Project Detail

Description

Battery switch over & charger circuit diagram

Background
The device is powered from a 12 volt 3 Amp switched mode power supply and also contains a 12 Volt 7 AH seal lead acid battery. Contained is a microprocessor and the 12 Volt supply rail which is used by a number of other sub system.

There are three main functional components to this project:

Battery Switch over:
1) The primary power source should be the power supply, unless is falls below 11 volts then the battery supply should be selected.
2) The change over from one source to another should not interrupt power supply to the application.
3) Once the power supply reaches above 11 volts it should return as the power source.
4) Protection from losing power out of the power supply (a diode) when the battery is the power source
5) Means of a microprocessor knowing which power source is in use (TTL level)

Battery Charger:
A boost converter maybe needed to step up the voltage in order to charge the battery
1) Current limited so that the charger can not consume more then 1 Amp from the power source
2) Micro controller must be able to disable the charger (TTL level)
3) Protection from a short on the battery terminals
4) Protection from battery being connected incorrectly
5) Boost converter must be at least 80% efficient

Some interesting boost converters
http://www.maxim-ic.com/appnotes.cfm/an_pk/2018
http://www.linear.co/pc/viewCategory.jsp?navId=H0,C1,C1003,C1042,C1031

Microcontroller Supply:
1) A simple regulator to supply constant +3.3 Volt supply to a microprocessor. This will not consume more then 200 mA.
2) Isolation from the main 12 Volt line, should there be a short in another component connect to the 12 Volt line the microprocessor will not lose power.

Deliverables:
1) Circuit diagram, can be in eagle cad or an image format (JPG, PDF, GIF etc). Dont mind if it is hand drawn as along as it is readable. No PCB is required
2) Bill of materials, part value, manufacture part number, manufacture name in Excel or CSV format
3) All parts SMD
4) Design must focus on a low manufacture cost
5) Energy efficiency must be considered.

Should not take more then 3 hours max to draw up this circuit diagram if you are experienced.