Charging batteries, both the small NiCad and NiMH, and larger lead acid types extends their usefullness for weeks, months, years. We will go over differing methods, and obviously differing costs and complexity.
The rechargeables, NiCad and NiMH, have AC and DC chargers available for them, as well as some purpose built solar chargers incorporating the solar panel as a lid to the battery storage compartment. The AC chargers can be used with an inverter from a storage battery of some type, usually a lead acid type that is charged with some other method. While this is not an efficient method at all, it sometimes is expedient to do. A DC input charger for the small batteries usually has a male lighter plug. This can be plugged into a car or truck lighter socket or power port as they are called now. There are also female lighter sockets available with alligator clips, that can be attached to any 12 volt lead acid battery to power the smaller charger, or anything else you might have that has a lighter adapter (camera, cell phone, spotlight, air compressor, etc.). Since the question will arise, generally you can't charge these small devices straight from a solar panel, even though the device is 12 volt input, and a solar panel is 12 volt output. There generally needs to be regulation of the solar panel, and we usually prefer an intermediary storage battery (5 amp hour SLA or larger), to provide stable current and voltage to the small device charger.
Charging lead acid batteries in a self reliance or emergency situation can take several forms, varying in cost and equipment. You can use the alternator in a vehicle to charge a battery, assuming you have fuel, and the engine starts and runs. If you only have one car and one battery, you can't take the battery down very far, or the vehicle won't start, and remember, starting batteries aren't designed to be deep cycled much at all, and won't last long. You can jumper to a deep cycle battery while the vehicle is running, and charge this way, though it's not the ideal method since the vehicle battery is mostly charged. This is a short term proposition only. You can use a generator with a plug in charger to recharge the storage battery(ies), and hopefully use some other AC devices while the generator is running. Some AGM batteries can be charged at a fairly high rate, which can be handy when running a generator. The Odysseys, for example, can be charged at their amp hour rating. Higher amp chargers are preferred when planning for this scenario, like 30 amp, 45 amp, or even higher for larger amp hour battery packs, to minimize generator run time. The Samlex chargers lend themselves well to this, as they can function as power supplies, so as the battery gets full, you can pull some power through the charger as well.
The obvious charging method to include, is a solar panel or panels. A typical deep cycle battery of automotive size or a little larger, is 80 to 100 amp hours in capacity. To use solar to recharge, we don't recommend anything smaller than a 20 or 30 watt panel with controller, which will give 5 to 12 amps in a day. This is not the 80 to 100 amps of the battery, so this works only for minimal battery use, but works in some applications. If you intend to use most of the power from a deep cycle battery on a daily basis, you need a couple of substantial panels. This gets fairly costly, but works. Solar is a good thing to have available, if you have sufficient planning and resources, but you must be aware of the limitations and have realistic consumption estimates. Now, using a 2.5 watt solar maintainer to keep your group 24 to group 31 battery charged until you need it (standby mode) is an excellent use of solar, and eliminates the maintenance charging issue, and sulfation issue. The 2.5 watt is large enough to maintain a 75 to 100 amp hour battery without a controller, but not large enough to overcharge. More info is in the solar tutorial.
There is wind, hydro, methane gas production, and all sorts of other sources of power generation for storage batteries, but these require infrastructure, maintenance, etc., and while viable, like full scale solar, are beyond the scope of this work.
A good NiCad, NiMH charger, preferrably AC and DC powered. A decent, even if small, generator, that you know how to use, and 5 gallons (minimum) of stabilized fuel for it. A quality higher amp battery charger that the generator you have is large enough to handle. At least one solar panel, even if it is only maintenance level size.