Custom Chargers for backup battery
 

This should be a quick and easy question.

Is it legal to build one's own custom circuitry to charge the backup battery from the main 12v battery system during the competition? FIRST distributes a rudimentary charging system schematic, but my specific question is whether or not it is legal to charge the backup battery during competition with a custom-designed battery charger.

Any insight into this issue would be greatly appreciated. I know that many teams are having difficulty with the longevity of the battery life of the backup battery. This should be one way to help out. If it is determined that it is legal, and our if team can get our design to work, I will be sure to try to post the schematics on this thread.

Re: Custom Chargers for backup battery
 

Michael,
<R53> 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 EX18-12 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 rule book is your friend. I suggest everyone download the manual to your laptop or shop computer. Acrobat reader has a nice search feature that helps me in finding answers quickly.

Re: Custom Chargers for backup battery
 

The rule on the subject of charging the backup battery:

<R53> 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 EX18-12 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)

It seems to say that you have to use one of the methods supplied by FIRST. However, it is not exactly clear. Try Q&A for a definitive answer.

And for the record, I don't think we've ever had any longevity problems. We could run most of the day on one backup battery, partly because it was rarely used.

Re: Custom Chargers for backup battery
 

I was referring specifically to circuitry that was DIFFERENT from the circuit that was provided by FIRST. I have read the rule book, but felt that its position on custom circuitry for the charging of the battery was a bit vague at best. I may have to get clarification directly from FIRST on this.

Re: Custom Chargers for backup battery
 

I have been unable to post at the FIRST question and answer forum. Is there no one that knows whether or not we are allowed to use custom cicuitry to charge the backup battery during competition?

Re: Custom Chargers for backup battery
 

Your issue is with the word "may", right? As in "[w]hen mounted on the robot, the backup battery may be charged" with the IFI device, vs. "[w]hen off the robot, the battery is to be charged" with the supplied charger. This is definitely one for the Q&A, because the intent of the rule-writer is non-obvious. Highlight (very explicitly) their choice of wording, so that the Q&A moderators don't give you a boilerplate RTFM response. In terms of posting to the Q&A forum, your team's main contact should have received login information by e-mail; ask them for the password to the account FRC135, or have them post on your behalf. If your main contact is not available, or has lost the information, ask here for another team to post the question for you.

Incidentally, here's the charger circuit from IFI (which you assemble).

Re: Custom Chargers for backup battery
 

The issue with the word "may" does not bring into the fact of using another board than the IFI one. The word "may" is meant as you may or may not charge from the 12V battery. If you chose yes you move on in the sentence told be told you are to use the IFI board.

Re: Custom Chargers for backup battery
 

I concur with Steve. There is a rule written telling you how you're allowed to charge the backup batt. If it's on the robot, you may charge it or not. If you choose to charge it, you're allowed to use the IFI circuit. All of the rules concerning the electrical system are very strict to keep robots from turning into short-lived arc welders. I'm pretty sure you should just assume that if they don't explictly allow something, then it's illegal.

Re: Custom Chargers for backup battery
 

I know from personal experience that the CMUcam will chew through a backup battery in about an hour. I'm not sure if the servos run off it, or if the CMUcam is actually eating up all of that juice, but the camera is almost unusable after an hour of twitching around tracking. I don't know about you, but I'm changing the backup battery out after each match.

JBotAlan

Re: Custom Chargers for backup battery
 

I seem to remember the spec is 200 ma, which is a lot for a little battery pack.

Re: Custom Chargers for backup battery
 

i cannot seem to find this whitepaper or w/e on ifirobotics.com.
can anyone give me a direct link?
this is about the backup battery chargeing while on the robot.
thanks

Re: Custom Chargers for backup battery
 

It's on the main page of ifirobotics.com under the "news or announcements" section and links to: http://www.ifirobotics.com/docs/firs...up-charger.pdf

Re: Custom Chargers for backup battery
 

If its only drawing 200mA, you should be able to get more than an hour off one of the IFI packs. Even with a 1A discharge, we usually get about 600-650mAh out of the packs, which is pretty good considered I believe they are rated at 700mAh.

Re: Custom Chargers for backup battery
 

The servo motors will run off the backup battery wether plugged into the brain, or the camera. This may be why it does not seem to last very long.

Re: Custom Chargers for backup battery
 

If you plug the camera from TTL to TTL serial port the camera will draw power from 5v channel on the RC which uses a 12v source.

Re: Custom Chargers for backup battery
 

Hate to bring up old conversation...

However at the Kokomo Workshops recently some members of other teams and i got into a discussion about wether custom charging circuits were legal.

We never really decided. However we did find that many teams had custom charging circuits on there robots and got through inspection with out a problem.

I just wanted to repost and remind every (incase i forget), that this should definitly be a question to ask this year... As it would be very beneficial to many teams.

Something a little off subject... but still partially related. I myself worked on a custom charging circuit and i do have to say that i learned a lot more by doing that then simply taking IFI's design... I think that we should see more custom circuitry in FIRST. Because right now the way it is you learn the very very very basics of DC electrical wiring. Unless you get into custom circuits you don't learn many things about electrical circuits and electricity as you should.

Re: Custom Chargers for backup battery
 

Larry,
No other charging circuits were allowed under the robot rules this year. Although I didn't like the IFI design, teams who wanted the charger feature on the robot for the backup battery had to use the IFI circuit. I recommend purchasing an additional backup battery and a valid charger for off robot charging.
Inspectors can't find everything so it is possible that robots made it through with chargers of their own designs.

Re: Custom Chargers for backup battery
 

You know, I'm still of two minds regarding this rule. And not just because I see it as being of limited utility.

Where it says "[w]hen off the robot, the battery is to be charged with the provided 7.2V backup battery charger", it specifies that if off-robot, a certain charger is the only one permitted. But when it says "[w]hen mounted on the robot, the backup battery may be charged from the EX18-12 primary battery by using the custom charging circuit available from Innovation First Inc.", it specifies that when on-robot, the battery may be charged by that method, but doesn't specifically preclude another charger, as it would have if "may only" were substituted for "may" in that sentence. The implication is that this does not specify the IFI charger to the exclusion of all else, but rather that you may choose between the IFI charger or an unidentified alternative. (Steve and I argued about this during the season, and I believe that we agreed to disagree....) That would mean, that as long as the battery is "on" the robot (secured with Velcro, perhaps), it could be charged in any manner (including, obviously, the IFI charger). Maybe (indeed, probably) they meant something else, but that's not what was written, and I don't remember a Q&A response clarifying that intention.

As for the usefulness of the rule, it's not as if we're charging potentially-explosive lithium-ion batteries here; the Ni-Cd battery is a robust and relatively safe type. I'm not sure we need this rule, as written. Maybe it would be enough to require any COTS charger rated for use with the appropriate battery type and voltage.

Re: Custom Chargers for backup battery
 

Tris,
I believe the "may" in the language is meant that you may or may not charge the battery in the robot. If you choose to charge in the robot then the IFI charger is to be used. The issue I have with the on robot charger is that a charger with no designed in safeguards will alllow the full charge of the main battery to be dumped into the backup battery. This is a dangerous condition as all additional current will be turned into heat at the backup battery. The other problem (the one that is more likely) is that the charger will allow a sneak path for the main battery to keep circuits on the robot energized at the full potential and current of the main battery or by the backup battery alone. In either case, it gives the team a false sense of security thinking that both batteries are fully charged when they are not.

Re: Custom Chargers for backup battery
 

Isn't that the first "may" in the rule? I'm considering the second one (i.e. the ninth word of the third sentence).

My point is simply that they failed to restrict alternatives (in the case of the on-robot charger) by only describing what's allowed, and not saying what's disallowed.

And of course, I'm pretty sure that they intended for teams to use only the IFI charge circuit for on-robot charging. It's just that in order to say that, they should have repeated "is to be" in place of "may", or qualified it with "only". Sure, it's lawyerlike to insist upon this point, but it's also crucial to consider all the possibilities when interpreting an engineering specification.

By the way, I should qualify what I said above about COTS chargers; I meant for that to refer to off-robot charging only....

Re: Custom Chargers for backup battery
 

Well regardless of whether or not it was that way last year. It should be asked in Q and A this year.

I was taking it as you may use their design or one of your own. The reason being under any of the other rules relating to custom circuits it does not limit charging the backup battery.

Despite the way it was meant to be i seen teams with the IFI Circuit, and Teams without it, but still charging the battery on the bot, but there were also the teams that didn't use either.

Either way i think IFI needs to modify there circuit design there are a few problems with it. The design we used also had a few problems, which is why this year depending on the rules we hope to make a newly designed safer charger.

And i still stick by what i said earlier, in FIRST we need more circuit experience and we need to do more advanced electrical stuff. I mean after all FIRST is about learning and inspiring others to learn.

Re: Custom Chargers for backup battery
 

From "The Robot Rev F"

<R53> 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)

Please note: You may charge on or off the robot. If on the robot you must use the IFI charger connected to the main battery. If off the robot you must use the provided battery charger. There are no other possibilities listed.

Re: Custom Chargers for backup battery
 

Al, you may want to double check your qoute....

The very last updated file which is revision F http://www2.usfirst.org/2006comp/Man...obot_Rev_F.pdf

says: "<R53> 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 other part is not mentioned, however you could have meant it to not be part of it and i not realized it.

Re: Custom Chargers for backup battery
 

Larry,
That is copied and pasted and is identical to yours. The "please note" is my editorial.

Re: Custom Chargers for backup battery
 

lol, ok, i thought of that after i made the post... lol, thats why i edited it. I figured you probably added that as your own

Re: Custom Chargers for backup battery
 

I contacted FIRST and they finally replied... this is what the rule was intended to mean

Hello Larry -

If the team is going to use the main battery to charge the backup battery then you can only use the schematics provided by IFI to design and build the circuit.

Thank you for your question.
FRC OPS SUPPORT

Re: Custom Chargers for backup battery
 

except that may not even be the case/and or option this comming season......

Re: Custom Chargers for backup battery
 

True... I still go with the prediction that something is changing in the control system...

I wonder if they will provide a charer in the KOP.... lol, doubt it

Re: Custom Chargers for backup battery
 

So, you run two backup batteries and swap them every match, keeping the spare on the charger between matches. Prior to the competition you load test the batteries to make sure that neither is laying down on you. You should also consider changing out the connector on the battery to the powerpole unit used on model airplanes. The connector that comes on the battery can develop intermitent contacts and this can lead to spurious RC controller resets.

http://www.powerwerx.com/product.asp...015&CtgID=3014

Eugene

Re: Custom Chargers for backup battery
 

Lol, well i think if that is the ruling for 2007 i'll stick with the IFI schematic and at least have some kind of charger on the robot. But i will probably wire a switch so i can turn the charger on or off then i could have a spot where i could test the battery without removing it... IDK, i am already thinking of my teams electrical board and i don't even know whats all going to be on it :rolleyes:

Re: Custom Chargers for backup battery
 

Larry et al,
As I have said in this thread and elsewhere, I am not happy with the design of the IFI charge circuit. I would rather have a charger that indicates a fully charged battery and takes up zero weight and size on the robot. The only way to get all of those items is to take the time to take the battery off the robot and stick it in a charger in the pit. Easy, safe, and sure that the battery is charged. I like the idea of conversion to power poles as well. (hates Molex)
On another note from an experienced inspector...When mounting the backup battery, don't velcro to the top of the RC. As convenient as that is it blocks the tally lights on the RC from inspection during on-field troubleshooting by IFI and field personnel.
As a suggestion to IFI, if there is going to be a change in RC design, I would allow the main battery to feed the servo power through steering diodes so that the backup has less current draw during normal match operation. This would allow the backup to feed camera servos and normal battery fail circuitry while allowing the main battery to power everything else during the period in which the robot is turned on during matches.

Re: Custom Chargers for backup battery
 

You mean make the back-up battery just a *gasp* back-up battery?

Re: Custom Chargers for backup battery
 

So this year it seems that a custom circuit is legal going by the phrase at the end which says "The use of this circuit is strongly encouraged."

I am still going to ask on Q & A when it gets up and going

Re: Custom Chargers for backup battery
 

We have built a few of the circuts according the diagram they do put out a nice voltage for charging. I will be adding one to last years robot to test for the backfeed i've read about.

We found it VERY diffucult to find all the parts, we needed to order from an online source who had high minimum order, thus we have a large quantity of extra parts.

For teams looking for a kit of required parts we have kits avaliable for a nominal fee. Please see link in my Sig for more info..

Re: Custom Chargers for backup battery
 

We will be posting a schematic of our charger circuit on www.frcsoft.com once we get it built and tested and stuff.

Re: Custom Chargers for backup battery
 

Do the backup battery chargers provided in the KIT have overcharge prevention?

Is it possible, for just PRACTICE/TEST purposes, to drive the 5V form the D I/O or the Analog straight to the backup battery connection slot so that we can get 5V out of the PWM pins? I don't see problem with this since there is not voltage regulators on the PWM lines(I know its custom circuitry but the backup battery circuity has just as much chance of failure, since individuals build it) The backup batteries are a pain when you are trying to play with the camera.

Re: Custom Chargers for backup battery
 

That's a good question. The chargers we received in past years have an indicator light to show full charge; this year's does not. I left a battery plugged in to this year's charger overnight, and the battery got very warm. I am going to use last year's backup battery charger instead.

Re: Custom Chargers for backup battery
 

Larry,
You may only use the supplied charger or the IFI charger for the backup battery.

<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.

Re: Custom Chargers for backup battery
 

OK, it has been resolved that if you are going ot charge the BU battery on the robot, you must use the IFI circuit design.

Could someone please post the circuit theory on that circuit?

Looking it over, it seems like it might waste more power than it provides to the BU battery.

Re: Custom Chargers for backup battery
 

You quoted it yourself "The use of this circuit is strongly encouraged." therefore meaning you do not HAVE to use the IFI design.

I will double check on that and post a question in the Q & A.

Re: Custom Chargers for backup battery
 

I believe the intent was that you use their circuit in lieu of not having an onboard charger. It seems very unlikely they'd let you connect any "non-official" circuitry to either of the power systems in such a manner, a mistake could result in dangerous damage to the backup battery and it's not easy for an inspector to look through every circuit to make sure its legal. I believe the correct interpretation is that you have an option of using the IFI circuit, or not using one, and they recommend you use it.

Re: Custom Chargers for backup battery
 

well, i see what your saying, which i agree partially, but the same thing is true for any custom circuit

Re: Custom Chargers for backup battery
 

Larry,
You need to read the rule again. You must use the IFI charger design. And it is "strongly encouraged" that use an onboard charger.

Re: Custom Chargers for backup battery
 

I'd like to take a stab at the theory of operations of the ifi supplied charger. Some things make sense... others leave me confused. Hopefully others can help correct and fill-in the pieces.

The LM317 regulator changes its output so as to maintain a 1.25v drop across R2 (220). Since the same current through R2 goes through R3,
Vout = 1.25 * (1+(R3/R2)) or about 8.3v. So far so good I think. R4 (22) limits the charge current that can be provided to the 7.2v battery. When the backup battery is near full, this would be ~45ma. This supply will be fairly constant as the regulator will hold Vout at 8.3v independent of input supply, as long as Vin is > ~9.5V

R1 (43) seems to be able to provide a constant charge current of ~110-130ma based upon the voltage differential of a 12v battery and the 7.2v battery independent of the regulated LM output. If or when the battery voltage drops to 10.8v - like under heavy discharge of a running robot - current available via R3 will drop down to about 80ma.

This is one of the things that is confusing... why go through the trouble of a regulated supply if the simple resistor is capable of supplying upwards of 2x or more than the regulated circuit is capable of?

So there is a total current available of ~150-170ma draw from the 7.2v circuit as shown?

Why D1? I'd be tempted to replace this with a LED until competition to show when the 7.2V is in discharge. That is, if you disconnect the 12v battery the regulator circuit input supply voltage will be back fed through R1. In addition, D1 will bypass R4 and 7.2V will be applied to (R2+R3). This equates to an approximately 5ma current constant discharge. So, if you are like us who routinely "safe" the robot by unplugging the main battery but don't disconnect the backup, then we'll end up draining/discharging the 7.2v battery through this charging circuit overnight. A small red led where D1 is would provide a visual reminder that this is occuring.

Many regulator circuits worry about sudden current discharges and transient spikes delivered back into the output pin of the regulator and put a protection diode across the input/output pins. D1 in essense is across the pins but in the reverse direction of what is needed so the regulator isn't protected. I guess if you burn out the chip you might not notice because you still have R1 delivering 100ma+ charging current?

Ok, pull the backup battery and what happens. In an open circuit with a load <~80ma through R1, the 'batt' point floats above 8.7V and the zener diode draws current to clamp it at 8.7V. The zener could see current spikes up to ~180ma as it does this.

I'm sure I have some of this incorrect and would like feedback.

thanks,
bud

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

Re: Custom Chargers for backup battery
 

Sketched up a parts layout for the charger tonight: (be sure to double check before using):

Re: Custom Chargers for backup battery
 

For me, D1 is the real fly in the ointment. It's hard for me to tell what it is doing to the current coming directly from the 12V battery.

If the regulator voltage is at 8.3V and the battery is below this, then I guess D1 is reverse biased, and has no effect. The only way D1 comes into play is if you disconnect the backup battery.... If this happens it shunts the 12V to the regulator output... which is innecessary since the D2 would shunt it to ground anyway.

IMHO D1 would be much better served protecting the backup battery from being discharged when the 12V breaker is off.

Phil.

Re: Custom Chargers for backup battery
 

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

I've posted a layout at: http://www.chiefdelphi.com/forums/sh...ad.php?t=52541

Re: Custom Chargers for backup battery
 

This circuit is improved!
But I feel there should be a current steering diode in the path from the 12v main battery to the backup battery. Without that, when the main is shut off, there is still current leakage back to all of the 12v circuitry.

Ray M.
Team 1138

Re: Custom Chargers for backup battery
 

Your link didn't come out... Here it is in full.

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

Yeah, the addition of that connector makes a HUGE difference!!!!

And now that I've seen this schematic... it looks familar, as if I've seen it before. Maybe we've all been pointing at an old schematic.

Phil.

Re: Custom Chargers for backup battery
 

I agree that an additional diode would help this circuit and prevent back feeding the rest of the robot.
The diode in the schematic is a Schottky diode with a 0.4 volt drop instead of the 0.6 volt drop of normal diodes.
What we all need to realize is the 317 regulator will provide all the power needed by the 7.2 volt buss (at 8.5 volts) until the main battery falls to about 9.5 voltsor less. When this occurs, the output of the regulator will fall and when the output is less than about 7 volts, the backup battery will begin to supply current to the backup buss at 0.4 volts less the terminal voltage of the 7.2 volt battery. The resistor in parallel with the diode will also pass some current in each direction. The effect is that servos will not be running on 6.8 volts or less so teams should not be surprised if there servo operated devices run slower at lower main battery voltage.
There is still no substitute for a fully chargd backup battery working alone.

Re: Custom Chargers for backup battery
 

So this might belong in a discussion of off robot charging, but I'll put here too. Last year we used VEX bats and VEX intelligent charger for our backup bat on FRC robot. Looks like we can still use the bats, but maybe not the charger, which seems far superior in regard to life expectancy of backup batteries.

If we are really forced to use the supplied wall wart to charge our backup bat, we probably won't risk ruining a VEX bat in the process. Seems like a step backwards - and I'm pretty sure DLavery wouldn't use the wall wart on his second car - (on Mars). :)

Re: Custom Chargers for backup battery
 

Now that we have actually built this circuit, and placed it on our machine, I am more convinced that it needs an input current steering diode.
When the main power breaker is turned off, the RC, Victors ,and such stay alive. Within a few seconds, the RC starts to indicate low voltage, but keeps on going.

Is this really what IFI had in mind?

Ray
Team 1138 Eagle Engineering

Re: Custom Chargers for backup battery
 

Ray, connect it through a spike. Power the robot off, the spike disconnects.

Can you put an ammeter on the backup battery and measure the charge rate? I get about 100mA even with a fully charged backup battery. If we use the charger, I will probably use a timer to turn the spike on and off to prevent overcharging.

Even without the charger, our RC will stay energized after the main breaker is turned off, until you hit the reset button.

Re: Custom Chargers for backup battery
 

We built a couple (one for a spare) and measured ~98ma charge current with a fully charged backup battery (~7.4v). Disconnecting the 12v and the RC we measured a ~5-6ma discharge current back through the charger circuit. We haven't pulled the 43ohm resistor, but the next one we build we leave out this resistor to measure how much charge is coming through the regulator and how much is unregulated through the 43ohm resistor.

Bud

Re: Custom Chargers for backup battery
 

That is a design feature. The IFI manual says it will shutdown in about four minutes if you forget.

Re: Custom Chargers for backup battery
 

The manual describes the behavior with the 2004 master code. In 2005, the timeout changed to something more like ten seconds.

Re: Custom Chargers for backup battery
 

This from the RC reference guide 6/13/06
The Robot Controller cannot start running from Battery Backup power only. It requires Main Power connected at the required voltage to initially power on. If Main is lost and a link with an OI connected to an Arena Controller had been established, the RC will continue to operate with only Battery Backup power for approximately 4 minutes before it automatically turns itself off. If Main Power is lost under any other condition, the RC will continue to operate with only Battery Backup power for approximately 4 seconds or until the RESET button is pressed. This is to ensure maximum Backup Battery life.

Re: Custom Chargers for backup battery
 

Hey, One question is anyone making the backup battery custom charger for sale??
just wondering!
natalia

Re: Custom Chargers for backup battery
 

Actually, if you're attending the San Deigo or LA Reagonal our team, 1388 will be handing out the boards and materals for them (some assembly required) and you can make them for your bot.