Tenshintai was the doll used by Yoruichi Shihouin to force Ichigo's release of bankai. It was invented by Kisuke Urahara, he achieved bankai in 3 days while it took Ichigo 2 days.

Since Kenpachi Zaraki is no longer their enemy, didn't they realize of helping him achieve his bankai? by letting him know about their invention?

Zaraki would be of great help to them if he achieves his bankai. Furthermore, i was also wondering how long would it take Zaraki to achieve bankai, if he would be allowed to use tenshintai (could he do it in a day???)

He must be able to talk to his sword to release his Bankai. I am not sure he knows how to do that since he doesn't know the name of his sword. Other than that I thought that all captains and lieutenants had to be able to release their Bankia before becoming gaining that title.

but it was mentioned that zaraki was so strong, he didn't need his soul slayer. but it was then... now, their conterparts are so strong, i wish they would give zaraki a more important role...

ofc he is important, they could just have him do some more stuff...

i thought ichi got bankai in 3 days as well..?

in my opinion, zaraki does not want to fight "with" his zanpaktou, coz he thinks that people should fight with their own strength, and zanpaktou is *just* a killing tool according to him. so even though yoruichi offers that to him i dont think that he is willing to accept the offer..

besides, tenshintai has not been properly proven to work yet. if it fails zaraki might die as well..

Yeh Zaraki needs to know the name of his Sword to be able to call it, and his swrod also needs to be willing to work with him, because in the episode where Ichigo fights Zaraki, Zangetsu says to him "Can you hear it Ichigo...Can you hear his sword screaming" or something like that.