Balancers are well understood now thanks to the proliferation of electric cars. There are two kinds of balancers, passive and active. Passive balancers work by creating a resistive load across any batteries that are more charged than the lowest charged battery. This is exclusively a power drain, but as long as the batteries are all the same size and have similar capacities this won't waste too much power. Active balancers are usually much more complex and expensive but they actively transfer power from charged batteries to low batteries through capacitors. There are still some losses but overall they are significantly more efficient than passive balancers.

If you get a balancer, regardless of the type, you need to make sure that it is capable of supporting the battery size you have. Balancers that are too small are going to burn up really fast. You should also try to get a balancer with cell health information. Again this is more expensive, but will usually be part of a higher quality balancer.

A couple of notes. I am assuming you have lifepo4 batteries and I assume you have 12v batteries (4s 12v = 48 volts). I also assume you got "smart" batteries since you said it shut off (you should never get non-smart lifepo4 packs for this kind of application, you are just asking to burn your house down). The "smart" part of the lifepo4 battery is that the battery has a built in balancer for it's internal cells (usually called a bms). These balancers are usually resistive unless the pack was super expensive and/or custom. What this means is your 48v system is actually a 16s 3v lifepo4 pack, but you are only balancing chunks of 4, but not those chunks of 4. This is where a balancer can be helpful. However, it is also possible that your battery packs have a "smart overcharge protection" where it effectively prevents the individual 12v battery from overcharge and they basically have created their own passive balancer system with this mechanic and an additional passive balancer will only reduce your total ability to charge. This works because each battery monitors its own internal voltage and shunts any excess voltage through a big ass resistor... Which is exactly what a passive balancer does. At that point you need to decide if the money should just be spent on a cell health monitoring system or an active balancer... And usually the price is not worth it for most people. Usually it's just more cost effective to buy a second set of identical batteries and wire them in parallel (one for one) with the existing batteries. Note, you don't have to match the existing batteries, you just need the new batteries to match each other, and to charge every battery (old and new) to the exact same voltage before wiring them together. (These big batteries, a single tenth of a volt difference can be 50-200amp discharge).