Basic Dual Battery Setup to run Fridge/Freezer questions (sorry in advance if these are dumb)

[luthj]

The easiest method in my view is to get a second battery (true deep cycle) for the fridge, and just wire a split-charge relay. If you have a bit of extra cash, a 50-100AH lithium battery pack will charge just fine from a 13.8+ volt alternator. It will be smaller, lighter, and won't suffer from partial charge issues like lead will. Lithium batteries do have issues with low temps (below freezing), so if you need to operate in winter temps, lead is likely your best bet.

Part of the reason I suggested a cheaper flooded battery, is that true deep cycle units will last a lot longer, but they are quite expensive in the group sizes, and selection is limited. In many cases, where the usage is not frequent, its more cost effective to use a cheaper "dual purpose" flooded battery. Put a good wall charger on it occasionally, and replace every 2-3 years. As opposed to a battery thats 2-3x the cost, and will only last 3-4 years tops.

Again, get a decent volt meter, and start poking around to get some data. You may not need to rewire everything, so confirm for yourself the voltage drops and alternator voltage.

 

[Photobug]

Again, get a decent volt meter, and start poking around to get some data. You may not need to rewire everything, so confirm for yourself the voltage drops and alternator voltage.

To test my alternator on my truck, Can I just test my battery lugs while the truck is running or do i need to put the multitester to the alternator?

 

[dwh]

Just moving to a different campsite every day won't fully recharge any lead-acid battery unless you drive for -at least- eight hours per day.

Chronic undercharge will kill any lead-acid battery prematurely. Deep discharging a cranking (starter) type battery (no matter if flooded, glass mat, or gelled electrolye, and no matter what group size) will kill it quickly. Deep discharging and chronic undercharging a cranking battery can kill it in record time.

I dunno WTH this "Alt-S" fuse is that supposedly magically raises voltage. Sounds like a unicorn to me. But while raising voltage can help, it doesn't make up for the hours it takes for the electrolyte in ALL lead-acid batteries to absorb electrons.

AGM is a sealed lead-acid battery with fiberglass fabric sandwiched between the plates to act as a sponge to keep the electrolye in better, more complete, contact with the plate surface area. That trick has both pros and cons.

Cranking batteries have thin plates, deep cycle has thick plates. You can crank with a deep cycle, but it will have a lower cranking amps rating. You cannot deep cycle a cranking battery. At least, not many times before it's toast.

There are "dual-purpose" batteries rated for both cranking and deep cycle. Odyssey is one, but it's a special design that requires proper care and feeding. Most dual-purpose batteries are called "marine". Those have medium thickness plates that can do a decent job of cranking, and also survive more deep discharges than a cranking battery. It's a compromise that can do both jobs, though can't do either as well as a proper cranking or deep cycle.


There are quite a few who run a fridge with a single battery under the hood and get away with it. Most also have solar.

 

[DaveInDenver]

To test my alternator on my truck, Can I just test my battery lugs while the truck is running or do i need to put the multitester to the alternator?

You can measure in many places and get useful numbers but at the battery terminals is the best place since that's where it really matters.

 

[DaveInDenver]

I dunno WTH this "Alt-S" fuse is that supposedly magically raises voltage. Sounds like a unicorn to me. But while raising voltage can help, it doesn't make up for the hours it takes for the electrolyte in ALL lead-acid batteries to absorb electrons.

Thet are stuffing a diode in the voltage sense line and forcing their alternators to bump proportionally. I think it's about the worst hack you can do without monitoring and knowing why and how. It'll take a stock system such as my Tacoma and they'll run >14V even at idle and near 15V at RPM. They will charge their batteries for sure and quicker, too, but most likely vent the whole time driving the family to Yellowstone for summer vacation, too. Which is of course the main reason why FLA and AGM are not fungible, you can't put the magic back into AGM when it's let out.

 

[dwh]

To test my alternator on my truck, Can I just test my battery lugs while the truck is running or do i need to put the multitester to the alternator?

Does it have an OBD2 port? I use a Kiwi3 OBD dongle and the Torque App on my phone. With my Ford, it shows both alt and batt voltages (unfortunately, doesn't show amps).

To test the alt voltage with a meter, if the engine is running, it doesn't really matter where you put the test leads. If you put them across the battery, you'll create a test circuit that bypasses the battery anyway.

 

[dwh]

Thet are stuffing a diode in the voltage sense line and forcing their alternators to bump proportionally. I think it's about the worst hack you can do without monitoring and knowing why and how. It'll take a stock system such as my Tacoma and they'll run >14V even at idle and near 15V at RPM. They will charge their batteries for sure and quicker, too, but most likely vent the whole time driving the family to Yellowstone for summer vacation, too. Which is of course the main reason why FLA and AGM are not fungible, you can't put the magic back into AGM when it's let out.

Oh, that. There's a plugin wiring harness for that. But only works on some (Toyota?) vehicles.

 

[Photobug]

Does it have an OBD2 port? I use a Kiwi3 OBD dongle and the Torque App on my phone. With my Ford, it shows both alt and batt voltages (unfortunately, doesn't show amps).

Yes and I have a cheap OBD2 reader and a USB dongle that I never got to work to my satisfaction in the past I do own the Torque App. Will try it again.

 

[Fletchxj]

Just moving to a different campsite every day won't fully recharge any lead-acid battery unless you drive for -at least- eight hours per day.

Chronic undercharge will kill any lead-acid battery prematurely. Deep discharging a cranking (starter) type battery (no matter if flooded, glass mat, or gelled electrolye, and no matter what group size) will kill it quickly. Deep discharging and chronic undercharging a cranking battery can kill it in record time.

I dunno WTH this "Alt-S" fuse is that supposedly magically raises voltage. Sounds like a unicorn to me. But while raising voltage can help, it doesn't make up for the hours it takes for the electrolyte in ALL lead-acid batteries to absorb electrons.

AGM is a sealed lead-acid battery with fiberglass fabric sandwiched between the plates to act as a sponge to keep the electrolye in better, more complete, contact with the plate surface area. That trick has both pros and cons.

Cranking batteries have thin plates, deep cycle has thick plates. You can crank with a deep cycle, but it will have a lower cranking amps rating. You cannot deep cycle a cranking battery. At least, not many times before it's toast.

There are "dual-purpose" batteries rated for both cranking and deep cycle. Odyssey is one, but it's a special design that requires proper care and feeding. Most dual-purpose batteries are called "marine". Those have medium thickness plates that can do a decent job of cranking, and also survive more deep discharges than a cranking battery. It's a compromise that can do both jobs, though can't do either as well as a proper cranking or deep cycle.


There are quite a few who run a fridge with a single battery under the hood and get away with it. Most also have solar.

Yeah, sorry, when I say "move to another site" I mean 8-10 hours of driving. When I go on a trip, it's an overlanding trip so we're cover miles/trails each day then we find a dispersed camping site each night. So, I would have a full day of driving pretty much every day.

 

[luthj]

The diode bump trick works fine. BUT you must confirm your systems voltage before you do something like that. In my case I have a switch to bypass the boost diode when needed. There are numerous diodes available, so you can pick the desired boost (from 0.25 to 1V is doable). On my van I went from 13.8 to 14.3V, and it made a big difference in charging my lead bank,

 

[dwh]

The main advantage to a better OBD dongle like the Kiwi3 is more bandwidth.

Also a good idea to get the right MIB file. My Ford is a gasser, but I use the MIB for a diesel, which has everything the gasser MIB has, plus a few useful extras. Though, there are a lot of parameters that just come up blank using the diesel MIB on a gasser.

 

[DaveInDenver]

Oh, that. There's a plugin wiring harness for that. But only works on some (Toyota?) vehicles.

It's just inserting a diode drop in the voltage sense, so should work with any externally sensed alternator. Seems like it would be true of most Japanese brands and doesn't Fiat-Chrysler use Denso alternators, for example? It's just that Toyota named the fuse "ALT-S" (e.g. alternator sense) in the box that people pull and replace with the diode.

Anyway, that's why I say you have to do it with some understanding. I've been meaning to do it with some Schottkys I have sitting around. Going to ~14.7V and ~13.6V (e.g. +0.3V bump to my measured voltage) I think would make a difference minimizing the under charging without as much over heating risk in the summer.

 

[DiploStrat]

Ouch, ! I read through all that stuff, but clearly didn't get it all, haha. Guess that's why I'm not an electrician. It's interesting because I think that there's a lot of misinformation out there (and I've been inundated with the bad info before coming here) so that it's burned into my brain that an AGM is a deep cycle battery, which is clearly not the case. I also heard multiple times that using a standard flooded cell starting battery wouldn't work to power a fridge (not realizing that there were deep cell FLA batteries too), hence my questions here. The voltmeter will be helpful, I'll track one down and see if I can get a good reading on the alternator.

It seems that there's going to be a decent amount of cost/rewiring in order to safely and consistently run the fridge.

As I said, you don't want to know the number of dumb things that I did over the years.

This sentence is key:

"... using a standard flooded cell starting battery wouldn't work to power a fridge." Delete the words "flooded cell" and you are correct.

A deep cycle battery will start your truck, it is just that it will not have the cold cranking amps for the same size and weight. It will, however, withstand deeper discharges better.

In your case, the key numbers are: Amp draw of your fridge. 1A = 24 Ah per day. 2A = 48 Ah, etc. A 100 Ah battery can probably take a 50 Ah discharge and still start you, but it may be close and it will have a shorter life.

If you can find a 200 Ah deep cycle battery that will fit under your hood, you should be fine. Else, I fear you are looking at a second battery and then you probably want to isolate it.

 

[luthj]

Depending on the alternator design, a reverse biased "flyback style" diode needs to be used in parallel with the forward biased diode. This is because some regulation schemes can cause a voltage spike with the bump diode, and the reverse biased unit suppresses it. Its a bit outside my purview, as I am not learned on the various regulation schemes used in alternators.

 

[DaveInDenver]

I never heard phrase ”stuffing diode” before.

LOL.

verb (used with object)

to pack tightly in a confined place; crowd together.​

https://www.dictionary.com/browse/stuff

I've kind of always felt working within factory fuseboxes qualified.