Powering BOE with 6 AA batteries

berudd

My son is working on a robotics project for school. He's going to use continuous drive servos to drive the robot. Originally, we were going to just have a second set of AA batteries to power the servos independently of the BOE's 9v power supply but a closer look at the rules revealed that this is not allowed. It's either a single 9v battery or a set of AAs. Concerned that the single 9V won't be enough to power two servos reliably or, at best, if it does I'll go broke buying 9v batteries.

So, the thought now it so get a 6x AA battery older and connect it with a 9v clip to the BOE. My question here is will higher mAh rating of the AAs harm the BOE or BS2 or will be OK? I know there servos can have a spike in the amps the draw when the start up. It would be nice if we can just power the servos off of the Vin source on the BOE but if the higher current draw might be to much, it would be easy enough to splice a couple of leads off the battery to power the servos directly from the battery pack.

My biggest concern is frying the BOE by plugging 6 AA batteries in instead of a single 9v.

Publison

6 AA would actually be be a better power supply. The Standard BOEBot uses 6 AA batteries. They both equal out to 9 Volts, but the AA's have more current to supply.

6AA will not fry a BOE board. Let's see some pics and videos!

Hal Albach

You can safely use 6 AA cells to power the BOE. The board will only draw the current it was designed to pull. The larger capacity of the 6 AA's just means that the project will run reliably for a longer period of time.

Mike Green

I strongly suggest using a battery holder with 4 AA cells. With good quality alkalines, this will provide 6V to the BOE. You can run servos off Vin at 6V and the BOE's regulator is fine with the 1V "headroom" between the 6V and its 5V output. The mAh rating indicates what the battery can supply and for how long. It doesn't say anything about what the BOE and/or servos actually will draw. You can run the whole thing off a battery pack with 4 C or D cells. These'll be heavier and physically larger, but will last longer. Make sure the battery holder's cable is wired properly for (+) and (-). You don't want to connect it backwards, even for a fraction of a second. You may be better off using a power plug (2.1mmID, 5.5mmOD) with the center to (+) and the outside to (-).

The BOE will run off a 9V power source, but it works best with 6-7.5V. Anything over that is just dissipated as heat. Servos work best from around 4.5V to 6V. You can run them off Vdd (5V), but that adds to the heat dissipation.

If you want to use rechargable AA cells, use 5 since they're 1.2V instead of the 1.5V of alkalines. 5 x 1.2 = 6! There are battery holders for 5 cells, but this won't fit in the space provided in the BoeBot's chassis if that's what you're using. You can get a holder for a single cell and glue it to the back top of the chassis then wire the two battery holders together in series.

Courtney Jacobs

Parallax suggests using a 4-or 5-cell AA battery holder to power a small robot like the Boe-Bot, driving with continuous rotation servos such as the ones we include in the Boe-Bot or ActivityBot kits. Our small hobby servos are rated for 4-6V (regular continuous rotation), or 6-8V (high-speed continuous rotation), but your suggestion of 6 AA x 1.5V = 9V. Using 6 AAs exceeds the recommended voltage limit for all small Parallax hobby servos; however, 9V is within the acceptable range of voltage for the Board of Education itself.

A 4-cell battery holder is included with the Boe-Bot robot kit, and can be installed according to the directions in the kit documentation (if using our small robot chassis). A 5-cell battery holder will fit as well, if installed in the orientation shown in the documentation for the ActivityBot robot (again, only if using our small robot chassis).

berudd

Thanks for all the input. One correction on my part. He's not using the BOE but rather the Homework Board. Not sure if that changes anything. I don't think it includes the 2.1mm power plug like the BOE does which is a bummer. It just has the 9v plug. So, we have to make sure we get that wired correctly. What I understand from looking elsewhere is that the red wire from the battery holder needs to be wired to the black wire of a 9v plug. Makes sense as we're using it on the opposed end of the transaction, if you will, from it's normal usage.

We're not using the Boe-Bot kit, the rules of the contest require it to be a bit more scratch built. Straight line speed of the robot is a factor so the thought was to run the high speed servos at 9v to eek out a little more speed. I had done some other Googling to indicate servos could handle a bit more voltage than what they are rated for. But, maybe a 5 cell pack is a little better option as it won't shorten the life of the servos.

AAs should be sufficient as the test runs for the contest will be very short. He doesn't need it to run for an extended period of time.

LoopyByteloose

9VDC may simply be too much for continuous rotation servos and cause damage to them. The Homework Board will run okay, but a bit warm over 9VDC. There are snap in dongles for the 9VDC that can be wired to anything you desire -- just becareful with the polarity as it is possible to get confused and reverse it. Don't rely on red being plus and black being minus in this context.

What you need to do is to research the absolute maximum voltage acceptible for your continous rotation servos and let that define your power design. Many will take 7.4VDC that two lithium ion 3.7VDC cells provide. And the Homework board will do fine with the 7.4VDC as well.

Another alternative is NiCad battery packs that are rechargible. They come bundled in a variety of combinations that might work well.

erco

If the rules allow any one battery, get a 7.4v LiPo and add 2-3 silicon dropping diodes to power the servos, but apply full voltage to the Homework Board. It has a voltage regulator.

berudd

Yeah, I know it could shorten their life but I did read some stuff of others running the up to 11v. The high speed servo is good to 8v so I'm probably willing to accept the risk of 9v.

AA batteries are fine for our needs. Cheap and effective.

MikeDYur

My rechargeable AA's are rated @ 2400 mAh, 1.2V x 5 = 6 volt's, you have the current to run motors. Seems to work for me.

_mike

LoopyByteloose

The situation with R/C servos is that they have an internal IC or transistor logic that interprets the PWM. I am sure you have noticed the Absolute Maxium values in IC documents. They have to be observed with devices as well.

The motor itself may easily tolerate 9VDC or more. But each manufacture adds a board that may have much lower voltage limits. If you do burn out a board, it can be removed and the servo can be adapted to be driven externally with an H-bridge... maybe at a higher voltage.