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



mrdarrett@xxxxxxxxx wrote:
On Mar 18, 6:17 am, Fred Bloggs <nos...@xxxxxxxxxx> wrote:

mrdarr...@xxxxxxxxx wrote:

On Mar 17, 11:48 am, Fred Bloggs <nos...@xxxxxxxxxx> wrote:

mrdarr...@xxxxxxxxx wrote:

<snip>

Ok, I built it. Didn't have a 5k pot handy, so I strung up two 1k
resistors in parallel and a bunch of 100 ohm resistors in series
until I got about 7.2V out.

Hooked it up to the battery, and I was surprised to see only 50mA
going through the battery. Typically, without the LM317 I got 100+
mA going through.

LM317 was cool to the touch (with heatsink).

I thought my power supply was underpowered, so I tried a 12V wall
wart (about 14V with no load) instead. But then, after going through
the LM317, voltage going to the battery was just over 7.2V. Not
comfortable with that, so I went back to my 6V power supply.

Any ideas why the current is so low with the LM317? Only
modification I made to the data*** schematic was I used a 10uF
electrolytic instead of a 1uF.

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.

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?
http://www.national.com/mpf/LM/LM317.html

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.

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.

.