Why A 12V Bulb Connected In Series When Charging

Why A 12V Bulb Connected In Series When Charging

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zakmuh

Original Poster:

454 posts

109 months

Thursday 17th May 2018
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Hi Guys,


I need your help to understand certain things when doing a DIY charger. So please help me with ALL THREE questions below and then add more if you like.

I've got this ATX power supply unit from the PC and intending to use yellow and black wires, which should give 12V 20A when connected. I know it needs a little bit more than 12V to charge but I need to understand the followings first.


1. Should I solder all the yellow together? Can I not use just one yellow wire?

2. Can I directly connect these wires to SLA 12V 7Ah battery? Then the charging time would be 35mins (7Ah/20A=0.35h)?

3. Why some people connect a 12V indicator bulb in series? Is this to reduce/increase charging ampere and what would happen if I charge with a bulb in series?


Any advice on this would be highly appreciated.


Cheers

GreenV8S

30,150 posts

283 months

Thursday 17th May 2018
quotequote all
That will (should) be a voltage regulated supply which is not ideal for an SLA battery charger. For this you would be better off with a current limited supply, preferably with automatic cut-off when fully charged.
A commercial 12V battery charger would be much more suitable and not expensive. Get one with a battery maintenance capability if you plan to use it long term. That way you can just connect it up, and know that it will charge the battery and keep it fully charged until you are ready to use it.

Mr2Mike

20,143 posts

254 months

Thursday 17th May 2018
quotequote all
The bulb in series is to add current limiting to the PSU. Too much current can either damage the battery or trip the supply.

You are correct that 12v is nothing like enough to charge a six cell lead acid battery, it will probably get you to a 10-20% charge level depending on battery temperature.

finishing touch

808 posts

166 months

Thursday 17th May 2018
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I used to fly model aircraft and had a 12v starter that I ran from a small motorbike battery intended for a bike with a kickstart.

My car battery chargers output was 7amps so I ran it through a 55w headlight bulb.

It would charge fully overnight (8 hrs) and when charged the bulb reduced to just a dull glow. A useful indicator.



Paul G

zakmuh

Original Poster:

454 posts

109 months

Friday 18th May 2018
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Thanks for your replies guys.

I thought PSU would give exactly 20A but when checked it with a bulb and a multimeter connected in series, it read only 1.3A. Its too much for me to understand for being the first timer on DIY charger.


Yeah, I'll be better off with a proper battery charger, but the one sold there is 12V 500mA. I need more than 12V?

Cheers

Edited by zakmuh on Friday 18th May 09:12

Mr2Mike

20,143 posts

254 months

Friday 18th May 2018
quotequote all
zakmuh said:
Thanks for your replies guys.

I thought PSU would give exactly 20A but when checked it with a bulb and a multimeter connected in series, it read only 1.3A. Its too much for me to understand for being the first timer on DIY charger.


Yeah, I'll be better off with a proper battery charger, but the one sold there is 12V 500mA. I need more than 12V?

Cheers

Edited by zakmuh on Friday 18th May 09:12
Current = Voltage/Resistance

20 amps is the maximum the power supply can safely deliver, but the actual current flow depends on voltage (12v in this case) and the resistance of the load. A bulb drawing 1.3 amps at 12v will have a resistance of 12/1.3=9.3Ohms.

And yes you need more than 12v to charge a nominal 12v lead acid battery. A fully charged battery will be closer to 14v than 12v.

zakmuh

Original Poster:

454 posts

109 months

Friday 18th May 2018
quotequote all
Mr2Mike said:
And yes you need more than 12v to charge a nominal 12v lead acid battery. A fully charged battery will be closer to 14v than 12v.
How come then the battery charger output voltage shown 12V with 500mA

Krikkit

26,500 posts

180 months

Friday 18th May 2018
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zakmuh said:
Mr2Mike said:
And yes you need more than 12v to charge a nominal 12v lead acid battery. A fully charged battery will be closer to 14v than 12v.
How come then the battery charger output voltage shown 12V with 500mA
Because it's designed for a 12V battery - the actual voltage is carefully controlled, and a digital charger will usually have different voltage strategies depending on the charge state of the battery in the first place.

500mA is how much current it can supply while charging, this usually takes hours so is quite low. Very expensive "rapid" chargers might give 4-5A (or more), but cost £££.

tl;dr: Charging batteries isn't straight-forward.

zakmuh

Original Poster:

454 posts

109 months

Friday 18th May 2018
quotequote all
I know its very complicated!

I was trying to convert a PC power supply unit to charge the battery rapidly but I don't think its possible. The max I can get is 12V, unless there is a way I can use orange +3.3V and blue -12V to make up +15.3V but not sure about the current rate (amp)

zakmuh

Original Poster:

454 posts

109 months

Friday 18th May 2018
quotequote all
I know its very complicated!

I was trying to convert a PC power supply unit to charge the battery rapidly but I don't think its possible. The max I can get is 12V, unless there is a way I can use orange +3.3V and blue -12V to make up +15.3V but not sure about the current rate (amp)

Edited by zakmuh on Friday 18th May 12:38

GreenV8S

30,150 posts

283 months

Friday 18th May 2018
quotequote all
Just get a battery charger designed for 12V SLA batteries, preferably with an automatic charging capability. They're commonly available and not expensive.

Mignon

1,018 posts

88 months

Friday 18th May 2018
quotequote all
The actual voltage required to fully charge a battery is approximately nominal voltage plus 20% so for a 12v battery that's 14.4 volts. The actual fully charged voltage of a 12v lead acid battery is higher than the nominal voltage anyway at 12.7 volts. The nominal voltage is only an approximation. It's generally recommended not to charge a battery at a higher amperage than 10% of the ampere hour capacity. For an 80 ampere hour car battery that would be 8 amps. I usually prefer half of that over a longer period so 4 amps overnight for example. It's less likely to lead to overheating of the cells.

Penelope Stopit

11,209 posts

108 months

Friday 18th May 2018
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zakmuh said:
I know its very complicated!

I was trying to convert a PC power supply unit to charge the battery rapidly but I don't think its possible. The max I can get is 12V, unless there is a way I can use orange +3.3V and blue -12V to make up +15.3V but not sure about the current rate (amp)

Edited by zakmuh on Friday 18th May 12:38
Take a read of this and Google about for more http://www.instructables.com/id/How-to-make-a-car-...

If you pull your power supply apart you will notice that all the yellows join up at the circuit board the same as the reds and blacks do

Mr2Mike

20,143 posts

254 months

Friday 18th May 2018
quotequote all
zakmuh said:
I know its very complicated!

I was trying to convert a PC power supply unit to charge the battery rapidly but I don't think its possible. The max I can get is 12V, unless there is a way I can use orange +3.3V and blue -12V to make up +15.3V but not sure about the current rate (amp)
All the voltage rails on an ATX supply share the same ground, so it's not possible to connect any of the positive voltages in series to increase the voltage.

You could use the negative voltage however, between the -5v and the +12v rails is 17volts. However, the -5v rail is very low current (maybe 0.5Amps) and 17v is now too much to charge a 12v lead acid battery, even at low current you will boil away the electrolyte if you leave that much voltage applied.

It's possible to raise the output voltage of some ATX supplies by changing the feedback resistors internally, but given your level of experience I don't think it's wise for you to be attempting to modify the PSU. A basic car battery charger is pretty cheap however, Lidl and Aldi regularly sell a pretty good charger for about £14.

GreenV8S

30,150 posts

283 months

Friday 18th May 2018
quotequote all
Penelope Stopit said:
Take a read of this
I don't recommend that as a DIY solution. It ignores some pretty important issues, and the quality of writing doesn't give me any confidence that the author pays attention to such details. You can get volts and amps out of it, but that doesn't mean it's going to make a good battery charger.

Penelope Stopit

11,209 posts

108 months

Friday 18th May 2018
quotequote all
Here's an interesting Youtube video...you may have already seen it https://www.youtube.com/watch?v=XQS4Fm4owLM

Mr2Mike

20,143 posts

254 months

Saturday 19th May 2018
quotequote all
GreenV8S said:
I don't recommend that as a DIY solution. It ignores some pretty important issues, and the quality of writing doesn't give me any confidence that the author pays attention to such details. You can get volts and amps out of it, but that doesn't mean it's going to make a good battery charger.
Agreed, no proper current limiting and no reverse polarity protection for starters.

Penelope Stopit

11,209 posts

108 months

Saturday 19th May 2018
quotequote all
This method may be of interest to you if you haven't already seen it and is possibly the best one yet
http://www.instructables.com/id/How-to-Make-12v-Ba...

Kccv23highliftcam

1,783 posts

74 months

Saturday 19th May 2018
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Just go out and buy one that won't overcharge and self regulate!

Penelope Stopit

11,209 posts

108 months

Saturday 19th May 2018
quotequote all
Kccv23highliftcam said:
Just go out and buy one that won't overcharge and self regulate!
I go along with this
Buy one and build one for fun