Circuit diagram for a rechargeable lantern


Postby Wouter » Mon May 17, 2010 10:07 am

I am looking for a circuit diagram to build my own rechargeable lantern. I intend using a 12 volt battery which will be charged by an AC/DC adaptor. The light source will be a 12 v energy saving bulb. Suggestions on making a lantern casing will also be appreciated.
Last edited by Wouter on Tue May 18, 2010 7:06 am, edited 1 time in total.
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Postby ericmark » Mon May 17, 2010 11:40 pm

Since pictures are not permitted not easy to give circuit diagram but a few pointers.

1) Lead acid batteries have two basic ways to charge them. The easy method is float charge and around 13.2 volts will maintain the battery in good condition without drying it out. The easy way to get 13.2 volts is with a 7812 voltage regulator and a red LED between common on 7812 and real common this will raise volts by 1.2 and show it is charging.
2) Stage charger this will charge the battery a lot faster it can be in 2 or 3 stages. 2 stage I built used a start button and gives constant current until a threshold voltage is reached at which point a latching relay swaps it to float charge. The commercial chargers as on my sons narrow boat charge at 25A until the volts reach 14.8 and then maintains it at 14.8 volts until the current drops to 5.5 amps at which point it drops voltage to 13.2 volts.

Nicad batteries are a completely different system.
1) Cheap system is to limit charge so it will take at least 16 hours to recharge. However really once charged the charger should be disconnected and although 24 hours instead of 16 may be OK giving the batteries a weeks charge instead of 16 not much good.
2) Second method is heat. When electric energy is being converted into chemical energy some heat is produced but quite low. However once fully charged the electrical energy is all turned into heat. So a sensor which will detect the heat is a common method. Often directly in the charger supply and either a manual start button or a button in the charger base is used to start the charge and a current relay is latched on. As soon as the temperature switch disconnects the battery the relay drops out and no more charge is put into battery.
3) Voltage sensing very complex but as the battery charges the voltage raises then falls and the electronics sense when voltage starts to fall and starts timer which then stops the charge not really a DIY option.

As to new Ni-metal-halide batteries not a clue how to charge I think you will find complex and either the 16 hour or heat method is used or a PIC controlled system.

As to energy saving bulb be aware there are those which need 12 volt and those designed for boats and caravans and can take a range of 11.5 to 14.4 volts and likely you will need to select the latter.

Do say how you get on please.
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