Simple LDO Current Regulator

Can any-one recommend a 3 pin current regulator with low voltage drop between input and output voltages?

This is to charge a car lead acid battery from a cigarette lighter as fast as possible so would like to use the 20A available from the cigarette lighter.

I have a LM317 however this has a ~2v drop so if vehicle voltage is 14.5 max charging voltage is 12.5 so this leaves a lot of capacity in the battery after charging.

If a 3 pin regulator had less that 1v drop that would be great but I'm struggling to find anything to accept max 16v high current and low voltage drop input to output. ...

Or does the simple solution I'm after not exist and I need something more complex like this proposed circuit - problem is it will be difficult for me to get hold of these components and difficult to build (in comparison).

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You need to boost the voltage, a cigarette lighter socket will give less than the host battery voltage because of drop along the cabling. If you want to charge a 12V lead acid battery you need MORE than 12V to do it, in other words you can't use one battery to charge another of the same type. If your vehicle battery voltage is 14.5V it means something is wrong, it should be nominally 12V while idle and about 13.8V while under full charge. Most batteries will buckle thier plates and be damaged if you try to force 20A charging current through them.

What you really want is a booster to lift the socket voltage to say 15V then a proper lead acid charging circuit after it.

Maybe the PB137 would be a suitable charge controller.

Brian.

Brian,

14.5-14.6 is a normal automotive charge voltage if the vehicle battery is nearly fully charged.

If I drop say 0.5v due to wiring losses and 1v in a LDO current regulator that gives a max charge voltage 13V which should hopefully get the battery to 90% charge which will be enough.

I believe automotive lead acid batteries charge at unlimited current but the current will drop at higher SOC due to limiting voltage ~14.5v.

A car could recharge a flat battery (say the vehicle was jump started) in ~30 min of running say the capacity is 100ah then the battery is pulling ~200A to recharge in that time. I'm not concerned about charging in the lower SOC at 20A.

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So just to be clear - I'm only expecting the battery to charge when the vehicle is running and the alternator voltage is 14V+

Hi,

that gives a max charge voltage 13V which should hopefully get the battery to 90% charge

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Yes, hopefully.
Instead of hope ... did you look at some charging curve?

They show about 20% at 13V charging voltage.

Klaus

Well the 20A request completely disqualifies the LM317 and makes any linear solution difficult at the least (20A*2V = 40W)

There are ways to make regulators that can go to zero dropout but if the circuit posted (two transistor current limit) is too complicated it's hard to imagine what to recommend.

A big resistor?

1.
It could be worthwhile to make a cable for charging automobiles through the cigarette lighter. Just a few Amperes for a few minutes can make a difference. Recently I put a weak car battery on a 6A charger. After ten minutes it was able to start the engine immediately, whereas beforehand it would not even crank.

2.
This is sort of relevant. My mother purchased an 'emergency car battery booster' that you plug into the cigarette lighter. We never used it, and I never tested its claims. It contained a battery pack whose voltage had to be several volts higher than 12V (I'm guessing several large nicads). After usage on the road you were supposed to recharge it at home. We had it around for several years, then it became too weak to hold a charge.

This sort of direct electrical hookup carries risks, which you should anticipate. Fire could result, ruined components, battery discharging instead of charging. Etc.

A car could recharge a flat battery (say the vehicle was jump started) in ~30 min of running say the capacity is 100ah then the battery is pulling ~200A to recharge in that time. I'm not concerned about charging in the lower SOC at 20A.

Click to expand...

It isn't the case that doubling the current halves the charging time. Unless this is a huge truck battery or from an electrically propelled vehicle, more than a few Amps will damage it. Consider that passing 200A through the battery could potentially (no pun intended) disspate over 2KW from the plates, it would simply boil itself dry. Pb batteries should be charged at constant voltage but with a current limit as well. In other words a ceiling voltage should be decided (~14V) but that voltage should be reduced to ensure the current doesn't exceed maybe 5A for a normal car sized battery. Even at 20A the battery will get hot and could be damaged.

I would still suggest using one or more PB137 ICs in parallel, with a heat sink each will deliver 1.5A and they have thermal shutdown and reverse discharge protection. They are named PB137 because PB is the chemical symbol for lead and 13.7 is the cut-off voltage, they are specifically designed for lead-acid charger circuits. They do have a drop-out voltage of around 2V like most regulators but the only way around that is to increase the voltage ahead of them.

Your other option is a boost switch mode supply with voltage and current feedback. That would be considerably more expensive and complicated and you would not achieve full current rating of the lighter supply anyway. Power has to come from somewhere, it you want higher voltage out you have to put higher current in.

Brian.

Something that I've done in the past, quick and dirty and somewhat tricky to get it right (but works) is a PTC thermistor in parallel with a power resistor. These two devices are then in series between the cigarette lighter and the battery being charged.

The key element is the PTC thermistor. When the battery is completely discharged, it will attempt to sink lots of current. The thermistor then heats up and goes into a high impedance state, and on if left on its own, the available charging current would be too low. That is when the parallel resistor comes into play. It allows additional current to charge the battery.

As the battery starts to charge and the voltage differential drops, the thermistor cools down and goes low impedance, allowing again some extra current until it heats up and goes high impedance again.

This cycle repeats itself several times until the battery is almost charged and the thermistor always remains in the low impedance state.

The circuit is very simple, but requires significant experimenting and measurement to get the values correct. You will have to have an assortment of resistors and thermistors for your experiment. It is also better if you have a current probe to actually measure the charging current. And the battery voltage, of course.

EDIT: the PTC thermistor cannot be a polyfuse. Polyfuses have a wearout mechanism with each heating/cooling cycle which would render them useless after a few days. I used ceramic PTC thermistors.