Battery Chargers

[slijin]

So it's that time of year again - we're coming up to kickoff and we're frantically scrambling to make sure our labs are ready for the onslaught of activity. One of the things we're doing for this time-honored tradition is getting some new battery chargers.

So before I start discussing my options, I just want to make sure that I understand the recommended specs for FRC battery chargers. My understanding from experience and the spec sheets for the ES17-12 and NP18-12 tells me that battery chargers should (must) meet the following specs:

• Nominal output voltage between 14.4V and 15.0V
• Nominal output current between 4A and 6A, despite specified max charge currents
• Capable of charging 12V sealed lead acid AGM/gel batteries

Do correct me if I'm wrong.

That being said, there are the two obvious options: the Schumacher SC-600A and AM's AM-0026. I believe I've also seen CTEKs around.

While doing some browsing, I also came across this option, which says that it can support up to 6 batteries at once, and the spec sheet says that they have to be charged in parallel from the charger's single output - can anyone offer some insight into how exactly this works and whether it's a viable/safe option for us?

Alternatively, does anyone else have any other recommendations? (besides FIRST Choice).

Thanks!

[Ether]

Do we know that both FRC batteries are indeed AGM and not gel? It's not specified in either of the datasheets.

[slijin]

Originally Posted by Ether

Do we know that both FRC batteries are indeed AGM and not gel? It's not specified in either of the datasheets.

I recall seeing a post from Al way back when mentioning that both batteries are AGM, although I couldn't find it when I tried searching, so I could be wrong there.

I stand corrected. The ES17-12 is a AGM, the NP18-12 a gel.

[Wetzel]

We have the three-bank charger from AndyMark, it has worked well for us. I much prefer the SB50 connectors to the clips.

Wetzel

[Al Skierkiewicz]

Quick correction, last years rules specified the NP18-12 which is also an AGM design. For most types of lead acid batteries the charger is a voltage type. This means that the charger adjusts the output voltage as a method of controlling the charge current. An 8 amp charger uses a higher output voltage than a six amp charger. The Enersys data sheet recommends..."• Cyclic use: Apply constant voltage charging at 2.40-2.50 VPC. Initial charging current should be set at less than 0.25CA." Stated another way, 17.2AH * 0.25 = 4.3 amps and 2.5v * 6 cells is 15 volts. It is my opinion that 6 amp smart chargers will not stay at the full 6 amp charge currents for very long when charging these batteries making 6 amp chargers OK for our use. In discussion with MK engineers several years ago, they felt that higher charger currents (and the implied higher charge voltages) might cause internal cell arcing between the plates. This might lead to permanent damage to the cell(s) in the form of a internal short. There are a variety of factors that play into this so testing at higher charge voltages would not reveal consistent results but the specified max charging current is what MK felt was safe. For our purposes, most teams using 4-6 amp smart chargers have very little problems in charging or battery life. The critical factor for our use is the deep discharge and current demand we make on these batteries during competition. Under normal FRC usage, I tell teams to expect no more than about 400 charge/discharge cycles before the battery can no longer supply current for competition matches. If your robot is regularly depleting the battery in one match, life expectancy is even less. About 80 % of the failed batteries I see at competition are due to mishandling (dropping).
The charger shown above can be used but I would not recommend (nor was it legal in 2012) to charge at 8 amps.

[Tristan Lall]

Originally Posted by slijin

I recall seeing a post from Al way back when mentioning that both batteries are AGM, although I couldn't find it when I tried searching, so I could be wrong there.

I stand corrected. The ES17-12 is a AGM, the NS18-12 a gel.

Just to be clear, according to the datasheet, the NP18-12 is AGM. Check the 2013 rules (once released) to determine whether the NS18-12 is legal. (Are you sure that NS18-12 is even a valid part number?)

[Michael Hill]

Another related question...how many years do other teams use the same battery? It seems like after just a couple years of competitions, we've got some batteries that are bad. It can't help charging at such a high charge current during the days of competition (we have maybe 8 batteries total on our team?)

[Michael Hill]

And by "bad", I'm referring to the fact that for a couple of our 3 year old batteries have an internal resistance double that of one of last year's batteries. I assume oxidation buildup on the plates.

[MichaelBick]

Many teams get new competition batteries every year.

[alb4h]

How many batteries do your teams take to competition?
Ann

[dcarr]

Originally Posted by alb4h

How many batteries do your teams take to competition?
Ann

We typically cycle them after every match. Around 8 total.

[Gregor]

Originally Posted by dcarr

We typically cycle them after every match. Around 8 total.

We have around 9. We also cycle them every match.

Keep in mind every rookie will get 2 batteries, and every veteran will get 1.

[dcarr]

Originally Posted by Gregor

Keep in mind every rookie will get 2 batteries, and every veteran will get 1.

Going on precedent, but without the 2013 KoP list we don't actually know for sure...

[Al Skierkiewicz]

Do not try to charge batteries in parallel please. I asked this question of MK and The Battery Association of America (I think that was the name) and both responded that the only way that was acceptable was if the batteries were all purchased at the same time, had been charged the same and had the same loads applied over their lifetime. And then it may work or it may damage all of the batteries. In other words if the batteries were assembled into a larger array and only used that way could other methods of charge and discharge be used. These batteries are not like the large arrays used on electric subs or in large UPSs where the acid is checked regularly and specific gravity is monitored cell by cell.
The best way to test batteries is to use an analyzer like the CBA III from West Mountain Radio. when they can no longer supply more than 15 amp hour, I would move them into the practice pile. When they fall to 10-12 amp hour, then it is time to recycle. If one cell dies before the others, recycle immediately, that cell is likely damaged internally. The Battery Beak from Cross the Road Electronics should also indicate when to remove the battery from competition.

[slijin]

Originally Posted by Tristan Lall

(Are you sure that NS18-12 is even a valid part number?)

Originally Posted by Al Skierkiewicz

Quick correction, last years rules specified the NP18-12 which is also an AGM design.

Typo on my behalf, I was referring to the NP18-12. Sorry!

Originally Posted by Al Skierkiewicz

The charger shown above can be used but I would not recommend (nor was it legal in 2012) to charge at 8 amps.

We won't be getting the BatteryMINDER, but if we had my intent had been to run it at 4A, not 8A. I understand and see the reasoning behind your perspective that 6A is a purely nominal maximum setting, but I still can't help but be mildly uncomfortable charging batteries rated for max charge currents of 5.1A and 4.3A at "6A" (let alone 8A!)

Originally Posted by Joe Ross

The problem with charging batteries in parallel from a single output is that each battery is in slightly different condition and state of charge. They try to work around this by giving the following conditions in the manual. I don't trust that you could ever guarantee this, so I don't think it's safe.

Additionally, notice that this is only to maintain the batteries, once they are fully charged. It won't charge 6 batteries.

Originally Posted by Al Skierkiewicz

Do not try to charge batteries in parallel please. I asked this question of MK and The Battery Association of America (I think that was the name) and both responded that the only way that was acceptable was if the batteries were all purchased at the same time, had been charged the same and had the same loads applied over their lifetime. And then it may work or it may damage all of the batteries. In other words if the batteries were assembled into a larger array and only used that way could other methods of charge and discharge be used. These batteries are not like the large arrays used on electric subs or in large UPSs where the acid is checked regularly and specific gravity is monitored cell by cell.

I thought as much - I was rather skeptical of the method when I noticed it, because I just couldn't see charging them in parallel as a safe method, given that batteries often tend to differ from unit to unit. Thanks for confirming.

Originally Posted by Michael Hill

Another related question...how many years do other teams use the same battery? It seems like after just a couple years of competitions, we've got some batteries that are bad. It can't help charging at such a high charge current during the days of competition (we have maybe 8 batteries total on our team?)

We buy a new batch of batteries every year, since most of our robots tend to err towards ridiculous power consumption. Our competition batteries tend to consist of mostly ones we buy that year, supplemented by ones from the previous two years that have good max charge records (we made a simple battery tester in-house that displays the battery voltage measured across a small load on an LCD screen; a "good" max charge record, with reference to our usage, tends to mean that it consistently charges up to 12.5V or higher).

Originally Posted by alb4h

How many batteries do your teams take to competition?
Ann

We bring 8 with us. During qualification matches, we'll often have as many as 3 at full charge (the rest are either charging, in the robot for practice/testing/matches, or being used for auxiliary 120VAC through an inverter), but we make sure every battery is fully charged (or as many as possible) going into elims, because of the rapid turnaround time between matches.

Thanks to everyone for the outpour of feedback

I'm still curious if anyone has found success with alternatives to the Schumacher and Dual Pro. Speaking of which, it seems that Dual Pro makes an IS3 along the RS3, but I can't seem to identify a difference between the two...