Small AGM backup for diesel heater start, vs heated Lifepo4.

A recent thread got close to this, but I didn't want to derail it. Folks with interesting comments on this were
@RV8R @HarryN @Tomthumb62

I have a small truck topper "popup canopy camper" made by Tune, similar to an Alu-Cab or GoFast Camper but whole top pops up, so not much insulation. Recently, three friends all got bit by colder than expected weather, below -5f. In all 3 cases, they couldn't get battery power to start their diesel heater. Two of them had power stations with -5f operating limit, pretty common, and stations just shut themselves off, no way to discharge the battery. Other guy had a Lifepo4 battery system, but also could not discharge, don't know if that was BMS.

In any case, I decided I need a small backup to get me just enough 12.8V power to start and run my diesel heater for an hour or so, about 34 Watt-hrs (14 to start, 20 more to run an hour to heat Lifepo4 or power station). So say 100 Watt-hrs total or just 8Ah would do it.

What are your thoughts on keeping a small AGM charged with a cheap Noco or such, vs. a Lifepo4 in an insulated box with heat pad or self heated? Reliability and weight and total cost? I'm thinking a small AGM if available would work better and easier?

Also, I need minimum 12v, 12.8 better, no matter what, 11A startup flow, can an agm do this? Or add some kind of boost transformer to ensure 12-13v?

Oh, it could be sealed lead acid, I just assumed AGM would be better. Thoughts?

Loves me some LFP, but this sounds like an ideal solution for AGM. I would prefer AGM as they are more freeze tolerant, and they lose almost no charge sitting there in the cold for months.

If your concern is that 12V AGM battery will be a bit too low in voltage, is 16 nominal OK?


I don't know how they will do for your (deep?) cycle application.

What would happen if you mixed agm and lfp? Since it's a simple system you could flip a "cold mode" breaker and enable agm in the circuit.

The cheap Chinese diesel heaters should run on 11 or so voltage. I remember trying to run 2 of these on very small gauge wire and had to stagger start them.

Agm and lfp are happy together.

Agm requires oddity charging recovery modes, but if only used for small tasks, and always kept full, will happily play with LFP charging profile.

Supervstech said:

Agm and lfp are happy together.

Agm requires oddity charging recovery modes, but if only used for small tasks, and always kept full, will happily play with LFP charging profile.

Click to expand...

Nice then. I see they have small $35 AGMs that are 12 to 15Ah, perfect as a cold weather piwer source to bootstrap up my diesel and then warm the main players up to functionality. Little AB switch would give options, and I could just let it charge from system once in a while. Thx!

Dave in AZ said:

Nice then. I see they have small $35 AGMs that are 12 to 15Ah, perfect as a cold weather piwer source to bootstrap up my diesel and then warm the main players up to functionality. Little AB switch would give options, and I could just let it charge from system once in a while. Thx!

Click to expand...

Also very useful if you need to jump start LFP in discharge protection.

I highly recommend PowerSonic brand especially, for small sealed lead acid batteries. With a battery tender 800mA too.

Dave in AZ said:

Nice then. I see they have small $35 AGMs that are 12 to 15Ah, perfect as a cold weather piwer source to bootstrap up my diesel and then warm the main players up to functionality. Little AB switch would give options, and I could just let it charge from system once in a while. Thx!

Click to expand...

Another thought i had was... this is a motor vehicle right? With a starting battery... couldnt you use jumper cables to jump the heater battery?

Supervstech said:

Another thought i had was... this is a motor vehicle right? With a starting battery... couldnt you use jumper cables to jump the heater battery?

Click to expand...

Yes, pickup truck camper. The start battery could provide a backup 8Ah to start and run diesel heater. But, imagine the scenario:
You're sleeping in your pickup bed. Wake up in morning, temps have dropped to -10f. Or maybe you drove there after work, and now it's dark and you're setting up camp. Due to cold, Your power station has shutoff, as has your LFP backup battery. You just need 35 Watt-hrs of 12-13v to get the diesel heater started and warm up your system 10 degrees so the LFP will discharge (most won't restart until > 34f once stopped though).

Would you rather roll over, turn a switch for a brick sized AGM sitting in your crowded pickup bed? Or get dressed and start dragging 30ft jumper cables around?

I'm just trying to decide if a small agm is better than a 100Ah LFP in an insulated box with a heater. Payload is limited to like 400 lbs total and a pickup bed is small... also, I don't like risking running down my start battery at -10f. I just haven't run any of the 3 batteries at -10f or below to know reliability and voltages etc.

Dave in AZ said:

Yes, pickup truck camper. The start battery could provide a backup 8Ah to start and run diesel heater. But, imagine the scenario:
You're sleeping in your pickup bed. Wake up in morning, temps have dropped to -10f. Or maybe you drove there after work, and now it's dark and you're setting up camp. Due to cold, Your power station has shutoff, as has your LFP backup battery. You just need 35 Watt-hrs of 12-13v to get the diesel heater started and warm up your system 10 degrees so the LFP will discharge (most won't restart until > 34f once stopped though).

Would you rather roll over, turn a switch for a brick sized AGM sitting in your crowded pickup bed? Or get dressed and start dragging 30ft jumper cables around?

I'm just trying to decide if a small agm is better than a 100Ah LFP in an insulated box with a heater. Payload is limited to like 400 lbs total and a pickup bed is small... also, I don't like risking running down my start battery at -10f. I just haven't run any of the 3 batteries at -10f or below to know reliability and voltages etc.

Click to expand...

Oh, i meant something wired in to flick a switch with. Not actual jumper cables...

Supervstech said:

Oh, i meant something wired in to flick a switch with. Not actual jumper cables...

Click to expand...

Hmm... well, you might be onto something. I do plan on running some 4AWG back there from starter battery, to allow either a
1. dc to dc charger for LFPs, probably Victron 12 12 30.
2. Or if I keep using my power station, a 12 48 converter to feed its PV input, need 32v min. Still haven't found a good victron quality 12 to 48 converter.

I don't want my starter tied into my house system at all, thus the dc to dc units to separate the two and keep me from ever running down start battery. However, I guess I could just run another on/off switch and a few more feet of cable to bootstrap my diesel heater if needed. I guess it comes down to < -5f drain on starter battery, vs another wee backup source. On the other hand, I know a guy who replaced starter SLA with a big Dakota Lithium, heated, and he uses that single battery now for all truck camping uses. Scarey!

Hi, I am continuing to add to that thread on the trailer.

The part that isn't completely obvious regarding your diesel heater is that when just running, the power draw is small, but there is a glow plug / igniter inside that pulls pretty hard during the warm up to light it up.

When the battery is cold ( any brand ) , the voltage drops and the ability to sustain a high enough voltage to not trip the heater low voltage off is hit.
Most of the Li power stations on the market cannot deal with this surge, probably some can but many cannot.

The typical budget AGM battery can't do it either, in spite of what is claimed. When I had my shop, one of the first customers to come in was using those full river house batteries that another place had installed and it just would not run.

Their "solution" was to have him keep the van engine running so that the voltage would be high enough if you can believe it.

My personal solution is to use a 24 volt AGM setup and a good quality DC - DC converter to run the heaters. Not a cheap one, like a $200 one.

Most stuff on the market is designed by people who live in places that don't get all that cold - if you want things to run when cold, you have to get serious and be willing to pay significantly more for things that work when it is cold. That is what separates the men from the boys.

_____________

I should probably try doing this with a single, good quality, 12 volt battery sometime, but since I have a proven solution, it isn't that interesting.

There are also some 24 volt diesel heaters and they will naturally be more tolerant to the voltage drop - because they have a wider input range.

My experience running a diesel heater is that the power wire is a little undersized for the amperage during warmup and causes an excessive voltage drop. For example, running my diesel heater on the full-length uncut power wires it came with, off a full size car battery, the controller reports it dropping to the low 10s or high 9s volts. I know what it takes to do that to a car battery.. it takes a couple hundred amps or more. The diesel heater is probably pulling no more than 20 amps. The low voltage is due to voltage drop across the power cabling. Fix that and the battery itself will not need to be as good.

@Vigo @HarryN
Good points on voltage drop, always good to mention in a heater thread, 90% of errors I see posted are due to this!

It takes 110-144 W, 10 to 12A, to heat up the glowplug at start. I have never seen it take more than 4 minutes, so I use 15 Watt-hrs as a conservative start number, then it will run on 10 to 20 W, I figure 1 hr is more than enough. That is where I get 35 Watt-hrs, 3 Ah so double that for SLA or AGM, 6Ah. The weak voltage on many power stations, as well as people using 5A ports, and poor car charger plugs, all give voltages too low--if it dips below 11.5 or so, most heaters throw an error.

And yes, the cheap thin and super long wire that comes with heater will already cause marginal voltage drop if you retain it!

I actually got a Pecron power station with a regulated 13.4V dc output, 30A xt60, to defeat these issues. And rewired my heater with 10AWG. All good there!

The point made about a small agm not supporting 11A start flow without dropping voltage below 12v, especially in severe cold, is a good one. I like the idea of 24V setup feeding a small converter... but I'm not buying a Victron just for this lol! I'd think a cheap one would suffice for 5 min of 10A, then 1A. But I don't want to be separated with the boys lol!

But this exact type of hidden issue is what is making me think, "maybe just a heated LFP instead is simpler..."

Dave in AZ said:

@Vigo @HarryN
Good points on voltage drop, always good to mention in a heater thread, 90% of errors I see posted are due to this!

It takes 110-144 W, 10 to 12A, to heat up the glowplug at start. I have never seen it take more than 4 minutes, so I use 15 Watt-hrs as a conservative start number, then it will run on 10 to 20 W, I figure 1 hr is more than enough. That is where I get 35 Watt-hrs, 3 Ah so double that for SLA or AGM, 6Ah. The weak voltage on many power stations, as well as people using 5A ports, and poor car charger plugs, all give voltages too low--if it dips below 11.5 or so, most heaters throw an error.

And yes, the cheap thin and super long wire that comes with heater will already cause marginal voltage drop if you retain it!

I actually got a Pecron power station with a regulated 13.4V dc output, 30A xt60, to defeat these issues. And rewired my heater with 10AWG. All good there!

The point made about a small agm not supporting 11A start flow without dropping voltage below 12v, especially in severe cold, is a good one. I like the idea of 24V setup feeding a small converter... but I'm not buying a Victron just for this lol! I'd think a cheap one would suffice for 5 min of 10A, then 1A. But I don't want to be separated with the boys lol!

But this exact type of hidden issue is what is making me think, "maybe just a heated LFP instead is simpler..."

Click to expand...

I am not sure if the victron DC - DC converters will work or not.

If you use 2 each small batteries in series, then each one only has to produce 5 - 10 amps to feed the converter and this isn't too difficult.

My theory is that once you put a LiFe battery in a vehicle, it is no longer a "car", it is now an "extension of your home".

Because of this, your plans should be to always keep the battery pack in between 40 - 110 F, what ever that takes is what you do.

It does not take all that much fuel to run those diesel heaters, so it might be worth considering to just set it at 40 - 50 F in the back and just keep it warm back there all of the time. This would keep your vehicle power system functional in quite tough ( cold ) weather.

___________

If you have room, maybe consider to take along a small generator - some of those have 12 volt / non regulated outputs, but either way you could use it to feed a small 12 vdc converter worst case. Fuel consumption of those things is pretty low in econo mode.

Dave in AZ said:

A recent thread got close to this, but I didn't want to derail it. Folks with interesting comments on this were
@RV8R @HarryN @Tomthumb62

I have a small truck topper "popup canopy camper" made by Tune, similar to an Alu-Cab or GoFast Camper but whole top pops up, so not much insulation. Recently, three friends all got bit by colder than expected weather, below -5f. In all 3 cases, they couldn't get battery power to start their diesel heater. Two of them had power stations with -5f operating limit, pretty common, and stations just shut themselves off, no way to discharge the battery. Other guy had a Lifepo4 battery system, but also could not discharge, don't know if that was BMS.

In any case, I decided I need a small backup to get me just enough 12.8V power to start and run my diesel heater for an hour or so, about 34 Watt-hrs (14 to start, 20 more to run an hour to heat Lifepo4 or power station). So say 100 Watt-hrs total or just 8Ah would do it.

What are your thoughts on keeping a small AGM charged with a cheap Noco or such, vs. a Lifepo4 in an insulated box with heat pad or self heated? Reliability and weight and total cost? I'm thinking a small AGM if available would work better and easier?

Also, I need minimum 12v, 12.8 better, no matter what, 11A startup flow, can an agm do this? Or add some kind of boost transformer to ensure 12-13v?

Oh, it could be sealed lead acid, I just assumed AGM would be better. Thoughts?

Click to expand...

Hi @Dave in AZ

I’m late to the Party here, & speed read the thread

I use to have a phrase for LFP ,,, the “3Cs”

Cost
Charge Complexity
Cold

I have cheap Chinese Diesel Heater experience also @ my off grid cabin.

As I finger peck out my response, I am in front of the supply air duct of my Propex HS2800 running off my Rolls 12vdc AGMs in my 300st Solar Cabin. & here is todays 12vdc energy use @ 7:42pm;



North of the 49th ,,, still snow on the ground & I am 100% self sufficient on Solar & Propane.

So Diesel Heaters;

- need 10 amps for glow plugs, then 2 to 4 amps to run after they have started.

- I assume @HarryN is correct extreme cold both AGM & LFP gets voltage sag ,,, I have zero LFP experience ,,, Canada

- I experienced voltage “pickyness” on those diesel heaters ,,, The trick is to apply a charge source of greater voltage ,,, I’m thinking Kisae in your case ,,, flash up the truck engine to Kisae to AGM ( In The Cold ) ,,, voltage sag issue gone ,,, that is what I did @ my cabin to get the glow plugs happy ,,, once the unit is “lit” ,,, no need to keep the 14.7v on - shut down the engine. On to next topic “Diesel”


Diesel Heaters;

Much different function over Propane furnaces;

Diesel, likes to get “lit” & go all night !! ( I resemble that remark ) ,,, but seriously - they are not an on/off device. If you want a thermostaticly controlled device ,,, Propane.

Maybe - AGM / Kisae / Diesel or Propane ?

Or both ,,, You might find the following interesting;

Planning for colder than LFP can take - Sodium-Ion instead, thousands of cycles instead of hundreds for AGM ?

That is a good point about propane @RV8R . I remember some conversations about the power needs ( especially the ignition power needs ) of your propane setup vs diesel heaters and the difference is quite significant.

HarryN said:

That is a good point about propane @RV8R . I remember some conversations about the power needs ( especially the ignition power needs ) of your propane setup vs diesel heaters and the difference is quite significant.

Click to expand...

Power Needs & Functionality

We could chat “High Altitudes” ,,, best that topic be kept for later.

The Propex uses about 2 amps ,,, diesel 10 amps to light them back down to 3 or 4.


edit; Just did a screenshot of the Propex propane furnace running;



So that 2.14 amps is the total system, but the majority is the Propex furnace.

It just shut off, so here is the system without the furnace running ,,, the difference is the furnace draw;