After several experiments with diodes and MOSFETs, I opted for the circuit below that satisfies both the need to have DC and battery power and the need to charge the batteries when DC power is connected.
The battery charger requires the ground output to be independent of the DC ground so my original design wouldn’t work. A DPDT relay connects both battery terminals to the battery charger and removes it from the supply lines when the DC power is plugged in.
I added D1 to ensure the battery charger shuts down when DC power is removed. D3 prevents the battery from keeping the relay coil energized.
The ATTiny25 monitors the battery voltage and will disable the charger once the battery is fully charged. It will also act as a timer to prevent charging over 15 hours.
The battery charger is an inexpensive LM2596-based constant current voltage regulator/battery charger that you can find on ebay.
For the past 5 years, this has been my prototyping station and it has served me well. It wasn’t designed to be portable and it is a bit top heavy so I decided to replace it. The new prototyping station will use a B&W indoor/outdoor case (Fry’s carry these) to make it more portable and more self contained.
It will have batteries as well as a plug in DC power source. The batteries are rechargeable so I am looking for a clean and effective way to switch between battery power and DC power yet be able to charge the batteries when the DC power is connected.
I found several circuit examples that would work but they rely on passive trickle charging to keep the batteries charged. This is fine for a gel cell or NiCad batteries but I’m using 2700mAH NIMH batteries and would rather have an active charging system to avoid damaging the batteries or the prototyping station.
After some research and experimentation, I came up with this circuit that uses a relay to switch between DC power and battery power. the DPDT relay connects the batteries to a smart charger circuit when the DC power is connected.
The noise suppression and filtering isn’t reflected in the circuit yet but will be added later. I will also replace the FP2 D3009 relay with one that can handle more current.
A relay doesn’t feel like the most elegant way to do this but I haven’t found an alternative that allows for the smart charger. The other circuit examples rely on isolating diodes and may interfere with the charger.
Has anyone built a circuit like this before? Any suggestions on this circuit?