Need help with charging circuit.

[Opencircuit74]

Hello, everyone, I am trying to make a rechargeable circuit for my Atari flashback 7. It has an input of 5v and 500ma, and I wanted to add two lipo 1200mah batteries to use it without plugging it in. I have a 5v regulator, as well as standard electronics parts such as resistors, capaciators, etc. the only items I have limited access to are NPN transistors and op amps. I was wondering if there was a way I could charge the batteries while it is plugged in, and use them when t is not. Thanks! On a side note I can also get NIMH batteries since they are a lot more suitable for this sort of thing.

[Electronics Wizardy]

You need a charge controller ic  for this  you can't really do it without one. There are lead acids where you can leave them plugged in all day. Lithium batteries are known to blow up if you do this. 

[samcool55]

I don't know much about circuits but i do know that working with NiMH batteries will be a LOT easier than LiPo's.

LiPo batteries need to be charged properly or they will combust so i wouldn't use them.

 

Also because i'm assuming you aren't very experienced, if it goes wrong and you used NiMH batteries it won't be a complete disaster and they are cheap to replace anyway  Once you are more experienced you might go LiPo but for now i would stick with NiMH batteries.

[manikyath]

piece of advice: dont do it with a simple 5v regulator, you're wasting so much power there, essentially you'd have to get a battery pack that's ~7v at its cutoff voltage to make it output 5v all the way down.

[Opencircuit74]

2 hours ago, samcool55 said:

I don't know much about circuits but i do know that working with NiMH batteries will be a LOT easier than LiPo's.

LiPo batteries need to be charged properly or they will combust so i wouldn't use them.

 

Also because i'm assuming you aren't very experienced, if it goes wrong and you used NiMH batteries it won't be a complete disaster and they are cheap to replace anyway  Once you are more experienced you might go LiPo but for now i would stick with NiMH batteries.

I have been working with electronics for about 5 years now, but I haven't done much with charging circuits. I am thinking I can get by with 4 NIMHs and the regulator on the output side. Any objections?

[cameroncoats]

Sparkfun Powercell / Adafruit Powerboost (or knock offs) would be perfect - charge control IC and a boost converter to get steady 5v from a 3.7v LiPo.

[DragonTamer1]

58 minutes ago, Opencircuit74 said:

I have been working with electronics for about 5 years now, but I haven't done much with charging circuits. I am thinking I can get by with 4 NIMHs and the regulator on the output side. Any objections?

NiMH would be better since they are safer, however you would still need some kind of shut off timer or smart charging circuit to prevent leakage from the batteries. Lithium batteries explode, NiMH will just leak their interiors and make a mess of everything when overcharged.

 

You will need either a timed circuit that charges them for a certain amount of time or a smart circuit like the MAX712/713 to actively monitor the charging process and cut off the batteries once fully charged. You will also need a circuit that can monitor the battery and cut them off from the circuit when they get low. I believe that the ICL7665 is ideal for this but it's been a while since I worked with it so I may have gotten its number wrong.

 

As for output regulation from the pack you will need some sort of buck converter. If you have 4 batteries in series that would be between 5.5V and 6V output nominal and for most regulators that is below their dropout voltage. What I suggest is adding two additional cells to the pack and adding a buck converter to drop it from 9V to the 5V you would need. Even when the cells are at their minimum voltage they should still be above the dropout voltage of the regulator.

[r4tch3t]

You could look at eBay for some lipo chargers, some even include a boost circuit. Should only set you back a couple of dollars. Essentially a powerbank is what you need. The NiMH is also a good choice.