Re: Be aware of battery and charger inconsistencies!
 

Your problem is that you are being logical. The Q&A is not always logical.

Re: Be aware of battery and charger inconsistencies!
 

The administrative manual says "Every team should know, understand and follow the safety rules found in the FIRST Safety Manual." That phrasing appears to be advisory (in that it does not create a requirement).

However, If event staff or officials feel that a hazard exists, they'll refer it to the regional director, who can determine the extent of the team's continued participation in the event, based on the hazard present and efforts to resolve it.

In terms of the competition, noncompliance with the safety manual carries no specific penalty, and certainly none related to passing or failing inspection or playing the game. But, if charging the battery in a particular way has rendered a battery unsafe, the inspectors and referees will prohibit the use of that battery in competition.

Re: Be aware of battery and charger inconsistencies!
 

We do take the teams' word on a lot of things; there's no way to inspect every part of each robot and everything in each pit in a reasonable amount of time. We take everyone's word that they didn't start working before build season, that they bagged their robot on time, etc. Part of the inspection form is a signature by both the team captain and a coach stating that they followed the rules; we even take their word that they didn't lie when signing. On all of those things I think the vast, vast, majority of FRC teams tell the truth.

However, we don't take the teams' word for a number of things. That's why spec sheets for non-standard parts are required, for example. Ask in the Q&A; maybe non-standard battery chargers fall into that category.

Good question; I don't know. I do know what I've been told to look for, though. LRIs, while not infallible, do have a lot of room to use their discretion -- or not.

Re: Be aware of battery and charger inconsistencies!
 

Sorry. That was my understanding of how chargers work. I should have done more research. I have edited my post.

Re: Be aware of battery and charger inconsistencies!
 

We're using the KOP chargers. (With anderson outlets of course) Haven't had too many issues with these. We do a 2 amp charge overnight (Treat your batteries right when you can!) and a 6 amp charge during competition time to boost the charge speed, and to 'top off' batteries. (I've noticed that a 2 amp charge leaves about 12.6 volts in the battery, whereas a 6 amp charge gets us to 13.1, even with two year old batteries. Our robots haven't been big power drawers until this year, though.)

Higher voltage (as long as it isn't surface voltage) is always better. Duh. It means less voltage dipping through the match, etc.

But if you're experiencing inconsistent performance based on battery voltage, You have a bad design. You will experience voltage drop throughout the match, during movement, so basically, if your performance is hinging upon a perfectly (not fully) charged battery, you really need to rethink how you've built something. (Though this late in the season... what can you do? Maybe use the battery reading in your programming to do something like an open-loop control, or you can slap on a sensor to do closed-loop control.) (This is my #1 concern with all these motor-powered catapults this year...)

Re: Be aware of battery and charger inconsistencies!
 

Your 25/60 number is close to the 1/2 number I found for converting RCM to an approximate Ahr. Since the RCM is measured at a constant current, regardless of battery, it isn't the same as the Ahr rating though it is close.


The peak currents that a typical FRC drivetrain will draw during acceleration is well off the charts provided by Enersys. Since the design of these batteries were optimized for their storage capacity rather than high discharge rate, their "output resistance" will be rather high, guaranteeing that you will get severe voltage dips.

Choosing your battery charger to give you a consistent state of charge is one way of dealing with the battery variations. As Toa states, a more robust solution would involve making your robots performance insensitive to the variations in your battery's state of charge or output voltage. FIRST has already provided us with an example; the regulated 12V output of the PDB.

Learning to devise strategies that are independent of variables such as your battery voltage is part of the STEM education you should be getting from participating in this program.

Re: Be aware of battery and charger inconsistencies!
 

it's easy to say stuff like this, but in reality it's almost never the case. Having a feedback loop requires you to have overhead in power, which i don't think many if any robots actually have. Not sure how much you have worked with control loops but it's pretty much never perfect. We have tried it, and concluded it was a waste of time for our needs. It was simplest and best to just slam it full power, but as you stated this relies on battery state. Our main issue is our kicker. it's less about being 100% full and more about having a consistent charge so it shoots the same every battery swap we do. with a good battery off these chargers we can accurately and consistently shoot it from the white zone for autonomous, this isn't the case with our old chargers. Sure you're right it's not an optimal design but we don't have a bunch of control system engineers at our disposal and it's hard enough making a robot work to bag much less something this advanced.

Re: Be aware of battery and charger inconsistencies!
 

As an LRI in past years you know that we must follow directions. We were told no battery charger above 6A. Need I say more. You know that I questioned the rule so that it could be enforced and the safety manual was quoted as law. As LRI's we have no choice but to enforce. We will ask that the charger be removed from the pit area.

Re: Be aware of battery and charger inconsistencies!
 

RCM is how many minutes it takes to drain a (fully charged) lead acid battery down to 1.75 volts per cell (10.5 volts for 12V battery) with a constant current of 25 amps.

Thus if you divide RCM by 60 you will get the number of hours it takes, under those test conditions.

If you multiply that number of hours by 25 amps you will get the Ahr rating of the battery, under those test conditions. That's where the (25/60) number came from.

You will get a higher Ahr number if you run a test with a lower test current.

To compare the capacity of an FRC battery to an automotive battery, you have to compare apples to apples. That means running the same test conditions on each battery.

Discharge curves for the NP-18-12 are available in the datasheet. From these curves you can get a rough estimate of the FRC battery RCM rating. It's very roughly 17 minutes. That number can be compared to the RCM of an auto battery to get a rough idea of the relative capacity of the two.

Re: Be aware of battery and charger inconsistencies!
 

By not letting the charger in the pits, you are not enforcing the rule. If a team limits the charger to be under 5.4 Amps, they are not breaking the rules. Ferraris aren't illegal to drive on the street because they have the ability to go over the speed limit. It's up to the driver to limit its speed.

Re: Be aware of battery and charger inconsistencies!
 

This is more akin to the purpose behind electrical rules for robot construction. This is about safety. Can most teams pick and use chargers safely? Yes. If they let teams pick and use any charger they want, will somebody blow up a battery? Also - yes. It is just statistics. It will happen some time and some where. The inspectors stand on this is reasonable and makes common sense.

Re: Be aware of battery and charger inconsistencies!
 

So dremels, grinders, and etc also shouldn't be used either because they have the potential to cause sparks and catch something on fire? Drills and other power tools shouldn't be used either because someone may get injured? this is complete nonsense. A properly used battery charger is no different than a properly used dremel. And so long as all rules are fallowed, safety ones included, there shouldn't be arbitrary limitations. Is there also speed limits for robots? because you know that just isn't safe. Robot's shouldn't be allowed to be powered on in the pit either, because of course you know something might happen and it's a confined space. There is common sense, then there is going to unnecessary and unwarranted extremes.

Re: Be aware of battery and charger inconsistencies!
 

Have you got an answer yet from Q&A?

Re: Be aware of battery and charger inconsistencies!
 

I am going to point you to this section of the Administrative manual

Re: Be aware of battery and charger inconsistencies!
 

Hint: Don't run your mechanisms at 100% power on a full battery, and scale appropriately. There are absolutely ways around this issue.