Is it safe to charge an aux deep cycle battery from the cars 12v supply on a long drive?

[DaveM]

Is it safe to charge an aux deep cycle battery from the cars 12v supply on a long drive?

I have a 12v group 24 deep cycle batt in my small tag-along gear trailer. The battery runs my fridge in camp but only lasts about 3 days on average. Looking for an easy way to top it off as we drive between locations. The battery has a permanent plug installed for the Genius chargers. I have a backup Genius extension cord that I was planning on tapping into my cars 12v system (strip and tap the Genius cord to the wires of a rear cargo area 12v acc plug). My question is, do I run the risk of overcharging and damaging the batt this way? Typical drive times might be 4-7 hours.

 

[john61ct]

Since a long run of fat copper gets expensive

consider putting a DC-DC charger at the target battery.

Its input voltage can be lower than you'd want to use directly, and with a lead batt you won't get higher amps anyway.

Note it takes 7+ hours to get back to Full, so also use solar or a mains charger when possible.

 

[mep1811]

I use this in my wife's Forester to charge the aux battery we use for the fridge. It does a great job .

Towed Battery Charger Plus for flat towing

RVi Towed Battery Charger Plus, charge line replacement for flat towing a vehicle. Great peace of mind for the towed battery while towing a car behind a motorhome. Often used with RVibrake. TBC+

rvibrake.com

 

[DaveM]

For a little more detail:

It is flooded deep cell, Interstate. I charge up at home via the Genius 3500. Once in camp I have a 100w solar panel w Renology Charge controller. That's enough to extend the batt an extra day or maybe two if there's lot of sun. But on longer road trip, with stops at motels (fridge running in trailer all night) and multiple camps, I often run low before were done. What I want from the car connection is to be adding a little back (even if it's just 10%-20%) ea leg we drive. I would not use the actual 12v port, was planning to tap the wires to the 12v port and run that wire and plug to just under the rear bumper. That will give me a plug I can plug my Noco Genius extension into and then into the battery.

 

[billiebob]

Just a thought, trucks with electric trailer brakes have a trailer battery which is charged thru the 7 pin connector.
Same with deck trailers running an electric winch.
Might be a system out there that works.

 

[Hilldweller]

For a little more detail:

It is flooded deep cell, Interstate. I charge up at home via the Genius 3500. Once in camp I have a 100w solar panel w Renology Charge controller. That's enough to extend the batt an extra day or maybe two if there's lot of sun. But on longer road trip, with stops at motels (fridge running in trailer all night) and multiple camps, I often run low before were done. What I want from the car connection is to be adding a little back (even if it's just 10%-20%) ea leg we drive. I would not use the actual 12v port, was planning to tap the wires to the 12v port and run that wire and plug to just under the rear bumper. That will give me a plug I can plug my Noco Genius extension into and then into the battery.

Most batteries hate you if you don't charge them all the way back up. Lithium is the exception.
How about some bigger/better solar? Renogy is running a Christmas sale right now. A 200w suitcase, the high efficiency one, will probably solve your problem. I have one and don't even think about shore power or generator when I'm offgrid.