So, how DO I charge a 6V SLA battery?

The LM317 will set the voltage. If the battery is already charged, it won't take much current at 7.2V.

So, first discharge the battery, then try it again.

I've looked around on the internet, and seems to me that you really want to be charging the thing at the 'float voltage' of 6.75V. If you set the LM317 to output 6.75V, and just leave it on there, it'll float up the right charge level after a day or so. You can also just leave the battery on the charger forever at this voltage, which is a plus.

The datasheet gives the formula for deriving voltage output. The thing basically tries to keep 1.25V between its output and adj pins. So, pick a resistor to go between the out and adj pins, and then find another one to go between adj and ground that is nearest to 5.4 times the first resistor value.

Regards, Bob Monsen

Thank you for the information.

Could you provide the links that point to a 6.75V charge?

Thanks,

Michael

Regards, Bob Monsen

Thanks for that. I'm starting to get dizzy with all the different voltage recommendations.

6.75V

less than 7.2V (

snipped-for-privacy@gmail.com wrote: > On Mar 9, 10:55 pm, ehsjr wrote: >

You have a 3-cell SLA with full charge at 25oC of 2.4V/cell makes for

7.2V, assuming the 317 was dead-on 7.200000V and the SLA had even 0.001R impedance, the 50mA would make for a battery voltage of 7.19995V, a 0.01R would make for 7.1995V, etc...there is nothing wrong with the '317 circuit.

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Get your head around the difference between "charging" and "floating" so you don't suffer any more confusion. The ideal charging protocol doesn't take place at a constant voltage, but rather the voltage changes according to the battery's state of charge (as evidenced by current draw).

You, however, do not have a sophisticated "smart charger" to tailor the charging curve. You'll operate at a fixed voltage. So you should pick a voltage according to how much time you want to take to charge the battery. If you want the battery to charge reasonably fast, use about 7.3 or 7.4 volts. If you have all the time in the world use about 6.7 or 6.8.

Yup! That's what my "warning" was about. :-( :-) (It deserves both a "frowny" and a "smiley".)

When you want to get a little more complicated, the circuit can be modified to include other features.

But modifying the circuit won't get rid of the dizziness. :-) You'll need to decide the charge parameters you want.

Ed

It's a bit more insidious than that, I'm afraid. I had my 3-year-old daughter ride the car to the park again (and my 5-year-old rode it back - my daughter was tired of pressing the "Go" button continuously. ;-) So the battery was a bit used, and read 6.47V.

Charging it with the LM317, I got 54.2 mA charging current. With no battery connected, the LM317 gives 7.17V.

Bypassing the LM317, I get 160 mA charging current, and 9.13V with no load.

Still confused as ever why the regulated current is ~ 1/3 of the unregulated current.

From a practical point of view, I'm happy leaving it on during the day unregulated, monitoring it every few hours. Overnight, I can use the LM317 set to 7.17V.

Michael

Your unregulated supply is too weak, not enough drive or voltage too low. Have you measured the differential voltage across the 317 with battery loading, this should be >=10.2V to maintain the output at reasonably large currents like 200mA or more, and don't forget about a

1U output bypass right at the OUT terminal, and it wouldn't hurt to have one across the input too, actually beefing up the unregulated storage capacitor maybe your answer. What is the AHr rating of the SLA?

You have to realize, I'm a newbie at this sort of thing, so I don't know the lingo quite yet.

Differential voltage across the 317 with battery loading... do you mean the voltage the LM317 puts out? That one varies. It's 6.75V now, but it was pretty close to the battery voltage +/- 0.02 V last night. 6.48V, say, last night.

Why should it be 10V? Wouldn't that fry my battery?

1U output bypass... do you mean a 1uF cap? C2 in this schematic?

I put that one (didn't have 1 uF so I put 10uF). It's a tiny one though, 10uF electrolytic at 50V I think. Less than 1 cm tall. Mouser P/N 647-UVR1H100MDD1TA

FWIW, I also measured the current upstream of the regulator circuit, and it was also about 50-60 mA at about 8V (I didn't write the numbers down last night before I went to bed). Seems that the regulator doesn't want to suck much current out of the P/S.

Thanks,

Michael

Forgot to answer your last question. It's a 6V, 9.5A-hr battery. I returned the other (6 A-hr?) battery to Home Depot when I realized that Wal-Mart sells SLAs for Power Wheels toys, and they're cheaper per Watt-hr than the Home Depot unit. With built-in self-resetting fuse, too.

Michael

Are you saying the voltage at the IN terminal of the 317 is 8V at

50-60mA??? No wonder the thing is dropping out. You want to keep that IN voltage up near 10V and no less..Are you using some kind of cheap 9V wal-wart, that's not enough, you need to go higher to a 12V or some popular voltage greater than 10V, and that voltage has to be specified at a current greater than your DC charging current. The 317 does not regulate properly if the IN to OUT voltage difference is less than 3V at sizable currents.

For that size you want something like this, right out of the datasheet, if the (-) output of the modular is not GND'ed, GND the (-) Batt node: View in a fixed-width font such as Courier.

. . . . ---------- LM317 . | |(+) ----- . ----|12V 1A |----+----|I O|--------+---+------> + Batt . | | |+ | A | | | . 120VAC | | === ----- [120] | . | wall plug|(-) |100u | | | . ----| |----+ +-----------+ === . | | | | | |0.1u . ---------- | | [560] | . | \\| | | . | 2N2222A |--[100]--+---+------> - Batt . |

Ok, gotcha, I need a bigger power supply. Thanks.

Page 21, "Current Limited 6V Charger". Thanks.

Thanks for all the tips

Michael

That did it!

I got a 9V nominal, 400mA wall wart (scavenged from some Mac modem in my garage junk pile) and now I'm getting 7.17V out, 400 mA into the battery.

So much for the theoretical idea that if I set the output to 7.17V, the LM317 will successfully regulate 7.50V down to 7.17... ha ha

Thanks again.

Michael

That would require a LDO. :(

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Low dropout regulator? Sounds like something high schools desperately need... ha ha

Interesting - I actually couldn't find anything on the datasheet mentioning that Vin-Vout must be >= 3V. This must be one of those instances where speaking with someone familiar with the device comes in more handy than just reading the data sheet.

Speaking of which - from personal experience - the LM317T is remarkably rugged. I accidentally switched the (-) and (+) terminals from my 9V wall-wart, and was wondering why the battery voltage started dropping... disconnected the battery, and the voltage was slightly negative (!!)... plus the LM317T was really hot... on a hunch, I checked the polarity of the input... WHOOPS!

Still seems to work though.

Thanks to everyone for the advice and tips

Michael

From the National Data Sheet...

Line Regulation 3V

You're right! Man I'm feeling silly now.

Page 5... Line Regulation 3V =98 (VIN - VOUT) =98 40V (Note 4)

Although at least one schematic on the data sheet seems to show the LM317 operating with Vin-Vout =3D 2.

5V Logic Regulator with Electronic Shutdown: Vin 7-35V; Vout 5V

How did I manage to get on your Good List? I thought you ignore Google Groupers like me.

MD