No, it is not OK to have a Li-Ion deeply discharged at all. Here is why: When discharged below its safe low voltage (exact number different between manufacturers) some of the copper in the anode copper current collector (a part of the battery) can dissolve into the electrolyte.
Lots of advice — some of it conflicting — circulates about whether to leave the battery on the charger all the time, and whether to let the batteries drain completely before recharging. For lithium-ion batteries, overcharging and completely discharging them can cut down on the overall life of the battery, Koenig says.

A BMS – battery management system is considered the actual brain of the battery and when designed with cutting-edge electronics, it performs numerous other functions that control and monitor the behaviour of the lithium battery inside the application in real time. Now, let’s discover the additional features of a smart BMS.

There are reasons why Li-ion is put to sleep when discharging below 2.50V/cell. Copper dendrites grow if the cell is allowed to dwell in a low-voltage state for longer than a week. This results in elevated self-discharge, which could compromise safety. Self-discharge mechanisms must also be observed in manufacturing. \$\begingroup\$ Device and battery can often be separated, that's why overdischarge protection circuit on the battery is added as a last resort protection against battery damage, but not to maximize battery lifetime. Just as lithium chargers have to stop at 4.2 V before the battery's overcharge protection will kick in. So being in a pinch a 12 volt battery charger will work to get some energy back into your dead golf cart batteries and revive them. The sole purpose here is to just get some energy back into the batteries so the golf cart battery charger itself can do the brunt of the charging and charge the batteries properly. . 47 150 363 119 56 208 7 324 25

lithium ion battery drain completely