# Battery Charging Box

<R19> The 12v Battery may only be charged by a 6 Ampere rated battery charger between matches. When
recharging Kit batteries, you may use the charger provided by FIRST or one with equivalent charging
current.

Should I interperet this to mean we can use any type of charger we wish as long as each battery is charged at a rate of 6 amps or less?

Sure, any 12V, 6A or less charger will work. I remember seeing one team with a device that charged 3 batteries at once. If you build a box to charge it in, I would recommend charging the battery upright rather than on its side (I think this gives a better charge because more of the lead is in contact with the acid). If your box is going to enclose the battery and charge, you might want to ventilate it. Good Luck

Well the question was asked in hope we could use say a 10 Amp charger. Using electronics to limit charging to 6 Amps or less to each battery.

Do you want to charge two batteries in parallel? If so I think it would be legal as long as you limited each output to 5 or 6 amps.

Yes, parallel for the most part. With the appropraite circutry to isolate each battery and provide resistance if only one battery is connected.

hooking two batteries in parallel is very dangeous. There would be nothing to limit the current from one to the other, and a charged battery could easily source a hundred amps into a dead one. When electronics fail they often fail as a short circuit - so your current limiting circuit could become an inferno.

you are really better off using one charger per battery, because the charger senses the voltage across its terminals, and will limit itself to the battery with the higher charge

besides, if you have two or three 6 amp charges, and one fails, you are not dead in the water - if your custom muliti bat charger fails, or if FIRST says ‘Hey you cant us that’ you will be in deep weeds.

in fact, the more I think of it, Im pretty sure FIRST would not allow any custom battery chargers in the pit area - if it fails the battery could explode - imagine the resulting lawsuit when 20 or 30 people are sprayed with hot sulfuric acid and molten lead! :ahh:

BTW - I assume you are not talking about charging the little backup battery this way too - you put 6 amps into that and you wont get out one chorus of ‘pop goes the weasle’ before it blows.

People had been creating battery charging devices long before FIRST made the ruling last year prohobiting them last year. Many teams have them, and I have yet to see a fire or explosion. Anyone want to share some schematics?

I planned on hooking the 7.2V battery in parallel with the 12V Exide to keep it highly charged

DON’T DO THIS! If you do it, please be far away from me in the pits. I hate the smell of burning electrolyte and have never very much cared for shrapnel. As a few informed people have said on these fori in the past, FIRST means just what the rules state. Charge a single battery with a single charger at no more than 6 amps or the battery will be damaged and people might get hurt. Think about the consequences of connecting the 7.2 volt battery in parallel with the 12 volt battery. Even if the 7.2 volt battery was charged the potential difference is 12-7.2=4.8 volts. This voltage will drop across the internal resistance of each of the batteries and assuming that the NiCad resistance is very small then 4.8v/.011 ohms=436 amps Is that enough to scare you? If the gods are smilling on you, the NiCad will short and the wires feeding it will go incandescent for a few seconds before they burn open, showering the pit with burning copper and vinyl insulation.
As Ken has stated above, under no circumstance charge multiple batteries in parallel unless the charging system was intentionally designed for that duty. Chargers of that type are usually prohibitively expensive for teams to own.

from an engineering perspective, you can buy 6 amp battery chargers off the shelf that have been extensively tested, that have built in safety features, and are UL approved - and they will charge the batteries as fast as is possible without damaging them

if you build your own custom charger, you cannot charge the batteries any faster, you will take away limited time from other (robot) work you could be doing instead and you will have a much greater risk of system failure - either not charging the batteries or cooking them to some degree

So as an engineer I would say, why custom design a functional system that you can buy commercially off the shelf as a commidity item, when all you have to gain is risk?

I would recommend spending your valuable time on some other design project that will make your team more competitive, or helpfull to other teams at the event - something that you cant buy at Wallmart because you havent invented it yet :c)

Can I ask a question related to battery charing? Why is it that most battery chargers I see usually require a power supply to make it work.

Are you asking about charging circuits you have seen? Most commercial chargers are stand alone boxes.

It was a joke, hence the wink.

The real reason for making it is to reduce clutter and organize the pit. I guess I could make a box to house the 6 amp FIRST supplied chargers. I thouht more than a handful of teams had battery charging boxes, but I guess not.

Mike,
Sorry to sound so strong in the reply but safety is a big issue for us. If I missed the smiley (and I rarely look at the graphics) I thought some other teams might think it was OK to do. You can’t believe what I have seen other teams do as far as electrical goes. FIRST also has intervened and that is why the strict new rules and electrical parts this year, in my opinion.

Yeah, I agree with this being my 5th year in FIRST I have helped fix my share of troublesome wiring. I think the new rules will help simplify inspecting the robots. Only disadvantage i can see is running a thicker wire up an arm or similar. Oh well, still for the better, kudos to FIRST.

At one time, our team had a cable called a high-octane cable. This cable, in theory, allowed us to charge one battery from multiple chargers. Needless to say, its use has been discontinued.

We are still using the battery charger we got in the 2001 season - our 6A charger emitted a cloud of bad smell last year (but oddly no smoke) and was disposed of. I owned one of those 6A chargers myself, but it too broke, in a similar manner. Both of these chargers I had seen had broken while charging very old FIRST batteries, made by Yuasa, probably from the 2000 season.

I have a question for the electrical wizards out there. How would a charging system be properly designed to charge multiple batteries at one? I would think a nicely setup diode layout would prevent one battery trying to charge another, even if there was a chance for something like that. I spent plenty of time last year trying to design one, but I couldn’t get enough diodes for the purpose. The problem becomes interesting, because each battery in the mesh will have discharged differently, and some batteries are older/stranger than others, and have different internal resistances.

basically you would have to wire a current regulator or voltage detector for each battery.

so there isnt really that much of the charger that would be common to all the batteries it is charging.

There are ways Venkatesh…
Actually there are a number of multiple battery charger for all kinds of applications. They generally use a common power supply and then branch out to fedd each battery through it’s own charge circuit. You want the charger to constantly check the state of charge the battery vitals and generally even the temperature of the individual batteries. I know that Motorola has eight wide charging stands for walkietalkies. These chargers give indications of the state of charge on each battery and can even flag batteries that are about to die. Some multi chargers are actually computer controlled devices that can produce a series of tests and give printed reports of battery by serial number.

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Originally Posted by Venkatesh
I have a question for the electrical wizards out there. How would a charging system be properly designed to charge multiple batteries at one? I would think a nicely setup diode layout would prevent one battery trying to charge another, even if there was a chance for something like that.
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The proper way to do it is to route a suitable supply through a Unitrode 3906 charge controller, set up with a pass transistor capable of handling the charge current and power dissipation requirements. As noted earlier, you can use a common supply and a separate 3906 and pass transistor for each battery. The 3906 has battery charge status indicators that can be set up on leds. We used two of the exide chargers this year with 50 amp connectors attached. We ran into problems with not enough batteries in the finals, and problems with chargers getting unplugged and students not reading the leds on them carefully. We will address the issue with a multi-battery charging bank design over the summer. It would have to be a custom design using a high power darlington pass transistor, the available unitrode 3906 designs on the web run at lower charge currents.