(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.

Re: Backup onboard charger not correct
 

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

Re: Backup onboard charger not correct
 

Must be referring to:

http://www.ifirobotics.com/rc.shtml

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.

Bud

Re: Backup onboard charger not correct
 

Here's the link,
http://www.ifirobotics.com/docs/firs...up-charger.pdf

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.

Re: Backup onboard charger not correct
 

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.

bud

Re: Backup onboard charger not correct
 

Ralph,
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.

Re: Backup onboard charger not correct
 

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.

Thanks.

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

Re: Backup onboard charger not correct
 

Paul,
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.

Re: Backup onboard charger not correct
 

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.

Re: Backup onboard charger not correct
 

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.

Don

Re: Backup onboard charger not correct
 

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.

Phil.

Re: Backup onboard charger not correct
 

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.

Bud

Re: Backup onboard charger not correct
 

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.

Re: Backup onboard charger not correct
 

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.

Re: Backup onboard charger not correct
 

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

http://www.chiefdelphi.com/forums/sh...ad.php?t=42591

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.

http://www.chiefdelphi.com/forums/showthread.php?p=562476&highlight=charger#post5624 76

Thanks.

Re: Backup onboard charger not 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.

Re: Backup onboard charger not correct
 

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.

thanks!

Re: Backup onboard charger not correct
 

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.

Re: Backup onboard charger not correct
 

The general feeling around here about <R56> 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.

Re: Backup onboard charger not correct
 

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.

Re: Backup onboard charger not correct
 

Ryan,
I could see how it might be interpreted that way but the rules simply state the IFI suppied circuit design must be used. I read that (and the inspectors last year were interpreting it) that it is and must remain unmodified.

Re: Backup onboard charger not correct
 

The rules say:

<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. (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.

This rule does not say that it MUST be charged with the IFI circuit. This wording appears to allow an alternate, on-robot charger. We should get an official ruling from USFIRST on this.

Re: Backup onboard charger not correct
 

I have already submitted a Q&A to the official FIRST forums. We should have our answer in a few days.

Re: Backup onboard charger not correct
 

Jerry,
Read that rule as "if you charge it on the robot it must use the IFI charger. If you charge it off the robot then you must use the supplied charger". There is no other alternative.

Re: Backup onboard charger not correct
 

Certainly that is the safest way to read the rule, but USFIRST has in the past been very specific in their choice between the words "may" and "must". If this rule specifically, definitively, was intended to disallow an alternate on-robot charger, is should use the word "must".

A poorly designed charger could lead to overheating of the batteries, so a specific charger design should be used here.

I'll be interested in their comments on this.

Re: Backup onboard charger not correct
 

I might be wrong...
I am assuming they are enforcing the use of the IFI circuit for safety reasons, but if they truely want to be safe, they should include something prebuilt in the kit because there is no guarantee that whoever builds the circuit will build it correctly.

Re: Backup onboard charger not correct
 

We have just built one, and will post some voltage and current measurements in the next few days.

I would not be concerned about overcharging, the current is limited by the resistors, it is only on for a few minutes at a time, and you should only be using this circuit if you have servos & cameras drawing power.

Re: Backup onboard charger not correct
 

We might be getting a new schematic... I can't find it on the IFI web site.

Re: Backup onboard charger not correct
 

Second that. The links in this thread are 404ing.

Re: Backup onboard charger not correct
 

Oh geez... right after I built the old defective one :D.

Re: Backup onboard charger not correct
 

Built mine on monday, didnt have too much trouble with it, oh well, that's life. However I do hope that they manage to produce a new circuit for us, the idea is still great and I hope that they do generate another circuit.

Re: Backup onboard charger not correct
 

does anybody have a working link to the pdf file for the backup charger? the one at http://www.ifirobotics.com/docs/firs...up-charger.pdf seems to be down

Re: Backup onboard charger not correct
 

Keith,
As mentioned above, I agree that IFI may be rethinking the design. Be patient, I am sure they are working on something.

Re: Backup onboard charger not correct
 

whops! I must have skipped past that page by mistake, thanks for pointing me in the right direction.

Re: Backup onboard charger not correct
 

Modified schematic now posted on IFI: http://www.ifirobotics.com/docs/firs...up-charger.pdf

Re: Backup onboard charger not correct
 

It's the same schematic, with the 43 ohm resistor changed from 1 watt to 2 watt. No, wait, there is a difference: the RC is now connected to the output of the regulator. There still isn't anything to keep the backup battery from backfeeding the 12 volt input, though.

There are two cosmetic changes to the print. First, it's got color. Second, it says So I guess we're not going to find out the official theory of operation any time soon.

Re: Backup onboard charger not correct
 

About the updated backup battery charger on IFI website:
http://www.ifirobotics.com/docs/firs...up-charger.pdf
Its better, the RC controller is connected right after the regulator now. The RC is still running off the 8.7V zener instead of the regulator though (although through the 43 ohm resistor). I think R1 could be removed. Let the battery charge off the output of the regulator. The Zener D2 could be left to protect the batery in case the regulator shorts outs. This would also take care of the discharge problem. when 12V is off.

I like how the manual highly recommends it, but IFI now says "use at your own risk".

Brian

Re: Backup onboard charger not correct
 

Ok after reading through this thread I've noted a number of objections to the backup charger design from the EE types. However a few people have actually built and used the circuit with no terrible ill effects. The chief complaint seems to be that when the main power is turned off, the backup feeds back into the 12v circuitry. And if I am understanding correctly, this problem doesn't seem to be taken care of with the new design. Other complaints seem to be concerning the charge rate; in other words the voltage supplied to the backup is barely more than what a fully charged backup battery would produce.

Have I summarized correctly?

So, I am thinking that the puny charge rate is fine, as the only real purpose of this circuit is to reduce draw down of the backup battery when using the camera. Other loads are pretty minimal and of negligable concern during competition.

As for the back feed issue:
I believe the problem can be solved by introducing a switch that disconnects the backup from the RC when main power is removed. Best scenario would be to route the pigtail through a relay that disconnects the backup from the RC when main power is turned off. I don't think this is feasible with a Spike, so next best solution is a manual switch. Orrrr, just unplug the backup as part of the robot power down procedure!

Re: Backup onboard charger not correct
 

Circuit makes more sense now. We have not scrutinized the rules with this in mind, but I think it can and should get power through a breaker & spike.

I have one built, and will power it up and take some measurements soon.

Re: Backup onboard charger not correct
 

There should be enough EE types hanging around here to agree on an alternate backup charger design. Once there is a consensus we can at least offer the design to Ifi - if not for this year, then maybe next.

Attached is a straw design, let the darts fly. I purposely removed the values of most of the resistors to start.

Theory of Operation - abbreviated.

1. output voltage should be set to 8.5V, there will be a .2v(?) drop across the forward biased steering diode D1 presenting 8.3v to battery to be charged.
2. D1 prevents discharge when 12v is disconnected.
3. R1 provides a non-charging current to the backup battery input (100ma)
4. R2 provides charging current to backup battery (45ma)
5. D2 prevents the non-charging current available from R1 from mixing with or contributing to the charging of the backup battery
6. D3 is zener crow-bar to prevent voltage runaway due to the charger failing in some manner
7. D4 shunts any current spikes around LM chip to prevent failure.
8. Change C2 to 1uf
9. R2 and R3 selected per Vout = 1.25v * (1+R3/R2)

Re: Backup onboard charger not correct
 

Bud,
I think you are getting closer. I would make the input cap 500mfd and add a fuse or breaker to the input side. The diodes will have a 0.6 volt drop if you use standard 1n4xxx diodes. Since the output cap is very small the diode across the regulator is not needed but wouldn't hurt. It might be a good idea to make the diode in series with the backup a Schottky to gain an additional 0.2 volts of available power supply. If you add a diode in series with the charging reistor then the series resistor to the load is not needed. That way the diode in series with the battery will only conduct when the regulator drops out or goes below about 6.8 volts.

Re: Backup onboard charger not correct
 

OK, after much speculation (and a couple posts to Q&A) Update #6 clarifies the wording to match the interpretation that if you chose to use a charger on the robot, it must be the IFI provided circuit.

Re: Backup onboard charger not correct
 

Updated circuit as follows:

. selected 0.1% resistor values available from mouser.com that yeilds 8.5v
. use germanium diode (or small power transitor w/Vce of 0.3v) to block reverse current and drop voltage to 8.2v through charging resistor which has a C/16 trickle charge current target (45ma).
. use 1N4xxx diodes to drop voltage to 7.1v (or less, say down to 6.8v min), load current through R1 provides up to ~100ma of helper current if battery backup voltage drops due to extra load or loss of backup battery capacity.
. dropped steering diode between battery and R1 - shouldn't be needed on normal battery operation at 7.2v, but this new circuit will add charging current via R1 as battery voltage drops below 7.1v
. changed .1uf cap to 100uf on input. this is quite large for most regulators, shouldn't have to go bigger -- would need to see the charger circuit input at regulator in use with oscilliscope to determine if there were additional problems that would be addressed by larger capacitor.

Still not happy with diode selection, I'd almost rather swap out the germanium diode with a small power switching transistor that had a typical Vce of 0.3v - it should do the same thing as the diode, namely block discharge current when 12v battery not present.

User could add slow blow fuse into circuit as optional component in front of regulator. Say 250ma fuse. Another optional component would be a switch to disable the whole thing. Another optional component set would be a LED and series resistor at point in front of D1 to ground to show when circuit was active.

Looking for help on D1/D2 selection to achieve design voltages.... or someone to redesign the whole thing. Its easy to get carried away, for example I thought of using a 2nd LM317 in load support branch. It would be in constant current mode at 7.1v. This would current limit that branch more effectively but changes all the resistors and output voltage target of the first LM317. This would replace the D1/D1/R1 components. But goal of the design is to just be a basic/simple on-board charging circuit to prevent loss of voltage from the backup battery due to camera loading. The backup batteries main purpose is simply to prevent a brown out interrupt on the PICs which would result in processor resets.

1N4xxx diodes list Vf as 1.0v, but this is at maximum load. 1N58xx schottky diodes list Vf as .55-.6v range. Just need to get to a 1.4-1.5v drop across a couple diodes or transistors but don't know which would be best choice.

I'm looking to order some components with another order something in next day or two so I can breadboard and try this circuit out/take some measurements.

Re: Backup onboard charger not correct
 

Where are you all buying the 1n5345 8.7V 5W zener? Digikey sell every 5W zener EXCEPT 8.7V :(

Re: Backup onboard charger not correct
 

Re: Backup onboard charger not correct
 

Hooked up the IFI charger circuit tonight and took some measurements.

First with a fully charged 12V battery, no connection to RC, and no backup battery connected:
Vin @ J1 - 13.2v Vcharge @ J2 - 8.7v Vout @ J3 - 8.6v
Circuit draws 102mA from 12v battery

Then I connected a discharged (7.7v, would no longer power camera) back-up battery. Backup battery was charging at 130mA, charging circuit was drawing 140mA from 12v battery.

Re: Backup onboard charger not correct
 

Bud, these circuit improvements that you've made are fantastic. I would recommend presenting your design to IFI, because as it stands you won't be allowed to use it in competition. My guess is that they won't update it again this season, but considering the work that you have obviously put into it, it's worth a try.

Re: Backup onboard charger not correct
 

That sounds pretty high for a 700maHr battery. Did it fall after a few minutes or stay at 130 ma? If it didn't I think you need to change the series resistor and bring it down into the 70-80 ma range.

Re: Backup onboard charger not correct
 

Do we have to built this circuit or can we do it the old way?

Re: (Earlier version of) Backup onboard charger not correct
 

Julie,
You can use the supplied charger. If you want to charge while the battery is on the robot, you must use the IFI charger as drawn on their website.