I am trying to charge a discharged 12 V Car Lead Acid Battery using a SMPS. According to voltage information provided for charging lead acid battery (https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery), I have set the output of SMPS to 14.2 V.

However, as shown in the diagram attached, when I measure the voltage across battery it is showing 12 V instead. However, at junction, the voltage is shown as 14.2 V. The junction is simply a joint between wires coming from battery and the charger wires.

wiring diagram

So, I had some questions:

  1. Is 12 V being read due to fact that as battery sinks the current, the voltage reading at its terminals drops?

  2. As per the information provided for charging battery, the voltage range is 2.30 - 2.45 V/Cell or 13.8 - 14.7 V for Car Lead Acid Battery. So is this the open circuit voltage of the charger, or the voltage which should be read at the battery terminals when the battery is being charged?

  • 3
    \$\begingroup\$ This kind of thing is not at all safe to do when you do not understand the issues. You should probably not be trying to improvise a charger, but rather purchase a suitable automotive battery charger. Really the first thing you should do is take the battery to an auto parts store as they will often test them for you so you can find out if it is recoverable at all or if you need to get a replacement and turn the old one in for reclaiming of the lead. \$\endgroup\$ Commented Oct 10, 2020 at 14:38
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    \$\begingroup\$ That schematic and those measurements tell me those wires are rather thin... \$\endgroup\$
    – user16324
    Commented Oct 10, 2020 at 15:37
  • 1
    \$\begingroup\$ The battery will only measure 13.8 volts when it is fully charged. If you connect your 14.2 volt supply to a discharged battery measuring 11 volts, the battery will draw as much current as your supply can produce, pulling the supply voltage down, and dropping some voltage in the connecting wires. \$\endgroup\$ Commented Oct 10, 2020 at 16:02
  • 4
    \$\begingroup\$ It could well be that the thin wires are saving your battery from too high a charging current, also depending on the size of the battery and the maximum current your SMPS can deliver. You may want to read the whole Battery University article you refer to, especially the bit about constant-current charging; what you are trying to do is not the way to charge a discharged lead-acid battery. \$\endgroup\$
    – ocrdu
    Commented Oct 10, 2020 at 16:07
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    \$\begingroup\$ How are you limiting the current? \$\endgroup\$
    – winny
    Commented Oct 10, 2020 at 20:21

2 Answers 2


What you are currently doing exactly opposite of what the article you linked to suggests charging lead-acid batteries.

The article says to use CCCV method which means constant current constant voltage. It means that the charger first limits the charge current to constant predefined level and the charging voltage is at the battery voltage level, until the battery voltage has risen to the predefined voltage level, so the charge current then starts decreasing.

Your SMPS is a constant voltage source, it will try to push as much as current it possibly can into your battery because battery has less voltage. Fortunately the resistance of wires has limited the current because there is voltage drop in them, but the battery or SMPS or wires may not handle the current if it is too much.


Charging a lead-acid batteries is rather complex matter.

First, the safety:

  1. Even a discharged car battery can output enough current to melt your wires and/or start a fire and/or burn your hands, if you make a small mistake and short the battery.

  2. An over-charged, over-discharged or simply damaged battery may produce hydrogen. It is explosive when reaches enough concentration in the air. Closed spaces are not good unless you really know what you are doing. Experiment in open air or at least near open window.

  3. Wear goggles or safety goggles. The battery acid is not immediately dangerous, unless it gets in eyes.

The 'net is full of safety tips regarding lead-acid batteries.

Then, the charging device.

You absolutely need to use a power supply that limits both the current and the voltage. Not all SMPS devices behave adequately when overloaded. A discharged car battery can easily sink ~200 amperes if the charger has enough power and tries to "up" the voltage to 14v. The SMPS may either decrease its output unless the current drops to a pre-defined limit, or completely turn off, or get damaged. Only the first variant is good for charging a battery.

The process:

Set the SMPS to limit the voltage to 13.8 - 14.4v and the current to less than 1/10 of the battery capacity. A 60Ah battery needs to be charged with 6A or less. The wires should be adequate for this current.

Watch happily as the voltage increases up to the set point (constant-current phase) and then current decreases (constant-voltage phase). That's what the CCCV abbreviation stands for.

You can consider the battery 100% charged when the current drops below ~1/50 of the battery capacity (say, 1.2 ampere for a 60Ah battery). A higher voltage setpoint will shorten the proccess, but is worse for the battery life.

Observe constantly the battery temperature. If the battery gets ~50C, disconnect the charger and let the battery cool down. You may as well try with lower current and voltage limits.

A damaged battery may as well not reach the voltage setpoint. You should give up charging if the battery gets ~120% of its nominal capacity (say, 12 hours x 6 ampere for a 60Ah battery) even if the voltage setpoing is not reached.

You may as well stop the process at some intermediate point. A battery charged to ~10% of its nominal capacity should start the car with no issues. Running the car for an hour or two should get the battery to adequate state of charge.

Well, good luck. Or just buy a charger. It does most of these things by itself.


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