DC-DC charger as an alternative to generator?

[rruff]

Adding more battery is a bit like opening a new bank account without any increase in income.

I think it's more like carrying enough food on a backpacking trip, so you don't need to kill animals to keep from starving...

Still don't know what the OP's use case is, though. Charging the batteries by idling the vehicle on a regular basis is not something I'd want to do...

One thing I don't recall being mentioned is the exhaust fumes.

 

[Ozarker]

They don’t though. It is not possible to cool through natural means below the ambient air temperature. If the ambient temperature is 35° with 50mph wind (windchill 18°F) you will not get frostbite. The same way your batteries or floor will not get below 35°.

WOW!

Not totally right on either issue...

Windchill is the result of how cold effects human skin in wind, it is a cooling effect, and while it does not have the same effect on other objects as it does on your skin, it can literally freeze your skin.

Asking Co-pilot;

You​

can you get frost bite from windchill

Copilot​

Yes, you can. Windchill accelerates heat loss from your skin, which can lower your skin temperature and, in extreme cases, lead to frostbite. Protecting exposed skin is crucial in cold, windy conditions...

Can objects be colder than the ambient temperature by natural means

Copilot​

Objects typically can't be colder than the ambient air temperature without some external influence. However, when the air temperature drops, certain conditions can cause objects to cool even further due to processes like radiational cooling or evaporative cooling. Think of a wet object on a windy, cold day—it can feel colder as the water evaporates and removes heat.

 

[crazysccrmd]

WOW!

Not totally right on either issue...

Windchill is the result of how cold effects human skin in wind, it is a cooling effect, and while it does not have the same effect on other objects as it does on your skin, it can literally freeze your skin.

Asking Co-pilot;

You​

can you get frost bite from windchill

Copilot​

Yes, you can. Windchill accelerates heat loss from your skin, which can lower your skin temperature and, in extreme cases, lead to frostbite. Protecting exposed skin is crucial in cold, windy conditions...

Can objects be colder than the ambient temperature by natural means

Copilot​

Objects typically can't be colder than the ambient air temperature without some external influence. However, when the air temperature drops, certain conditions can cause objects to cool even further due to processes like radiational cooling or evaporative cooling. Think of a wet object on a windy, cold day—it can feel colder as the water evaporates and removes heat.

Based on everything I’ve been taught on Arctic exposure/survival and from personal experience you can’t get frostbite if the ambient temp is above freezing, regardless of a windchill value. You will absolutely lose heat faster but the exposed skin won’t drop below the ambient temperature and cause true frostbite. There are other cold weather effects that will cause injury but the flesh will not physically freeze. I’ve spent a lot of time with exposed skin while windchill values were well below 32F and have never seen anyone suffer frostbite.

 

[Dave in AZ]

Based on everything I’ve been taught on Arctic exposure/survival and from personal experience you can’t get frostbite if the ambient temp is above freezing, regardless of a windchill value. You will absolutely lose heat faster but the exposed skin won’t drop below the ambient temperature and cause true frostbite. There are other cold weather effects that will cause injury but the flesh will not physically freeze. I’ve spent a lot of time with exposed skin while windchill values were well below 32F and have never seen anyone suffer frostbite.

Well, nothing like taking one anecdotal frostbite statement, and extrapolating it into a General Theory of Heat Transfer. That there is known as the Redneck Scientific Method.

 

[crazysccrmd]

Well, nothing like taking one anecdotal frostbite statement, and extrapolating it into a General Theory of Heat Transfer. That there is known as the Redneck Scientific Method.

 

[Martinjmpr]

I can only assume that the OP will be parked for long periods, or at least will not be driving very far.

I asked earlier what the intended use case was for energy consumption and driving because I hadn't seen this mentioned. I thought simply adding batteries might be sufficient nearly all the time.

One issue with direct alternator charging is the often too-low voltage for LiFe.

Sorry, I thought I mentioned that earlier but I guess I wasn't specific enough.

Our recent trip to Oregon is probably a good example of the kind of travel we'll be doing. We left Colorado on a Friday, and spent 3 days traveling to Southern/central Oregon (near Crescent lake.) Our overnights were free boondocking spots, so the only charging the batteries got was from the rooftop 100W solar panel and whatever is pushed through the 7 pin trailer plug. It does have power to the charging circuit but I'd guess it's pretty low amperage just based on the fact that it's running through some pretty small wires. If I was to guess I'd be surprised if it was getting more than 5 amps through the trailer plug.

Once we got to Oregon we were camped there for 5 nights (Sun - Thur) with no hookups. I brought an extra 100W solar panel but that was useless because (a) it was cloudy and overcast the first two days and (b) even when the sun was out, we were beneath a 150' canopy of trees and probably got less than an hour of direct sunlight per day, so solar power was useless to us.

This was when I came up with the idea of just running jumper cables from the truck to the batteries and running the engine for ~ 45 minutes or so. This would give enough of a charge to the 12v FLA batteries to keep things running for a day.

Once we left the campground we stayed overnight with some friends on Medford and they did let us plug into their home so we were able to fully recharge the batteries overnight. Next day we headed home, again it was 3 days/2 nights to get home, boondocking with no hookups and no charging other than what was pushed through the 7 pin.

There were a couple of nights it would have been nice to run the furnace but there wasn't enough power in the batteries to run the fan. We made do but I want to do better for our upcoming trip next month to the Florida Keys.

TL;DR: Would like the ability to be "off grid" for at least 10 days and need the ability to keep batteries (2 x 100AH LiFePo4) at a reasonable SOC for that period.

ETA: Biggest draw is the 12v refrigerator/freezer.

 

[Martinjmpr]

he supposedly quiet inverter generators have very limited 12vdc outputs, if at all. To charge batteries requires 12v chargers of some type with enough output amps and mid fourteen volts. More stuff. Fuel. Unpacking, repacking. Wires, connectors. Been there and sometimes am there still.

Yes, this is what's making me lean towards alternator charging. Trying to keep things as simple as possible and with as little equipment as possible.

Just went out and started my truck and checked the voltage at idle. Seems to stay right around 14.4v.

 

[gator70]

Yes, this is what's making me lean towards alternator charging. Trying to keep things as simple as possible and with as little equipment as possible.

Just went out and started my truck and checked the voltage at idle. Seems to stay right around 14.4v.

Or generator, they are cheaper and output twice the charge

 

[rruff]

TL;DR: Would like the ability to be "off grid" for at least 10 days and need the ability to keep batteries (2 x 100AH LiFePo4) at a reasonable SOC for that period.

ETA: Biggest draw is the 12v refrigerator/freezer.

Typically a fridge will use ~300Wh per day, but more if it's hot. Do you ever measure the energy you consume? I have a cheap monitor for this.

A LiFe 100A 12V battery will have at least 1200Wh of capacity, and draining them that low does not hurt them. IOW, there is no reason to charge them unless they are near empty. They weigh about 20 lbs, and I bought one recently with BT and cold temp cut off for $170 shipped. 3 of them should be enough to run your fridge for 10 days. Maybe add a 4th and call it good.

I'm just thinking that a DC-DC charger that you mainly only use while driving would be a good solution. Running your vehicle regularly when parked just to top off batteries, not so much. But having that as a fringe emergency measure would be fine.

 

[Dave in AZ]

one recently with BT and cold temp cut off for $170 shipped

Which was that, and do you recommend? I was about to buy one Will Prowse just tore apart and liked, WattCycle, but the day after his review price went from $207 to $250. Looking for a decent BT 100Ah mini. Thx

 

[rruff]

I haven't tested it as a bought it right before a trip, and now that its all snug and cozy I probably never will. It is a mini and the BT works... although it doesn't seem to get the watts right. Supposed to be a JB BMS. I like having BT for trouble shooting bad cells.

It's even cheaper now, $160 shipped with coupon. Since Amazon links are wacky I won't try, but search for this.

Elefast 12V 100Ah Bluetooth LiFePO4 Lithium Battery​