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Old 15th November 2006, 07:47 AM   #1
Wynand is offline Wynand  South Africa
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Default 12V Battery Charger for Car

Hi. I know this isn't audio, but maybe it can apply to the guys building car amps.

Could anyone tell me how I should charge a car battery and give a circuit to do so. I've got to build a battery charger for my dad.

Preferrably I'd like something simple with a LM338 regulator (because i've got one), and just a LED to say when it's finished.

Q1. What do I need to supply to the battery, a constant voltage or a constant current?

Q2. What voltage, or what current?

Q3. Will it need to switch between charging types?

Q4. When it's finished charging, could I use a relay to disconnect the battery so it doesn't charge/discharge too many times.

Thanks for helping
Wynand.
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Old 15th November 2006, 07:51 AM   #2
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Constant current is generally preferred for charging batteries.

I'm not sure of the best charge rate for car batteries, but I would imagine the current is in the 1 - 10A range or thereabout.

Others will have more information to offer.
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Old 15th November 2006, 07:55 AM   #3
Wynand is offline Wynand  South Africa
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Thanks Duo.
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Old 15th November 2006, 08:13 AM   #4
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Most of the commonly available battery chargers that I've seen are nothing more than a transformer and rectifier..maybe an ammeter.They don't even have any filtering capacitance. (The battery takes care of that.)
But they aren't meant to be connected to the battery 24/7,just long enough to charge them..
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Old 15th November 2006, 08:20 AM   #5
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Yes, most typical car battery chargers are extremely basic, and they do get the job done.

The charging system a car uses isn't too elegant either.

Really, large lead acid batteries don't need very precise charging conditions.

On the other hand, lithium ions and nickel metal hydrides must be looked after very carefully
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Old 15th November 2006, 08:20 AM   #6
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That probably all I need. I'll just want a relay on to switch the bettery out of circuit when its reached full voltage.

...but to me it sounds too easy?!?
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Old 15th November 2006, 08:24 AM   #7
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I see there's a circuit in the LM338's datasheet using a LM301 opamp, but it doesn't specify if it's for a lead-acid battery
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Old 15th November 2006, 10:24 AM   #8
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You musntn't switch the charging off when the battery has reached full voltage; voltage is not an indicator of a fully charged battery.

Current draw at a specified voltage is more likely to tell you when it's done. For example; say the battery has reached 13.4V(not a bad normal voltage) during its charging but is still drawing 3A current. You take away the charge because the voltage is good, and it drops then to maybe 10V... 10V is pretty bad, you don't want it to go that low, but just as an example.

Generally in the case of lead acid, you can put them on a regulated voltage charger, and leave them indefinitely because once the battery reaches equalibrium with the charger voltage, it will only draw enough current to maintain its charge. This is not overcharging.

Think of uninterruptable power supplies; they leave the battery on a 13.8V supply constantly, day and night. This is to maintain the charge but cannot overcharge it.
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Old 15th November 2006, 11:39 AM   #9
Wynand is offline Wynand  South Africa
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Ok. Been thinking

LM338 to regulate the voltage, followed by a LM338 to regulate the current to 5A.

2.25V/Cell floating voltage = 13.5V for 6 cells.
2.30V/Cell floating Voltage = 13.8V for 6 cells.

Vout = 1.25 * (1 + R1/R2 )
13.75= 1.25 * ( 1 + 2200R/220R )

Thats the V reg and then the I regulation is a simple on in the datasheet.

I see your point on the switching off part.
Maybe a small Ammeter in series is a good idea.

I'd like to be able to leave this circuit connected in the car. So I rekon a diode on the output would be needed. So I'll have to raise the voltage by ~0.6v.

I'm thinking of a switch on R1/R2 so you can boost the voltage a bit for quicker charging.
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Old 15th November 2006, 10:12 PM   #10
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This is good as far as projects go, but such a regulation scheme for charging car batteries is entirely overkill...
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