# Charging a lead acid battery

#### jethro99

Joined Oct 31, 2020
13
When a constant voltage power source is applied to a lead acid battery the initial charge current is relatively high. The current will then get lower and lower as the battery state of charge increases. Voltage of course stays the same given the constant voltage power supply.

Using Ohms law, it would seem that the internal resistance of the battery is increasing as the battery gets charged. But since a battery is not a typical electrical load as I normally think of an electrical load, is it correct to say that the internal resistance of the battery is changing or is there some other term that is to be used?

Effective resistance maybe? Equivalent resistance maybe? Or ?

Joined Feb 20, 2016
3,906
Don't forget the internal battery voltage that is opposing the charging voltage.
So, if you want to apply Ohms Law, maybe use (charging volts - battery volts) as the voltage used in the calculations.

#### crutschow

Joined Mar 14, 2008
27,709
The internal impedance of a near fully-charged Pb-acid battery is very low.
As dendad noted, you can't use Ohm's for the DC voltage value since the internal battery voltage is opposing most of the applied voltage, giving a very low net voltage across the internal resistance.

To use Ohm's law you could impose a small, low-frequency AC voltage from a generator to the battery through a DC block capacitor and measure the change in current from that AC voltage.

#### jethro99

Joined Oct 31, 2020
13
Thanks for the replies.

#### KeithWalker

Joined Jul 10, 2017
1,912
A lead/acid battery charger is usually a fixed voltage source of around 2.4V per cell, but with either a built in constant current supply or a series resistor to limit the output current to a safe level.
When a discharged battery is first connected to the charger, the voltage at the battery terminals will be limited by the charger current limiter. As the battery charges, the terminal voltage will gradually increase until it is equal to the maximum charger voltage, when no more current will flow.

#### Ian0

Joined Aug 7, 2020
3,214
To use Ohm's law you could impose a small, low-frequency AC voltage from a generator to the battery through a DC block capacitor and measure the change in current from that AC voltage.
You can use the ripple on a rectified supply as the AC test signal.

#### Ian0

Joined Aug 7, 2020
3,214
A lead/acid battery charger is usually a fixed voltage source of around 2.4V per cell, but with either a built in constant current supply or a series resistor to limit the output current to a safe level.
When a discharged battery is first connected to the charger, the voltage at the battery terminals will be limited by the charger current limiter. As the battery charges, the terminal voltage will gradually increase until it is equal to the maximum charger voltage, when no more current will flow.
The "chemical" voltage keeps increasing as the battery keeps charging. When the battery is fully charged, the chemical voltage stops increasing. If the chemical voltage is less than the charger voltage, current will keep flowing indefinitely. This is what happens with a float charge voltage of 13.65V, which is above the fully-charged voltage. The current that continues to flow electrolyses water into hydrogen and oxygen, which recombine to form water in a VRLA battery. A flooded battery gradually loses water if left on float charge, and will fail if not topped up.

#### sdowney717

Joined Jul 18, 2012
705
I just replaced an old dumb charger which slowly cooks batteries with an IP22 Victron 30 amp. Uses bluetooth to tell you things about the charging. IT is a 4 stage, so after float at 13.8, it drops to storage at 13.0 volts. The old charger boiled the water out in a few months, and even though has 3 bank charging, it cooks all banks. Float is set to 13.8. I did notice, if a bank is disconnected, the output volts rises to 17.5 vdc. It seems if one battery goes down, it would compensate by raising the volts on all 3 banks cooking good batteries. To send charging to 3 banks, It uses diodes for each bank from common output source.

#### Tonyr1084

Joined Sep 24, 2015
6,253
Dumb chargers are good for an overnight boost of a weak car battery but they're not intended to maintain battery voltage. Their construction is simply a transformer and rectifier. Maybe an amp meter too. Since the home voltage can vary - the voltage output of a dumb charger will vary as well.

Long time ago I built a car radio into a portable radio. At home it worked well but that was because the house voltage was about 113 VAC. When I took it to the work-place it didn't work because the voltage was too high. AC outlets were pushing 122 VAC, which translated into too much voltage; and the radio didn't like the higher voltage. Would not play unless I modified the power source. Got my hands on an autotransformer and used that to knock the voltage down just a little, but now the darn thing was getting too heavy.

I now have a car battery powering a car radio in the garage/shop. The battery is kept charged via a buck converter set to maintain 13.6 volts DC. 'The converter is only good for a few amps so it's not good as a charger, but once the battery IS charged it can maintain it just fine. And the car radio, which isn't played all that often, is happy with a battery voltage of 13.6 V.