One simple way to do this is to use a series of comparators that power LEDs. You will need a stable voltage to compare against so you would need to place a voltage regulator on the batteries with a voltage out that is lower then what you plan on the batter being.
You then can use some voltage dividers off of the battery to get different voltages to compare the battery to. Since you have already stepped down the battery voltage with the voltage regulator you will also need to use a voltage divider on the battery to get it in the range that you are comparing against.
Basically if you have a battery that ranges between 6v and 10v, you will want a voltage regulator around 5v. You can then set the comparator voltages to be 3v, 3.5v, 4v, 4.5v and then use a resistor divider to divide the batter voltage by 2. So your new range is 3v - 5v and if it is above the 3, 3.5, 4, 4.5 then the LED associated with that comparator will turn on.
NOTE: 6-10v is just a random range I picked to illustrate my example, don't expect for your batteries to be like that.
The biggest problem with this method is the poor accuracy you get. Batteries don't discharge with a linear voltage drop, so it can be hard to perfect what is considered 25% charge, or 50% charge, or what ever values it is you are looking for.
The nice thing about this method is it is very simple to understand and build.