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


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!


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

What POWER rating is the supply you have ?

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!

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.

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.


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