So i am working on a project where an arduino, 12v led string, and a rechargable battery are in a fully enclosed box (a very small box I might add).
I was thinking of using a lithium rechargeable 9v battery as the power source with a wireless induction coil but I need help with a smart charging circuit or pcb. Id prefer to buy a premade circuit that is cheap and simple. Id like to be able to charge the battery and use it to run the arduino at the same time without any overcharging or overdischarging.
Any help is greatly appreciated 
Paracelsus:
So i am working on a project where an arduino, 12v led string, and a rechargable battery are in a fully enclosed box (a very small box I might add).
I was thinking of using a lithium rechargeable 9v battery as the power source with a wireless induction coil but I need help with a smart charging circuit or pcb. Id prefer to buy a premade circuit that is cheap and simple. Id like to be able to charge the battery and use it to run the arduino at the same time without any overcharging or overdischarging.
Any help is greatly appreciated
A 9 Volt Lithium Battery?
Like Sparkfun [retired] 9Volt
Actually, only 8.4 Volts max, really just a 2S battery.
Why not 2 x AAA or 2 x AA in series?
A very simple and cheap 8.4 Volt Li-Ion Charger ...
8.4Volt_Charger
No balancing.
Paracelsus:
So i am working on a project where an arduino, 12v led string, and a rechargable battery are in a fully enclosed box (a very small box I might add).
I was thinking of using a lithium rechargeable 9v battery as the power source with a wireless induction coil but I need help with a smart charging circuit or pcb. Id prefer to buy a premade circuit that is cheap and simple. Id like to be able to charge the battery and use it to run the arduino at the same time without any overcharging or overdischarging.
Any help is greatly appreciated
What powers the LED string? Can't you extract your Arduino supply from that, rather than adding an extra battery?
And you should not charge batteries in a fully-enclosed case, either.
@mrsummitville, that's not a particularly good choice for a Li-ion battery charger. Fine for a lead-acid battery.
And you should not charge batteries in a fully-enclosed case, either.
But civilian aircraft lithium batteries are mandated to be used in a sealed case!
Back to the question - how does a 9V or even 8.4V battery work with a 12V LED load?
4S LiFePO4 battery (= 12.8V) would be a much better match and its safer than other lithium
chemistries. You always need a balancing charger for any lithium battery pack, but these
are readily available from RC model suppliers.
If a boost regulator is being used anyway to generate 12V, then presumably the battery
voltage is pretty arbitrary. 2S or 3S packs are the most popular, and 2S LiFePO4 are
common as 6V receiver packs.
MarkT:
But civilian aircraft lithium batteries are mandated to be used in a sealed case!
Don't lithium batteries produce gas like other battery chemistries?
They can swell and also vent in extreme situations
OldSteve:
Don't lithium batteries produce gas like other battery chemistries?
If they do, stand well back and don safety goggles! No, there is no gas evolved
in the normal cell action. The cell has to be sealed completely as any moisture
or oxygen getting in will immediate react. Lithium chemistries are very sensitive
to and damaged by overcharging and over-discharge and are normally only used
with protection circuits to prevent both.
MarkT:
If they do, stand well back and don safety goggles! No, there is no gas evolved
in the normal cell action. The cell has to be sealed completely as any moisture
or oxygen getting in will immediate react. Lithium chemistries are very sensitive
to and damaged by overcharging and over-discharge and are normally only used
with protection circuits to prevent both.
Thanks Mark. I've learned something new. I already knew about the risks associated with lithium batteries, but not the fact that they produce no gas whilst charging.
And I guess I should take care with my 36V 16Ah lithium electric bike battery when riding in the rain. (Not that I do it often.) I don't trust the seal on the top of the case to be waterproof. There's no rubber or other sealing material.
How do you know ?
Did you do a teardown to find out what's inside ?
Would make a funny youtube entry, a bike with an ejector seat.
MAS3:
How do you know ?
Did you do a teardown to find out what's inside ?
Yes, I've pulled the top of the case off, to reverse the polarity of the wires to the connector. The present battery is a replacement for the original, which died after 4 years of heavy use with many charge/discharge cycles.
I don't know how well the individual cells are sealed, but they're probably fine. I'm not really too concerned.
Would make a funny youtube entry, a bike with an ejector seat.
Yeah maybe, but it won't be me. 
(I can just picture myself flying into the air, trailing smoke and flames.)