Discussion in 'Power Electronics' started by pidja105, Apr 27, 2016.

1. ### pidja105

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1
Oct 16, 2015
Hello,
I have 12V 7Ah lead acid battery, I have power supply (classic power supply without charging circuit) 12V 2-3Amps, how to use supply to charge battery?

1,096
104
Oct 26, 2011
How simple?

3. ### Alec_t

2,933
799
Jul 7, 2015
You need a power supply voltage of at least 13.8V to charge a 12V lead acid battery.

davenn likes this.
4. ### Colin Mitchell

1,417
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Aug 31, 2014
"You need a power supply voltage of at least 13.8V to charge a 12V lead acid battery."

13.8V will give you no charging current.

5. ### ChosunOne

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108
Jun 20, 2010
Wow, you mean for the last 40 years I've been using ~13.8V as a standard to see if my customers' 12V Lead-acid batteries would charge after I've left the premises? that I've actually been leaving them with a system that wouldn't work? Amazing that all those thousands of customers didn't complain about my service sooner or later! Also amazing that their batteries continued to work through all the occasional power outages year after year without the batteries dying from lack of being re-charged from a charging circuit that topped out around 13.8 VDC.

All sarcasm aside, a lead-acid cell that is fully charged will read ~2.2VDC, OCV (Open Circuit Voltage, i.e.. no load connected); so a 6-cell battery will read ~13.2V when fully charged. The "12V" is a "nominal" designation for a battery that seriously needs charging if it actually reads 12V, OCV.

For an accurate OCV reading to assess charge, the battery needs to lie inactive (no charging or discharging) for a couple of days, but you can get an approximate reading after a few hours. Taking an OCV reading right after it's disconnected from charger or load won't give an accurate reading.

13.8VDC will push a charging current through a lead-acid battery, even if it's fully charged already--which is why you need some kind of regulation on chargers: So they don't overcharge batteries.

I'm curious, Colin: Why did you think it wouldn't?

EDIT: @Pidja, some power supplies and batteries actually operate at a slightly higher voltage than their ratings indicate. If your "12V" (I assume it's actually "12VDC"? ) power supply actually has a constant 12VDC output, it's not high enough to charge your "12V" lead-acid battery.
If you have a digital multimeter (DMM), check the output of the supply. If you don't have a DMM, I recommend you not try any DIY electronics before you acquire one.

Last edited: Apr 28, 2016
6. ### Colin Mitchell

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Aug 31, 2014
The 13.6v "floating charge" or "floating voltage" produced by the battery will oppose the 13.8v and almost NO CURRENT WILL FLOW. (maybe 50mA) - certainly not 2 amps.
You need to have more than 13.8v "up your sleeve" so you can charge at a decent current and then stop the charging when the battery voltage reaches 13.6v
A simple power supply of 12v will not do this because you do not know the unloaded voltage.
A battery charging circuit (power supply) must have a LOW impedance so the supply will deliver a current to the battery when its "floating voltage" is below 13.6v and this current can be 2 - 4 amps for a car battery. When the "floating voltage" reaches 13.6v, the current must reduce to less than 1 amp. This mans the number of turns on the secondary must be counted - down the last half-turn, and even the leads to the battery and ammeter resistance must be taken into account.

Last edited: Apr 28, 2016
7. ### cjdelphi

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Oct 26, 2011
I float charge at 14.2v never had a problem