Testing an Aux batt solution...

[Snakebitten]

Is there any down side to leaving the AGM battery in parallel to the LFp battery?

Not sure I understand the question.
Replacing the auxiliary battery is the same procedure whether it's another 7AH AGM or it's a 20-30AH LFP.

Although admittedly with the LFP you should be prepared to either do some modifications to the underseat battery storage compartment to fit the LFP into, or accommodate an alternative location.

I placed the LFP in the underseat storage space adjacent to the AGM storage space. Then added 12" cable extensions to the factory cable ends. Took maybe 10-15 minutes.

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[HammaMan]

It's like the seat mounting brackets were designed to be able to slide wiring underneath them

 

[hpwjr]

What I was meaning was is there any potential downside to leaving the AGM battery in and therefore it would be better to remove it (though it could be left in) or is leaving the AGM in a total nonissue and therefore just make it parallel to the installed LFP?

 

[HammaMan]

No, the aux battery remaining or not is of no concern. Personally I'd mod the box and install it in there. Just do keep in mind that even when the aux batt's B+ is disconnected from the aux batt, it's still connected to the primary AGM so don't let that wire contact ground.

You can disconnect that wire inside of the passenger kick panel to make playing in the aux batt box 'safer'.

 

[hpwjr]

Thanks. I might get to this Wednesday.

 

[Snakebitten]

What I was meaning was is there any potential downside to...........

I can prove how senior I am getting. My response above was me answering the question "Is there any downtime........

?

 

[hpwjr]

Got it! Been there. Done that. Fortunately it’s accompanied by the bliss of forgetfulness so that I can’t even remember how many times I’ve misread something.

 

[Jmitchelltfo]

What I was meaning was is there any potential downside to leaving the AGM battery in and therefore it would be better to remove it (though it could be left in) or is leaving the AGM in a total nonissue and therefore just make it parallel to the installed LFP?

If your OEM AGM is still good and you prefer to leave your pickup as unmodified as possible, I would just leave it. It's an additional 7.5ah on top of the LFP you choose too..........so I don't see any drawbacks, if you have the storage space.

 

[Spiffy]

Its so excellent to be past my battery issues and trust that my truck is going to remote start in these 12 below mornings. I was just about to have drone mobile installed. It's does the same thing as ford pass does with exception to having 45 min runtime. It does that by restarting 3 times in a row. And it provides geo fencing and precision location.
But now I can save my money for the next time my truck breaks. Lol

 

[dochawk]

Is there any down side to leaving the AGM battery in parallel to the LFp battery?

generally speaking, when you put two batteries in parallel, current will flow between them.

and even once they equalize voltage, they are going to have different discharge characteristics, so whichever discharges (thus lowing its voltage) faster will then draw from the other.

And that's without going to the internal resistances of the batteries, which will be different.

 

[scott011422]

If we are adding the LFP to, instead of replacing the Aux battery, What about using a supercap? Granted, More expensive than the Amazon LFP battery, but not as affected by cold weather.

 

[HammaMan]

A supercapacitor provides zero utility in this instance. While a supercap could in theory be used in place of the aux batt, it provides zero reserve capacity which is what we're achieving here.

 

[hatallica]

To nerd out a bit on @HammaMan's answer, the voltage across the capacitor may be characterized as:
Q = V x C
and
Q = i x t
Where Q is charge, V is voltage, C is capacitance, i is current, and t is time.

V = Q/C = (i x t)/C

For a fixed current draw over time, the voltage across the capacitor would linearly drop towards zero. In other words, if 13.2V is considered 100% "State of Charge", then 12.2V is about 92% SoC and 6.6V is 50% SoC. That is what is meant by the capacity being basically useless.

By comparison, batteries maintain a fairly steady voltage across a wide SoC range. An AGM that is 100% at 13.2V is still about 12.2V at 50%. Unlike capacitors, sources vary on using voltage to determine SoC, so I kindly request a bit of grace here.

Similarly a bit of grace on the simplified Q = i x t. It makes the point without getting into integrals.

 

[scott011422]

Replacing the aux batt with a supercap isn't what I had in mind. I was going more on the notion that the trucks didn't need more capacity, but needed to basically charge faster. Adding the LFP battery does this, being much more capable of absorbing power.

In my way of thinking the LFP is being greedy and sucking up all the power. Then when the truck is off, that larger power level is flowing out over time and topping off the underhood and aux batteries. Obviously the supercap doesn't have anywhere near the capacity to replace the aux battery, But thought that adding to the aux battery, once the two batteries were fully charged, would a supercap then be enough. I'm not necessary looking to keep my truck sitting long term. I would just like my batteries to be full during day to day driving. Charge levels above 12.6 are a rarity for me. I average 12.3 most days.

Also, For us northerners, Supercaps don't really care if its cold.

 

[HammaMan]

The LFP is using so little capacity that even if it's sub freezing, the energy needed to top it up is trivial. Based on my very loose calcs based on the papers regarding the chemistry, ~500 sub freezing top ups may end up reducing the LFP's capacity prematurely. Typically LFP life is rated such that after 3-6k charge cycles, it only has 80% of its rated capacity left. In the case of my 30ah batt, 80% life left after several thousand cycles would make it a 24ah battery in a 30ah housing. A 30ah AGM is much larger in size and only has a 15ah reserve capacity.

If you're below freezing and you only get around 500 charge cycles before hitting 80%, how many times does this occur assuming the BMS even allows sub temp charging? Let's say you have 90 days below freezing and you make 2 trips a day 5 of 7 of that 90 day window. That's 128 cycles a year which gives you 4 years of usage before hitting 80% capacity. That doesn't really matter because it's still going to charge and rest above 13.2v. In 8 years you'll be down to 60% capacity which is Just beginning to approach an AGM's reserve capacity. You'll still get the higher resting voltage AGM charging functionality which is the real benefit here.

It's why I rave about the LFP tech as a whole despite its cold limitations. Even abused it's still a much better chemistry both in storage density, total energy output, and its long-life. People are used to 3-5yr batt life with substantially degraded performance towards the end of its life. LFP isn't analogous to AGMs or other SLAs.

Now if a batt has proper cold temp cutoff in the BMS, seat heater material is quite ubiquitous (this is a series/parallel style, wire in series for "low" mode) and so are programable controllers to connect to a KEYED circuit. Wrap it around the perimeter of the cell housing, place the temp probe around the top or bottom of the batt (not touching the heating element), wrap in a thin insulation material and let it do its thing. Every time the key is on and the temp is within whatever range you program, the batt now has auto warming (this is precisely what the "self heating" batteries have in them).

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