12V automobile battery charger

Question

12V automobile battery charger

rqjoe (Aeronautics)

(OP)

30 Jun 17 22:18

Hi folks. Wondering if anyone here is familiar with design of older 12v car type portable chargers? I have a RN Industries one that will run small motors and reads 15V across the clamps but it does 'nothing' when connected to a battery. The voltage across the clamps drops to whatever the battery itself is once connected. And no reading on the ammeter on the charger. (the ammeter does move properly when running motors.) Any ideas why it runs the motors but doesn't charge a battery? thanks.

itsmoked (Electrical) · Answer 1 · 2017-07-01 06:28:36

find a voltmeter and measure the unconnected leads. If the voltage is lower than the battery voltage you won't get any charging but it would certainly turn a motor.

Keith Cress
kcress - http://www.flaminsystems.com

rqjoe (Aeronautics) · Answer 2 · 2017-07-01 14:21:20

Thanks. But, sorry, I meant I did test with unconnected leads and that is where it is around 15V. But when I connect to the battery posts, volt meter just drops to whatever the battery was and does not come up as if no charging is going on.

IRstuff (Aerospace) · Answer 3 · 2017-07-01 16:59:59

Then, it's broken, most likely the power transistor(s) is(are) fubar. Someone possibly shorted it, or did something to kill it.

TTFN (ta ta for now)
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itsmoked (Electrical) · Answer 4 · 2017-07-01 19:53:36

Interesting.. Is it a new charger?

Keith Cress
kcress - http://www.flaminsystems.com

rqjoe (Aeronautics) · Answer 5 · 2017-07-02 19:53:12

No. old style manual charger from the 70s.
Yeah really odd. Runs a small DC motor & ammeter shows the inrush jump then about 3 amps for the running motor. But when I clamp it up to a battery it just does nothing at all.

Thought maybe my battery was bad, but I tried another charger and it charges the batt fine. Also tried a known bad batt and neither charger did anything to that one just as I'd expect. I tried all sorts of jumpering solder areas, connections, and switches but nothing helps.

Only conclusion I can come to is one of the coils is bad in the transformer, but how does it run the motor is still then unexplained.

Oh well.... thanks for the help guys.

IRstuff (Aerospace) · Answer 6 · 2017-07-02 20:13:51

not a transformer problem; if that were hosed, the charger wouldn't work at all.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

itsmoked (Electrical) · Answer 7 · 2017-07-03 05:10:42

Could it be a center tapped transformer with one half open so the charger is only putting out 10V? That leaves the motor to still run at half speed which is not as easily detected when full speed hasn't been recently observed. That is the motor would spin on 6V but the battery wouldn't begin to charge. The 15V you're measuring is the completely unloaded output which would crash once a load is applied.

If the 15V is when it's hooked to the battery then the battery is truly toasted.

I asked if it was new because a lot of new chargers don't start charging unless they think they're hooked to a sound battery. They don't even apply charge unless they've measured a steady acceptable voltage. I think it's to prevent any sparking near the hydrogen monster.

Keith Cress
kcress - http://www.flaminsystems.com

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12V automobile battery charger