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6V Sealed-lead-acid battery charger

Posted: Tue Apr 19, 2011 9:22 am
by Yerry
Okay, somehow my Science Olympiad team's $400 digital scale had blown its charger circuitry. It uses a 6v, 1.2 SLA battery. Right now, the "fix" involves putting two batteries (we have a spare) in series across a 12V el-cheapo Hargior Freight trickle charger.

I found a bunch of charger circuits that use the LM317, but all use British/European transistors, like the BC547. At the low voltages & currents involved, is there any way of using a pair of 317's, one for voltage (6.6V) and one for current (120mA)? I'll also consider other stuff that works, of course, like a 2Nxxxx substitute for the BC547; I just happen to have a bag (100+) of surplus 317's someone gave me.

Thanks.

Re: 6V Sealed-lead-acid battery charger

Posted: Tue Apr 19, 2011 10:13 am
by CeaSaR
The BC547 is a low voltage, low power, NPN small signal transistor with a gain in the
neighborhood of 100. You can use a 2N3904 or 2N2222 depending on the power needs.
If you can do with 100 - 200 mW then the 2N3904 will do, else use the 2n2222 as it
can handle about 600 mW. Here's my search.

CeaSaR

Re: 6V Sealed-lead-acid battery charger

Posted: Wed Apr 20, 2011 12:45 pm
by haklesup
Any regulated (or unregulated) PS greater than 6V will charge that battery.

WRT the LM317

The Data sheet only shows a max 3V input to output difference so to use the LM317 to drop 12V to 6.6V you should use several in series. Those other circuits probably do that in various ways or boost the output current above 1.5A

http://www.fairchildsemi.com/ds/LM/LM317.pdf See figure 3 dropout V vs Temperature at various load currents.

if the current is significant, it's going to get pretty hot by itself. I believe you can parallel this device up for current sharing if you don't want to get the equivalent in a TO-3 package and accompanying heat sink.

Re: 6V Sealed-lead-acid battery charger

Posted: Wed Apr 20, 2011 8:06 pm
by sofaspud
The LM317 with a max 3V input/output differential? Is that being read correctly? I thought the differential was more like 40V.
3V sounds more like the minimum differential.
How much current does this digital scale draw? I was thinking it wasn't that much, and the easiest/best charger would be simply a
regulated voltage. The Panasonic 6V 1.3A SLAs I have advise 7.25 ~ 7.45V for cycle use and 6.8 ~ 6.9V for standby use.