Power to the BOEBot

Pierson

I have a slightly front-heavy BOEBot, with a Ping/servo·and another smaller servo added for tilting a tiny wireless video camera.

The camera says it requires at least 8-volts, and has a 9-volt·transformer and a·9-volt battery clip.

I'm thinking of replacing the Boebot's battery pack,·the 4 AA batteries, with some heavier, rechargeable nicd batteries. What I have is a battery pack, 7.2 volts and 1,500 mAh. I'm thinking of dissecting the pack and carefully reassembling it where the AAs were.

What I need to know is what the maximums the BOE can take. The board says that the jack can take between 6-9 volts. In the manual, I read that if you use a wall wart, it needs to be at least 1,000 mAh. Elsewhere, I read that the BOE can take up to 30 volts!

Will my battery pack, as is, power the BOE-Bot without damaging it?

Mike Green

The board and the manual say that you should use a power source between 6 and 9 volts. Less than 6 volts might work, but leaves little "room" for the built-in voltage regulator to do its stuff. The voltage regulators used can take 30V without damage, but don't really work at or near that level. They quickly "shutdown" because the difference between the applied voltage and 5V is dissipated as heat and the regulators don't really have heatsinks. They get hot and their "failsafe" thermal shutdown turns them off. Stick to what's printed on the board.

Your 7.2V battery pack is right in the middle of the recommended range. The recommendation for at least 1000mA (not mAh) capacity is due to possible peak demands from servo motors. The 1500mAh capacity is the average current the batteries can supply for an hour. If you draw 1/2 that current, the batteries will last about 2 hours. If you draw twice that current, the batteries will last 1/2 hour, etc. It's not exactly that, but that explanation is close enough. Batteries "self-discharge", that is, they act like there's a small load attached all the time. They'll discharge to nothing in anywhere from a week or two to a month or two depending on the battery and the chemistry used. Very high current loads cause any battery to act as if its capacity is less than that specified. Usually batteries are specified for a 10 hour rate (an average current of 1/10 the capacity for 10 hours).

The BOE-BOT has two settings for its servo power connection. One (Vin) connects the servos to the power supply. The other (Vdd) connects the servos to the 5V regulated supply. Servos are designed to operate on voltages up to about 6V. Their lifetime is shorted by operating them at higher voltages (sparking internally wears out the motor brushes faster than normal) although they're commonly operated off 7.2V power packs where it doesn't make much difference.

Pierson

Thanks for the info. The 7.2 might be too low for the camera (8 volts), but it's also too high for the servos. Maybe not the best solution.

I also have a 9.6 volt, 1,000 mAh, smaller battery pack as an option. It would supply more ballast to the rear of the BB than the 4-cell AA, definitely power the camera, and any extra life the batteries would have compared to the AA would get chewed up by the Ping, Ping servo, camera, and camera servo. So, being that it's at, presumably, at the 1,000 mA range, and only a little bit over the 9-volt limit, would it make a good candidate?

I realized I haven't given any thought as to how I would handle the camera's power connection. Would Vin be at 9.6 v if I connected that battery?

Thanks again.

allanlane5

The BS2 would work fine on the 7.2 or 9 volt battery packs. However, if you DO use the 9-volt battery pack, you need to insure you're driving the servo's off of Vdd, NOT Vin. I don't think the Servo electronics will survive a 9-volt level.

Where almost all servo electronics will survive the 7.2 volt level, with reduced motor life.

Pierson

Good point. I will have to watch that jumper position.

Thanks.

Chris Savage

There are 9V rechargeable packs available and as pointed out, if you have the servo power jumper in the V_DD position, the servos will only get 5V. We actually have a BOE-Bot setup to do this very thing and simply adding a 9V battery to the back allows it to run the came separately for quite a while. Take care.

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Chris Savage
Parallax Tech Support

Steve Joblin

why don't you just tap off different volts along the series of connected batteries... depending on where you connect to the battery pack, you can get a variety of voltages

Pierson

That's a great idea!

7.2 volt pack (6 cells):

4 cells x 1.2 v = 4.8 v (guessing that each cell is 250 mA, if so...) @ 1,000 mA <--Not enough
5 cells x 1.2 v = 6.0 v @ 1,250 mA <--That'd be good for the Bot without the camera, not too harsh on the servos

9.6 volt pack (8 cells):

5 cells x 1.2 v = 6.0 v @ 1,000 mA <--Bot ok, servos okay
6 cells x 1.2 v = 7.2 v @ 1,200 mA
7 cells x 1.2 v = 8.4 v @ 1,400 mA <--Camera good to go

I like that idea.

I have a smart charger that will automatically determine the number of cells that are hooked up to it. It also allows you to change parameters like maximum charge rate, etc.

Another thought would be using TWO 4-cell AA battery holders, and putting NiMH batteries in it, wire it in series, and·take power from·cells·5 and 7.

The cells I have are rated 1,600 mAh.

I think my conclusion about the mA rating is wrong. Each of the nicd cells must be the same mA rating, either 1,000 mA or 1,500 mA, it's not additive.

Post Edited (Pierson) : 1/8/2007 10:06:49 PM GMT

boeboy

Wont you have to worry about them discharging unevenly?

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lets see what this does... KA BOOM (note to self do not cross red and black)

Pierson

To Chris: That would be the simplest way to power the camera, and I can simply turn the camera off (unplug) when it's not being used.

To boeboy: Hmm, you might be right. My smart charger might be able to handle that, maybe. With nicd, I'd want to make sure they were all discharged anyway before I charged them. With nihm, I don't think it would be an issue. Hmm.

Zoot

This may not be the exact approach you were looking for, but I've been using the Sharp pq05rd21 and pq09rd21 voltage regulators (5v and 9v). They have 4 pins in a T220 package -- the 4th pin is an on/off control, so you can turn the power off. They're good up to 2-3 amps from 12V. They're low dropout, too. (DigiKey has them).

So, you could have one 10-12v battery pack. Run your 9v electronics off the 9v regulator and turn it on and off with a Stamp pin to conserve power if you want to. Use a 5v for the servos (if you want to turn the juice on and off) or use the Vdd from the BOE as has been suggested above. An advantage here too is you would have battery power for the 'bot and servos even after the battery pack is too low to run the camera.

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When the going gets weird, the weird turn pro. -- HST

Pierson

Thank you, Hunter.

I'm open to all solutions, but will probably pick whichever is easiest and uses the parts I have, unless I really feel like spending more money, which can change from day to day.