Lipo alarms and/or voltmeter and/or wattmeter.
The wattmeter tells you the present pack voltage and how many aH you've used. Lipo alarms sound when any cell goes below minimum.
In practice, the pack goes down slowly until it reaches 36v (10S) or 44v (12S), then accelerates rapidly downwards, so a pack voltage read-out is fairly safe as long as you don't go below 36v/44v; however, if you forgot to charge half your pack, which is easily done, you could start off with 18v +21 = 39v, and within a short time the 18v one would be bricked., so you have to use a bit of intelligence as well. You should know that a charged 10S pack is 42v, so when you see 39v when you start off, alarm bells should be ringing in your head.
When you charge your packs, you should always look to see what the individual cell voltages are, so you pick up any abnormal behaviour or duff cells. None of this is difficult, but you have to be sensible. Never make assumptions or ignore unusual cell voltages.
