I power a 12V fridge in my car either via the car battery (when the vehicle is running) or via an AGM battery. There is an Anderson outlet in the car connected to the car battery to which I attach a 3 port Anderson hub. When the vehicle is running, the fridge runs off the car battery. When the vehicle’s ignition is switched off and the car battery disconnects, I attach an AGM battery to the Anderson hub if I wish to continue powering the fridge. Sometimes I forget to disconnect the AGM battery when I restart the engine and I later discover that the AGM battery is fully recharged.

Does having both the car battery and AGM battery connected simultaneously in parallel in this way create any issues or incompatibilities or is it perfectly acceptable to routinely leave both batteries connected to the Anderson hub whether the engine is running or not?

  • 1
    \$\begingroup\$ So are you back feeding the car electrical system from another battery while the car does not power the fridge? And then when car does feed power it charges up the extra battery? Sounds like a setup that should not exist, as there are reasons why power outputs and power inputs have different type of connectors to make sure they are not plugged incorrectly. But it does depend on what curcuitry is between the car and the connector, as some systems do make it possible to charge extra batteries from the car. You need to know if this is okay in your specific car. \$\endgroup\$
    – Justme
    Apr 25, 2022 at 8:29
  • \$\begingroup\$ @Justme I might need to do some research regarding my setup. All I know is that the car battery is isolated and stops providing charge to the fridge when the ignition is switched off. So I need to connect the AGM battery to continue to run the fridge. If it is bad practice to have both batteries connected with the ignition on then I'd like to understand what the potential consequences are. What is the worst case scenario? Is it dangerous? Could it cause overcharging and damage to one or both of the batteries? \$\endgroup\$
    – Guru Josh
    Apr 25, 2022 at 9:39
  • 3
    \$\begingroup\$ If the AGM battery goes empty, and you start the car, and the empty battery gets connected directly to a full 12V car battery, how many amps of current will flow? Does it burn a fuse, weld some contacts, or melt wires between batteries? \$\endgroup\$
    – Justme
    Apr 25, 2022 at 9:43
  • \$\begingroup\$ @Justme I see the potential for this problem to occur though I always start off with a fully charged AGM battery which should prevent this from happening. \$\endgroup\$
    – Guru Josh
    Apr 26, 2022 at 4:43

4 Answers 4


The battery voltage is higher than 13V (close to 14V generally) when the engine is running. Generally, it's safe to re-charge AGM batteries through car alternator + regulator system. But the thing is, AGM batteries show higher than 14V (sometimes even 15V) when fully charged so you'll unlikely get your AGM battery fully charged when it's connected to alternator + regulator or any 12V outlet in your car. And for longer life, you should fully re-charge your AGM battery. So this requires a proper charger.

You shouldn't leave the AGM battery connected to any 12V socket in a car, regardless of that the socket is connected directly to the battery or through ignition, when the engine is not running. Because you don't want your AGM battery to discharge through the car or supply any electrical sub system of the car.

  • \$\begingroup\$ The car battery ceases to provide charge to the fridge when the ignition is switched off so I assume that it is isolated from the Anderson hub circuitry and I can connect the AGM battery to it. I always put my AGM battery on a charger when I get home so make sure it is fully charged up so I am not concerned that it is less than fully charged in the car. I am more concerned that the practice of connecting car and AGM batteries in the car is either dangerous or detrimental to one or both of the batteries. \$\endgroup\$
    – Guru Josh
    Apr 25, 2022 at 9:48

AGM and Lead Acid batteries are technically the same when it comes to their base chemistry, as long as both batteries have the same voltage at resting they can be connected in parallel, when your engine is running it charges both of the batteries to ~14.6V and after you turn off your car it goes down to resting state, if one of your battery has lower voltage it will bring the other battery down to that level too, if you don't wanna use the AGM to anything else than fridge and wanna keep your staring battery isolated from your AGM battery you should get split charging relay, that way you'll always have a full AGM after shutting down your car.

I have been running 3 AGM's and 1 lead acid in parallel for 4 years now and they work like a dream, all sit at around 12.6V at rest.


The problem has wide enough occurrence and few general solutions:

  1. Get the fridge battery (together with the fridge) in parallel with the car battery when the engine is running, disconnect when the engine is cut off.

You need a single power relay controlled by the ignition or an "automatic charging relay" controlled by the car battery voltage.

Pros: Simple to automate, fail-safe charging, no danger to deplete the starter battery

Cons: The Anderson outlet is nowhere powerful enough, one needs thick direct cables between the batteries and a means to disconnect the AGM+fridge part when not needed (probably Anderson powerpole 180A). The AGM battery will get only to 80-90% of its full capacity.

  1. A DC-DC battery charger

Pros: the charging current and voltage can be precisely controlled in order to top up the AGM battery (it generally requires higher end-of-charge voltage) and the current can be limited to the socket capability. Easy to disconnect when not needed.

Cons: Expensive. The bulk charging of the AGM battery (when it is fairly depleted) will be slower, limited by the charger and the Anderson socket.

Good luck searching for these keywords.

  • \$\begingroup\$ When the AGM battery is disconnected and the fridge is only running off the car battery, power to the fridge cuts off within a minute of the engine being switched off. So I am guessing that my car is fitted with some kind of ignition relay device. I am not concerned that the AGM does not reach 100% SOC as I generally connect it to an AC charger or solar panel at my destination anyway. So I suspect I am already implementing solution 1? The DC-DC charger solution sounds like it might be the optimal solution. \$\endgroup\$
    – Guru Josh
    Apr 26, 2022 at 15:30

From what I've read,

  • since it appears that your Anderson connection goes dead when your ignition is off, you don't have to worry about draining your car starter battery.

  • since you charge your AGM battery at home, you don't have to worry about your AGM battery unless you run it dry.

Me, I run two vehicles and two trailers for a few hours each week and I've been looking into how to keep the unused tow vehicle battery charged up without having access to a 110 volt line during the week... And also the battery of the tow vehicle that gets most of the use since it doesn't always get a full charge from being run a few miles every weekend and then sitting for most of the week.

My current solution has been to charge two batteries in parallel in the toy hauler Trailer (it gets hooked into a 50A 220 line split) and then charge the little trailer and the Yukon, by running a 110 line from The toy hauler Trailer, and alternating weeks between the little trailer and the Yukon battery.

At one point I had to replace all four batteries before I came up with the idea that I shouldn't ignore the batteries.

I used to run an extension off of the toy hauler battery to charge the second battery, but from what I've read it's better if I put One charging lead on the toy hauler battery and the other charging lead on the second battery. It's about leveling out the resistances which means that each battery gets a good charge without being over taxed because of the resistance and balance, or some such.

So basically I've got the charger positive lead going to One battery, and the charger negative lead going to the second battery, or the end battery in the chain if I have more than two batteries.

And of course I should be using the same gauge wire all around, which I think is 8 gauge but the 8 gauge is a pain in the butt for me to deal with unless I run clamp screws or wire nuts.

And of course both tow vehicles use side mount terminals on their batteries which I really learned to hate, so I've got auxiliary battery posts that I can use with a quick release battery clamp for the auxiliary charging battery.

Side note: The toy hauler is actually converted to run 7 dryers and washers for homeless people at a local church on Saturdays.

The small trailer is from a movie set and it's a shower trailer. When the on-demand heater got replaced, someone thought it was a great idea to put in a 110 volt on demand heater instead of replacing it with a 12 volt on demand heater running propane, so now we always have to have 110 volt to run the showers heater and enough water pressure to get water out of the shower heads.

We used to be able to run the shower trailer without any 110 line voltage at all for one day or about 8 showers out of a 100 gallon tank, or 16 showers before our gray water tanks get full.

Serves me right for letting someone else handle the on-demand water heater replacement.

The room lights only go off in the main power shut off.

So of course during covid when the water heater got replaced, someone forgot to turn off the main power, and then ran the 12 volt water pump, and ran the 12 volt battery dry, which meant that the water pump ran off of the charging system, which burnt it out, which meant no more lights but at least we had hot water.

Until I figured out what happened.

Which is why I run a separate battery charger to charge the small trailer, instead of the built-in battery charger and no charging if the pump is running.

If course the small trailer (and the toy hauler battery) battery is a pain to unhook and drag out (I'm not skinny enough) or I'd just pull it out and drag it somewhere I can plug in a charger when the trailer isn't in use.

We've had one or two people come out to look at the trailers, but don't ever comes back to fix the 12 volt system by replacing the onboard charger, since it's a low priority.

I probably should run a relay to the lights, so that they get shut off if the main power switch is left on, but I don't want to run a sensor to the 110 volt line, and I'm not sure I trust timers.

Maybe I'll throw an LED light on top of the main power switch to remind the operator at the power is still on, because it's hard to see the room lights once you button everything up and the Sun is up.


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