Battery charging/alternating to socket dc

Hi everyone

All new to the site and pretty much a novice at electronics. I'm having a small pet project where I want to have a battery from which the device runs in normal mode, but the ability to get 5V in, and in that case I want the battery to charge from that current, and the device switch to that power source.

I'd also want to be able to get some info from the device controlling the battery (pretty much what you can get in a laptop) so there has to be a micro controller there.

I'm pretty sure that this is something one can buy from digikey etc but I don't have a clue what it's called so if someone does it'd be of great help if that someone would share it with me.

I did have a look at microship but to be honest can't I figure out if those devices would do what I want.

Thanks in advance

Your question is a bit too general.

The only figure that you quoted is 5V. How much current does the device take? Does the electronics run from 5V or do you want the charging to be at 5V or both? Do you need the circuit to be able to run without the battery when running from an external supply? How long do you want it to run on the battery? How fast does it have to charge?

Laptop batteries are always at a much higher voltage than 5V. They are often 3 or 4 Lithium Polymer cells in series, so about 11 or 15 volts.

You always need a larger voltage to charge a battery than you get from it when using it. If you need an accurate voltagefor your load, you need a voltage regulator after the battery. It is easiest if the lowest battery voltage is higher than the accurate voltage you need.

You have to regulate the battery charging, but the method of regulation depends on what battery chemistry you use (Lead acid, Nickel Metal Hydride or Li-Ion).

I hope this helps.

It did help, and thanks. I tried to leave out as much as possible to shorten the post but I realize that my lack of knowledge made me leave out the wrong/too much. Trying again:

The device I want to run is a small ARM-based computer using about 2W and accepts 5V (5.5 and over is said to burn it so I need the voltage to be pretty stable). What I want to do is to be able to run it both from a battery and socket power. When running from an external source I want to charge the battery at the same time.

I'd use an adapter when getting electricity from an external source so in would be somewhere from 5-18 V depending on what works best. I haven't decided yet if I'd want to have a regulator accepting a big span of incoming volts, but for now I'd just go with one and maybe in the future extend the capabilities.

For battery I'd go for a small/flat one, I think around 900-1700 mAh to get at least a theoretical 2-4 h uptime on battery. I haven't decided what type but I guess from what you wrote about lowering the power in 6-7 V out from the battery is what I'm looking for. These preferences makes me think that a Li-Ion or NiMH is my way to go.

About the charging would I want to be able to connect to something (I guess a microprocessor) to be able to obtain info about the battery. Of course the quicker charging time the better but that isn't really crucial. The info that I ideally want is: Are we running on battery or not, how charged is the battery and if discharging at what rate (to calculate how much longer until it's dead).

So there are still a couple of ifs and buts and maybe I'm trying to solve this problem backwards? My thought was that the circuit for charging/regulating the battery/voltage would be the tricky part and when finding out that part get a battery that suits what I find. Should I rather first find the battery I need and then try to hunt down info and most likely help about it?

Also, does these "smart" charger circuits have a name so I can try to read a bit about it? Is this something common enough to be able to buy ready or is this something very custom that'll need either a whole lot of money or time? I'm fumbling in darkness and would like to at least know a little bit about it (it's not the main goal of my little pet project but having a general idea would be nice).

I would start here **broken link removed** as it provides many chips for power switching, battery charging and voltage/current/temp monitoring.

This is an example app note that includes all the circuitry to charging from the powersource, and running on batteries when not plugged it to the wall - i dont think this particular one includes monitoring - but im sure they do one https://www.electro-tech-online.com/custompdfs/2008/10/MAX8713.pdf

This one may be useful to you too, its all about charging batteries from a USB socket, if thats your thing

https://www.electro-tech-online.com/custompdfs/2008/10/AN3607.pdf

Ok. But just to make sure I understand what it I need:

Before battery:
Charging circuit that will take a set V in (I'll use an adapter in front with the correct voltage)
Battery monitor that keeps on eye on things, turns off charging when battery is full etc.

After battery:
Regulator adjusting vout from the battery to what the board wants

One thing that feels confusing is how to take the electricity from the external source when that is plugged in, or does one take it via the battery always? If so how to regulate the voltage since vin to battery is higher than vout?

This sounds like a very standard setup, used by basically all handhelds and laptops, does it have a name?

I guess these are very basic questions but like I said, I'm all new to the world of battery charging/electronics.

you are effectifly always taking it from the battery i think - your regulator goes after the battery.

OUTLET>CHARGER>BATTERY>REGULATOR>LOAD

nisse said:

One thing that feels confusing is how to take the electricity from the external source when that is plugged in, or does one take it via the battery always? If so how to regulate the voltage since vin to battery is higher than vout?

Click to expand...

If the lowest voltage from you battery is higher than the output of the regulator by more than about 0.5 V and you are using a low dropout regulator, you can put a Schottky diode from the battery to the regulator. Then you can put another diode from your main input supply to the regulator.

That way, whichever voltage is biggest will feed your regulator and therefore your circuit.

If you don't have the diodes, your battery will charge slower when the circuit is running. With Li-Ion batteries that can be a real problem if you let the batteries go really low, as the chargers only output a tiny current in those conditions. If you have no way of turning off the load, the battery never gets charged.