# Classic home made simple lead/acid Battery charger

Thread Starter

#### Axel Stone

Joined Apr 8, 2015
12
Hello, i've been searching the internet for some SCR battery charger circuits, and i found this one

Here is the image if you don't want to visit that link

i want to use it to charge my four lead acid batteries in parallel ( 12v 100AH)
i studied electricity in the past, but i forgot most of it since my job is completely different, and it's been really long time, so i came here to ask few things:

1- i want to know if this circuit is correct or not , i do not see any capacitor after the rectifier etc.
2- About the transformer :
the op mentioned a 15v transformer :

Vs=15 x 1.414= 21v
21v-1.2v=19.8v

edit: deleted something stupid that i wrote, any way, is'nt 19.8v too much for this circuit

3- P=UxI
20x40=800 VA

am i right ?

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Thread Starter

#### Axel Stone

Joined Apr 8, 2015
12
i couldn't edit my post, but what other changes are necessary for this circuit

#### frpr666

Joined Feb 2, 2010
50
It's correct you don't see any capacitor after the rectifier. If there was a cap you couldn't switch-off the main SCR. What is the reason to charge the batteries in parallel?

#### MrChips

Joined Oct 2, 2009
21,647
I don't see how you can turn off SCR1.
To turn off an SCR you have to reduce the current to zero.

#### GopherT

Joined Nov 23, 2012
8,012
I don't see how you can turn off SCR1.
To turn off an SCR you have to reduce the current to zero.
I think the idea is that, once the battery is fully charged, current will essentially stop flowing into the battery and there will be a big enough voltage drop across ZD1 to turn on scr2 and when that happens, Scr1 will turn off.

#### MrChips

Joined Oct 2, 2009
21,647
I think the idea is that, once the battery is fully charged, current will essentially stop flowing into the battery and there will be a big enough voltage drop across ZD1 to turn on scr2 and when that happens, Scr1 will turn off.
While charging 4 x 100Ah batteries?

#### GopherT

Joined Nov 23, 2012
8,012
While charging 4 x 100Ah batteries?
Scr2 is a sensitive gate and requires only 200 uA gate current to turn on. Current to the battery is essentially limited by the 3A rating of the transformer. I am guessing this is for a big lead acid battery.

Maybe the OP can let us know.

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Thread Starter

#### Axel Stone

Joined Apr 8, 2015
12
i built two electic go karts , i power each one with two batteries

i used two 24v 6.7kw starter motor and they are really fast.

now i only need little help with this. i am going to use a 40A transformer for it, would that be okay ?

#### DickCappels

Joined Aug 21, 2008
6,533
There is a chance that the valleys in the voltage out of the full wave bridge will be close enough to zero volts for the SCR to turn off.

#### MrChips

Joined Oct 2, 2009
21,647
There is a chance that the valleys in the voltage out of the full wave bridge will be close enough to zero volts for the SCR to turn off.
Right. I forgot that this circuit has no smoothing capacitor.

Thread Starter

#### Axel Stone

Joined Apr 8, 2015
12
so guys what does all of this mean ? i am no expert with chargers really.

it would be fun to build my own charger instead of buying it. little help is really appreciated. and thanks all for your time.

#### frpr666

Joined Feb 2, 2010
50
I think the schematic is all right. I would add a fuse that would protect the transformer and diodes and the SCR when accidentally sorted the output wires. If you are going to use about 40A and the diode drop is 0.7V there will be 0.7Vx40A=28W waste heat. 28W each diode and one SCR.

#### #12

Joined Nov 30, 2010
18,210
I don't think so. 12 amp SCR on a 40 amp charger.
You're going to need a bigger SCR.

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#### #12

Joined Nov 30, 2010
18,210
Try this one. It is rated for way over the voltage you need, but it's cheaper than the low voltage versions and has less waste heat than most of them.

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Thread Starter

#### Axel Stone

Joined Apr 8, 2015
12
Thanks every one
you were very helpful.
i will have to read some info about resistors and capacitors so i can calculate the right values that will work with 40 amps

thanks again, have a nice day all of you. and sorry if i made some grammar.

#### frpr666

Joined Feb 2, 2010
50
of course you will need bigger diodes D1-D4 and bigger SCR1. R3 should be at least 1W type. There is no explicit current limitation for the current flowing to the battery but I think it is usual for this kind of chargers.

Thread Starter

#### Axel Stone

Joined Apr 8, 2015
12
just something confusing me, as this doesn't has a fixed regulator

op mentioned that he used a 15v transformaer, so it means that it outputs Vs=15x1.414=21v
and minus two diodes voltage drop which is 0.6v/diode equals 1.2v

so the rectified current is 19.8v , so does scr1 regulates it and gives to the battery 13.8v only ?

#### MrChips

Joined Oct 2, 2009
21,647
just something confusing me, as this doesn't has a fixed regulator

op mentioned that he used a 15v transformaer, so it means that it outputs Vs=15x1.414=21v
and minus two diodes voltage drop which is 0.6v/diode equals 1.2v

so the rectified current is 19.8v , so does scr1 regulates it and gives to the battery 13.8v only ?
You mean the rectified voltage.

Note that without a smoothing capacitor the rectified voltage is the positive cycle of a sine wave from 0V to20V.

No. SCR1 does not regulate the battery voltage.

The voltage across the battery will be determined by the battery. Think of the battery as a storage capacitor. The charging voltage will be at the battery voltage with some degree of ripple depending on the internal resistance of the battery. What is more important is the pulsating current being used to charge the battery. This current will pulsate from 0A to the max charging amperage determined by the state of discharge of the battery and how much current the charger can supply.

As the battery becomes charged the battery voltage will rise until the full charge detection circuit kicks in and shuts down the charger.

Thread Starter

#### Axel Stone

Joined Apr 8, 2015
12
guys, i feel like a total noob now
can anyone be nice and tell me the right value for each one of these if i was to charge at 40A rate
R1 R2 R3 R4 R5 VR1 C1

i tried to calculate R1 and got 345ohm . but i don't think i did it right

also if i use 15v transformer it will give me 21v , so 21v would be too much to charge a 12v battery

after my stupid calculation : to get 13.8v 40A , the voltage i want to charge my batteries at , i found that i need 10.61V transformer 600va
13.8+1.2=15v 1.2v is the voltage used by the two diodes that rectifies the voltage( i learned in the past that silicuim diodes have 0.6v tension when they conduct)
15/1.41=10.61 v
i don't think that such transformer does exist
can anyone explain this to me please ? i never thought it would be so complicated

#### Dodgydave

Joined Jun 22, 2012
9,310
R1 is just for the led,
why dont you just get a 15V transformer and a 60Amp bridge rectifier ,and connect the output to the battery, like a normal car battery charger, without all the scrs,they're just to switch off the charger automatically when the battery is charged,
your transformer has to be rated at 40 amp if your looking to charge at that higher current.

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