Unregulated DC to regulated DC

[Demis]

Hey all!

For a university project I want to convert an unregulated DC input to regulated (steady) DC output ideally being able to control the amount of the steady output voltage as I need to charge a 6V lead acid battery.

I am not exactly sure to what kind of circuit I should be looking but if there is a component which is readily available from the market that would be ideal for now.

I will appreciate any kind of help! )

 

[ronsimpson]

What size of battery?
Do you have a data sheet.
Charge current?
If you charge smartly; the circuit will limit the current, and limit the voltage, and maybe when the battery is full, shift to trickle charge mode.

There are battery charging IC. Google search or search on DigiKey.com (any part vendor sight) also try TI.com, linear.com etc.

 

[audioguru]

A voltage regulator is not used to charge a battery. Instead a battery charger circuit or IC is used that limits the current and shuts off when it detects a full charge.

 

[Mr RB]

Actually A.G, a voltage regulator makes a pretty good lead acid battery charger I have made quite a few of them over the years.

An LM317 can be used as a voltage regulator, and will limit the current at about 1.8A by its internal current limit. Charging SLA batteries it won't stay long (if at all) in current limited mode, it's only a small part of the Charge cycle.

You can also use a resistor before the LM317 voltage regulator, which will limit peak current into the battery.

So Demis could use a common LM317 as a battery charger depending on some things we need to know like battery size and the power supply type and voltage etc.

 

[dr pepper]

I've seen circuits with the lm317 that have adjustable regulation of both voltage and current, great for a lead acid.
I'll see if I have anything on disk.

 

[ben7]

Actually A.G, a voltage regulator makes a pretty good lead acid battery charger I have made quite a few of them over the years.

An LM317 can be used as a voltage regulator, and will limit the current at about 1.8A by its internal current limit. Charging SLA batteries it won't stay long (if at all) in current limited mode, it's only a small part of the Charge cycle.

You can also use a resistor before the LM317 voltage regulator, which will limit peak current into the battery.

So Demis could use a common LM317 as a battery charger depending on some things we need to know like battery size and the power supply type and voltage etc.

Yeah, though, as long as you limit the maximum current. Or else the regulator might overheat. Using the maximum current limiting, internal to the LM317, makes me fidget! LOL

You can use a constant current trickle charger too. Some types of batteries, like lead acid for example, are fine with trickle charging. The idea is that the 'trickle' charge current is small enough that the battery won't get damaged much when it is fully charged. The trickle charger however would take a long time, especially if it is a big lead acid battery, like a car battery.

 

[audioguru]

A lead-acid battery loses its charge in about 2 months when it is not used. A trickle-charger is not used to charge the battery, instead it is used to keep it charged when it is already charged. The trickle-charger simply makes up for the losses.

 

[Mr RB]

Yeah, though, as long as you limit the maximum current. Or else the regulator might overheat. Using the maximum current limiting, internal to the LM317, makes me fidget! LOL
...

Yeah I agree it's generally not considered good form to use a regulator running on it's internal limiting. But this is not the overtemp shutdown limit in the regulator, and an LM317 has a separate independent current limit built in which can run safely constantly depending on device dissipation.

For a 12v charger, my experience with a 7Ah battery shows it spends little time on the current limiter, just the first 10 minutes, then stabilises at 14.4v, where it spends an hour or two getting the bulk charge (current constantly reducing during bulk charge). Float charge is then done at 13.8v for a few hours. Total time on the current limiter is very short and well within the safe dissipation of the LM317.

You can get a similar (and safe) effect by putting a resistor before the LM317, so the resistor sets the max current during those first few minutes of charging.

Alternatively, you can put the resistor after the LM317 and before the battery which also limits the peak initial current, but that affects the voltage regulation. If the resistor is before the Lm317 it only has an effect on the initial peak current, then after that the voltage regulation to the battery is unaffected.

 

[alec_t]

I found this on the web:

The principle is that the ADJ pin current path is initially ungrounded so the charge rate is initially current limited (set by R1, ~1Ω). When the battery voltage reaches a pre-set limit the SCR fires, grounding the ADJ pin current path, and the charger then operates in pre-set voltage mode.
This could be the basis of a smart charger.

 

[Mr RB]

The problem with that circuit is that the series resistor is still in circuit even in voltage regulator mode, so it drops volts and the regulation will be poor.

Putting a resistor in front of a simple LM317 voltage regulator will still limit max current but allows perfect voltage regulation at other times.

Even better, the old trick of one LM317 as a current limiter, feeding a second LM317 as a voltage regulator. That gives excellent current and voltage regulation and only needs the two regulator ICs and 3 resistors + caps.