Charging an extra 12 volt lead battory from the car 12 system.

Hi!

I have in mind to install a second 12 volt battory in the car that will be used for things like cold box, air compressor etc..
The idea is that this extra battory will be charged by manually connecting it to a 12 volt outlet in the car when the engine is running. Then using this extra battory for external stuff will never drain tha starter battory of the car.

Google? Yes! Can someone suggest some good search criteria?

Search criteria for what?

Good question! Search expression to find the charger being fed from the car charging the external extra battory.

When your extra battery is a lead battery, you just need a resistor to limit max current. > 10W, I'd suggest.

Thanks but no. That is not the way to fully charge a lead battory. It wil take forever that way. Such a battory of some 40 - 60 Amp hours chould be harged by some 4 - 6 Amps up to the nearly fully cherged state.

I search for a ready made charger. Simple home made solutions I can do myself but I search for better stuff.

Maybe this can help: How to charge all lead acid batteries; how to charge SLA lead acid batteries, a tutorial for engineers about lead acid chargers and charging.

Thanks! I'll check them up. A step up is needed and then the "usual" lead charger....

I'll come back.

You find DC/DC-chargers are on the end of that page :slight_smile:

I found this one, 12V 3A DC UPS controller for Cars and Automobiles, Uninterruptible power supplies for cars, 12 volt BBU (battery backup unit)
However the current is on the limit. I would need 4 Amps continiously.

A common/cheap approach is a large Schottky diode between +car battery and +house battery.

Can also make an "ideal diode" (Google it) with a P-channel fet and a double-transistor.
The Uno does something similar between USB+ and 5volt.
Leo..

Railroader:
I search for better stuff.

If you can't find what you want at your local car accessories supplier, go a little further down the road to one that specialises in caravan or 4WD stuff. This cannot be hard.

Wawa:
A common/cheap approach is a large Schottky diode between +car battery and +house battery.

Can also make an "ideal diode" (Google it) with a P-channel fet and a double-transistor.
The Uno does something similar between USB+ and 5volt.
Leo..

I get it. However I see one issue. If I connect the extra battory and it is a bit low it will charge without current limiting from the original car battory Circuit, with alternator running or not.

Nick_Pyner:
If you can't find what you want at your local car accessories supplier, go a little further down the road to one that specialises in caravan or 4WD stuff. This cannot be hard.

Yes, that must be a business to check up. Campers and house cabins often use extra battories.

Railroader:
I connect the extra battory and it is a bit low it will charge without current limiting from the original car battory Circuit, with alternator running or not.

Any proper automotive/marine dual battery system separates the batteries. It is a vital part of their job - prevent the cranking battery from charging the second battery, and absolve a deep-cycle battery from cranking duty. Having said that, I have heard of a system that will allow some discharge from the cranking battery down to a safe voltage. I imagine they would be for people who didn't put in a decent secondary battery, and perhaps not such a great idea.

Some more info. I intend to put this device, the extra battory and a coldbox in the luggage compartment of the car. A 12 volt outlet is present there. That means that the alternator and starter battory is some 4 meters away….

Still I don't see the problem. You can spend $$$ on a ready made device. Or you can build a sophisticated charging circuit with a shunt+resistor+FET+Arduino. Or you can build the industries most used standard charging circuit (=resistor) with primary battery protection (=diode/FET+resistor). YOu'd just need to put the intended numbers down and decide.

I can surely build just anything, just that I'm looking for something ready and spend my time in my mechanics shop.

LOL .. I'm quite sure soldering 2 2-pin-parts would take less time than debating the storry on the forum :slight_smile:

You like charging using resistor. That's the most ineffektive and slowest method to charge a lead cell. For a battory connected all the time it might work. I want the charging to be quick and do as much charging as possbile in as short time as possible.

I doubt that Your "2 2-pin-parts" would even come close. I set up the requirements out of my idea how to use it in the total. You don't know,and You don't need to.
Working with lead batttories for many years I've seen things and have lots of respect for the energy they contain.

If you like a constant current source, use the first schematic here: Konstantstromquelle – Mikrocontroller.net - one FET, 2 resistors. RL is battery-to-be-charged, RS you'll need to calculate - and it needs to be > 10W+cooling. Charging will stop at Uext. >= Uint.-0.7V
Oh, and you cannot use this for tricklecharge - that would requre just a resistor+diode. Anyway, you could do fnny stuff with voltage comperators, relais and some more resistors. Or a stepper that turns a variable resistor, those good old 2kg wirewound :slight_smile: