There's another thread going on about 48V batteries that I've been adding thoughts to. I think it may be helpful to suggest a dead-man high voltage cutoff (HVC) switch that I've been working on. I believe that a crude failsafe is better than nothing at all.
The heart of the circuit is a lowly 555 timer:
Fundamentally the timer is a high powered set/reset flip flop. It's set then the trigger (pin 2) falls below 1/3 VCC (pin 8), which drives the output (pin 3) high. The FF is reset when the threshold (pin 6) reaches 2/3 VCC. Timing circuits are produced by combinations of RC circuits connected among the timer's pins. However we can use it as a voltage controlled cutoff switch.
The plan is simple: the 555 triggered on turns on the charger. The battery reaching it's target voltage turns off the charger. To keep the numbers simple let's drive the 555 with a regulated 12 volts.
The trigger is trivial. Tie pin 2 to a pullup resistor to Vcc, with also a normally open switch to ground. So as a default the trigger is deselected. Pressing the button triggers the part and turns on the charger.
The high voltage cutoff is equally as simple. Measure the battery voltage and set it to trigger the threashold at 8 volts (2/3s of VCC). Done with a simple voltage divider. Say you want to be real safe and cut off the charger a 4.0 volts/cell. So the battery would cut off at 48 volts. So you need a voltage divider that will translate 48 volts to 8 volts. Fortunately there are a million voltage divider calculators out there. I use this one:
Since the upper resistor is going to have the higher resistance, let's fix R2 at 1K ohm and see what R2 needs to be to translate 48V to 8V. Result is a 5 to 1 ratio. So 1K for R2 means 5K for R1. Finally we need to quickly compute the power. The current is 48V/6000 ohms = 8 mA. The power is 48V*8mA = 384 mW. So 1/2 watt resistors will do the job. If we wanted to use 1/4 watt resistors, we'd have to get the amperage below 250 mW. 250 mW/48V = 5.2 mA. So let's reduce the current to 5 mA. 48V/5mA = 9600 ohms. Doesn't really give good round resistor numbers. 12K would give us R1=10K and R2=2K. 48V/12K is 4 mA. Looks like a winner.
So tie a 10K resistor to the battery + terminal, a 2K resistor to ground, and tie them both together and feed the divider to pin 6 of the 555. According to this tutorial:
other connections should be reset (pin 4) to Vcc, ground (pin 1) to ground of course, and control Voltage (pin 5) to ground via a 0.01 mF capacitor. Pin 7 discharge is an output (open collector) and is unused in this application.
So there's the dead man switch. Press the button, and the output of the 555 goes high. When the battery reaches 48V (in this example), pin 6 resets the FF, the output goes off, and most importantly stays off. Also note this particular circuit needs to be driven with a regulated 12V power supply to ensure that the threashold voltage is a steady 8 volts.
The most trivial way to drive the charger with the 555 is using an SSR. I bought a handful of 40 A overkills from Marlon P. Jones for $5 a piece on sale. Drive the input with any DC voltage from 3-32V and it'll switch up to 240V @ 40A supposedly. Way overkill for anything that plugs into a wall.
None of this precludes using other monitoring or control. It's literally a dead mans switch that protects the battery even if everything else blows up. See the other thread for suggestions on CC/CV charging, switching power supplies, and using Arduinos as control circuitry.
BTW it's always possible to just watch it. I just recharged my Volt module today using a Meanwell 48V 12A supply. Biggest problem I had was trying to figure out how to keep the battery from sparking when connecting it to the supply. It seems that a precharge resistor and a bypass switch are necessary to charge the output caps of the supply so that it won't spark.
Hope this helps,