Power Changeover Circuit Ideas?

Greetings.
I am putting together a stationary power supply (plugs into a wall socket) for my robot. I plan to use this instead of the on-board batteries when the robot
is powered up for extended periods of time. The supply will connect to the robot through an umbilical cord, so I obviously do not plan on the robot physically moving during these periods.
I would like to be able to switch between the batteries and stationary supply while the robot is powered up. One idea I had was to use a DPDT relay or switch to toggle between the two, but I need a way to supply current while the contacts are flying. I wondered if just a big capacitor wired in series with a resistor to limit the inrush current would do the trick.
The batteries (4 x 12V SLA) and stationery supply output 48V. The DC-DC converters that supply the various voltages on the robot are rated from 36 to 72V, so they can take a fair amount of input sag without cutting out. The robot typically draws less than 1A at 48V.
I am fairly new at this, so I suspect that there are better solutions out there. Any ideas?
Jeff.
P.S. Until I get the "Delete Sender" button working on my email program, replace the references to spam with "bucky" to email me directly.
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Jeff Shirley
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Jeff,
I have been working on a robot with two power sources as well, a battery and a hydrogen fuel cell. I do exactly as you suggest, and it works fine. I use a 2200uF cap to keep the Stamp lit up during the switch over.
Jonathan

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How about a diode in series with each power source? Adjust the stationary supply so its output voltage is slightly higher than the maximum battery voltage. The diodes will cause the power to be drawn from whatever source is higher. When the power supply is disconnected, the batteries are higher and supply the load. When the supply is connected, it is higher so it takes the load. (You could also design a circuit to charge the batteries while connected to the external source.)
One advantage of this method is that you never really disconnect the batteries, so if you have a power dip or something while plugged in, your robot doesn't reboot itself, it just runs on the batteries until the power comes back.
John Kasunich
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Hi John.
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I like it. Any pointers for a charger design (48V)?

Good point. I live in California, and you know how reliable the power is here. ;^)
Jeff.
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Jeff Shirley
snipped-for-privacy@mindspring.com
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I'm not a charger/battery expert by any means. Chargers can be very simple, or very complex. It depends on the battery chemistry and the performance you want/need. The simplest charger is a single resistor. Assume for instance that you have a 48V battery and a 52V external supply. A 1K resistor from the supply terminal to the battery terminal (before the diodes) will deliver 4mA ((52-48V)/1K) to the batteries. This is at best a trickle charge, and would take forever to actually charge the battery. A lower resistor could increase the charge current and shorten the charge time, but with a serious risk of overcharging and damaging the batteries. More sophisticated chargers use special time/voltage/current profiles to charge the batteries quickly but without overcharge or other damage.
John
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I have found a fair amount of information on charging SLA batteries. Methods include constant voltage, constant current, and varying both voltage and current for fast versus float charging. I modified my external supply to output 57.6V (2.4V/cell), which seems to be a maximum based on my reading.
I have a microcontroller reserved for monitoring battery condition, so my idea is to use one of the PWM outputs to modulate the charging through a MOSFET and a choke. That way I can implement the charging strategy in software.
Jeff.
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Jeff Shirley
snipped-for-privacy@mindspring.com
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Sounds good. One caveat - provide some kind of current sense and shutdown in hardware for your FET. If the code burps and accidently leaves the PWM output full ON, it would only take milliseconds for the current to ramp up high enough to pop the FET or do other damage. What you describe is basically a switching power supply, and unless you have lots of experience in this area, a pure software control is likely to let the smoke out of lots of parts before you get it working correctly.
John Kasunich