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Old 9th May 2022, 1:45 pm   #21
The Philpott
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Default Re: Car battery charger current shunt location ?

Sounds like too powerful a charger for a 7Ah lead-acid. I'd say you want one that's sub 1Ah...actually i think i have a '12v' 400mAh wall wart somewhere if you're interested. Don't underestimate those 13A rotary timeswitches (made by smiths..?) as they provide a very useful battery saving service and at these low loads the contacts don't weld closed (famous last words..)

You charger registering 13v off load and 17v when charging said battery is...very odd. If it was the other way round it would make sense! On seeing this anomaly i would be reaching for another voltmeter straight away..

Dave
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Old 9th May 2022, 2:34 pm   #22
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Default Re: Car battery charger current shunt location ?

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Originally Posted by The Philpott View Post
You charger registering 13v off load and 17v when charging said battery is...very odd. If it was the other way round it would make sense! On seeing this anomaly i would be reaching for another voltmeter straight away..

Dave
As there is no smoothing off load the meter will read the average DC which will be more or less the same as the AC from the transformer.

When the battery is connected it acts like a capacitor and it will be effectively reading the Peaks. When the Battery is fully charged this will be pretty much the AC multiplied by √2

There is a bit more to it diode drops and the way a battery charges but essentially thats it.

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Old 9th May 2022, 3:21 pm   #23
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Default Re: Car battery charger current shunt location ?

- I have never encountered this with my unsmoothed charger feeding pulsed DC (perhaps this was because of the type of test meter used)
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Old 9th May 2022, 3:59 pm   #24
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Default Re: Car battery charger current shunt location ?

I still have one of those simple chargers with a moving iron ammeter reading to 5A, bought in the early 1970's, suitable for both 6V and 12V batteries. It has a 5A fuse but I have never managed to blow it by inadvertently shorting the croc clips. Possibly it provides protection if the croc clips are connected the wrong way round to provide a series connection instead of a parallel one. To limit the charging current for charging small SLA batteries I made up a simple lamp board fitted with 12V bulbs of various wattages to adjust current.

Unlike modern chargers, which incorporate voltage sensing circuitry to only produce an output when a battery within the correct range of voltage has been connected up, these old chargers can be used to revive seriously discharged or apparently dead batteries that read 0V. A Ph.D colleague who had been a chemist working on battery research told me that the SLA that I had salvaged from some scrap equipment that measured 0V might be brought back to life by connecting it to a charger via a high resistance, and this did work for me. It was s 12V 7.5A Sonnenschein SLA and I used a 12V 2.2W MES bulb as the resistance. After a week the bulb started glowing dimly, showing that charging current was flowing, and after several weeks about half the original Ah capacity was restored. You can't do this with a modern charger as it will not recognise the battery's presence.

My copy of a 1940's book on accumulator charging says that, for discharge below 1.8V per cell (10.8V for a 12V battery), the amorphous lead sulphate that is produced in the course of a normal discharge, changes to a hard crystalline form that is difficult to change back to lead/lead oxide.

Last edited by emeritus; 9th May 2022 at 4:05 pm. Reason: typos
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Old 9th May 2022, 4:19 pm   #25
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Default Re: Car battery charger current shunt location ?

Thanks for the replies / info.

Problems is there's a lot of conflicting info online regarding batteries and charging them, Although i have a decent grasp on electronics my battery knowledge is next to zero as iv'e never really messed around with them,

There's a ton of videos on YouTube with guys showing you how to make a 12v charger using a 19v laptop charger with a bulb in series to limit the current. Which i'm advised isn't a good idea.


As for the charger voltage i re-checked it using my fluke meter and it's registering 16v during charging.

Judging by the advice / info in this thread it seems I'd be better off buying a modern charger


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Old 9th May 2022, 4:58 pm   #26
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Default Re: Car battery charger current shunt location ?

There is also rather a lot of twaddle on the internet, so you need to know what you're doing before you can decide who to believe. It's a catch-22

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Old 9th May 2022, 7:55 pm   #27
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Default Re: Car battery charger current shunt location ?

I recently had to charge my stepson's 12V 72AH battery using my old school 'made in West Germany' charger. Overnight the indicated current fell nicely so I decided to measure the terminal voltage 13.8V exactly! I was very impressed how such simple technogy can achieve such a good result!

As others have said though I wouldn't let this anywhere near a 7AH cell.
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Old 9th May 2022, 8:38 pm   #28
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Default Re: Car battery charger current shunt location ?

Thanks to everyone who replied, Iv'e learned a bit from this thread, I'll invest in the appropriate charger,

Cheers
Jay
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Old 9th May 2022, 8:53 pm   #29
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Default Re: Car battery charger current shunt location ?

Hi.
Two things to explain. On an old transformer and rectifier charger the voltage off load (not connected to the battery) will appear to be low, this is because there is no capacitor acting as a reservoir, put a 470uf across the leads and you'll see a much higher voltage, the battery acts like a capacitor and the voltage will rise well above the max the battery will stand and begin to gas. A 4 amp charger on a 7 AH battery will destroy it easily as you have seen the volts have risen to 17v. A boost charge should never be above 15.1v and then for a limited time. In cold temperatures 14.4v as a float above 10 to 15c battery temperature 13.8v float charge, AGM and other sealed batteries have lower boost charge voltages than wet batteries and those sealed types should "never" be charged on a conventional charger like you have.
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Old 9th May 2022, 9:14 pm   #30
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Default Re: Car battery charger current shunt location ?

I use 5AH SLA batteries for some ham radio activities, these having been retired from powering my very old lawn strimmer and I charge these using of my bench PSU's which I can set for both max I and V.

On another forum recently, a guy was asking for advice about a "smart" charger he had bought from a supermarket for about £14. He described it as working as follows; Start >

Displays 3.6A constant, until voltage rises to 14.4v
Then drops the amps to 0.8A (and the voltage falls to 13.4)
Voltage then rises slowly back to 14.4
It then drops it back to a float of 13.5v @ 0.1A
and displays that its Finished.


That seems quite good to me. I'm normally wary about smart chargers, but that one would appear to be sensible and smart so good for modern batteries

B
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Old 9th May 2022, 10:25 pm   #31
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Default Re: Car battery charger current shunt location ?

Quote:
Originally Posted by Bazz4CQJ View Post
I use 5AH SLA batteries for some ham radio activities, these having been retired from powering my very old lawn strimmer and I charge these using of my bench PSU's which I can set for both max I and V.

I have a 30A - 0-15v power supply but it doesn't have current limiting
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Old 9th May 2022, 11:14 pm   #32
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Default Re: Car battery charger current shunt location ?

I now charge small SLA batteries using a Celtec electronically-controlled charger that has car, motor cycle, and small SLA/trickle charging options. Previously I used to use a 12V 6W car sidelight bulb with my old charger to limit the current to under 0.5A.
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Old 10th May 2022, 9:47 am   #33
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Default Re: Car battery charger current shunt location ?

Quote:
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I have a 30A - 0-15v power supply but it doesn't have current limiting
This needs something a little stronger than a warning, maybe a full Achtung! with optional Klaxons.

Using regulated power supplies to charge batteries is a high-risk occupation. Power supplies weren't designed to get power IN at their output terminals. In most cases this can blow output transistors as the reverse Vbe rating isn't enough. Posh power supplies have 'down programming' a shunt transistor to pull the output down if the voltage knob is wound back. Inevitably these aren't beefy enough to pull a serious battery down.

In HP we had a steady repair queue for power supplies needing to be fixed. Nearly all in winter when people had tried to use them as battery chargers!

If charging a batttery, include a series diode

David
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Old 10th May 2022, 1:16 pm   #34
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Default Re: Car battery charger current shunt location ?

-I fear the risk of you ending up looking like Freddy Krueger!

The ALCS (2-24Ah Lead-Acid) charger by Sunpower or Ansmann is available on eBay and elsewhere, not particularly cheap, but worth a look. Region of £25 inclusive from one seller. (Typical output for a 12v battery is 600mA)

They auto-switch between 2v/6v/12v/24v by interrogating the battery they have been connected to. (edit- i believe it does have trickle facility at end of charge.) The only thing it might have trouble with is a completely dead flat battery- it wouldn't know what V output to provide.

I have one, but I need it unfortunately.

Dave

Last edited by The Philpott; 10th May 2022 at 1:29 pm.
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Old 10th May 2022, 5:22 pm   #35
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Default Re: Car battery charger current shunt location ?

Lidl have chargers in at the moment advertised as for car or motorcycle batteries down to 1.2AH, auto 6/12v selection, “Pulse charge mode” for flat batteries (the box says from 7.5V min for a 12V battery) etc. I think it was £13.99, cheaper than having to replace a battery due to overcharging.

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Old 10th May 2022, 7:41 pm   #36
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Default Re: Car battery charger current shunt location ?

Note that a lot of the simple transformer /rectifier charges are very dependent on the actual mains voltage (which can vary widely) for correct charging current and end point voltage

Ed
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Old 10th May 2022, 7:55 pm   #37
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Default Re: Car battery charger current shunt location ?

Cheers all

I've spotted one on eBay for £12.99. Has auto cut off and a float function and various forms of protection, At that price there's no point trying to cobble something together, It's a no brainer,


Looks like one of my posts has been moderated / deleted, I guess the schematic of the homebrew charger I posted was a bit too shocking!!!



Thanks to all who have replied, But think we can wrap this one up now
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Old 11th May 2022, 3:00 pm   #38
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Default Re: Car battery charger current shunt location ?

We don't want to lose forum members, or their families, pets etc. Nor do we want to see anyone fed to the ravening lawyers.

I see Dave's Freddy Krueger jest is now orphaned by the deletion, but most will be able to figure it out.

David
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