Battery Chargers
 

This past season we used the 6amp charger on our batteries from andy mark. This gave us up to 13 volts for a full charge but the FRC batteries can go up to 15 max. Our team knew this would happen and knew that our batteries would be destroyed by the end of the season. Well we were right. So over the off season we plan to develop a battery charging cart that will charge the battery at 2 Amps for a while, then 4 Amps, and then 6 Amps to get our batteries at full complicity and keep them living longer. Does anyone have any ideas on how we can make this possible?

Re: Battery Chargers
 

Your batteries were zapped after one season? I think there's a bigger problem here.. We've had 8 batteries now, and after 2 seasons none of them show any signs of slowing up any time soon... Nearly all of them charge up to about 13 every time.. Do you leave them on the charger over night? Was your robot intensely power draining?

Re: Battery Chargers
 

Charging a battery to 15v will not make it last longer, it is the other way around. The 6 amp charger will go green at 13v but will charge up to 14.6 if you leave it to maintain the battery. The best way to charge a battery is by charging the battery at 6 amps then slowly lowering the amperage as it gets closer to a full charge.

If you want to charge to 15v every time get one of these (http://www.hobbyking.com/hobbyking/s...er_w_acc_.html (It dosen't list lead acid charging in the ad but it does have it). It has adjustable endpoints and charge rates.

Re: Battery Chargers
 

This year our robot drew a lot of current and it killed the batteries when we had to keep recharging them constantly. Also i know that 15 volts wont last longer. My point is that we are not managing our batteries right if we arent getting 15 volts. The 6 amps wont get it there because doing a 6 amp charge does not allow a peak and level type of charge that those types of batteries need. You do 2 amp until it peaks then 4 until it peaks then6 to finish charging it's.

Re: Battery Chargers
 

This charger from HobbyKing is about half the price when you factor in shipping weight, and also charges at 6 amps. I have one for charging Li-Po batteries at home, but I brought it to a competition, and it works great for FRC batteries. http://www.hobbyking.com/hobbyking/s...cessories.html

Keep in mind for either one you will still need to find a power supply. Laptop chargers as long as they are >6A and 12V and ATX power supplies both work great.

Re: Battery Chargers
 

This thread may be of use, our team purchases from this company:
http://www.chiefdelphi.com/forums/sh...d.php?t=110031

Re: Battery Chargers
 

I have that charger but it only changers at 50w vs 150w on the other one(Meaning on a 12v battery it will charge up to 4.1 amps on the 50w or 12.5 amps on the 150w) A boat battery makes a great portable power source.

Re: Battery Chargers
 

No it's the other way around. once it peaks you lower the current, lowering the voltage under load allowing you to put more power in. http://batteryuniversity.com/learn/a...d_acid_battery

Re: Battery Chargers
 

Mind you, many other things (current draw on the robot, cycles of use on the robot, physical abuse seen on and off the robot, storage conditions) can also determine how long a battery will last before it's due to retire from the match rotation.

On 2815, our standard procedure is to charge at 2A at home and overnight at competitions, then 6A during the day at competitions to ensure we have enough batteries ready. We also started using a Battery Beak to spot-check battery health; if you don't have one, another team nearby might.

For the capacity of these batteries, 6A until automatic cutoff shouldn't have serious negative effects...but we figure it's better not to tempt fate.

And even if your batteries have weakened to the point that they're not optimal for match use, never underestimate the utility of batteries for powering your electrical board during testing, or for powering an inverter for laptops in the stands. (I've seen a scouting team with a small cart that held a robot battery, an inverter, and a network switch for their laptop-based scouting network. Kinda slick.)

Re: Battery Chargers
 

Something is fishy.

We constantly charge our batteries. We have 10 'competition' batteries, with another couple that we keep to charge the inverter on our cart. We draw plenty of current from the battery, and it's not unheard of for us to see the batteries drop to 6-7 volts when we're driving on our practice field. Nothing you've said leads me to believe that your batteries should be dying.

Our batteries fresh off the charger come in at just a touch over 13 volts, but let sit for a couple minutes for the surface charges to drop, they go down to 12.8 - 12.6. When our batteries are not on the robot, they are always on charge. I question your assertion that the batteries should be at 15 volts. I have never once seen an FRC battery come off a charger at 15 volts, and would worry if they did.

I think we need some more info. Are you running matches on the batteries without fully charging them? Are you overheating them?

Again: these batteries (generally speaking) should not die after a season of FIRST use if handled correctly. We have heavily used batteries from 2008 still going strong, and our last 2007 ones just failed this past year.

Re: Battery Chargers
 

While you can safely charge AGM 12 V batteries at 15 V in certain conditions (smart charger, proper current and relatively low ambient temperature), they won't hold 15 V for long, if at all.

Test them under a small load like the Battery Beak does to get a more reliable voltage measurement. Test them under a moderate load like the CBA tester does to get an idea of voltage performance throughout a full discharge cycle.

Here's some data for the MK batteries, and the old (no longer FRC-legal) Exides.

Also, Al will likely be around shortly to explain more thoroughly why various forms of overcharging are contraindicated.

Re: Battery Chargers
 

Ok thanks guys ill check into these and Billfred the issue is that when you constantly do a direct 6 Amp charge and not 2 and 4 it damages the batteries. The second we take them off the charger they start dropping voltage even when not being used. That is the effect of the constant 6 amp charge.

Re: Battery Chargers
 

Can you answer some more basic questions?

1. How many batteries did you use this season?
2. Did you have a process for use such as a serial number and a cycle schedule?
3. Were you fully charging batteries before taking them off the charger?
4. Were you putting them back on charge immediately after use?
5. Did you Battery Beak them before and after use?
6. Did you maintain their charge through the pre-season?
7. Did you fully charge them before first use?

You easily can kill batteries by the method you described if you are not cycling (keeping an even usage) and charging them appropriately.

I'm paging our battery experts. Hopefully they will chime in soon.

Re: Battery Chargers
 

What voltage are you discharging your batteries down to? I suspect you are over-discharging the batteries and causing damage.

Charging the batteries to 15V will not kill them, 12V Lead Acids today are designed for this. Also, charging the batteries at 2A, will not do anything for you over a standard 6A charge.

Other things which will kill your battery life:
- Storage in hot areas
- Letting your batteries sit uncharged for months at a time
- Charging/Draining batteries while they are hot or cold (est. >90 & <40)

Re: Battery Chargers
 

Guys,
There are a few things about batteries you need to know. A battery might show something above 12 volts (no load) when it comes off the charger due to a condition known as 'surface charge'. This voltage will drain back down to the 12 volts as soon as you start to draw current from the battery. That is why no load battery testing is inconclusive when checking the state of charge or the condition of the battery. Charging SLA/AGM cells requires that a voltage higher than the cell voltage be forced on the battery. The higher the voltage, the more charge current is pushed into the battery to reverse the chemical reaction that takes place inside. The manufacturers state that currents in excess of six amps will damage the battery. However, the smart chargers provided in the KOP and equivalents will back down from the six amp maximum as needed during the charge cycle. So setting the charger at 6 amps does not mean it will provide six amps during the entire charge cycle. Any additional current in excess of what can be used to reverse the chemical reaction will generate heat. In our batteries this excess heat can cause boiling of the electrolyte within the cell, deformation of cell plates, leaking of the battery and explosion in rare cases. That is why inspectors will ask you to remove battery chargers that are in excess of six amps and are not smart chargers. We regularly will find teams trying to use 50 amp car chargers in the pits. These are extremely dangerous for use in the pit and will guarantee early battery failure with no warning. While the KOP charger does allow current selection below 6 amps (i.e.4 and 2 amps) selecting these lower settings will not measurably extend the life of the battery but if you have the time to wait, the lower current may be a more efficient charge method.
The best method of checking battery state of charge is under a controlled load. The Battery Beak is an excellent simple charge tester that you can carry in your pocket. For really accurate checks that mimic the manufacturer's specification, use a West Mountain Radio CBAIII.

Re: Battery Chargers
 

Stinglikebee

1. 6 Batteries
2. Yes
3. Yes
4. Yes
5. Yes
6. Yes
7. No

Re: Battery Chargers
 

Adciv
They aren't reaching 15v like we want. It goes to 13 for a charge and the second we take them off they drop to 12.6-12.8v and after 1 match they are below 12v. When we get low on voltage we drop to around 6-7v when the compressor runs.

Re: Battery Chargers
 

Here is a picture that we drew out in paint to try to demonstrate how the batteries are supposed to be charged for maximum performance.

Re: Battery Chargers
 

I'd like to know where your charge curve came from. It is very different from any I've seen before. The typical one I've seen is on the link below (scroll down for lead-acid).
http://www.maximintegrated.com/app-n...dex.mvp/id/680

Re: Battery Chargers
 

It charges fine when I set it to 12v/6A. It just gets a little warm, that's all.

Re: Battery Chargers
 

did you check the actual amperage on the screen. You can input up to 6A but it will only go up to 50 watts as it is the lower limit.

Re: Battery Chargers
 

Again your charge graph is wrong. The higher the charge current the higher the battery voltage. if you want to put the most amount of power in while staying at the same voltage you lower the charge current which lowers the battery voltage. This allows you to put more power in while staying below cut off voltage.

To test this check your battery voltage while it is charging at 6A then check it again right after unplugging it from the charger. The voltage will drop proportionally to the charge current.

Re: Battery Chargers
 

This is absolutely standard behavior for the batteries we use (and pretty much all 12 volt lead acid batteries).

All batteries all drop below 12 after a full match. If you are running a compressor, it will draw down the available voltage, as will any load placed on the battery. Batteries will drop when a heavy load is placed on them. With a completely charged battery, running 4 cims at stall or near it (say under full acceleration) will drop a fully charged battery down under 10 volts temporarily. Once that load is removed, it will return back up near it's previous point.

To me, it sounds like you're expecting some type of different behavior out of the battery, but everything I've read so far suggests these batteries are just fine.

Could you further clarify what you mean when you say your batteries have gone bad? Using them in the manner you are describing: charging to a surface charge of 13 (true charge of 12.6-12.8) then running a match, then recharging should absolutely not hurt your battery at all.

Indeed, if you read the specifications for the batteries we use (available at AndyMark to save you time) you'll see that the maximum charging voltage is 15 volts. However, once your battery is fully charged (amperage going into the battery is no longer increasing), your charger will drop to a floating voltage charge of 13.5 to 13.8 volts to keep the battery topped up.

These batteries will not read measure 15 volts acrossed the terminals when you are done charging them.

Re: Battery Chargers
 

I was seriously hoping Big Al would come in this thread.

Re: Battery Chargers
 

Guys,
I am afraid you are confusing things a little because of the common terms. In SLA/AGM batteries, in order to force charge current to flow, the external power supply must have a supply voltage higher than the "cell voltage". So when you are measuring a battery that is connected to a charger and the charger is supplying current, you are measuring the supply voltage not the battery voltage. You must remember that the battery has an internal resistance and that you are measuring the battery cell voltage plus the voltage across the resistance. The voltage drop across the internal resistance will increase as the charge current goes up. However, there are limits on the conversion of that current into reversing the chemical reaction within the battery. No matter how much current you attempt to force into the battery, that current which is in excess of what can be used for charging is lost as heat in the internal resistance of the battery. Once fully charged, all excess current is converted to heat.
So when charging at 2 amps, the supply voltage is set by the charger to produce 2 amps of charge current, to start. This will fully charge the battery, it will just take longer and produce less heat. To pick an arbitrary value, say you measure 12.8 volts on the power supply at 2 amps. At 6 amps, that voltage will increase to perhaps 14.5 volts, the battery will still fully charge, but in less time than the 2 amp setting. However, the increased current will also generate increased heat in the internal resistance. The internal resistance is 0.011 ohms. It is a simple matter to make a calculation. At 2 amps, 0.044 watts will be dissipated in the internal resistance. At 6 amps close to 0.4 watts will be dissipated. The remainder will be used in the charge chemical reaction (which also produces some heat). If we used a constant current charger then at the time that the chemical reaction is finished, all power will be dissipated by the internal resistance of the battery or nearly 72 watts. Thankfully, SLA chargers do not use a constant current charge method and most smart chargers never will charge at the rated current for their entire charge cycle. Please keep in mind that different battery chemistry have different cell voltages. Our batteries are very close to 2 volt/cell. Your car battery is more like 2.3 volts per cell so it will show 13.8 volts when fully charged. Alkalines are 1.5 volts per cell, NiCad is 1.2 and lithium is 3 volts per cell.
As I explained before there is always the "surface charge" that will confuse your readings if you remove the battery from charger and then immediately measure the terminal voltage. It is likely to measure as much as 14 or 15 volts but it is a lie. Leave the battery for a while or connect it to a load and the terminal voltage will fall back to 12 volts. Make that an excessive load like six motor drives and the terminal voltage will fall well below 12 volts, again because of the voltage drop across the internal resistance. The Battery Beak switches in different loads to make a quick calculation for terminal voltage, capacity, and internal resistance. The CBAIII simply charts the terminal voltage vs. time while connected to a dynamic load. The resulting chart will mimic the discharge graphs supplied by the manufacturers and will make Amp Hour calaculation based on the data.

Re: Battery Chargers
 

Al, what is the ideal charge rate for these batteries? The data sheet for the ES17-12 says no more than 5.4 amps. How much of a risk are we at by using the 6A initial charge rate? I've read somewhere that the issue of gas buildup really only happens when you overcharge, and that higher charge rates shouldn't hurt it much at all (even up to C/2) provided you don't overcharge. I ask this because I've been tossing the idea around of building a (ridiculously overcomplex) battery cart using my own charging circuit (using a lead-acid battery management IC of course...I'm not going to screw around with the charging profiles). However, I do have to tell it the max charge rate. What are your thoughts?

Re: Battery Chargers
 

Mike,
The manufacturer's spec is based on constant voltage dumb charging, so 5.4 amps is something they are comfortable with. Since the KOP charger is a smart device it uses typical lead acid techniques like delta V or some other profile. This where the charger is pumping current periodically and then testing the the battery without charge current. It compares the level of the battery now compared to previous samples and adjust the current (voltage) accordingly. The only improvement on this method would be to monitor internal temperature as part of the charge cycle. As the battery nears and reaches full charge the delta V levels off and actually starts to descend at full charge. At this point most chargers will stop charging or move into a maintenance charge cycle that is similar to trickle charging.
Something to keep in mind is that dumb chargers are usually unfiltered, rectified line voltage so the current meter is actually only reading average current but the peaks are much higher. Although the manufacturer that I spoke with a few years back was reluctant to agree, it is my thought that excessive charging currents (which require higher voltage to produce) may actually cause arcing between the closely spaced plates in AGM cells. If this is actually occurring, then the damaged plates can have reduced area and therefore reduced capacity or in rare cases, the plates may short internally. We know that some teams have experienced shorted cells, but in many cases that I have examined, the case showed signs of rough handling. In some cases there was no external damage visible so I have to conclude either a defect in manufacture or an arc internal to the cell. In a few of these cases, a charger defect was discovered. As you investigate charging circuits you will discover that all of them have some power device that feeds the charge current to the battery. Often these are similar devices to the MOSFETs you would find in a Victor or Jaguar. If that device should short on failure, you would deliver full power supply voltage to the battery under charge. I can't tell you that this was the case in the battery failures that have been reported simply because there are so few failures.
As to gas generation, high charge currents produce heat in the electrolyte in the glass mat. If this temperature is high enough, the electrolyte will start to boil. Also due to the electrical action some of the water will actually disassemble into separate hydrogen and oxygen atoms. Combine high temps, boiling electrolyte and hydrogen formation and the internal pressure of the battery will cause some venting through the vent plugs in each cell. In our batteries this is nothing to worry about as the gas production is relatively small and confined to the battery in most cases. As charging tapers off, the gases recombine and are reabsorbed by the glass mat.

Re: Battery Chargers
 

I am trying to say that it is safe and good to do a 6 Amp charge but not to start at directly. You want to start at 2 amp and escalate your charge to 6 amps because doing a constant 6 amp charge damages the batteries. My team along with other teams have tested this and the batteries that go from 2 to 4 to 6 amps last a lot longer than the constant 6 amp batteries. The voltage is also higher on the 2-6 amp charged batteries. I'm just asking for ideas that we can use to get a charger or build on that you can switch from 2 to 4 to 6 amps when you want.

Re: Battery Chargers
 

Devin,
There are so many variables in battery life. The greatest reduction in our battery use comes not from over charging but the high demands of discharge during a match. While our batteries are capable of extreme currents (600+ fully charged) they are really designed for discharge life of 1/10 C or about 2 amps. I have not read any articles that relate the stepped charge you are proposing to extended battery life. It won't hurt, I just don't think it will help that much value. You might consider reverse engineering the KOP charger to see what they do for the different charge rates and come up with a switch to perform that function. The reason the KOP charger has the different positions is to allow charging batteries of lower capacity than the KOP batteries

Re: Battery Chargers
 

Ok we will start putting together ideas and we dont have a huge load on our batteries during the match.

Re: Battery Chargers
 

Devin,
This is a common misconception. A CIM motor has a stall current of 130 amps. This is what it draws when ever you start moving. A four motor drive puts a momentary 400+ amp load on the battery. Anytime the robot runs into something or your driver changes direction or if your design uses tank with four wheels on the ground, your battery is seeing a big load. You can't help it.