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):