In-circuit 9VDC Charger

Thread Starter

onebigfellah

Joined Apr 3, 2012
9
Hello all.

I need to develop a battery charger for an in circuit 9 VDC battery.

Details:

The battery is constantly in use by the PCB it is attached to. The PCB has been designed to detect tampering in our system by means of mechanical switches. When the PCB loses power from the PC it is connected to, the PCB then looks to the battery to provide power for tamper detection.

The battery is currently a 9 VDC Lithium.

I have two possible power sources:

120 VAC, which can be adjusted to 9 VDC by use of a transformer.
12 VDC, which would have to adjusted through a PCB design.


Need:

For someone to help me design a circuit that would provide a stable electrical voltage / current to the battery and PCB but would prevent the battery from discharging when power is off to the PCB.

I look forward to seeing what comes up from this discussion.
 

wayneh

Joined Sep 9, 2010
17,496
You need to first choose a specific, rechargeable battery. The charging and maintenance strategy will depend on the chemistry (ni-cad versus NiMH, etc.) and on the number of cells. "9V" covers a range of actual voltages.

In general, I think a nicad is easier to deal with but Li-ion has the highest power density (and cost).
 

chrisw1990

Joined Oct 22, 2011
551
question, why not buy an IC? theres a few charger ICs out there for batteries and such? see Farnell, that could be an easy solution in a compact footprint.
 

Audioguru

Joined Dec 20, 2007
11,248
The 9V lithium battery is not 9V, it is only 7.4V. But it is 8.4V when fully-charged then its voltage slowly drops to 6.4V when its internal circuit disconnects the load.

The manufacturer says it has a protection from over-charge circuit inside (that does not work) because they say if you use an ordinary charger then the battery might EXPLODE! They say to use their expensive charger.

You must NEVER trickle-charge a lithium battery.
 
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