I am a retired forester interested in electronics as a purposeful hobby;presently trying to build a very low-cost solar LED lamp;there are several such lamps in the Indian market targeted at the middle-class customer and not the poor / tribal communities;secondly these lamps provide a mini solar panel and the LED with a life of perhaps 20 years and a battery that lasts for less than three years and no provision for replacement of battery !the purchaser has to throw this light or give to his children as a toy once the battery is dead;i am trying to make a light with a provision for a replaceable rechargeable battery;I have identified a low-cost[Rs.20] Lead-acid rechargeable 4 volt ,500mA battery to power 4nos pirhan LEDS [20mA] and i propose to charge the battery with a mobile phone charger where power is available and with a solar panel where power is not available;it is a not for profit venture;i welcome suggestions from the August members of this website.I served in tribal areas for twenty years .Thanking All.