Power Supply/Charger

[mwtheplumber]

I am building a project which involves a remote control transmitter. I want this to run off either a 9v supply or 9v battery. I would like transmitter to run and the battery to be charged by the 9v supply when a mains supply is available and default to battery power when no mains supply is available. I would be grateful for advice on the best type of battery and charger/supply to use for this.

 

[audioguru]

Your request is missing a very important specification: How much current?
We also need to know how often the transmitter is transmitting from the battery and how long must it last.

 

[MikeMl]

And we also have to know the battery chemistry.

 

[audioguru]

And we also have to know the battery chemistry.

There is a small choice for a rechargeable 9V battery. An old Ni-Cad that is probably not available any more or a modern Ni-MH.

 

[mwtheplumber]

Thanks for your replies. Quiescent current is 6μA, transmitting current is 30mA. Would anticipate using a Ni-MH battery. Would expect up to 600 transmissions in a period of 7 hours. Hope this helps.

 

[MikeMl]

How long does a transmission last?

How long after a power failure do you want it to keep running?

 

[audioguru]

An Energizer "9V" Ni-MH battery has 7 cells so its voltage is about 9.8V when fully charged. Its voltage drops to about 8.4V for most of its discharge time then slowly drops to 7.0V when it is almost dead.
It can supply 30mA for 5.8 hours but it does not have the "new" cells that hold a charge for 1 year so it is dead in about 2 months even if a charge is not used.

 

[ChrisP58]

One more thing to keep in mind is that you cannot charge a battery with it's nominal voltage. You need a higher voltage to push current into it. How much more voltage depends on the cell chemistry.

So, you'll need to make sure that your transmitter can withstand the peak charge voltage of your chosen battery.

 

[mwtheplumber]

How long does a transmission last?

How long after a power failure do you want it to keep running?

Less than a second.

 

[MikeMl]

And the answer to the second question is????

 

[mwtheplumber]

Ooops Sorry, if possible for 4 hours.

 

[MrAl]

Hi,

One of the main choices involved is what kind of battery to use.

NiCd is hardier than NiMH. NiMH really requires a special charger that can detect when the cell is completely charged.
Another good choice is Li-ion, but again there you have to charge with a charger that is made just for Li-ion.
Lead Acid is more of a pain than anything else.

Simple Li-ion chargers can be made from cheap LM317's. NiMH chargers really need a special charging chip. NiCd doesnt require anything too special, maybe just a current limiting resistor.

 

[audioguru]

A battery drain of 30mA for one second every 4 hours needs a very small battery. Tiny rechargeable Lithium batteries are made for remote controlled helicopter toys but not Ni-Cad nor Ni-MH.
Maybe this project can be powered for many years from three little 3V Lithium disposable coin cells in series or the little battery used in a car remote.

Does anybody make a "9V" Ni-Cad battery anymore? Energizer and Duracell don't.
My Sound Level Indicator project is 9 years old. Its original "9V" Ni-Cad battery failed after 1 year and was replaced by a Ni-MH battery that has been charged with a simple resistor for 8 Years. The battery is not used often and its trickle charge current is 1/40th of its mAh rating.

 

[alec_t]

30mA x 1s x 600 = 5mAH in 7 hours. Personally I'd probably try a pre-charged 12V battery on a simple permanent trickle charge of a few mA, and a LDO 9V regulator.