Question on charging 12v car battery with step-down transformer, safety issues


I have an AC->DC step down transformer that outputs at 12v (max 13v, can be adjusted with potentiometer). It is rated at 10A.

1) If I were to connect it to my dead car battery would it even charge? If not, how should I go about charging it from the mains?

2) If it charges, would it charge safely? If not, do I need any resistors or fuses?

3) How should I monitor the charging or know when it's good enough for cranking?

Thank you!

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You can't charge it with 13V, you really need at around 14V.

What POWER rating is the supply you have ?
pantss (author)  steveastrouk4 years ago
I have 2 step-downs with specs:
Output Power Rating: 74 W
Input Voltage: 85 VAC to 264 VAC, 120 VDC to 370 VDC
Number of Outputs: 1
Output Voltage (Channel 1): 12 VDC
Output Current (Channel 1): 6.2 A

I have another one which is largely similar, with 3 outputs and 10.0 A - I can't find the power rating at the moment.

I hope I'm answering your question. Thanks again!
pantss (author)  pantss4 years ago
Ok, found it.

The bigger step-down is as follow:

1. Output voltage: 12VDC
2. Output error: ± 1%
3. Output current: 0~20A
4. Wave and noise: 150MP-p
5. Output power: 300W DC
6. Efficiency: 82%
7. Voltage range: 10-13.2
8. Input voltage range: 88~132VAC 176V~264VDC 47~63Hz
9. Input current: 6A/115V 3.5A/230V
10. Impact current: 20A/115V 40A/230V (Cold-start)
The bigger of the two should manage it. Its a beefy charger, but I think it will be OK, because the output voltage limit is just about ideal.

Try it, ideally with a good high current meter in series - most little DVMs won't handle 20A.
pantss (author)  steveastrouk4 years ago
Thank you Steve.

May I know what should I be looking out for on the meter? Should the amps be high or low when charging?

Also, when should I stop the charging process?
The meter needs to be a 20A rating at least - that's what the supply can give you. You'll see a very high current for a minute or two I suspect, but then it will decay exponentially as the battery charges. You terminate charge when the open circuit voltage reaches ~13V -that's why I worry that the supply isn't going to be quite high enough output volts to charge fully. Stop when the charging current drops to ~0.2 A - that's pretty well guess work.

A plain powersupply is NOT a charger, it CAN do the job, kind-of - even "official" car battery chargers don't do it properly, unless you pay big bucks.

pantss (author)  steveastrouk4 years ago
Much obliged, Steve!


Couple the 13 v and 2 v outputs together that should give you the extra push you need