Panic-not Jon..
By Default, there is a feature in your Deltang receiver called Low Voltage Cutoff (known as LVC, for short).. This 'looks' at the battery voltage and, assumes from the value, how many Lithium-cells are present (even if you are not using Lithium batteries!).
It will then monitor the voltage, and if it should go below the threshold set for 'however-many' cells it thinks there are, it will shop the loco, and flash the LED to tell you. - If you take-heed of this, and recharge the batteries, all will be well.
Aside:
LVC catches out those who do not disable it / have it disabled, who use a PP3 9volt battery. these are just above one of the thresholds, so do not 'last very long', as the receiver shuts-down after a very short time!
This is if NOT using Lithium batteries.
Here is a rather scabby set of 18650 cells (3S) with a board on..
View attachment 261908
2S boards, are smaller (obviously):
View attachment 261909
You might find this information useful?:
Information about how to use battery protection circuit boards with lithum ion batteries wired in series, with circuit diagrams
www.rctrains.co.uk
PhilP.