Lead Acid Battery charge controller using already available power supply unit

[samEEEf]

My plan is to make a charge controller to auto charge a 12v Lead Acid battery. Here is the idea -

1. Power supply will be adjusted to deliver a safe voltage above 12 volt. Say 14v. Note that ampare rating of the power supply is 20A (safe for a 120AH battery)

2. Power supply will be connected to the battery terminals and the charging will be started happening with a high current depending on the terminal voltage of the battery. Say the battery voltage is 11v to be charged.

3. Battery voltage will be increasing with respect to time...

4. Battery terminal voltage will be monitored with a MCU ADC.

5. When the voltage reached to the upper safe threshold value (for example 13.8v), Power supply connection to the battery will be disconnected using a controlled switch (relay or MOSFET).

6. After disconnecting the power supply connection from the battery, the voltage will go down a little (for example 13.5v). Even if there is no load connected to the battery, battery will be discharging (at self discharge rate) with respect to time.

7. The MCU ADC will still monitor the voltage of the battery voltage.

8. When the battery voltage goes below a predefined value (for example, 12.5v), the switch (relay or MOSFET) will again connect the power supply to the battery and the battery will be started getting charged.

9. The controller will protect battery to be overcharged and discharged following the above steps.

Need opinion from the experts about the above process of controlling charging of a Lead Acid battery.

 

[treez]

you shoudl use a current regulated smps...then change to voltage regulated control when the voltage gets to about 13.8v.

..but actually its a four stage process.
1....trickle it up
2....big current in
3....Maintain at voltage 13.8V till current dies down to v small
4...maintain at the flow voltage

 

[Warpspeed]

Sam, I did something very similar but on a much smaller scale and for my own special purpose.
It has been working extremely well now for well over a year.

What I wanted was to keep the starting battery alive for a vehicle that is very infrequently driven. Float charging at a constant 13.8 volts still causes a gradual reduction over time of battery capacity, which eventually kills the battery.

I used a 15 volt dc one amp wall pack in series with a 12 volt 18 watt filament lamp connected to the battery through a relay contact.

Across the battery is a programmable digital voltmeter with relay output that is programmed to charge the battery up to 14.2 volts then open the relay.

The battery then self discharges (plus LED display voltmeter load) down to 12.6 volts, where the relay closes for a recharge.

Its good because the filament lamp glows very dull during charging, but the resistance of the filament increases at higher charging rates, keeping the charging current within reason, but also giving a visual display of charging activity.

Over time, the recharge cycles become very short and the discharge periods between recharges very long.
It exercises the battery in a way that seems to completely prevent sulphating, and gradual slow loss of capacity.

Its been a great success, and highly recommended for something like a standby power generator starting battery, that may sit for months and suddenly need to reliably source huge current.

This is the programmable voltmeter I used, perfect for this:
**broken link removed**

 

[samEEEf]

Sam, I did something very similar but on a much smaller scale and for my own special purpose.
It has been working extremely well now for well over a year.

What I wanted was to keep the starting battery alive for a vehicle that is very infrequently driven. Float charging at a constant 13.8 volts still causes a gradual reduction over time of battery capacity, which eventually kills the battery.

I used a 15 volt dc one amp wall pack in series with a 12 volt 18 watt filament lamp connected to the battery through a relay contact.

Across the battery is a programmable digital voltmeter with relay output that is programmed to charge the battery up to 14.2 volts then open the relay.

The battery then self discharges (plus LED display voltmeter load) down to 12.6 volts, where the relay closes for a recharge.

Its good because the filament lamp glows very dull during charging, but the resistance of the filament increases at higher charging rates, keeping the charging current within reason, but also giving a visual display of charging activity.

Over time, the recharge cycles become very short and the discharge periods between recharges very long.
It exercises the battery in a way that seems to completely prevent sulphating, and gradual slow loss of capacity.

Its been a great success, and highly recommended for something like a standby power generator starting battery, that may sit for months and suddenly need to reliably source huge current.

This is the programmable voltmeter I used, perfect for this:
**broken link removed**

Similar design except the bulb in series with the battery.
The bulb is equivalent to 8ohm resistance. If battery internal resistance is 4-5 ohm or less, will the battery be charged? There will be significant voltage drop in resistance compared to the battery whose internal resistance is much lower than that?

 

[treez]

sorry and this is the charging stages that i meant...
http://all-about-lead-acid-batterie...attery-charging/three-stage-battery-charging/

 

[Warpspeed]

Similar design except the bulb in series with the battery.
The bulb is equivalent to 8ohm resistance. If battery internal resistance is 4-5 ohm or less, will the battery be charged? There will be significant voltage drop in resistance compared to the battery whose internal resistance is much lower than that?

Ah ...That is the trick.....

The hot resistance will definitely be around 8 ohms, but the stone cold filament resistance is more like 0.6 ohms.
I know, because I just now measured the actual lamp.

It makes a fairly effective current limit, in that under light load the filament is barely red hot and the resistance is still quite low. But if you short out the battery charger, the lamp blazes brilliantly, and limits the fault current to roughly 1.5 amps to protect the power supply.

So its a current limiting resistor that goes away under normal light load.

It does work rather well, controlling the charging current to what the wall pack can comfortably handle, and still providing fault current limiting.

I tried out a few different wattage lamps to get it right, but settled on an 18 watt parking lamp as being just right for this.

 

[samEEEf]

Thanks Warpspeed for the clarification.

You chose the wattage of filament lamp using trial and error method. 18w fits with your battery capacity. But what is the math of getting right lamp for the required Battery capacity. Also need to know what technique is used in professional design for commercial products?

 

[Warpspeed]

It was not selected on battery capacity.
The power source is a one amp wall pack, and the lamp wattage was selected so that the charging current during a typical charging cycle was around 800 mA or so.

No calculations, just a bit of experimentation to get the system working reasonably well with the power source I was using. I would have been quite happy with 500 mA.

Commercial units are sized for the application which varies considerably.

For instance, a fork lift battery runs for an eight hour work shift and has a sixteen hour recharge period before it gets put back to work again.
Really no point in over sizing the charger.

A UPS may run for months without using the backup battery, so a really small charger will work fine even on a very large battery bank.

Some of the battery powered floor sweepers / scrubbers (used in shopping malls) have the requirement of needing to be charged from any standard wall outlet. So ten amps of mains power is the limit, no matter how large the battery.

No hard and fast rules. Every situation is different.