I am wiring my robot, and I was wondering what I should go with for a power source. It needs two inputs: 12v and 7.2v dc. The 7.2v goes to a Vex PIC microcontroller, and the 12v goes to a wireless router. I have a 7.2v 3000mAh Vex NiMH battery and a 12v 7Ah sealed lead acid (Heavy.) I would rather use only one battery, and I would prefer the 7.2v.
Is it possible to have a circuit that makes the 7.2v go to 12v just for the router? My dad had the idea of converting the 7.2v to 7.2v ac, running it in to a transformer to get 12v ac, and then using a bridge rectifier. I'm sure it isn't that efficient though.
If the router requires 1a, then you might want to stick with the 12v battery. I tinkered with something similar (below) and used a rechargeable 12v battery from a drill.
How about buying two of these (7.2v 3000mAh Vex NiMH battery) wire them in series.
You would then have 0V to 7.2V and 0V to 14.4V.
Now 14.4 volts "may?" be able to run the router directly, but if not, you could always add three 1N4007 diodes to lower the voltage down to ~12.3Volts.
In the case of the 12v drill battery, I would have to find a 12v to 7.2v power converter that could support at least 2 or 3 amps, maybe more. Anyone know of one? The wifi router is the only thing on 12 volts; everything else is running out of the Vex PIC controller, motors, switches, and an Arduino Uno with a SEEED Ethernet Shield.
I know my dad has some 14.4v and 18v drill batteries that I could probably use.
Personally, I think LarryD's suggestion is the best; you gain the low weight you are seeking, and can easily center-tap the battery to get your 7.2v output for whatever else you are running. Dropping the 14.4v down to 12v @ 1A isn't very problematic - you can easily find a DC to DC converter to do that. Or - have you tried running the router off 7.2 VDC? It might just work...
cr0sh:
Personally, I think LarryD's suggestion is the best; you gain the low weight you are seeking, and can easily center-tap the battery to get your 7.2v output for whatever else you are running. Dropping the 14.4v down to 12v @ 1A isn't very problematic - you can easily find a DC to DC converter to do that. Or - have you tried running the router off 7.2 VDC? It might just work...
Whoops, looks like I posted as that post went up. Yeah, that sounds better; more powerful, easier, and lighter. I think I could throw $30 in to another battery, since I already have the charger and they are nice batteries for robots.
zoomkat:
Are the 7.2v batteries susceptible to damage if discharged too low?
My dad just brought up that point with me. One of the batteries is going to drain really fast. Is there any way to run them in parallel to the Vex PIC, and in series to the wifi router, at the same time? I would think you can using a few diodes.
zoomkat:
Are the 7.2v batteries susceptible to damage if discharged too low?
My dad just brought up that point with me. One of the batteries is going to drain really fast. Is there any way to run them in parallel to the Vex PIC, and in series to the wifi router, at the same time? I would think you can using a few diodes.
Yeah, your dad knows. This is a serious problem with rechargeable cells in series,
especially NiMH. If the cells aren't all equally balanced and equally healthy, and
well-matched to begin with, then during loading the "better" cells will actively
discharge the lesser cells, ie run them flat and can actually drive them into
reverse-polarity. I've done this myself by way of experiment. If you buy a
good-quality series pack from a hobby RC company, they will have chosen the
cells to match each other.
For your app, I would use 2 separate battery packs for the 2 loads. Lots of
batteries, but your robot can probably handle them. Alternatively, I would use
the single 12-14V pack to power both loads, and drop the voltage to the
Arduino board using a series R or even a 5V, 2-5W zener. Not the most efficient
scheme, but the Arduino current requirements will probably not be too large.
Like the buck converters, UBECs are efficient in providing lower voltage output from higher voltage sources. Most can output 5v or 6v (nice for servos) and provide high current output.
My dad thought of another way to power the router. He was thinking I could use those rechargable AA batteries. I found some, but the specs arn't that good. It says 2050mAh, but I don't know if that is for all of them combined, or each cell. That would be for these. There are also these, but they provide lower mAh. Again I don't know if the power is combine or for each cell.
If you are considering the typical consumer rechargeable batteries, go to walmart and look at the batteries/chargers in the camera section. Ray-o-vic is another popular brand that is available. I think a charger and 4 AAs is ~$14.
AKSoapy29:
Alright. Looking at these, they say the cells are 2000mAh. Does anyone know if that is 2000mAh each, or total?
The info says "•AA cells are 2000 mAh" which would indicate to me they are talking about a single cell. More than one cell is usually referred to as a battery. I've got the below charger and the recharge time is rated at 3-5 hrs. Bottom is a radio shack holder (not the best, but quick/easy to get.