Thanks justDIY.
Then I will make a constant voltage type with LM317T.Need only two resistors.
Then the battery will draw its own needed current.
My battery says STAND BY VOLTAGE 6.8V – 6.9V what is this?
Do I have to adjust the charger to 6.8V?
As justDIY points out, the 7.4V is a cyclic charge rate [ when you want to charge the battery quickly, when charged, remove it from the charger and use it elsewhere]
The STANDBY VOLTAGE is the 'float charge voltage', thats when the battery is left connected to charger and used to power the 'system' when the mains power fails. When the mains is restored the battery charges up to the float voltage of 6.8V.
If you used a cyclic 7.4V charger permanently connected to the battery it would overcharge the battery and shorten its life or ruin it.
What’s the transformer do I have to use?
A 12Vac transformer would enable you use either cyclic or float, it depends upon how you plan to use the battery. Remember to use the correct VA rating for the transformer to suit the current charge rate.
Thanks.