# Simplest current limiting method for battery charging

I need to charge 12V car battery (from main battery), but I have to limit current, because power cables are quite thin and I don't want to draw too much power from main system (in case battery is empty). What would be simplest solution (without ineffective linear regulators)? I though about PWM controlled LC circuit, but maybe there are controllers (ICs) made for that? I would like to charge the battery at no more than 3A.

Those cheap chinese converters have current limiting option through potentiometer, but I want something less delicate.

• What batteries are those and what voltages they have? Commented Dec 27, 2021 at 9:03
• regular lead car batteries Commented Dec 27, 2021 at 9:04
• Given your requirements stated (which are in no way sufficient to make a better judgement), a 1 Mohm resistor would likely suffice. Might even get away with something much smaller in value too. Commented Dec 27, 2021 at 9:04
• well a resistor (1 Ohm) would limit the current, but it would also wouldn't be very efficient. I could get ~18W of charging power and ~4W of losses. But it very depends on what is batteries internal resistance. Commented Dec 27, 2021 at 9:09
• Connect a car headlight bulb in series - 55W @ 12V is 4.5A so close enough. Used bulbs like that after rewiring cars so any fatal shorts weren't... Commented Dec 27, 2021 at 11:37

1. Charge a 12V car battery from the “main battery”. <=> Assumed here the main battery is the battery connected to the car starter engine and alternator.
2. Use of thin cables, to not draw to much power in case “aux” battery is empty. Here is a problem, as thin cables should not be used to present a high resistance to limit the current. This is wrong and dangerous! More on that later.
3. Simplest solution without inefficient linear regulators. <=> Linear is inefficient, but are simple and can be very robust.
4. Something less delicate than “Chinese converters”.

My thoughts of what you will need:

1. Charging/equalizing cables compatible with the maximum current expected to charge the Aux-12V battery. Surely anything of at least of 4 mm^2 or 12AWG, for at least 20A and a couple of meters long, but 6 mm^2 or 10AWG is good up to 30A; and 8AWG goes up to 40A safely, without overheating. And please, do protect the Positive cable(s) with Fuse(s) rated accordingly.
2. Switching device: For something Robust and Fool-proof, use 12V relays as switching elements; they are more forgiving than bipolar or mosfet transistors. Auxiliary relays can operate 30 to 40A, so you may be ok if you use a current limiting feature (later). You will need two relays, eventually a third one.
3. Current limiting circuit: The simplest and a robust solution is to use headlight lamps as power resistors. A more elegant option is to use sensing resistors (0.6~0.7V of voltage drop at max. current) monitored by a driver transistor to control a series-pass power transistor, heatsinked.This is essentially a current limit, but causes a minimum voltage drop of about 1.0V.
4. A Control circuit, to measure voltage differential between batteries and absolute voltage in Aux-Batt, and act according to these voltages. For example: (A) If voltage differential is low enough, the current-limit circuit (or lamps) could be bypassed by above Aux-relays, enabling a complete charge and in shorter time. (B) If Aux-Batt voltage is too low, another Aux-relay could disconnect the battery. (C) If Aux-Batt is fully-charged, a third Aux-Relay could disconnect the charging circuit. Hysteresis and interlocking logic could be used to prevent erratic operation of relays.

Finally, I suggest you to check if a dedicated VSR - Voltage Sensitive Relay as those used in trucks and motor-homes could solve your needs. I believe it does. Here are two links of different brands - a VSR only or as a VSR kit. I found also a good reference about Split Charging Methods - here. See below a typical VSR assembly diagram:

Let us know what you would prefer to do - use a Split charging system as those VSR-based, or to make and tinker your own system “VSR-like”, where the above list is a reminder.

What about a 2-transistor constant current sink? Use an NPN as the control transistor. If you put in a 0.2 Ohm resistor, the NPN will choke the pass transistor at about 3-4 A. If current is lower, the pass transistor will be fully on and can be a low resistance N-MOSFET.

This is probably a little bit more efficient than using only a resistor of around 1-2 ohm.

However, I think the 55W, 12 V bulb in series as suggested by @SolarMike is actually a more elegant way to achieve similar behavior. The transistor version allows setting the current limiter more deliberately, though.

Don't reinvent the wheel.

They sell a ready-made solution: 12 V to 12 V battery charger.