Let's Make Robots!

Robots charging robots

ByJAX

I've had a recent glut of "good ideas" recently and one might help along my swarm dream. I always wanted a bunch of little robots that could assist each other in some way. The aforementioned idea would take a Joule Thief running off of a fully-charged robot's battery pack and use it to give a dead battery on another robot a quick boost to help it get out of the dark and back to a better lit area (bots would normally recharge in the sun).

For those not into chasing links around the 'net, here's my Ctrl-C/Crtl-V version of what a Joule Thief is:

"'Joule thief' is a nickname for a simple switched-mode power supply that is easy to build and can use nearly all of the energy in a battery for driving light loads, even far beyond the point where other circuits consider the battery "dead". The circuit uses a variant of the blocking oscillator as an unregulated boost mode switching power supply."

For instance, a Joule thief (from hereon refered to as "JT") with only one AA cell with, say, 0.25V, can power the white LED in this cicuit for hours:

 

Another interesting example, and semi-pertanent to this post, is this JT Charger on Make online.

The model I chose uses a ferrite toroid in the transformer design. Some have made JT's with only coiled wire but I choose something more robust. There are a million Instructables and blogs of people who've made them but isn't a lot of data about them that is meaningful (to me). I plan to use 4x1.2V NiMh cells on the robots, probably four from a NiMH 9V with ~350mAh. Maybe button cells, I don't know. Space is key. Anyhow, I know that to charge each 1.2V NiMH cell you need to supply 1.5V+ per cell or recharging just won't happen. To acheive full-charge on the cell (or the whole battery pack when cells are connected in series), you need to be putting in at least C/10 (Capacity of one cell divided by 10) amps into it or it will never reach its peak. I also know that when briefly exposed to a fast charge, the dead cell(s) will act charged for a moment. But just a moment.

 

Robot application notes:

  • Robots will "die" around 90-95% of the full charge voltage. Enough to where extended searching for light could potentially render them helpless but not enough to where they couldn't hold still and signal for help for a meaningful duration of time.
  • The "reviving" robot will do the ever-difficult-to-logistically-figure-out docking maneuvers to charge and switch on the JT for awhile. Enough to impart the memory charge but not enough to drain the responding robot into dying itself.

 

My presumptions are:

  • With a dead 1.5V battery a JT can put out 50V. Voltage is not a question for charging it seems with this circuit.
  • Although there doesn't seem to be any real detriment to going far above the required 1.5V per NiMH cell, I feel less is more in this situation (read, closer to 1.5V the better).

 

My questions are:

  • If I strapped a JT to the underside of a small robot would EMF make it bug out? The circuit uses a basic transformer by my elementary undstanding. Noise is inherent in that beasty, yes? Something about it being common noise, the unshieldable kind?
  • Is there a way to wind the JT to make its output more predictable in terms of voltage/amps?  If 0.25V going in gets >50V coming out, what does 4.2V going in do? Again, my feeling that less is more here. Maybe digging for some hard transformer math would be good for me, but to be honest, I'm hoping one of the Big Brains here knows off the top of their surely enormous head.
  • I'm also hoping to have the PICAXE on the bots to stay powered up while the charging sequence is in process. Is this an unreasonable request?

Sorry if these questions are retarded or if the idea is old news. I think I'm going to show off my electronic inexperience here...

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JAX's picture

For some reason I never considered that option. Like jumping a car, sans alternator. Slow, but I remember it working.  I think I was aiming for a faster completion of the charging but it's just sooo simple. I'll have to try.

What do you mean by things depending on the robot design?

OddBot's picture
If your robot is very efficient then this could work. Considering that a NiMh or NiCd battery maintains close to 1.2V until it is nearly flat I am not sure how much power can be imparted from one battery to another.
OddBot's picture
If your robot is very efficient then this could work. Considering that a NiMh or NiCd battery maintains close to 1.2V until it is nearly flat I am not sure how much power can be imparted from one battery to another.
JAX's picture

Mmm... good point. I definitely don't want to run the batteries down to the point that there's that voltage difference you spoke of. Hmm. I think I might give the supercap thing a try. I have some 4.7F @ 5.5V supercaps. It would be nice if they were more like 6.5V but maybe the 5.5V can shove a meaningful amount of electrons into the pack. Thanks OBwon.

OddBot's picture

Since not everyone knows what a JT is you should explain it better in your question. You will get more responses and better answers. People asking questions in the forums often fail to give enough information about their problem. Schematics should be included.

JAX's picture

Fair enough. I always think everyone knows everything I know and more. Especially on this board holy holy some smart cats on here. I'll cough up more for y'all to work with here is a sec. Thanks for the tip, Odd-san.