(Earlier version of) Backup onboard charger not correct

I looked at the (on robot) battery charger circuit dated 1/9/07 on the IFI site.

This Circuit is not correct and could in fact blow some parts… Note the forward biased diode bridging across the regulator, that goes nowhere…

Can we design our own circuits to keep the battery charged ?

I have had big problems with battery life when using the CMUcam and Servos in the past… especially in practice and setup, which are much longer than a game match.

I am not able to find any diagram listed as jan 07 for the charging circuit. Can you post the link?

Must be referring to:


which has a link to:

Battery Charging Circuit (pdf) 1-9-07

Yeah, it doesn’t look right. I’d expect a steering diode in the path from +12v to the backup battery? Right now there is only the zener diode trying to clamp off the 12v while the charge current from main battery is just being limited through the 43ohm 2w resistor. Maybe I’ve had a brain fade and its ok - but it doesn’t look right it to me either.




Here’s the link,

It looks fairly sensible to me, really. The regulator puts 7V on the Vout. The 43 Ohm and 22 Ohm resistors act as a voltage divider between the ~12V battery and this 7V. At 12V, this would keep the output around 8.7V. Then you have the 8.7V reference to clamp the voltage at 8.7V. The diode is, I think, to protect the regulator when the battery voltage is extremely low or non-existent. There’s a constant load of atleast .08A on the 12V battery, but as long as this circuit is after the main switch, it shouldn’t be a problem.

EDIT: Okay, thinking more, D1 seems reversed. But the circuit makes sense to me without it.

My concern would be the main battery gets drained or just disconnected and now the backup battery is directly connected back into the main circuit. True, it can only supply approx. 180ma back into the main +12v circuitry, but I’d really prefer that the backup battery not do that. Or is there something here to prevent that from happening.


That is the schematic from last year and as I have said before, I don’t like the design of this circuit. In analyzing what takes place, the 317 puts out 8.26 volts. V out= 1.25(1+R3/R2) With this in mind, the drop across R4 is about 1 volt with no load on the battery so R4 limits current to under 100ma which is a good charging current for a 700maHr battery pack. However, the 43 ohm resistor R1 will provide some current as well, perhaps more than the battery should be charged at. The zener diode is obviously there for protection should the backup battery be removed and will clamp at 8.7 volts.
If the circuit is intended to only charge the battery, there should be no path for current from the main battery except through the charger. If the intent is to operate the 7.2 volt input from the main battery, then I would use either the zener changed to a 7.2 volt device (and recalculate the value of R1) or set the regulator to 7.2 volts and eliminate the backup battery. If the intent is to perform both tasks then a more complex circuit is required.
The only limit on the backup supplying power to the RC is where this circuit is wired and R4. Since the RC goes to sleep at under 8 volts, there is likely very little current being drawn except by the fans.

I agree, I don’t really like the circuit as well.

Can we modify the circuit? According to Rule <R63>, we are not allowed to alter the power pathway between the battery.

According to Rule <R56>, it states that we are allowed to use the custom charging circuit from IFI. Does that mean that we are ONLY allowed to use the IFI circuit? Can we use an alternative circuit, or modify the circuit (such as putting an extra diode at the 12V terminal to the circuit to prevent the 7.2V battery from driving the 12V electronics when the 12V power is off?)

Any help/clarification would be appreciated.


Paul C. Tan., P.Eng.
Electrical Engineer / Mentor

If you use the IFI charging circuit, then IFI makes it mandatory to use it as it is drawn with no mods. If the RC should fail and they determine the cause relates to backup battery it may void the warranty.

<R56> The 7.2v backup battery may be charged on or off the ROBOT. When off the ROBOT, the battery is to be charged with the provided 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.

There is no other provision beyond these two.

I just built one of these devices, and it was not the easiest thing to get working. Apparently the diode is in the correct direction because it is charging our battery at a measly 10mA, which is not good at all, but I think I was using a bad battery. I had to substitute some of the resistors to other ones of very close value, like the 221 Ohm resistor (just try to find it at Radio Shack :D) I substituted for a 220 Ohm resistor, so nothing major. When hooked to the robot I can run the camera from it without any problem, but if I add the backup battery and turn off the main switch/breaker, the back feed into the robot is horrible. All the spikes stay lit and the controller tries to run. I could not find an 8.7V Zener diode at Radio Shack, so I am going to order one of those. My voltage out hovers around 8.6 to 8.7 volts, and with the camera it drops to 6 volts. With the back feed problem, we may not be able to use the charger and a simple 3A diode that they refer to would work great to keep the back feed from draining the battery.

We also built the circuit last year, and it did work, but we had some odd experiences with how the RC powered down after a match and such, so we discontinued using it.


What we really need here is somone at IFI to post a “Theory of Operation” for the charger.

I’ve read half a dozen “this is how it works” posts today, and none of them (spanning several years) fill me with confidence. It really does seem that a basic series steering doide would have been a good idea to prevent the battery from powering other devices on the 12V bus. So I wonder why they didn’t/haven’t add(ed) one.

It’s unlike FIRST to insist we do something based on “faith”, and that’s the feeling I get with this charger.

Time for IFI to step-up and explain this circuit.
Or at least add some text to the schematic so it’s funtion is clear…

“An un-commented schematic is just as bad as un-commented code.” Me.


I wonder if just adding a switch into the circuit at the 12v + point would violate R56. At least then we’d have a manual method of disabling the circuit rather than physically unwiring it if there were issues we wanted to isolate.

As currently written, it would appear that adding a switch would be a violation of R56 though.


Please, please, IFI, do explain.

I showed the circuit to another fellow Electrical engineer, and we both agree that it is either the MOST brilliant circuit ever designed, and it is using the components in ways we did not think about, or there is indeed something fishy in the circuit. Either way, I am most interested in finding out more about the way the circuit operates.

Paul C. Tan., P.Eng.,
Electrical Engineer / Mentor.

What a relief to find this thread. I thought I was losing it. The circuit doesn’t seem right to me at all. I search on IFI’s forums and didn’t find anything. I’m going to post a question about it there.

There is another thread on the backup charger and I’ve been reading/bouncing back and forth in both.


I posted an attempt at operation theory - not that I know what I’m doing. Lots of questions that I’m hoping folks can help answer/correct.



You can switch the power (12v) with a spike. Then when the robot is shut down, the charger is off. Drain to the backup battery in this case would be through r3/r2, about 5ma.

You can switch the power (12v) with a spike. Then when the robot is shut down, the charger is off. Drain to the backup battery in this case would be through r3/r2, about 5ma.

Excellent idea. I’m sure there is spike already in the mix somewhere that is on if not on all the time, then most the time. The better idea would be a steering diode below the ‘batt’ point in the schematic… but the schematic isn’t likely to change at this point so the spike at least addresses 1/2 the discharge/back power issue with the current charger design.


Where in the rules or on Innovation First does it say that we cannot modify this circuit to work the way we need it to work? I found nothing so we took it as a chance and modified it a bit. Added a steering diode to prevent the back feed, and we are going to add a 1 AMP fuse to it as well.

The general feeling around here about is that the backup battery may only be charged by the circuit from IFI, and not from a custom circuit. I could have sworn there was a Q&A that backed up this interpretation, but I’m not seeing it under section 8.3.

Maybe this will be clarified in Team Update #6 :D. I have 2 things that I’m not sure about, this, and the radio problem.