Rechargeable Emergency Lamp

Frequent blackouts are really a pain in the neck. One time there was a blackout and I realized I didn't have any backup lighting in the house aside from my cellphone, great. My wife then came up with candles but it was a bit hazardous. I thought of buying an emergency lamp from the hardware store but it so happend that I was designing a backup power system for our home security alarm. I could use the same backup power for an emergency lamp so I designed a board that can work for both.

The Battery Charger

The battery charger is an entirely separate project so I will include it here in a single step only. I attached the schematic and the PCB layout. You should be able to build your prototype.

This is the battery charger board for my home security. It was also designed to work as the charger for the emergency lamp. It can get power from an AC line through a 12-0-12 center tap transformer or an alternate power source such as solar panels or wind generators. Maximum output current is 400mA, just right for my 6V 4AH battery from panasonic(LC-RB064P).

In the second picture, I did a little modification. The heatsink was replaced and I used 1K in series with 120 Ohms for the adjustment resistors. The output voltage should be 6.8V - 6.9V so you can charge the 6V batter for an indefinite time without worrying about overcharging. But I wouldn't recommend to charge the battery 24/7. For our emergency lamp application, after using it for more than an hour or so, charge the battery for at least overnight or 8 hrs. Also, charge the battery for at least once a week if you haven't used it to prolong the battery life and minimize self discharge.

To learn more about Lead Acid Battery chargers go to: http://www.batteryuniversity.com/partone-13.htm

Downloads

sch_battery_charger.pdf

layout_bottom.pdf

layout_placement.pdf

Gather Components

After building your battery charger board, here are the rest of the parts you need:
1) 6V Bulbs w/ socket - 2pcs.
2) Power switches - 2pcs.
3) SPST push button switch
4) LED's - red & green (or any color you want)
5) 12-0-12 Center tap transformer, 1A.
6) 0.5 A fuse w/ holder
7) AC cord
8) Assortment of wires
9) Nuts and bolts
10) Plastic Chassis
11) 2 Ohm 5W resistor
12) Hot glue
13) Reflector sticker
14) Heat Shrink tubes
15) 6V, 4AH Sealed Lead Acid Battery
16) Double Sided adhesive tape

Mount Parts in the Chassis

1)Plan the proper component placement. Design a layout for the placement of switches, LED's and the rest of the componets such that they do not interfere during mounting. A pencil and a steel rule will come in handy.

2) Mark hole centers and prepare to drill the holes.

3) Drill holes and mount the switches, LED's, transformer, fuse holder, charger board and bulb sockets.

Wire Them Up

After mounting the parts in the chassis, start wiring them up. The wiring is quite simple, you can refer to the schematic but I made a few modifications to the alarm output of the charger. The light bulbs will be our load instead of the alarm. The two bulb sockets are connected in parallel.

The (+) terminal of the alarm output goes to the center terminal of the SPST switch. I connected a 2 Ohm, 5W resistor on one terminal and connected it's end to the other terminal w/c connects directly to the bulb sockets. This becomes current limit function to dim the lights and save battery. 2 ohms is not much but there is a noticeable dimming effect.

One terminal of the AC cord goes to one terminal of the transformer primary and the other goes to the fuse, connected in series with the power switch and then back to the other terminal of the transformer primary.

The wiring is going to get ugly when we close the chassis so make sure to insulate them properly and organize them with cable ties.

Final Testing

Test everything before closing the box. Insert the 6V bulbs and plug the assembly to the AC socket. Measure the B+ and B- output of the charger (batteries disconnected) and make sure you get 6.8 - 6.9V. Turn on the switches and check if the LED's and the light bulbs turn on. Also test the battery save switch.

Keep an extra eye on the AC line wiring and make sure the insulation is good.

To make sure you did a good job of wiring, while all switches are on, hold the entire assembly and shake it a bit to make sure you don't have intermittent connections. Watch out for shorts!

Close the Box

1) Close the chassis
2) Add reflector stickers
3) Add markings

Final Note

Always charge the batteries immediately after using the lamp, this will prolong the life of the SLA Battery. Also, if you're expecting long hours of blackout, turn-on the battery save switch. If you're expecting an even longer blackout, remove one light bulb.

You can go to http://nevwe.blogspot.com for more of my projects.