strange battery pack issue

xanadu

To power my BOE-Bot platform I have a bench power supply, and a Parallax 4 AA battery pack with fresh batteries. Both outputs tested at 6.3v.

I'm running a Spin program to turn two continuous rotation servos very slowly (aka move bot forward). The servos will not turn with the battery pack, only the bench top power supply. The servo jumper has been set to both Vin and +5v.

Both the bench power supply and the battery pack are plugging into the DC jack of the Prop Protoboard. Any ideas what could possibly cause that? I just put another set of fresh 1.5v dry cells in there and still get the same result.

xanadu

The problem appears to be Panasonic Carbon Zinc batteries. Brand new at full voltage just don't work.

Mike Green

Servos can draw quite a lot of current under load and carbon-zinc batteries have a relatively high internal resistance and low peak current capacity compared to alkalines or any kind of rechargable. See the graphs on the datasheet and compare to alkalines.

Duane Degn

When you say "don't work" what do you mean.

Does the Prop keep resetting? It might be possible to use those batteries if you add a couple of big capacitors to the Vin and/or 5V lines. Servos seem to use the most current when they first start moving. The caps might be enough to keep the Prop from resetting when the servos start to move. I've had several servo project where some extra caps solved the reset problem.

Recharables, as Mike mentioned, can provide more current. I'm pretty sure they're better than alkalines in the amount of current they can provide. Plus you can recharge them. You might need a five or six cell pack to use with a PropBOE though.

Edit: The PropBOE uses some super regulator that can take from 4V to 16V. Wow! An extra cell or two would still improve the amount of current the pack could provide.

xanadu

Just when I thought I knew something about batteries... When Alkaline batteries are specified I thought that meant as opposed to NiMH or NiCads. I had never seen or heard of these or just don't remember.

Thanks Mike that datasheet pretty much explains it all.

Duane, the Prop powers up fine, and then the servos turn about 2 degrees and stop, then repeats. It would take about 5 minutes to move an inch at that rate if that helps you get an idea. The Prop doesn't reboot during this time, although I did just notice that the green power LED on the Protoboard dims which should have been big clue about the batteries. That's a good idea to use caps, but I have a feeling these batteries will end up in an LED flashlight or something.

Now I know why this 20 pack was $6. I usually only use NiMH or LiPo but the charger was full of dead batteries already. Kind of funny that in a pinch I go out and buy more 'dead batteries' lol.

Thanks guys

Duane Degn

xanadu wrote: »

When Alkaline batteries are specified I thought that meant as opposed to NiMH or NiCads.

Oh wow! I just realized what you mean by Carbon Zinc batteries. It's been so long since I've seen or thought about them, the name didn't register in my brain. I didn't think they were still sold since alkalines work so much better.

I don't think they have much of a shelf life so whatever you're going to use them with, you ought to do it soon. At least they were cheap.

There was once a time when almost all batteries were Carbon Zinc. Carbon Zinc batteries reminds me of the free flashlight Radio Shack used to give out around Christmas time (back when I was a teenager). They made their money from selling the 5 D-cell batteries the flashlight required. I sure thought their Christmas catalog was fun to look through back then.