Annoying newbie battery charging circuit thread

[victormeldrew]

Hello...sorry to be the newbie.   
Feel free to reply with a simple link to useful information, or easy to follow principles... I'm able to work things out for myself, as long as I know where to look, but the more I search the more I get confused and I'm sure this is pretty simple, so I apologise if it's annoying! Also, apologies for the long post, but I wanted to get everything down that you might need to know to point me in the right direction.

I'm building a basic toy device which is for all intents and purposes a mini robot, with a simple PIC/servo/button/led type circuit. For the purposes of this, the circuit can be considered a black box, connected to a battery of 3-5V. It is a fairly low drain circuit (250mA peak, about 50mA average). Battery life is not crucial, but I'd like to get a few hours at a time. I've powered circuits using mains DC and removable batteries before, but not done a rechargeable device.

I want to make this a rechargeable device, with the battery semi-permanently fixed in-situ. I don't want to remove the battery to charge it, instead it will be charged by plugging in a power supply. I don't mind if the circuit is powered during charging or not, but if it is, it needs to be safe for my PIC (my circuit already includes a smoothing capacitor and VR though)

What I have so far (and would like to use, as they're already in the parts box):
My Circuit
Simple 9V DC power supply (not a battery charger)
Power connector, 3-pin (so I have a connection that opens when the jack is plugged in, if you know the type I mean?)
5V Voltage Regulators (7805s) for my main circuit, and charging if necessary
Whatever resistors, caps, etc I need...
Stripboard for my circuit board.
Basic skills...

What I don't have yet, but can/will get:
Some kind of battery pack
Some kind of charge management IC (I can breakout a SO-23 or whatever, so size/type is not important)
or...whatever other circuitry I need to charge the battery.
Whatever datasheets...that I'm usually able to follow.

What matters to me is safety, ease of construction, and circuit size, with cost being the least important.

What I need to know and can't seem to fathom (too many results rather than too few!) is what my best option is for battery type - and what I need to do in my circuit to ensure that, when plugged in, the power supply will safely charge the battery, and/or power the circuit while that is happening, and give charging state feedback via an LED.

It seems to me that a 1-cell Li-Po would work, and I can find lots of cheap Li-Po batteries and suitable charging controller chips. I can't find Ni-MH charging ICs cheap (3 times the price?), but I can find the batteries easily enough (I could use 4xAA for example). I could use my circuit's PIC for charging control but I'd rather not as the program is very simple and so I'm using a basic PIC at the moment with no ADC, and very little spare program memory. However, I can spare a few lines for simple timers if needs be, and already have an LED that could be used for feedback during charging, and have 1 spare digital I/O pin. However, if it will be better overall to use a bigger better PIC and use that for charging, I can if needs be (it's not much more money to get one with ADCs and another few I/O pins.

The benefit of the charge controllers though, seems to be that all the required connections and logic are already there for the charging and the feedback LED. BUT, if a simpler and cheaper circuit is possible, I'll do that.

So, to my question(s)

1) What battery type would you use?
2) And so, what would be the best/cheapest/smallest charging circuit to use? If I'm using a charger IC, I have recommended circuits in the datasheet that I can understand.
3) Most importantly, what will that mean I need to do to connect everything up correctly? The IC's recommended circuit is simply connected to the battery, but no advice on what to do about the existing circuit beyond that (if anything) to ensure it doesn't interfere with charging, and doesn't blow anything up.

I'm sure I'm making this more complicated than it needs to be, so please let me know if I'm being stupid!

Help? 

[Gertlex]

From my experience (rather than pure engineering logic), my inclination would be to use LiPos, because that's what I've been using recently, and charging them is more straight-forward than charging NiMH batteries was, in my experience.  With LiPo's I would purchase rather than buy a charger, but that's my inclination to not mess with LiPo charging circuits.  You could certainly keep the LiPo on the bot at all times.  Would you be manually connecting the bot to the charger?

[victormeldrew]

Yes, there will be a break-on-insert power connector, and a power supply (which I already have a few of each in the parts box). I think I'd like to use an IC like the MCP73831 (or similar, depending on the battery pack I use):
http://uk.farnell.com/microchip/mcp73831t-2aci-ot/li-ion-li-poly-charge-controller/dp/1332158

But the datasheet (linked to on that page) shows a recommended circuit (Section 6) with the battery connected across two of the pins...but my worry is whether my existing circuit should be disconnected from the battery during charging?

[victormeldrew]

In case this helps anybody else, yes the circuit needs to be disconnected from the battery during charging.

I think I've sorted out a rough circuit that will work.
Q1 might need to be a relay, depending on current.
SW1 just simulates plugging in the power to the charger circuit.
Q1's base is pulled high, unless +V is present on Q2's base (only when charger is plugged in), when Q1's base is pulled low, leaving just the charger and the battery in the circuit.

[Soeren]

Hi,

So, to my question(s)

1) What battery type would you use?

Based on your self-declared noobness, I'd recommend NiMH, as they're less temperamental charging - you can always shift to lithiums when you gain more insight or in your next project.

2) And so, what would be the best/cheapest/smallest charging circuit to use? If I'm using a charger IC, I have recommended circuits in the datasheet that I can understand.

A taper charger is the cheapest/simplest.
A comparator in SO case or eg. a PIC10F2222 (TO-23-6 case) could be used to make a fully automatic charger, while still keeping it fairly simple and cheap.

3) Most importantly, what will that mean I need to do to connect everything up correctly? The IC's recommended circuit is simply connected to the battery, but no advice on what to do about the existing circuit beyond that (if anything) to ensure it doesn't interfere with charging, and doesn't blow anything up.

There's no reason to disconnect the battery when charging - you charge your cell phone while it's on, right?