Integrated Battery Charging?

[BlackAndChrome]

I had this idea for a completely sealed dive light that I am building.

Charging system is fully integrated using a TI bq2000 NIMH fast charger when 6v DC is applied across the acorn nuts at the bottom of the light. A mechanical relay disconnects the terminals when DC input is not applied (eliminating underwater shorts). A diode will be put on the coil of the relay to ensure that charge polarity is not an issue. Rough schematic shown below.

I'm a Computer Engineering student so I have no trouble assembling circuits according to schematics, unfortunately as a freshmen I have immense amounts of trouble designing such schematics and understanding datasheet lingo. That being said if there is anyone that knows there stuff that would be willing to help implement this that would be great. It's approaching finals so I really hate to ask any of the seniors that I know for help, then again they have already assured me that a charging schematic would be simple...:thumbsdow

I've read the bq2000t implementation guide like 6 times and just keep getting more confused. Here is the datasheet
http://www.datasheetcatalog.org/datasheet/texasinstruments/bq2000t.pdf

and implementation
http://focus.ti.com/lit/an/slua064b/slua064b.pdf

page 8 offers a example schematic that is obviously not something I am going to get into a flashlight unless I get boards made (which I don't plan on spending money on). Can it be done smaller/more simply?

Perhaps I would be better building that as an external circuit and having 3 probes on the bottom of the light, ground, relay coil, and V+ for an external charging circuit? But then I guess I would lose ∆T/∆t termination!

In that case can someone explain what needs to be modified on the example circuit to give 2A charge rate, for a 4 cell NIMH pack, without thermal termination (-∆V instead)?

Or perhaps I should just suck it up and buy a decent digital charger?

-B&C

 

[CKOD]

The DS2715 doesnt look so hard to implement in its linear mode, 1 to 10 cells.
http://datasheets.maxim-ic.com/en/ds/DS2715.pdf

maxim/dallas stuff can be unobtanium in production quantities, but student samples are usually easy enough to get.

SOIC-16 is nice and friendly, and on breadboard like this http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=9162CA-ND you could probably fit in most the resistors and smaller caps. With just bulk input cap (the 47uF in their figure 6) and the Q1 mosfet located on a heatsink, maybe the input schottky, being the only off-board components.
For simplicity you could drop D2/R2 if you dont want a trickle charge, LED1 and R4 if you dont need a status indicator(guessing you probably do want some sort of status indication though, :nana: ) The D1 schottky eliminates the need for a mechanical relay, for short circuit protection on the input pins, and Q2 doesnt need to be that big, just something to pull down Rt when input voltage is applied.

With it being linear, obviously youre going to want it to not be feeding it a ton of voltage that Q1 is going to have to burn up. probably ~7v or so since you'll need ~1.6v/cell(could be less, depends on the battery). Going from .8v/cell with a 2A charge current is 7.6W dissipated, which you should be able to dissipate with a large flashlight body, but is getting up there. Obviously make sure your thermistor is located in good contact with the batteries, and not so good contact with the transistor spewing tons of heat.

Dont know what youre trying to fit this in, but hopefully this is helpful for you.

 

[BlackAndChrome]

Thanks for the help! I just put in a sample order for some of those chips, we'll have to see how long they take to get here but I only have a week and a half of school left so if I lose access to labs and ECE parts shop before I get it implemented then I'll probably spring for the external charging. In that case I would love to experiment with them for future projects!

Current looking at one of these...
http://www.xheli.com/thac6bachwac.html

If I go with external charging how can I do 2 port charging without a third pole to close the relay? I realized that with external charging with my schematic above, once the relay closed it would short the battery or input source and battery current would hold the relay closed. I'm not sure how digital chargers work but I'm going to guess that...

1. they don't have any voltage on the terminals until a battery voltage is realized (no voltage to close a relay)
2. they go low periodically to check idle battery voltage (although I suppose -∆V could be measured without low cycling)
3. they would not appreciate the voltage drop of a diode or the lack of ability to sense a battery.

Ideas?

-B&C

EDIT:
Here's an idea that keeps from water shorting and eliminates significant forward voltage drop but it still has the problem of initial battery sensing

 

[CKOD]

That circuit above is a handy little circuit especially for reverse polarity protection, however I dont think it'll work for you. In this application, your battery would be on the load side, and the charger would be where the battery is in the drawing. Normally with whatever load, and no battery present, the source of the mosfet (load in the diagram) would be pulled to ground, and the mosfet would be off. Apply input voltage in the correct polarity, and it flows though the body diode, and as the source rises above the gate, the p-channel mosfet enhances and turns on, lowering resistance further. However, with a battery as your 'load' there would always be a -Vgs potential and the mosfet would stay on.

With the battery on the drain as shown, obviously the body diode would defeat what you want to do.

Though if you put a reed relay in the light body, and a magnet on the plug, have a ~10k or so resistor to pull the gate to the source on the mosfet to make it turn off normally, and have the reed relay pull the gate to ground, which would turn on the mosfet.

only issue I can see is that with 4 batteries, discharged, you have ~3.2v which is kind of low to enhance gate on the mosfet, so make sure to use a logic level mosfet, and find one with good ratings for < 5 Vgs. Ive seen some rated at 3v and even 1.8v for Vgs