Dual Battery with Wall Plug

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Jon Hylands's Avatar (by Gravatar)Jon Hylands posted on
May 12, 2007, 8:45 am
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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      Jon@huv.com      http://www.huv.com/jon

  Project: Micro Raptor (Small Biped Velociraptor Robot)
           http://www.huv.com/blog

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BobH's Avatar (by Gravatar)BobH posted on
May 12, 2007, 12:40 pm
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Re: Dual Battery with Wall Plug


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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Jon Hylands's Avatar (by Gravatar)Jon Hylands posted on
May 12, 2007, 1:34 pm
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Re: Dual Battery with Wall Plug

On Sat, 12 May 2007 09:40:16 -0700, BobH


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      Jon@huv.com      http://www.huv.com/jon

  Project: Micro Raptor (Small Biped Velociraptor Robot)
           http://www.huv.com/blog

BobH's Avatar (by Gravatar)BobH posted on
May 13, 2007, 11:21 am
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Re: Dual Battery with Wall Plug


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


Jon Hylands's Avatar (by Gravatar)Jon Hylands posted on
May 13, 2007, 6:51 pm
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Re: Dual Battery with Wall Plug

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      Jon@huv.com      http://www.huv.com/jon

  Project: Micro Raptor (Small Biped Velociraptor Robot)
           http://www.huv.com/blog

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