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Old 9th Jan 2021, 10:22 pm   #1
MALC SCOTT
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Default Car battery charger project.

Hi, i have a new old stock Douglas MT-5-AT battery charging transformer which i intend to build a battery charger with. It has a secondary of 2v-6v-12volts. Handy for charging accumulators, motor bike and car batteries.What type of rectifier should i use? a bridge or just a single diode? Cheers, Malc.
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Old 9th Jan 2021, 11:20 pm   #2
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Default Re: Car battery charger project.

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Originally Posted by MALC SCOTT View Post
Hi, i have a new old stock Douglas MT-5-AT battery charging transformer which i intend to build a battery charger with. It has a secondary of 2v-6v-12volts. Handy for charging accumulators, motor bike and car batteries.What type of rectifier should i use? a bridge or just a single diode? Cheers, Malc.
Does it have a center tapped LV secondary?
If it has a non-center tapped LV secondary, I prefer a bridge rectifier.
What kind of rectifier did it use originally?
I repaired several battery chargers, installing silicon diodes to replace the old style selenium plates.
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Old 9th Jan 2021, 11:20 pm   #3
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Default Re: Car battery charger project.

Malc. First of all , due to one poster, car projects are seen as projects non grata as the posts will degenerate.
Possibly a few high tech battery bods on here, but I'm an olde BT blokie who looked after tech kit and battery rooms. transformer drops to charging volts, rectify and as battery is a large capacitor, it'll smooth out the chopped volts. One thing to look out for is an old battery suddenly taking current ( fuse?) and any old battery getting hot, because a hot battery is not WOT you want. They go BANG.
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Old 9th Jan 2021, 11:24 pm   #4
MALC SCOTT
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Default Re: Car battery charger project.

Hi, this will be built from scratch. The t/x has never been used. There is no centre tap on the t/x.
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Old 9th Jan 2021, 11:49 pm   #5
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Default Re: Car battery charger project.

Bridge, I reckon.

Also, transformers done for battery chargers used to be designed with deliberate leakage inductance paths so that their raised output impedance acted to limit current into discharged batteries. If this was stated to be a battery charger transformer, then it might be this type.

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Old 10th Jan 2021, 12:11 am   #6
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Default Re: Car battery charger project.

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Bridge, I reckon.

Also, transformers done for battery chargers used to be designed with deliberate leakage inductance paths so that their raised output impedance acted to limit current into discharged batteries. If this was stated to be a battery charger transformer, then it might be this type.

David
The newer battery chargers, the switch mode type, cannot turn on unless the battery has a small amount of voltage in it, to prevent exploding.
They're light as a feather and work well. I use them for back up batteries.
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Old 10th Jan 2021, 12:17 am   #7
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Default Re: Car battery charger project.

Agree, a full wave four diode bridge, either a ready made bridge or four diodes.
The diodes should be generously rated, and not run too hard.
Some battery charger transformers did indeed have a high leakage reactance so as to limit the current into a badly discharged battery.
Others relied on a series resistance to limit the current. Sometimes a low ohms wirewound resistance was used, but more usually the stray resistances of the connecting wires, the ammeter, the fuse holder, and the fuse would suffice.

I would consider it good practice to fit TWO fuses, one in series with the output to protect against short circuit, gross overload or a wrongly connected battery. And another fuse between the transformer secondary and the rectifier input, to protect against a rectifier failure.

A cheap moving iron ammeter is usual to monitor the output current. Remember that this will indicate the RMS current and therefore be a bit optimistic as regards time to charge the battery, which is related to AVERAGE current, not RMS.
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Old 10th Jan 2021, 1:11 am   #8
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Default Re: Car battery charger project.

I now have a modern battery charger that can be set for charging car, motor cycle or SLA batteries. Before I got it I used to use a simple 1960's trickle charger and connected a tungsten bulb of an approriate wattage in series to limit the current when charging SLA batteries. The brightness of the bulb used to give a good indication of the state of charge.
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Old 10th Jan 2021, 1:34 am   #9
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I now have a modern battery charger that can be set for charging car, motor cycle or SLA batteries. Before I got it I used to use a simple 1960's trickle charger and connected a tungsten bulb of an approriate wattage in series to limit the current when charging SLA batteries. The brightness of the bulb used to give a good indication of the state of charge.
One of the electricians I worked with asked why a battery charger, set for 12 volts, only measured only 10 volts without a battery connected. I reasoned that the discharged battery acted as an electrolytic capacitor and removed the ripple. After the battery was fully charged, the ammeter showed minimal current.
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Old 10th Jan 2021, 1:46 am   #10
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Default Re: Car battery charger project.

I wrote up one I built but the post did not survive for long! I was trying to get as much current into the flat battery as possible for emergency starting after being in lockdown for weeks on end. I managed about 20A with a single-winding transformer, and the only problem was the heat generated, first in the bridge rectifier block, where I progressively increasd the heatsink made of cast-off bits of sheet ali and painted matt black (blackboard paint is ideal). Next I had to add a heatsink to the transformer! Fortunately this is never needed for longer than 10 minutes
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Old 10th Jan 2021, 2:06 am   #11
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Default Re: Car battery charger project.

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Originally Posted by Julesomega View Post
I wrote up one I built but the post did not survive for long! I was trying to get as much current into the flat battery as possible for emergency starting after being in lockdown for weeks on end. I managed about 20A with a single-winding transformer, and the only problem was the heat generated, first in the bridge rectifier block, where I progressively increasd the heatsink made of cast-off bits of sheet ali and painted matt black (blackboard paint is ideal). Next I had to add a heatsink to the transformer! Fortunately this is never needed for longer than 10 minutes
Scrap fans taken from computer power supplies are ideal for cooling rectifier heat sinks. They're readily available for next to free.
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Old 10th Jan 2021, 3:50 am   #12
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Default Re: Car battery charger project.

I use an homebrew charger, which I like very much. It's a very simple transformer + bridge (Si) rectifier to which I have added a trip which ends charging at ~14.8 volts, which I think is necessary for modern "Maintenance free" batteries. I've encountered vintage chargers which would drift up to voltages where vigorous gassing would ensue. The whine of a Pye inverter and/or the smell of sulphuric acid immediately take me back to 2m AM days of the 1960's.

I've got meters for both current and voltage; I really like to be able to see what's going on. I wouldn't go near many of the chargers currently on sale.

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Old 10th Jan 2021, 6:28 am   #13
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Default Re: Car battery charger project.

Wet lead acid batteries are a thing of history. "Dry" modern batteries are very similar except the electrolyte is contained in fibreglass matting that prevents spillage. Motor accidents very often had major health risks for fire fighters and rescue personnel from leaking batteries.

Imagine screwing down a shingle/slate/roofing tin with a 10 AH lead acid battery. I would rather use a "weightless" Lithium Ion electric drill.

I DO agree with metering of battery chargers, ALL types. Modern chargers tend to have a horrible screen full of jumbled jagged numbers with sometimes unknown designations, so I still tend to use D-Arsonval movements that can be "read" from a couple of metres away.

Joe
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Old 10th Jan 2021, 10:35 am   #14
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Default Re: Car battery charger project.

Hi Folks, as mentioned earlier most battery chargers used a bridge rectifier, so a modern bridge would be ideal. It will give a little higher peak current due to the lower rectifier drop but this should make little difference to the charging.
A cheap MI ammeter is a good additions as would be a low value (2R?) high wattage resistor so you can set a trickle charge for maintenance. In older transformers this was sometimes catered for with a trap on the transformer.

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Old 10th Jan 2021, 11:36 am   #15
MALC SCOTT
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Default Re: Car battery charger project.

Hi, many thanks for your helpful comments. My son who is a steel fabricator has made a nice robust case for this project. I have a 25a metal clad bridge rectifier and several 25w metal clad resistors to experiment for trickle charging. Just need an ammeter and a voltmeter now. I will post some pics when construction begins. Cheers, Malc.
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Old 10th Jan 2021, 12:14 pm   #16
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Default Re: Car battery charger project.

I too would favour analogue ammeter/voltmeter for a charging regime. Apart from improved readability at commencement of charge, i am not convinced that (all of) the pretty multi-coloured online digital purchases are going to have the level of long term robustness needed for a portable charger. It would be a shame to be left with an awkward shaped hole in the panel if the module failed and was found to be out of stock.

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Old 10th Jan 2021, 2:24 pm   #17
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Default Re: Car battery charger project.

Quote:
The newer battery chargers, the switch mode type, cannot turn on unless the battery has a small amount of voltage in it
I have a modern one (A Ring from Halfords, emergency purchase to get SHMBO's car working), indeed it needs a few volts to start normally, it has a "hospital" button for recovering dead flat batteries which works to try and get something out of a scrapper.
 
Old 10th Jan 2021, 3:56 pm   #18
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Default Re: Car battery charger project.

A bridge-rectifier is definitely the easy/cheap option: you can get nice aluminium-encapsulated 25A bridges with blade-connections at remarkably low prices.

One important thing - you need to have some mechanism to limit the voltage-rise as the battery reaches full-charge: it's oh-so-easy to forget to disconnect a charger, and come back the following day to find the terminal-voltage has reached 16.5V and the battery is vigorously gassing! OK, vintage batteries can be topped-up with deionised water to replace that which has been electrolysed-away but if a modern 'sealed' or 'maintenance-free' battery is made to gas then the lost electrolyte-volume represents irretrievably-lost charge-storage-capacity.

The 'cut-off' point is also temperature-dependent [many modern vehicle batteries have an embedded temperature-sensor so the charge-level can better be controlled].

The 'cheap and easy' way to prevent overvoltage killing the battery is to stick a Zener-diode-feeding-a-big-power-transistor in parallel with the charger's output-terminals: a 2N3055 on a heatsink, with a 14.5V Zener, will stop things going above 15V. Alternatively, a suitable Zener in series with one of the little 'Sonalert' beepers - so when the battery-voltage tends towards overvolting the beeper starts squealing!
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Old 10th Jan 2021, 5:35 pm   #19
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Default Re: Car battery charger project.

There's a bit more to charging sealed lead batteries, shoving loads or amps down them at first shortens their life. From the attached bumf at first a CC charge, then a CV charge is the way to go, so some sort of regulation would be needed, either that or a few switchable resistors, apologies if this is teaching elderly relatives to masticate and imbibe chicken arse bullets.

I have a few dead Alcatel 12v PSU's (PCB with with big diodes and simple 723 regulation) and some gurt big switches if you'd like em Malc, drop us a PM if so.

Andy.
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File Type: pdf charging_sealed_lead_acid_batteries.pdf (61.1 KB, 74 views)
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Old 10th Jan 2021, 6:36 pm   #20
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Default Re: Car battery charger project.

Quote:
Originally Posted by Julesomega View Post
the only problem was the heat generated, first in the bridge rectifier block, where I progressively increasd the heatsink made of cast-off bits of sheet ali and painted matt black (blackboard paint is ideal).
Be careful with blackboard paint, some of it has metal particles in it so the resultant painted surface will allow magnets to be stuck to it.
I found this out the hard way.

Peter
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