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?

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?

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 (Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking (It dosen’t list lead acid charging in the ad but it does have it). It has adjustable endpoints and charge rates.

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.

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.

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.

This thread may be of use, our team purchases from this company:

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.

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. BU-403: Charging Lead Acid - Battery University

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.)

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.

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.

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.

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.

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)

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.


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

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.

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.

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).

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