Battery Charging Station, need some help!

[michael.hill]

I'm pretty new to electronics, so I don't really have a huge amount of experience with this kind of stuff.  I've been reading the Art of Electronics, listening to the Amp Hour and watching Dave for a while.  I'm really stuck on a design I'm working on for a FIRST robotics team.  I've tried to highlight my biggest questions in bold.

Throughout our competition, we have to keep batteries charged for our next match, and the time span for matches can be anywhere between an hour and a half during regular competition to about 10 minutes during the tournament, so we always have to have multiple batteries charged and ready to go. We are allowed to use two types of batteries, the MK ES17-12 and http://www.mkbattery.com/images/ES17-12.pdf the Yuasa NP18-12http://www.yuasabatteries.com/pdfs/NP_18_12_DataSheet.pdf.  We will rotate battery use, but overall we'll use them for about 2-3 years before they're no good anymore and end up on a practice robot (and that's fine with us).  Overall lifetime isn't exactly a top priority for us.  At any given competition, we'll probably take 8 batteries with us. I would guess each battery sees maybe a total of 50 charges/discharges throughout its entire life. This is because we charge them at maximum charge rate of 6 Amps using this charger http://www.walmart.com/ip/Schumacher-6-4-2-Amp-Charger-Maintainer/21642120 (You can get them anywhere, really).

Now our current issue is that we're pretty unorganized when it comes to battery maintenance, so we want to build a "cart" that will store batteries in slots for easy access and charging.  We try to keep up with checking the internal resistance of these batteries before packing up to make sure we're not packing any "bad" batteries.  What I would like to do is have an "all-in-one" solution battery cart that will automatically sense which battery is in a slot based on an RFID tag attached to each battery and a sensor in each slot.  I'd also like to be able to display a "percent charged" on a battery-by-battery basis on an LCD screen.  My biggest question is how I go about doing this.  I have some ideas, and hopefully some smarter guys can tell me if they'd work or if they're half baked.

I think instead of making my own Ni/Cd (Edit: Sealed Lead Acid) battery charging circuit, I'm just going to crack open the ones we already use and run LEDs to the front panel of the battery cart. That way, I can just throw in a  beefy power strip inside the case and let AC power run to the chargers.  In order to measure the internal resistance, I'll have to break the connection and use my own circuitry for that. Should a use a solid state relay for that? To drive the LCD and deal with RFID stuff, I'll probably use an arduino.  Should I just use a wall wart to get my DC voltage to run the arduino (since I already have a power strip inside my case)?  I also assume I'll have to worry about heat because it will be an enclosed space, so I'll need to put some fans inside as well. Any other thoughts on if this will work or anything else I should consider? Also, is all I need to do to measure 6A is to put a low ohm resistor (like 0.001 Ohm) in series with the charging circuit and measure the voltage drop and use Ohms law?  

Thanks,

Michael   

[Paul Price]

Measuring a battery;s hidden potential is not so easy as one would like. I have some microcontroller based ways of doing this but this requires some good MCU programming skills.

What I would like to do is have an "all-in-one" solution battery cart that will automatically sense which battery is in a slot based on an RFID tag attached to each battery and a sensor in each slot.
This is really cool and nice to have, but very much trouble and money and time to achieve. (am I wrong about this?)

Should a use a solid state relay for that?
You could, or you could make a discrete circuit that would be cheaper and easier to integrate into your Battery Cart.

Should I just use a wall wart to get my DC voltage to run the arduino (since I already have a power strip inside my case)?
 No problem with that, perfectly good choice with the right wall wart.

Any other thoughts on if this will work or anything else I should consider? Also, is all I need to do to measure 6A is to put a low ohm resistor (like 0.001 Ohm) in series with the charging circuit and measure the voltage drop and use Ohms law?[/i]
This is an excellent idea the .001 Ohm, and use an op amp to magnify the current to a voltage acceptable for the full scale A2D input of your Arduino.

I think with a microprocessor it might be possible to measure the "fitness" of a battery by having a MCU connect the .001 ohm resistor directly across the battery for a few mSec and measure the battery voltage at the beginning and end of this stress test. You could try this and see if the voltage drop of the battery from the first to second reading readily indicates how fit the battery is.

You could also with a MCU simultaneously charge a number of battery packs and monitor their charge acceptance and charge state automatically in you Battery Cart..and give a readout on a 7-seg LED array or a cheap 4-line by 32char display.

[michael.hill]

Thanks for the reply!

We currently use this device: http://www.crosstheroadelectronics.com/Beak.html to measure the internal resistance and state of charge.  I think I can probably manage to come up with my own circuit that does the same thing.  I'm just worried about bringing in an 18 Amp load into a PCB (I'm trying to read up on PCB standards and stuff).

You could, or you could make a discrete circuit that would be cheaper and easier to integrate into your Battery Cart.

What kind of discrete circuit would allow me to break the connection to the charger? Or are you saying to make my own discrete charging circuit with battery management ICs and whatnot?  I've looked into Lead Acid Battery Management IC App notes, and most of them seem to have trouble with providing 6 Amps (or am I missing something?)

[Paul Price]

Nothing is impossible. It is not difficult to carry large currents on a PCB, your PC power supply does this all the time. It is that size matters(width anyway).

If you have the time and the courage, you could an attempt to  build your own charger/diagnostic Super Duper Battery Cart.
The parts are out there and my idea is that you could use individual 1 1/2 in by 1 i modules to supply a regulated 5A amp supply to power each individual charging circuit..no problem and only cost something close to < $10 US  per charging module. The real total cost would depend on the charging currents per battery and the number of battery packs that must be charged at any one time.

If you want, you can salvage high-current power supplies from old used desktop PC's or other electronic equipment and this would cut the cost dramatically. The real problem is to how to find all the current necessary to feed all the chargers at once.

How many battery pacs at once?  Choose the number and you can name the price.  What about size of this? I  was imagining something the size of a golf caddy up to maybe the size of  R2D2.

I have built a mult-battery charger circuit that charged 4 battery packs simultaneously and measured the capacity of the battery by monitoring the charge acceptance and end of charge voltage droop.