Bootleg Battery Charger Heart Transplant

Following on from my previous post where I looked inside a cheapo battery charger from eBay – I have since gutted the charger. All that I kept was the case and the battery contacts.

Gutting the unit eliminates a potential fire hazard. I replaced the circuitry with a dedicated lithium ion battery charger IC instead.

I’m using a MAX1555 and it is a single chip solution which requires a minimal number of external components. It is a single cell charger and has a maximum charge current of 280mA.

The chip has dual inputs, allowing you charge a battery from either a USB port or a DC plug pack. When charging from a USB port, the charge current is limited to 100mA. Whereas, the DC plug pack input allows for a charging current of 280mA. Also, this version of chip has a charge status indicator which can be used to drive an LED.

I’m not going to use the 240VAC socket anymore, and I’ve replaced it with a micro USB socket. This change lets me use a USB wall socket adapter which can deliver 1500mA at 5VDC. However, my new circuit will only require 280mA. (I’m already thinking about a 2nd iteration of this project which will use an IC with a higher charger current to charge faster.)

Schematic for the new battery charger using a MAX1555 IC.

Normally, you’d connect the USB socket to the USB pin on the IC. Since I won’t be connecting the charger to a computer’s USB port, but a dedicated USB charger, I am not worried about limiting my current draw to 100mA. I’ve connected the micro USB socket to the DC pin which will allow me to charge the battery at 280mA. (The MAX1555’s USB pin has been shown broken out in the schematic and I did break it out on the prototype, but I never used the input.)

Since, this was a one day project, everything was assembled on some protoboard. I had some SOT-23 breakout boards which made soldering the MAX1555 package easier.

The next time you’re on eBay, order some SMD breakout boards and keep them handy in your junk box.

Everything wired up ready.

Everything managed to fit quite neatly inside. I got lucky with the plastic case’s mounting standoffs being the same dimensions as the protoboard’s mounting holes.

Everything neatly fits inside the case. To support the micro USB socket, I printed a small bracket.

To support the micro USB socket and close the hole left behind from the AC adapter, I printed a bracket to hold the USB socket in place.

Printed bracket to support the USB socket.

One thing I have noticed is that the battery contacts are not very reliable. I think this is because there is not a lot of tension on the spring contacts. I’ll have ago at trying to improve this to increase the charger’s reliability.

Anyway, just a quick little project which will hopefully get a lot of use.

Everything ready to go and less likely to start a house fire.

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