Briefly, if you are not familiar with the rating used on rechargeable batteries, the units are "Ah" or "mAh". This means Amperes Hour or milli-Amperes Hour, respectively. The rating indicates the amount of current the battery can consistently output for one hour of use. Those are theoretical numbers, because it depends on how fully charged the battery was, and the age of the battery. So, when you have an 850mAh battery, it means that it can output 850mA for an hour. So, if your locomotive needs 850mA to do its work, then you will get about an hour's worth of running time out of that battery. However, in the real world, small-scale locomotives use a lot less than that. Also, you don't run your locomotives with the same speed continuously. You vary the speed, and sometimes the engines are sitting still for a little while.
Other things that drain the battery more quickly are the speakers of sound-equipped engines, the length and drag of the train the engine is pulling, grades, and to a lesser extent the LEDs for the lights. The locomotive's motor and internal gearing will also have an impact on the duration of the battery's charge. It is always best to have a modernday efficient motor in the engine, and to make sure the gearing is clean and correctly lubricated. Some locomotives offer a built-in smoke unit. Those things are power hogs, so they are not likely to be powered by small internal batteries. But, despite all of these considerations, you always want to try to fit the largest battery you can into your locomotive, and try to double them up, if possible.
The S-CAB batteries, if fully drained, need about 8 hours to fully recharge. Charging time for these types of batteries is not a linear curve. The batteries charge quickly at first, but then slow down as they get closer to being full. So, if you are in need of the engine but its battery has completely drained, you can charge it for 2 hours, and then use it again (with limited duration). However, to be sure that the battery is fully charged, 8 hours is needed. In my typical scenario, I let my engines recharge overnight.
My layout is completely dead-rail. So, I pick up my engine and place it on a piece of flextrack which has a 13.7-volt power supply hooked-up to it (see photo). I have a spare Radio Shack power supply (used for testing or charging automotive components or batteries), to which I connect two alligator clip-equipped wires to a piece of flex track. I made it fancy by buying and connecting an LED read-out display that shows the voltage on the line. But that's it. Just turn it on, and let it sit for 8+ hours. After turning the power on, I use my throttle to change the headlight LED of the engine, which helps me to verify that the wheels are indeed picking up power from the power supply (at this point in time you do have to worry about dirty wheels/dirty track issues, of course). I then leave the headlight on to remind me that the engine is being charged. On my previous layout, I had a section of track set aside that was connected to this power supply underneath. I simply drove the engine onto the track and turned on the power supply to let it charge overnight. I believe the BPS requires about 200mA per engine to charge the battery. So, if you have five battery-equipped engines, you'll need at least a 1-amp power supply for this.
My latest layout has no wires soldered to any track on the layout. However, it is a bit of a pain to set up the parts shown in the photo above, and I have to handle the locomotives, which I prefer not to do. My layout's hand-laid track is a bit more delicate than a spare piece of flextrack, so I didn't want to just attach those rough alligator clips to the track. I decided to build something that could just lay on top of the rails. This first photo shows the parts I used.
I bent two pieces of thin stock brass into a 90-degree L-shape. I used the short edge of the metal ruler as a metal brake. I soldered a wire to each piece of brass. Then I used five-minute epoxy to glue those to the outside edges of a block of wood. The C-clamp in the photo above was used to hold the brass to the block, while metal weights were placed on top of the block to act as a clamp for the down-force. The block was just the right size of the track. In other words, the idea is to have the two flat surfaces of the brass touch the tops of each rail.
And here is the final set-up. The 13.8-volt power supply is connected via two green clip-on wires to the brown wires I soldered to the block of wood. The block of wood, of course, is too light, so I simply place some metal weights on it after I have positioned it on the rail. When I turned on the power supply, the headlight of the engine turned on, confirming that it was getting the 13.8 volts, so the battery in the engine is now charging. I now have a portable charging station!
While my layout's track work is under construction, or the layout is otherwise not ready to have equipment sitting on it, I built this simple battery-charging station. I cut a piece of plywood big enough to hold the piece of flex track that I have that is long enough to hold my longest engine (the RS-1 in this photo). I used 5-minute epoxy to glue the flex track to the board. I reinforced the rail to plastic ties connection with some superglue, as I have found the S-scale flex track to be too easily damaged, and this board may see some rough treatment over time. Next, I used some heavy-duty double-sided tape and installed the laptop power supply I had left over, to this board. It puts out 12 volts and at least 1 amp of current. I then soldered the output wires to the rails. Now I have a battery charging station for when the layout isn't set up to handle engines.
(external link: Use high-quality chargers!)