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Battery Charger Design

Wishing won't make it so.

As previously stated, 0.9V/cell ==> 4.5V is more than fully discharged. As I stated before, I

I think you should check that assumption against the specs. Never assume when you can use real info. At this level of discharge you should probably pay some attention to equalizing the cells. Small overcharge can do that.

Not unless you change your charge time requirements AGAIN!!! Quit using the phrase, "as long as possible." There is no MATH do deal with that requirement. Everything must have a NUMBER. And pick the number according to REAL requirements AFTER you've thought about it. Arbitrary numbers are no better than no numbers.

Yeah, and I want Taylor Swift to show up at my door and call me "daddy", or in my case, "granddaddy". But we're both likely to be disappointed. Wishing DON'T make it so.

regardless. There is no "regardless" in electronic design. REGARD is the essence of design. Since this is for a research project, the

That's a nonsense sentence..."It doesn't have to be perfect as long as it works perfectly". A charger is unimportant until you find that the project fails because of the charger.

A 317 can be made to limit current. There are "headroom" requirements that you probably meet. A 317 is serious overkill for this application. With 9V input and 4.3ma charge, you can get by just fine with a resistor. even if it takes 24 hours? Or is there

Yes, you should be aware of the "division" button on your calculator. That is the only

The short answer to the question you asked is, "the path you're suggesting will not take you where you said you wanted to go." It's not because of the 317. It's because of the simple math you didn't do.

This is gonna be my last post...hopefully...on the subject. If you won't listen, I can't help. I apologize, but you've completely depleted my supply of tact.

THEMOSTIMPORTANTPARTOFAPROJECTISTHE SPECIFICATION/REQUIREMENTS DOCUMENT.

If you don't know what you're building, you can't design it.

You're all over the map. First it was 3 hours...now 4-5. You want it to charge "as fast as possible" then 8-hours then 24 hours. It's very simple math. 150 mah / 4.3ma ==> 34.883721 hours plus some based on the charge efficiency. If you can't/won't do that level of analysis, there's not much I can do to help you.

I gotta go to the store now and buy more tact. I'm sending YOU the bill.

Reply to
mike
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"mike"

** Experience with NiMH cells tells me that at the C/50 rate little if any charging occurs and at the C/100 rate - no charging occurs at all.

Only charging at current levels above these rates does the business.

So, in the OP's example, the first 1.5 to 3mA does nothing but tread water.

A second issue is being able to tell when the dish is cooked - ie, if one cell in a pack requires 3mA to tread water and another only 1.5mA, then the dish will only ever be cooked in parts.

Far better to charge moderately fast - like 5 hours at the C/4 rate ( 38mA in the OP's case ) and KNOW when the dish is fit to serve.

** ROTFL !!

... Phil

Reply to
Phil Allison

Yes, the LM317 set for 4.3 mA will work. It is not the best solution. Here's a simple circuit with a 317 and a TL431, which is better than just the LM317 constant current circuit. The 317 is set to charge the cells at C/10, which for your cells is ~14 mA. The TL431 is set to steal current when the battery voltage rises approaching full charge.

----- + ----in|LM317|out-+ ----- | Adj [85R] | | +-------+------+------+-->|---+ | | | [680R] | | | | | __|_/ | | / / \ P | + /431\--->0 1M Battery ----- T | | | | Gnd ----------------------+------+-------+

The 85 ohm resistance sets the LM317 to provide about 14 mA. The pot feeding the 431 is set to make it conduct when the battery reaches ~6.5 volts. Prior to it conducting, the full ~14mA current, minus the ~ 7uA the 1meg pot draws, goes to the battery. When the 431 conducts, the 680 ohm resistor limits the current through the 431 to a max of ~ 9.5 mA, leaving about 4.5 mA to go into the battery.

Note that you could change the 85 ohms to 18 ohms to get a charge current of about 70 mA, and the 680 to 100 ohms to allow about 65 mA through the 431. But the sheet at the url you posted mentions only the general** 14ma and 4.3 mA rates so depart from that at your own risk.

**The 70 ma rate is specific for a completely discharged battery for 3 hours at 20 C

Ed

Reply to
ehsjr

"ehsjr"

** No it won't - d*****ad.

That rate is only the " trickle charge ".

You better go look the term up.

... Phil

Reply to
Phil Allison

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but am

It seems (from the other posts) that this battery charger bit is not part of the research, just a project hurdle to get the colored cubes working ?

the battery spec. says it is a drop in replace for other similar ni-cad & nimh batteries and so i presume there are existing ready made battery chargers that can charge this battery ?

couldn't one just buy a ready made charger match for this battery and use that (**or the guts**) to solve the charger problem ?

I have a box full of various rechargeable battery chargers for nimh/nicad from 4.5/6v/9v and for a variety of devices like camera batteries, RC batteries , rechargeable cells, etc. with many output ranges.

maybe you just want to build one from scratch but that sounds like a project all it's own.

my $0.02

robb

Reply to
robb

"robb"

** Wot a silly presumption.

The battery in question is a *solder in * replacement for MEMORY BACKUP purposes .

So it fits on a large PCB and is supplied with a trickle charge by the same board.

** No.

.... Phil

Reply to
Phil Allison

rechargeable

questions.

another.

solution

I kind of have to agree. The very poor assumption that you can get a=20 full 50 mA for 3 hours out of a 150 mAh cell scares me. If you want=20 more than a few cycles of battery life 200 mAh or 250 mAh is a better=20 choice.

Reply to
JosephKK

Hi Ed, thanks for the schematic. This looks appears to be a good solution and I'll be prototyping this. I contacted VARTA directly and their recommendation is exactly the same scheme as originally proposed, using a regulator with constant current, but with a timer. Thanks for the help here.

Reply to
thorin92

Sorry, when I mentioned 50mA for 3 Hours this is the worst case. The actual circuit nominally draws around 20mA. When I tested the Battery lasted for around 5 hours.

Reply to
thorin92

You're welcome. You might want to consider adding an LED that glows when the TL431 steals current, to let you know that the charge is complete.

Ed

Reply to
ehsjr

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