Dead Battery over the weekend

http://ebrforum.com/showthread.php?1671-2016-SX-TPS-Reset-and-frequent-high-idle

Related post above.
I took my ZX on a camping trip... I really wanted to take the EBR but I didn't trust the battery/regulator situation. When I returned home to a used Reg/rec in the mail, I started to replace it and I found that the battery had discharged to 3V. I charged the battery on a battery tender branded Lithium battery charger over night. This morning the battery was charged at 13.6 V I installed it with the "new to me" r/r and... I saw the battery voltage vary between 15 and 16 VDC at idle. So two R/Rs are behaving the same, the battery is dying after two days of sitting, I suspect it's the battery now. Is there something I'm not thinking of? can a loose ground wire cause this? A few weeks ago I went to lock the bars and wound up turning on the parking lights... all day... draining the battery. I had to get a jump start... I notice that the fan that vents the tank? I think? comes on periodically when the bike is off.
I ride it to work... it's parked in the sun... Ideas?
Thanks all.

It's supposed to charge at that rate for the LI-Ion battery.
https://shoraipower.com/charging
It not a 'fan to vent the tank', you're hearing the fuel pump and it's supposed to do that.

If the battery is doing in 2 days:
1) Could be a bad battery. Li-Ion batteries HATE to be drained past 12v much less 3v! A Battery Tender is NOT a charger. Get a Shorai one.
2) Could be a drain. Take off the battery negative and check for a draw with your ammeter..