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Old 8th May 2022, 4:13 pm   #1
Radio Scotland
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Default Car battery charger current shunt location ?

I have an old 4 amp car battery charger with an analogue amp meter that i'd like to covert to a digital A / V Meter. I'd like to use the chargers built in shunt but not sure where it is. Am i right in thinking it's built into the meter ?



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Old 8th May 2022, 4:39 pm   #2
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Default Re: Car battery charger current shunt location ?

It will most likely be a moving iron meter that doesn't use a shunt at all.

Greg
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Old 8th May 2022, 4:52 pm   #3
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Default Re: Car battery charger current shunt location ?

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Originally Posted by G.Castle View Post
It will most likely be a moving iron meter that doesn't use a shunt at all.

Greg

Cheers.

So the current passes through the meters internal coil ?
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Old 8th May 2022, 5:49 pm   #4
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Default Re: Car battery charger current shunt location ?

Yes, that's it The wire will be quite thick on the coil around the movement if it can be seen, that's the clue.

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Old 8th May 2022, 6:22 pm   #5
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Default Re: Car battery charger current shunt location ?

I would recommend testing the digi-meter before modifying the charger to accommodate it. The quality of them is quite variable and you don't want to be left with a pile of bits.

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Old 8th May 2022, 6:44 pm   #6
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Default Re: Car battery charger current shunt location ?

Cheers

Iv'e bench tested it and it all seems to be working. There's a small device inside a white plastic box strapped to the right hand side of the transformer. It's connected to the transformer centre tap and the other end of it goes to the chargers negative lead. Any idea what the purpose of it is ?
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Old 8th May 2022, 7:31 pm   #7
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Default Re: Car battery charger current shunt location ?

Thermal cut out?
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Old 8th May 2022, 8:14 pm   #8
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Default Re: Car battery charger current shunt location ?

Thermal cut out, most likely.

Trying to digitise one of these is not a trivial task. The current is very pulse-y so which current do you want? Peak, mean, average, RMS? the only unabbiguous answer to "What is the current?" is a waveform plot.

The transformer in these things is also rather special, employing a lot of leakage flux to make the output current self-limiting. Connecting a charger toa dead flat battery looks pretty much like a dead short, and the current limiting is needed to stop the thing going up in smoke. The thermal switch is purely there as a last-ditch thing to limit the trnsformer temperature.

Electric welders with big iron transformers employ flux shunts moved by a damned great knob moving the shunt with a screw thread. The magnetrons in microwave cookers are unstable loads, looking like negative resistance, so the day is saved by mag shunts in their power transformers to set a current limit. This is why people trying to use ex-cooker transformers as cheap EHT supplies for big RF power amp valves are met with monumentally lousy regulation.

It's a common technique because it's robust and cheap and it works.

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Old 8th May 2022, 9:57 pm   #9
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Default Re: Car battery charger current shunt location ?

Interesting info on charging transformers mr wrangler. None of which i knew, Well i had the digital meter installed and although the voltage reading was spot on the current reading was a mile off, Even after adjusting it via the onboard potentiometer, So i stuck the old meter back in, At least that gives a fairly accurate reading,

The digital meter was just one i had laid around, Was worth trying i suppose.

And yeah i would agree the device on the transformer is a thermal cut out


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Old 8th May 2022, 10:35 pm   #10
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Default Re: Car battery charger current shunt location ?

Curious as to how voltage regulation works in these as there's no dedicated voltage regulator ?
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Old 9th May 2022, 12:43 am   #11
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Default Re: Car battery charger current shunt location ?

I really like to have both a voltmeter and ammeter on (all) battery chargers, so that you get a full picture of what's going on.

With these old fashioned type of chargers, most/many/all of them would eventually charge the battery until significant gassing went on (~15V ?). The transformer design limited the current, but was not fully effective on voltage. That did not matter too much in the days when you could take out the top caps and pour distilled water in, but it may be more of an issue with modern batteries. There may be debate about the exact max voltage to avoid gassing but it's probably between 14.4 and 14.8V and auto-shut off is now desirable.

Of course the odour of battery acid and the inverter whine from a Pye Vanguard evoke some happy memories of /P contests on 2m, high on the Moors of West Yorkshire.
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Old 9th May 2022, 1:17 am   #12
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Default Re: Car battery charger current shunt location ?

Quote:
Originally Posted by Radio Scotland View Post
Curious as to how voltage regulation works in these as there's no dedicated voltage regulator ?
As Bazz has just said, there isn't any!
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Old 9th May 2022, 1:19 am   #13
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Default Re: Car battery charger current shunt location ?

Yeah it's just an old school charger from the 90's, Basically just a transformer and rectifier. No over charging protection,

What i'm wondering is when i check the chargers voltage output it's around 13 volts. But when i measure it whilst it's connected to my battery it's around 17 volts. Is that normal ? Don't wanna ruin my battery.
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Old 9th May 2022, 1:21 am   #14
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Default Re: Car battery charger current shunt location ?

There won't be any regulation as such on a thing like that. It's about as basic as it gets and I wouldn't waste my time on it - good for charging the old school lead acid batteries from back in the day, as basically said in the above post by Bazz. It 'is what it is' and I'd leave it and use it as it is, just keep an eye on it and if charging a standard lead acid technology type battery, switch it off and disconnect it when the terminal voltage reaches 14.4 volts. Anyway, I like those old moving iron meters!
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Old 9th May 2022, 1:24 am   #15
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Default Re: Car battery charger current shunt location ?

Quote:
Originally Posted by Radio Scotland View Post
But when i measure it whilst it's connected to my battery it's around 17 volts. Is that normal ? Don't wanna ruin my battery.
Maybe the battery is fully charged and it's over charging it.
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Old 9th May 2022, 2:44 am   #16
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Default Re: Car battery charger current shunt location ?

There is no voltage regulation. It tries to act as a regulated current supply.

This is why it's a 4-amp charger! It's deliberately weedy so that once the battery is fully charged, the rate of doing damage is fairly slow so you have a bit of time to notice it's done and turn it off.

Back in the day, you could buy fast chargers from firms like Crypton. These had a little lead-cased temperature sensor you dangled into the electrolyte of one cell so the charger could sense when things were getting a bit warm and it would back off the current. They also had a boost mode if someone pressed a button while someone else operated the starter. Commonly used by sales showrooms to ****** batteries.

There were also 'trickle chargers' less than 4 amps which could be left on indefinitely.

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Old 9th May 2022, 5:52 am   #17
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Default Re: Car battery charger current shunt location ?

Regarding battery voltage, there's a recommended minimum below which lead-acid should not be discharged (10.7V ??). When you start to charge, they tend to rise to ~13V fairly quickly and then more slowly up to ~14V and beyond. There's a mountain of information about this on Google.

I suspect that there were lots of chargers made in the 60 and 70's (and probably "budget brands") that would take a battery well above 15.5V, but 'gas-off' lots of water in doing so. Using those chargers on modern batteries needs to be done with care.

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Old 9th May 2022, 11:59 am   #18
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Default Re: Car battery charger current shunt location ?

Thanks for all the info folks it's much appreciated, BTW this kind of charger is still sold believe it or not.

The battery is just a small motorbike battery I use it to power my Yaesu 817 when portable, Here's hoping it's not been fried by my antique charger..

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Old 9th May 2022, 12:18 pm   #19
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Default Re: Car battery charger current shunt location ?

4A to a motorbike battery is risky. THere are dedicated chargers for small Gel cells. Voltage limit at 13.8v to float the battery once charged and a current limit to control things on the way up.

David
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Old 9th May 2022, 12:25 pm   #20
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Default Re: Car battery charger current shunt location ?

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4A to a motorbike battery is risky. There are dedicated chargers for small Gel cells. Voltage limit at 13.8v to float the battery once charged and a current limit to control things on the way up.

David
Sounds like I should buy a more modern / safer charger. It's a lead acid 7-AH Battery and I've only just recently bought it. So don't wanna wreck it.
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