I've had a backup 12 V 7 amp Dakota Lithium battery failure recently. The battery was recharging and when I went to remove it from the charger the volt meter stopped reporting voltage. When I plugged it back into the charger it was indicating the battery was not charging. When I applied the voltmeter to the terminals I got 0 volts.
I contacted Dakota Lithium and reported the problem and was seeking warranty replacement. I was advised to reset the BMS of the battery. I was sent this video link https://www.youtube.com/watch?v=0Cd-86MrP3c and it worked.
I hope this helps someone else before giving up on the battery being dead.