When to replace the aux batt and how to know?

[Davexxxx]

I had a lot of the same electrical gremlins as many had, until I saw a different strategy re charging. Rather than including the BMS in the charge circuit, charging directly to the posts of the engine batt, waiting for float on the charger, disconnecting and immediately resetting the BMS.

That worked, for more than a yr. Once n done. No special battery monitors, no forscan changes, fan speed 5 didn't seem to work for me anyway. Just a single charge, as outlined above, thats it. To be clear, it has never left me sitting, failed to start, or even acted slow to start, even when it was issuing warnings.

A store parking lot hit and run, did minor damage to the driver's side, bed side and while in for repair / replacement, sat outside for a month last winter. Body shop couldn't get the engine batt to take a full charge and replaced the engine batt with a new OEM. (H7-800CCA AGM)

Once back in my possession, I did the same charge, BMS reset and has been trouble free, until a couple weeks ago. Did that type charge again and good to go, until another low batt warning last night. "System off to conserve battery".

So, looks like I need to do something, especially with winter coming but what?

I'm aware of the aux LFP mods. If there was a direct drop in, that would be it but I won't be paralleling another batt to the aux and I'd rather not carve on the housing to accept a single.

Other than disconnecting both engine and aux batts after a charge and monitoring SOC over time, or running a load test on each independently, is there a way to isolate and know which is the problem child?

Again, engine batt is a 1 yr. old OEM, the aux is original from 2023. It does not have the updated BMS but from what reading I did on that, it didn't turn out to be the sure cure that was hoped for.

How best to proceed?

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[PB2021]

I got 4yrs from both batteries. Both are AGM. Changed at same time. I do charge the vehicle if it sits for days. Although I had not been receiving battery save messages I was finding it took longer and longer to see full charge on charger and SOC would drop rather quickly even when it was being driven. At change up I charged each new battery fully outside the vehicle before drop in. Received several new updates immediately in the following days after changing both after over a year without any updates. I still charge when truck sits but now it charges to full very quickly and holds SOC well while underway. Both are receiving charge at the same time when you are charging main 12v. If you changed the main and didn’t change the aux I would go ahead and change aux. If it’s weak due to age/use your newer main bat will be losing charge as it tries to keep charge in the aux bat (when not on charger). As it weakens it can act like a parasitic draw on the main 12v and even if the main 12v is healthy the result will be less capacity in reserve without charging. Thankfully the aux is a little cheaper to replace because not replacing may result in premature aging of your relatively newer main 12v.

 

[PowerBoostSoCal]

Where is the AUX battery located?

 

[Davexxxx]

Where is the AUX battery located?

Under the right rear seat.

 

[Porpoise Hork]

Good rule to follow is if you're replacing the main 12v battery then replace the aux battery as well. Otherwise could lead to wierd quirks in the BMS reporting the correct SOC levels.

 

[Davexxxx]

Good rule to follow is if you're replacing the main 12v battery then replace the aux battery as well. Otherwise could lead to wierd quirks in the BMS reporting the correct SOC levels.

I know replacing together is the rule of thumb but how does the rest of your statement work with the LFP aux replacement some have done. i.e. not just different ages but different chemistries in parallel.

Also a rule of thumb is not mixing sizes but Ford did that.

Another would be the Ford strategy of staving LA chemistry, known to shorten lifespan.

Seems like none of the rules of thumbs apply?

Right now, they're on the charger, as described above. Once done, I'll disconnect them from the truck and monitor voltage. If nothing can be discerned overnight, get a load tester and see what I can find that way. The engine batt is under full warranty. The aux is beyond any.

 

[PB2021]

This site may assist in assessing different charge and maintenance characteristics of AGM vs Li-ion before you decide to add lithium to your setup
https://batteryuniversity.com/

 

[Porpoise Hork]

I know replacing together is the rule of thumb but how does the rest of your statement work with the LFP aux replacement some have done. i.e. not just different ages but different chemistries in parallel.

Also a rule of thumb is not mixing sizes but Ford did that.

Another would be the Ford strategy of staving LA chemistry, known to shorten lifespan.

Seems like none of the rules of thumbs apply?

Right now, they're on the charger, as described above. Once done, I'll disconnect them from the truck and monitor voltage. If nothing can be discerned overnight, get a load tester and see what I can find that way. The engine batt is under full warranty. The aux is beyond any.

The mixed chemestries only comes into play when charging but most LFP batteries have their own internal BMS when they detect most if not all cells at 100% and the supply voltage is high enough it should enter balancing to bring up anyt stragglers. If all calls are properly balanced then the LFP's BMS shunts the incoming voltage preventing it from overcharging the cells. At that point the AGM Aux battery would continue to accept the voltage as normal with the LFP rejecting the supplied power to it. Yes, the charging strategy for AGM and LFP are different and could shorten the LFP's lifespan but the LFP BMS should mitigate most of that.

If the aux battery capacity were for example reduced due to age/sulfation then the piggyback LFP would feed power into it as well as suppy to the truck's systems after key off. If the truck BMS sees the voltage drop below a certain point then it will activate the isolator next to the primary 12v to contnue to supply power to the truck preventing the aux battery voltage from being pulled too low causing damage. Thus possibly resulting in random dips in the overall SOC level on the 12v side. With a healthy aux battery and a parallel LFP this reserve time is much longer preventing the SOC dips over a longer period of time.

 

[Davexxxx]

BTW, my aux is a slightly different part number than I saw quoted in another post.

BHAGM-AUX1-B

B, rather than A

Dimensions are 5.25 long, 2-7/8 thick, 4.75 tall.

 

[Porpoise Hork]

There's not any third party batteries that are a direct fit, at least that I could find. I ended up replacing mine with this one Mighty Max Battery YB12A-A. It took a little adjusting the strap, base, and leads but I got it to fit under the stock cover. It's rated at 12AH vs the stock 7.5AH and with a 20AH LFP it's still an improvement over stock.

 

[mrblonde]

I'd like to order that mighty Max battery you suggested. Could you describe the adjustments that are necessary for it to fit?
Thanks

 

[Porpoise Hork]

There's a pair of tabs on the base that need to be snipped off. They help hold the stock aux battery but the strap does this so not really needed with the taller battery. The plastic cover on the positive side where it meets against the top of the batter needs to be trimmed away so it will reach the mounting hole on the battery. I used electrical tape to cover the now exposed metal the plastic had covered. Since this battery uses squared nuts as retainers it's an absolute bitch to hold them in place to thread the bolts into (the OEM one's work here) so I shoved a some paper under them to hold them in place. You'll also have to hold your mouth just right getting the positive terminal to line up. The strap just need to loosen the back retaining bolt almost all the way out so it will allow the front bolt to line up and reach the mounting hole. Then both can be tightened down.

Oh and since it is just a little taller than the OEM it pus the positive connection closer to the lid so it will fit a little tight against it but the cover will snap back in place but will flex just a little in the back corner.

 

[mrblonde]

Great! Thanks.

 

[Davexxxx]

After noting the charger had gone to float last eve, disconnected and took a reading at each bat.

12.8 something, According to the charts, 100% ish.

Just now, engine bat was @12.7 something, so 90%

Opened the rear door to check that batt, of course woke things up and V dropped to 12.46 at each. 75%

I'll check again in an hr. or so.

I'd like to discern something without having to take them out and load test them but something is different than before. Nearing a yr. twice, with no issues, by doing the charge procedure cited above, to getting warnings in 2 weeks. No difference in driving frequency. It was nice to not have to think about it.

 

[Porpoise Hork]

It takes between 6-8 hours sometimes longer in the key off state for the BMS/BCM to properly update the battery SOC level. Also when you open the door it wakes up multiple modules. If you want to get an accurate resting voltage you'll need either a bluetooth battery monitor or leave the hood and door open but use a wrench or something to flip the latches closed so the truck thinks they shut. Then you can come back in an hour or so to check resting voltages in the truck.

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