).
Additional information for an implicit question about how to charge, or when the absorption charging mode should be terminated?
There are 3 stages or modes applicable for normal charging:
- Bulk mode: Charging current is limited up to a “safe“ value, while the battery voltage increases. It is a constant current (CC) mode. When current starts to reduce, the battery is charged at aprox. 80% of rated capacity.
- Absorption mode: When the battery voltage reaches the “absorption charging voltage”, it enters the absorption mode, operating in constant voltage mode. Depending of literature sources bulk mode shifts to the next mode when the charging current reduces to about 10% to 20% of bulk current value or 3% to 5% of AH. At this point, battery would be fully charged (100%).
- Floating mode: The constant voltage mode value is lowered to the “floating charging mode”. It can be maintained for extended periods (days) without overcharging the battery, but for that, the precise value should be fine tuned for ambient/battery temperature.
The recommended current for bulk charging (first mode) is 10% of rated AH. Some articles cite up to 20% as maximum to be considered still within “slow charging” class. In this case (6V x 5.5AH), the maximum current for your battery should be 1A, while 0.6A would be better.
Yuasa’s smart chargers
Recommended charging for lead acid batteries - Battery University
Chargetek’s charging and equalization - additional information