I am trying to limit the amperage to a device in a parallel circuit. To be specific:

My friend is trying to do the van life thing, basically he has an extra battery in the back of his car, and he has an inverter hooked up to it. So my idea to keep that battery charged is to tap into the car's ignition circuit, and run a wire from the front of the car to the back where the extra battery is (This battery's only job is to power the inverter). That line will connect to the positive side of the battery, and then it will be grounded to the frame. This way when he is driving and the alternator is generating power, it will charge the battery. But, I don't want to strain the electric system in his van, so I only want to send about 5 amps to the second battery. So using a resistor is not a good option, because resistors simply convert the excess heat to thermal energy right? I have some things that I don't know the answer to, such as how many amps would flow to the second battery if I directly hooked it to the ignition circuit? Is there a way to figure that out? And is there some kind of device that I don't know about that can only let regulate the amount of amps? Thank you, I'm trying to learn more about circuits, but I still have some areas where I get stumped. Any help is appreciated

 

I see a van with a dead battery and possibly worse in your friend's future.

You'll also need some warning or auto-disconnect when the auxiliary battery drains too low.

you want the main battery connected to the alternator to charge but you also want the second battery to charge slowly but how do you plan to prevent the main battery from powering the inverter when both batteries are connected to the alternator?

 

The Alternator will have no trouble Charging 2 Batteries at the same time,
if it is in proper working order.
It may take ~45-minutes to an Hour to charge a completely dead-Battery.

What is needed is a "Continuous-Duty-Solenoid",
which will be energized only when the Ignition-Key is ON.
This Solenoid will connect the 2 Batteries directly together when it's Coil receives Power.

YOU MUST USE 4-Gauge Battery Cable from front to back.
with the Solenoid connected in between the 2 Batteries.

Carefully protect the 4-Gauge Battery-Cable against receiving any sort of physical damage.
If You ignore this, or do a poor job of it,
the Van could very easily BURN TO THE GROUND.
THIS IS NO JOKE.
.
.
.

 

I see a van with a dead battery and possibly worse in your friend's future.

You'll also need some warning or auto-disconnect when the auxiliary battery drains too low.

you want the main battery connected to the alternator to charge but you also want the second battery to charge slowly but how do you plan to prevent the main battery from powering the inverter when both batteries are connected to the alternator?

I see what you're saying, I will be sure to incorporate a low charge warning. As for your question, the main battery will not be connected to the inverter. The second battery will only be connected to the alternator/ignition circuit when the key is in the on position. Unless you are referencing something I don't understand?

 

I see what you're saying, I will be sure to incorporate a low charge warning. As for your question, the main battery will not be connected to the inverter. The second battery will only be connected to the alternator/ignition circuit when the key is in the on position. Unless you are referencing something I don't understand?

The regular battery is connected to the alternator when the car is running. You want the second battery to be connected to the alternator when the car is running. How can that be possible without a path that connects the second battery to the first battery? And, how can the inverter be powered by only one battery.

 

This calls for a battery separator.
Both batteries are connected to a solenoid that only activates when the vehicle battery voltage reads above 13.5 or so.
Here is one of many

 

I would recommend using a battery isolator with a manual switch (or you can fashion some control circuitry, but manual is a good starting point).

Put the isolator between your normal electrical system and the auxiliary battery. When the engine is off, turn off the isolator. That will prevent the auxiliary battery from draining the main battery. Once you start the van using the main battery, then turn on the isolator and let it charge. The electrical system should have no problem handling this. But you will need to remember to turn off the switch when you are not using the van otherwise the isolator will slowly drain the battery day. If you power the switch from the auxiliary battery, you at lease won't drain your main battery should you forget. You can also power the switch from the accessory power from the ignition switch so that the isolator can't be turned on unless the key is turned to at least the ACC position.

I had an old Ford Bronco that I put in a dual battery system and I wanted to be able to connect either battery to either the main or the auxiliary electrics. This involved four isolators. I also had some things (like the clock and also the the control circuit for the isolators themselves) that I wanted to have power at all times even if all four isolators were turned off, but still didn't want to drain one battery with the other. So I simply used two diodes to give me my control power (the diode drop was no problem for me). I made a nice panel in the interior that had four rocker switches on it to control the isolators (and a bunch of other things, too). To make it so that the isolators didn't draw current and kill the batteries when the truck wasn't in use, I simply routed by control power through the key switch (which I had to do anyway) and used that to go to the switches. Then all of the isolators would turn off when I took the key out but as soon as I turned the key then they would go back to whatever state I had them set to. If I needed to run comms through the night, I simply turned on the key and configured one battery to be the auxiliary and no batteries to the main. In the morning, I would then switch the other battery to the main and start the truck.

The only potential downside was that as the auxiliary battery drained, the control power would eventually be drawn from the main battery. Since the control power was powering the battery isolator coil, this would eventually drain the main battery. But this took a couple of days and so never actually caused a problem. But I had always intended to put in a mod that would prevent that from being a possibility and just never got around to it.