Alternator capacity and running fridge on 12v DC while driving

[Wellibe]

I know there's another active thread on this topic, but I didn't want to hijack it with my slightly different question.

My small trailer has a 1.7 cu ft three-way fridge. The sticker inside says it pulls 9.6 amps when running on 12v DC. I'm considering whether I should run on DC while driving to conserve propane. Or whether I should just load the fridge, turn it off, and not open it until I reach my destination.

My tow vehicle has a 150 amp alternator and is wired to charge the house battery through the 7-pin connector (via a 10 gauge wire running from the TV battery). On the surface, it looks like I should have plenty of alternator capacity to run the fridge and charge the house battery while driving. I'm not sure what all my TV draws add up to, but I know the two radiator fans could pull almost 40 amps if they are running.

The question is complicated somewhat by the fact that I will be doing some camping without an electric hookup for 2-3 days at a time. Currently I do not have a generator or solar panel. So on some drive days I will be starting out with the house battery partially depleted. I'm not sure how depleted; I'll mostly be using 12v just for LED lights and the roof fan, as needed. But I guess some of the systems like the furnace fan and fridge controls might also pull some from the battery.

I have one 12v, group 24, deep-cycle lead-acid battery.

I'm wondering if I'm going to put too much strain on my alternator by running the fridge and charging a depleted house battery at the same time. I'm also wondering whether a 4 - 8 hour drive will be enough to charge the battery. Any thoughts?

[Jkwilson1989]

In general, you won’t get but 7 or 8 Amps to go to the trailer while driving.

Air temperature might be a concern since that affects performance of the refrigerator as does the air movement on the road. On most 3-ways, the 12V mode has greatly reduced cooling compared to shore power and LP.

With a group 24 and boondocking for 2-3 day stretches, I think you’d be wise to carry a small inverter generator.

[Wellibe]

Originally Posted by Jkwilson1989

In general, you won’t get but 7 or 8 Amps to go to the trailer while driving.

Air temperature might be a concern since that affects performance of the refrigerator as does the air movement on the road. On most 3-ways, the 12V mode has greatly reduced cooling compared to shore power and LP.

With a group 24 and boondocking for 2-3 day stretches, I think you’d be wise to carry a small inverter generator.

Thanks. I'm new to all this - can you explain why I would only get 7 or 8 amps going to the trailer while driving? If that is the case, then obviously running the fridge on DC while driving is a no-go.

I've been pondering whether I should just go ahead and get a generator or solar panel. Even without running the fridge on DC while driving, if the house battery is only getting 7 or 8 amps, it seems like it isn't going to charge fully during a short drive day.

[Tha_Rooster]

Not a expert but isn’t the center pin of a 7 pin for auxiliary power?

[Persistent]

" My small trailer has a 1.7 cu ft three-way fridge. The sticker inside says it pulls 9.6 amps when running on 12v DC. I'm considering whether I should run on DC while driving to conserve propane. ... I have one 12v, group 24, deep-cycle lead-acid battery...."

This is a substantial load for a group 24 battery or a 10 gauge line to TV alternator. Use the propane to run the refer.

"... Or whether I should just load the fridge, turn it off, and not open it until I reach my destination. ..."

Certainly a viable option. 4 hours in moderate weather should be fine. Maybe 8 hours would be fine. Test it to find out. Put a case of bottled water in it and see how long the temp stays down.

"... My tow vehicle has a 150 amp alternator and is wired to charge the house battery through the 7-pin connector (via a 10 gauge wire running from the TV battery). ..."

10 gauge wire over that distance is not enough for any significant battery charging. There will be too much voltage drop. Increase the wire gauge from battery to 7 pin connector to 8 gauge, 6 gauge would be better. Also increase the ground wire from 7 pin to TV chassis to 6 gauge. (There is more current in the ground than in the 7 pin 12 volt pin.) Do the same for the TT wire harness. The 7 pin connector will then be the limiting component.

Make sure there is a circuit breaker near the connection to the TV battery to protect the alternator and 7 pin connector. I don't know how big it can go, but maybe 30 amp breaker. Check TV specifications.

"... The question is complicated somewhat by the fact that I will be doing some camping without an electric hookup for 2-3 days at a time. Currently I do not have a generator or solar panel. So on some drive days I will be starting out with the house battery partially depleted. I'm not sure how depleted; I'll mostly be using 12v just for LED lights and the roof fan, as needed. But I guess some of the systems like the furnace fan and fridge controls might also pull some from the battery.

I'm wondering if I'm going to put too much strain on my alternator by running the fridge and charging a depleted house battery at the same time. I'm also wondering whether a 4 - 8 hour drive will be enough to charge the battery. Any thoughts?"

Shut "off" all 12 volt devices that are not needed to maximize battery charging. 12 volt refer is going to interfere with charging. I do not recommend both. Charging will suffer.

It takes 14 to 18 hours of using a high quality voltage regulated charger to fully charge lead acid batteries. Full clean charge is needed every week or so for long battery life.

6 hours from engine generator through wire harness may restore to 80% of charge. Only testing will tell.

I wish you good luck and happy trails ahead!

[TXTiger]

Most likely your refer runs on propane. Only the electronics use 12v power, ie: the circuit board. You will not be drawing much power for the board. When parked, however, your other electrical systems, lights, television if you have one etc. will be drawing the batteries down which could affect the operation of the circuit board on the refer. You may looking into getting a generator.

[Wellibe]

Originally Posted by Tha_Rooster

Not a expert but isn’t the center pin of a 7 pin for auxiliary power?

Actually it's the blade contact on the upper right of the tow vehicle plug.

[Wellibe]

Originally Posted by Persistent

" My small trailer has a 1.7 cu ft three-way fridge. The sticker inside says it pulls 9.6 amps when running on 12v DC. I'm considering whether I should run on DC while driving to conserve propane. ... I have one 12v, group 24, deep-cycle lead-acid battery...."

This is a substantial load for a group 24 battery or a 10 gauge line to TV alternator. Use the propane to run the refer.

"... Or whether I should just load the fridge, turn it off, and not open it until I reach my destination. ..."

Certainly a viable option. 4 hours in moderate weather should be fine. Maybe 8 hours would be fine. Test it to find out. Put a case of bottled water in it and see how long the temp stays down.

"... My tow vehicle has a 150 amp alternator and is wired to charge the house battery through the 7-pin connector (via a 10 gauge wire running from the TV battery). ..."

10 gauge wire over that distance is not enough for any significant battery charging. There will be too much voltage drop. Increase the wire gauge from battery to 7 pin connector to 8 gauge, 6 gauge would be better. Also increase the ground wire from 7 pin to TV chassis to 6 gauge. (There is more current in the ground than in the 7 pin 12 volt pin.) Do the same for the TT wire harness. The 7 pin connector will then be the limiting component.

Make sure there is a circuit breaker near the connection to the TV battery to protect the alternator and 7 pin connector. I don't know how big it can go, but maybe 30 amp breaker. Check TV specifications.

"... The question is complicated somewhat by the fact that I will be doing some camping without an electric hookup for 2-3 days at a time. Currently I do not have a generator or solar panel. So on some drive days I will be starting out with the house battery partially depleted. I'm not sure how depleted; I'll mostly be using 12v just for LED lights and the roof fan, as needed. But I guess some of the systems like the furnace fan and fridge controls might also pull some from the battery.

I'm wondering if I'm going to put too much strain on my alternator by running the fridge and charging a depleted house battery at the same time. I'm also wondering whether a 4 - 8 hour drive will be enough to charge the battery. Any thoughts?"

Shut "off" all 12 volt devices that are not needed to maximize battery charging. 12 volt refer is going to interfere with charging. I do not recommend both. Charging will suffer.

It takes 14 to 18 hours of using a high quality voltage regulated charger to fully charge lead acid batteries. Full clean charge is needed every week or so for long battery life.

6 hours from engine generator through wire harness may restore to 80% of charge. Only testing will tell.

I wish you good luck and happy trails ahead!

Thanks for the detailed response. I really appreciate your explanations.

I'd hate to have to replace the charging wire. It was a real PITA to run it in the first place. I bought a 7-pin wiring kit from e-trailer; it came with a 10 gauge duplex wire (one wire is for the charging pin and the other for brake power). I did install a 40 amp breaker on the charging wire less than 2' from the battery (also came with the kit).

The battery definitely does take a charge while driving. The day I took delivery of the trailer, I checked the battery gauge before I left and it read 1/3 charged. I'm guessing they had the battery sitting in a storage room all winter. I drove 4+ hours home and the gauge read fully charged when I got home.

After the trailer sat in the driveway for 8 days, I looked at the gauge and it was down to 1/3 again. That's when I learned how to turn off the stereo's LCD display. I charged the battery overnight using my house's 15 amp connection. I plugged in through my EMS and made sure I didn't unplug until the current dropped to 0 amps. Then I disconnected the battery terminals just in case there's some other phantom draw that I don't know about.

The fact that it drew down so far in 8 days with very minimal power being drawn makes me wonder if the battery really was fully charged when I got the trailer home. I suppose I should get a multimeter so I can check the battery voltage directly instead relying on the mickey-mouse gauge in the trailer.

[Wellibe]

Originally Posted by TXTiger

Most likely your refer runs on propane. Only the electronics use 12v power, ie: the circuit board. You will not be drawing much power for the board. When parked, however, your other electrical systems, lights, television if you have one etc. will be drawing the batteries down which could affect the operation of the circuit board on the refer. You may looking into getting a generator.

No, it is definitely a three-way fridge. It has a propane knob, a 120v AC on/off switch, and a 12v DC on/off switch. Though I don't know why it has the capability to run on 12v; seems like that function is useless.

[Old-Biscuit]

The 12VDC element will maintain a COLD fridge when traveling

You vehicle alternator will handle it just fine

Folks have been using those 3-ways for decades

Don't over think it

[Wellibe]

Thanks to all for the replies. I'd still like to get some clarity on JKwilson's statement that only 7-8 amps will make it to the trailer, despite the alternator's capacity. Is that because the wire won't handle any more current?

[Winemaker2]

Originally Posted by Wellibe

Thanks to all for the replies. I'd still like to get some clarity on JKwilson's statement that only 7-8 amps will make it to the trailer, despite the alternator's capacity. Is that because the wire won't handle any more current?

Likely he is factoring in V drop which is less for larger wire and more for smaller wire size.
Charging takes a V differential to drive Amps into a batty to charge it. Thats why you will read 13.8 - 14.4V when charging a 12V batty and only 12V to 12.6V when fully charged batty is at rest. It also takes a looong time to charge the last 10% +/- vs the first 10% if you started at 1/3 - 1/5 charge.
V drop also dependant on length of wire run includi g the Neg return path. Look up/ search measuring V drop with a DVOM

[Persistent]

"...
I'd hate to have to replace the charging wire. It was a real PITA to run it in the first place. I bought a 7-pin wiring kit from e-trailer; it came with a 10 gauge duplex wire (one wire is for the charging pin and the other for brake power). I did install a 40 amp breaker on the charging wire less than 2' from the battery (also came with the kit). ..."

Run a second 10 gauge wire along side the kit wire. It will halve the voltage drop. Don't change the circuit breaker. Don't for get adding to the ground connection wires.

"... The battery definitely does take a charge while driving. The day I took delivery of the trailer, I checked the battery gauge before I left and it read 1/3 charged. I'm guessing they had the battery sitting in a storage room all winter. I drove 4+ hours home and the gauge read fully charged when I got home. ..."

I assume the refer was off during the trip home. Battery charging would be expected. You can increase charging rate for deeply discharged batteries by increasing wire size.

The 4 light LED battery state of charge displays are only accurate under "static" conditions. (No charging or discharging for 3 or 4 hours.) Terminal voltage will slowly drop from charging or increase from discharging. 100% immediately after charging does not mean batteries are fully charged.

"... After the trailer sat in the driveway for 8 days, I looked at the gauge and it was down to 1/3 again. That's when I learned how to turn off the stereo's LCD display. ..."

This is normal. Fully charge, then disconnect the battery to get up to 6 months of healthy storage time. Recharge before battery terminal voltage drops below 12.4 volts. Recharge means 14 hours charging using a high quality voltage regulated charger like the one that came installed in your TT.

"... I charged the battery overnight using my house's 15 amp connection. I plugged in through my EMS and made sure I didn't unplug until the current dropped to 0 amps. Then I disconnected the battery terminals just in case there's some other phantom draw that I don't know about.

The fact that it drew down so far in 8 days with very minimal power being drawn makes me wonder if the battery really was fully charged when I got the trailer home. I suppose I should get a multimeter so I can check the battery voltage directly instead relying on the mickey-mouse gauge in the trailer. ..."

A inexpensive digital multi-meter will come in handy for many electrical management activities.

Lead acid batteries have three basic profiles. They are Charging, Discharging, and Static.

The static profile (not charging or discharging for 3 hours) will tell you the state of charge. Table below translates static voltage into percent of charge.

Charging voltage will be higher than static. It depends on the combination of charger and battery bank. Voltages should start low depending on how deeply discharged. It will slowly rise to 13.6 or so and then remain constant for the remainder of charge. Some chargers increase to 14.4 volts for a while then drop back to 13.6 for remainder of charge. It takes 14 to 18 hours to fully charge lead acid batteries. It is in the chemistry.

Discharging voltage will be lower than static. The higher the discharge current, the lower the voltage. Voltage will slowly return to static when load is removed.

"static" state of charge.
100% 12.7
90% 12.5
80% 12.42
70% 12.32
60% 12.2
50% 12.06
40% 11.9
30% 11.75
20% 11.58
10% 11.31
0% 10.5


Battery University https://batteryuniversity.com/

How does the Lead Acid Battery Work? https://batteryuniversity.com/learn/...ased_batteries

Charging lead acid batteries https://batteryuniversity.com/learn/...d_acid_battery

AGM https://batteryuniversity.com/learn/..._glass_mat_agm

How to Charge and When to Charge? https://batteryuniversity.com/learn/...o_charge_table

How to Store Batteries https://batteryuniversity.com/learn/...tore_batteries

Summary of Do’s and Don’ts https://batteryuniversity.com/learn/..._battery_table

[Wellibe]

Thanks very much for the information, guys. That's very helpful.

I checked the battery's voltage today, six days after I charged it and disconnected it. It read 12.76 volts, so I'm pretty sure I got it fully charged.