Dual Battery with Wall Plug

Hi guys,
I've got this robot I'm building:
http://www.bioloid.info/tiki/tiki-index.php?page=BrainEngineering+BrainBot
This robot is eventually going to be going back to the Brain Engineering Lab for them to use, and I have one electrical problem I haven't been able to solve yet.
It has two NiMh batteries, one powering the bus, and the other powering the electronics (gumstix plus cameras).
I have a crossover cable I put together that I use for powering both sides at once, from a wall power supply (the bioloid kit comes with a 12 volt, 5 amp power brick).
http://www.bioloid.info/CrossoverCable.jpg
They want to be able to "hot-swap" batteries. It takes minutes for the gumstix to boot up (its running Linux), and I want them to be able to run all day without ever having to shut it down.
I came up with a technique they could use to do that, and at the same time this technique will allow them to run the robot off an external power supply when they are working on the bench and don't feel like running down their batteries.
Basically, I would include a second set of power plugs, hooked up to the existing power plugs on the board. They could plug in the crossover cable, which effectively hooks the two battery packs up in parallel. They would then remove one pack, plug it into the crossover cable socket, and then remove the second pack. The robot is running off the first pack they removed at that point. Then they can plug in the two fresh packs, and unplug the crossover and they're ready to roll.
I think that should work fine. The problem comes in when they want to run it on the bench. I can tell them until I'm blue in the face to remove the battery packs before you run it off a wall supply, but you know and I know eventually that isn't going to happen, and they're going to have a 5 amp supply plugged in with both batteries also plugged in (in parallel, because of the crossover cable).
I'm assuming that's going to cause problems for the batteries... What can I do that will allow me to have the crossover cable in place, the 5 amp power supply plugged in, and the batteries also plugged in?
Thanks, Jon
-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com http://www.huv.com/jon
Project: Micro Raptor (Small Biped Velociraptor Robot) http://www.huv.com/blog
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Jon Hylands wrote:

Jon, If you isolate the batteries with diodes, it will protect them. Each power connector gets a diode on the + line, with the cathodes of the diodes tied together and feeding your electronics. This will cost you about 0.7 Volts at the electronics, but will prevent the wall supply from charging the battery packs and will prevent shorts on an unused power connector (that is protected by a diode) from causing problems. If 0.7 volts is too much to live with, a schottky diode will reduce that to about 0.4 V.
Good Luck Bob
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On Sat, 12 May 2007 09:40:16 -0700, BobH

Bob,
Thanks, I appreciate the advice. Just to be clear, under normal operation, the batteries are not tied together (except on the ground side), so I'm assuming I would just put the diode between the + terminal of the connector and the rest of the + trace that goes to the circuit, one for each battery?
Thanks, Jon
-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com http://www.huv.com/jon
Project: Micro Raptor (Small Biped Velociraptor Robot) http://www.huv.com/blog
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Jon Hylands wrote:

Hi Jon, I am not clear on where the wall power supply ties in. I looked at the photos of the board and the Y cable, but did not see it. There are a couple of things that the diodes could do for you.
You had mentioned one configuration with two battery packs in parallel. Paralleling batteries is a mixed proposition. Yes you get extra capacity, but if the charge state of one pack is substantially different when you connect them, the pack with the higher charge will discharge into the other pack until the voltages are equal. There may also be a temperature related thing here too, but I am not sure how severe. Adding diodes separating the battery packs will prevent this equalization. In normal use, the packs will equalize their voltages because the pack with a higher voltage will supply current to the load until the voltages are equal, then both will share the load. The reason that this is preferable is that charging is not 100% efficient. Does it exceed the power lost by the voltage drop across the diode? I think so, but in all honesty, I would have to measure it to be sure.
Protecting the batteries from overcharging by leaving them connected in parallel with the wall supply.
If the wall supply hooks up to the + power rail on the PC board, putting the diodes on the board with the anode connected to the connector + and the cathode to the + power rail will solve that. If the wall supply hooks up on something similar to your crossover cable, the diodes will have to go in the cable.
By the way, that 'bot is a really nice looking piece of work, I am impressed!
Regards, Bob
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On Sun, 13 May 2007 08:21:03 -0700, BobH

Ahh, yes, I understand now... So, what I need to do is make the crossover cable plugs be "downstream" from the diodes, so the diodes protect the batteries from the crossover cable. Makes perfect sense.

Thanks, I appreciate it. I should have some decent video of the thing walking around by the beginning of June...
Later, Jon
-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com http://www.huv.com/jon
Project: Micro Raptor (Small Biped Velociraptor Robot) http://www.huv.com/blog
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