Way to limit current to charge battery with car battery while engine is on?

Hello, I was thinking of doing a little project with a arduino wifi board since I already have all the needed components laying around. I was going to put a silicone heating pad on my car's oil pan to warm up the oil for cold starts in order to put less wear on engine components.

I have a small 8ah 12v agm battery laying around so I wanted this to supply the power rather than the car's battery. However, I want the car's battery to charge this battery when the engine is running. It would be easy for me to make a connection from a key on relay to the 2nd battery but obviously it's not good to connect 2 batteries directly with different voltages and the smaller battery max charging current is rather low as well. I'm guessing a diode can limit the power in one direction but how can I limit the current and make this work?

05silgto:
Hello, I was thinking of doing a little project with a arduino wifi board since I already have all the needed components laying around. I was going to put a silicone heating pad on my car’s oil pan to warm up the oil for cold starts in order to put less wear on engine components.

I have a small 8ah 12v agm battery laying around so I wanted this to supply the power rather than the car’s battery. However, I want the car’s battery to charge this battery when the engine is running. It would be easy for me to make a connection from a key on relay to the 2nd battery but obviously it’s not good to connect 2 batteries directly with different voltages and the smaller battery max charging current is rather low as well. I’m guessing a diode can limit the power in one direction but how can I limit the current and make this work?

It is normally a good idea to put some load on the main battery before starting the cranking. So you should use the oil warmer shortly before and disconnect it while cranking.

Regarding the other aspect: the batteries -you put a Schottky diode to avoid discharging the small battery during cranking and a current limiting resistor in parallel. The voltage difference of the schottky diode will suffice to let practically all the charging current flow into the main battery only.

If the smaller battery discharges fully with a heater load across it, it will become ruined. You absolutely
require an under-voltage cutout on the battery to prevent this from happening.

What currents and times are you expected both for discharge phase and recharging phase?

RIN67630:
It is normally a good idea to put some load on the main battery before starting the cranking. So you should use the oil warmer shortly before and disconnect it while cranking.

Regarding the other aspect: the batteries -you put a Schottky diode to avoid discharging the small battery during cranking and a current limiting resistor in parallel. The voltage difference of the schottky diode will suffice to let practically all the charging current flow into the main battery only.

In most situations this may be true but my car always pulls a lot of amps to start and needs a almost fully charge battery to start. The stock battery recommended for this car barely cuts it as it is and my heating pad pulls 10amps.
I remember when I redid my fuel system, I was running the fuel pump(which pulls about 13 amps) with the engine off for several minutes before starting the car to look for fuel leaks. That alone was enough to prevent the engine from cranking over and no this is not with a bad battery, I’ve had the car for 8 years and I’m not the only one with the car that has experienced this. The most common fix is to buy a optima battery but i refuse to spend $300 on a battery.

MarkT:
If the smaller battery discharges fully with a heater load across it, it will become ruined. You absolutely
require an under-voltage cutout on the battery to prevent this from happening.

What currents and times are you expected both for discharge phase and recharging phase?

Even if its a agm battery? Here is a Link to the battery. If that's true, I can probably easily set up a voltage divider circuit to the microcontroller and have it cut off when the voltage drops enough through a relay.
The heating pad pulls 10amps, would probably run it for 10-15min. Looks like the max charge current of the battery is 2 amps.
Not sure how much time is expected on the charging phase, it will often vary, I will probably have to hook up a battery minder to it routinely in order to keep it charged.