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falconmaster
16-01-2008, 18:47
Hello all,

Does anyone know where you connect the 12 input for the backup battery charger that FIRST put out the schematics for? If you connect to the breaker panel the battery tries to run the robot electronics. Do we connect it on the battery side? Help!
falconmaster
16-01-2008, 18:53
Help!
kE7JLM
16-01-2008, 19:54
I would guess it goes on the breaker side, because FIRST would want everything off when the breaker is off.
jee7s
16-01-2008, 20:16
The 12 V input to the charger circuit should be attached to a small (20A) breaker from one of the breaker panels. (According to my reading of the wiring rules.)

The output of the circuit goes to both the RC backup input and the backup battery. When you connect this, you'll probably splice into the backup battery connector cable, or attach the backup battery connector cable to the charger board, then the charger board to the RC.

-JEE
falconmaster
16-01-2008, 22:39
When I connect it like you said, the backup battery tries to light up the motor speed controllers and spikes. This would drain the battery when you turn the robot off. Has anyone out there actually done this that could help us?
Alan Anderson
16-01-2008, 23:47
The IFI-provided circuit has the small issue of powering the entire robot from the backup battery. A simple addition will fix it: just put another 1N5820TR diode in series with the power input. That will prevent the circuit from back-feeding the breaker panel.
jee7s
17-01-2008, 01:06
Hmm...Al's on the money. It makes perfect sense both that the charger would try to power the robot, and that the diode will fix it.

I was basing my comments on the schmatic. My team has never used on board charging before. We'll be sure to include this modification.

-JEE
Tom Line
17-01-2008, 07:16
Yep. We built the circuit last year, and then chucked it out. You're much better off going to a hobby store and spending $25 on a high capacity 7.2 volt pack for RC cars and a nice charger actually comes with it. Once we did so, we never once had a backup battery issue, even when running with the same battery all day (with camera).
SuperBK
17-01-2008, 10:53
We also put a diode on the 12V power into the backup charger to prevent the backup battery from trying to power the electronics. Did that last year and plan to do it again.
mneary
17-01-2008, 18:28
It would seem that FIRST should modify the official schematic... where do we send the suggestion?
d235j
20-01-2008, 13:11
It would seem that FIRST should modify the official schematic... where do we send the suggestion?
A suggestion should be sent to Innovation First (as they provided the original schematic).
organman42
20-01-2008, 13:21
I heard that there was a new circuit for this year's backup battery charger. Is this true? If so, where can I get the schematic? (I have the one from last year already) Thanks
RyanN
20-01-2008, 13:24
If, and when, a new circuit is posted, it will end up here: http://ifirobotics.com/rc.shtml

Please read the thread before you post, because you should have noticed that there has not been an update to the circuit yet...
eeeeweeezeee
22-01-2008, 00:44
yep, the circut is more than a little flawed. using about 1/3 of the parts in the schematic, I made a small variable power supply today that I am a lot more happy with. now, i just charge that battery however I want (within the margins of ni-cad common sense) and I can also use the simple circuit to power various other devices from any DC source between the desired end voltage and 30 something volts. to make the LM317T into a variable power supply you add a 240ohm resistor in between the Vout and ground pins and then connect the ground side of that to a 5k potentiometer if I remember right. look up "lm317t" in Google and you will see lots of simple power supplies like this.
Al Skierkiewicz
22-01-2008, 08:11
yep, the circut is more than a little flawed. using about 1/3 of the parts in the schematic, I made a small variable power supply today that I am a lot more happy with. now, i just charge that battery however I want (within the margins of ni-cad common sense)

Please note my emphasis in the following rule...
<R49> The 7.2v backup battery may be charged on or off the ROBOT. When off the ROBOT, the battery is to be charged with a 7.2V backup battery charger. When mounted on the ROBOT, the backup battery may be charged from the 12VDC primary battery by using the custom charging circuit available from Innovation First Inc. or any similar charging circuit (note: IFI will provide the design for this circuit on the IFI website, however teams must obtain the parts for this circuit and assemble it themselves). The use of this circuit is strongly encouraged.

NiCad battery technology requires some method of limiting charge current to prevent battery failure. Don't fall into the trap that you need to charge the battery on the robot from the main battery. A few extra battery packs and the IFI wall charger will get you through any competition.
PhilBot
22-01-2008, 10:25
Yep. We built the circuit last year, and then chucked it out. You're much better off going to a hobby store and spending $25 on a high capacity 7.2 volt pack for RC cars and a nice charger actually comes with it. Once we did so, we never once had a backup battery issue, even when running with the same battery all day (with camera).

Even with a better battery, there's still the issue of someone remembering to change it periodically. The longer it lasts, the longer between changes... so kids tend to forget.

Having it charged on-board means never having to remember to change it. Granted, without the camera this year, the batteries should last longer....

We built a PCB for the charge circuit and got 3 boards made (just the boards) for $60.

I attached the ExpressPCB scematic and PCB files on this post: http://www.chiefdelphi.com/forums/showpost.php?p=669666&postcount=27

We'll be building a new one this year....
eeeeweeezeee
22-01-2008, 12:04
NiCad battery technology requires some method of limiting charge current to prevent battery failure.

you're right, and I forgot to mention it but I also have a very large 300ohm variable resistor just for this. we already have plenty of backup battery packs and chargers, but every year I am disappointed by the chargers given out in the kit of parts. with at least two of them, their internal components have failed catastrophically. the exact description is the charging indicator turned a color its not supposed to, and there was a muffled THUD! and the light went out. I don't quite trust the ones provided by IFI so if I can make one myself and it fails, at least it is part of my learning experience as a high school student.