Back-up battery charging circuit from IFI

[Don Wright]

After a fun weekend with the back-up batteries, we were told about the back-up battery charger on the ifi website that charges the back-up battery from the robot's main battery.

Has anybody made this circuit and if so, what are your thoughts, suggestions, etc. We want to make one in the fix-it-window before the Detroit Regional, but any lessons learned, advice, or whatever is appreciated.

Thanks.

Link:
http://www.ifirobotics.com/docs/firs...up-charger.pdf

[Eldarion]

We learned that the 8.2V zener is absolutely necessary.

Luckily we tested the output with a multimeter before putting it on the 'bot...

[The Lucas]

The backup battery should be charged initially at bot startup or your camera will have problems.

[Don Wright]

The backup battery should be charged initially at bot startup or your camera will have problems.

It will be. But we just want it so that the back-up is actually what it is called, a back-up battery for powering the RC when the main battery dies.

We just want the charging circuit so that the camera servos and other servos don't drain our back-up batteries during testing and a match.

[Matt Krass]

We used it, the only problem we had was a slight trickle through of power to the RC when the breaker was open and the backup battery connected, it wasn't even really a problem, somewhat amusing is all. I don't think we ever had to remove our backup battery and everything worked great for us, it gets a thumbs up from me.

[Don Wright]

Anybody else?

The circuit looks a little "interesting" to myself and some co-workers, so I'm looking for some experiences of it working.

We are probably going to try it anyway...

[X-Istence]

At my school we used the Vex backup batteries, which are more powerful, and last longer. Also they can be quick charged using a standard RC car charger. We had three we rotated, and it worked perfectly .

[kaszeta]

Originally Posted by Don Wright

Anybody else?

The circuit looks a little "interesting" to myself and some co-workers, so I'm looking for some experiences of it working.

We are probably going to try it anyway...

I've got one that I wired up, but in the haste to ship it never got onto the bot. It seems to work fine in bench tests.

[Barry Craig]

Originally Posted by Matt Krass

We used it, the only problem we had was a slight trickle through of power to the RC when the breaker was open and the backup battery connected, it wasn't even really a problem, somewhat amusing is all. I don't think we ever had to remove our backup battery and everything worked great for us, it gets a thumbs up from me.

My team had the same thing happen. We also discovered that if you push the robot for a few yards while the breaker is tripped and even when the Anderson connectors are unplugged, the same thing happens.

[Cody Carey]

Originally Posted by Barry Craig

My team had the same thing happen. We also discovered that if you push the robot for a few yards while the breaker is tripped and even when the Anderson connectors are unplugged, the same thing happens.

The reason that happens is because the motors are acting as generators, powering your breaker... I wonder,

If you push your robot too fast, could you burn anything out?

[coldabert]

One recommendation I would make after seeing the schematic is to mount the LM317T (regulator) to a heat sink or piece of copper. This is the unit handling the power dissipation and it may get too hot.

And of course you could blow something by pushing your bot. Voltage is voltage, and current is current (i wont say resistance is resistance because negative resistance gets tricky) But, the motors would have to spin faster than they do with the power on.

[Mark McLeod]

Originally Posted by coldabert

One recommendation I would make after seeing the schematic is to mount the LM317T (regulator) to a heat sink or piece of copper. This is the unit handling the power dissipation and it may get too hot.

We originally thought that might be the case, but after testing the completed circuit it turned out heat dissipation isn't a problem at all. A plastic housing will work fine.

[DonRotolo]

Originally Posted by coldabert

One recommendation I would make after seeing the schematic is to mount the LM317T (regulator) to a heat sink or piece of copper.

Note that the tab of the LM317T is electrically hot, meaning it has output voltage (and not ground!) on it. It needs to be electrically isolated from everything else.

We built the circuit, and saw the same issues as others: The backup battery 'powering' the bot, making for some anxiety but no real issues. We used it for testing, but took it off for competition.

Anyone wants a PC Board etching pattern, PM me.

Don

[X-Istence]

My team wanted to reduce the amount of extra electrical components on the robot, so we used the Vex batteries which are also 7.2 Nickel Cad, which are allowed under rule R68 (IIRC), which allows any replacement backup battery. You get longer runs with them, and charging them is the same as charging RC batteries, so they can be quick charged in 15 minutes, so with two there should be almost no downtime.

They do add a bit more weight, but it is negligible compared to what you get in return, which is extra power for the camera and servo's as well as anything else that gets it's power from the RC through the backup battery.

Edit: Seems I had already told my story, sorry for the double posting

[Alex G]

Originally Posted by Matt Krass

We used it, the only problem we had was a slight trickle through of power to the RC when the breaker was open and the backup battery connected, it wasn't even really a problem, somewhat amusing is all. I don't think we ever had to remove our backup battery and everything worked great for us, it gets a thumbs up from me.

There are two problems with the schematic from IFI. The first one is that if the 8.7V zenner comes off or breaks, you risk supplying a full 12V to your RC and backup battery.
ANother problem is that of the main battery is dosconnected, the backup battery will end up "replacing" the main battery. It will supply power to the motors through the 43 ohm resistor. At 7.2 V, the power dissipation inside that resistor is about 1.20W. You will end up frying the resistor.
My recommandation is that you add a diode(any diode that can take over 1A will work)before the output.Disconnect the "+" ouptut, and connect the anode of the diode to it. Connect the cathode to the wire you just disconnected. Or just build a new circuit with the extra diode. This will prevent power from the backup leaking back if the main power drops too much, or the main battery is disconnected. However, this modification will not protect the backup if the 8.7V zenner fails.