A 6V solar panel is what you need if you want to trickle charge a 6V battery. We have both rigid and flexible 6V solar panels, ranging from 50 mA current (0.3 Watts) to 1300 mA (12 Watts). The current that is required to safely trickle charge a battery depends on the capacity of the battery (mAh, or milliamp-hours). Most rechargeable batteries will have the battery capacity listed right on the sticker. See our FAQ page for more info on trickle-charging
You can also pair a 6V charger (or a 7.2V flexible Powerfilm panel) with a 6V battery for your own experiments with solar powering low voltage devices.