Possible "backup solution" for failed 12-volt battery??

[nlions]

I live my life creating backup solutions to problems I haven’t had as my working career was in business continuity and of course it drives my family crazy. I’ve not had a 12-volt battery problem yet with my 12-month-old MME but I was thinking of a relatively inexpensive way to ensure you could always get in your car, at a minimum, if your 12-volt battery drops out of the working world and assume it's at the worst possible time which varies by your level of paranoia. So, with too much time on my hands, and little electrical engineering knowledge I was thinking perhaps by using identical MME 12V batteries, wire them in parallel, and put a Remote Controlled Disconnect Switch (below) between the 1st (primary) and 2nd (backup) and have the normal position of the 2nd battery as not connected. Then, theoretically, if a failure of the primary 12v battery occurs and you cannot open the frunk, you can use the remote (if you have it with you) to connect the 2nd battery into the parallel wiring. Theoretically that should allow the 12v backup battery to provide power as normal. You could add an A/B switch, like boats use, in between the Primary and Backup so when you got the frunk open you could take the primary battery out of the wiring configuration altogether. I know you would have to keep the backup battery charged with a trickle charger/etc. but what has my lack of knowledge missed here?

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[G.O.C.]

Or maybe monthly or biweekly use the interrupt switch to alternate which battery is in use so that both stay charged without the need for the trickle charge. Reading the amazon listing you might need two disconnects anyway.
You would also need to somehow anchor the second battery in the frunk, protect the terminals from something else in the frunk accidentally shorting the auxiliary battery terminals, plus some type of cabling system to split and run the batteries in parallel.
I'd hate to lose the frunk space so I think I'll pass.

 

[tonytaylor53]

How would you energize the solenoids? I'd keep one side hot through a fuse, and switch the ground(s). Also consider that keeping a solenoid energized 24/7 requires a higher spec than what you've linked, if your system needed one on the main battery too.

I think all of this is more trouble than it's worth, and it'd be simpler to just replace the 12V every 2 years if you're that worried about it.

 

[JohnFoxeSheets]

Or maybe monthly or biweekly use the interrupt switch to alternate which battery is in use so that both stay charged without the need for the trickle charge.

I suspect doing that would drive the BMS crazy and could result in unexpected behavior.

But yeah, if you wanted a backup 12V, you gotta keep it charged one way or another.

 

[Maquis]

So you’ll have 2 batteries that fail in 3 years instead of one? Brilliant!

 

[Snakebitten]

So you’ll have 2 batteries that fail in 3 years instead of one? Brilliant!

Not necessarily.
Some of us Powerboost F150 owners replaced our auxiliary AGM battery with a cheap LFP. (~$60)

The auxiliary battery is under the passenger rear seat and is a small 7AH AGM that is in parallel to the H7 AGM under the hood. The purpose for the auxiliary battery is that Ford isolates/protects (battery isolator) the electronics onboard the truck from the High amperage draw of a ICE starting event. So for the brief moments that the traditional bendix/flywheel starter is engaged, the starter and underhood AGM are electrically isolated from the rest of the low voltage bus and the 40+ modules are powered by the little 7ah AGM.

Swapping to the LFP, which is 25AH for similar size to the AGM, we've found the higher resting voltage of LFP actually propping up the combined resting voltage of the two batteries in parallel.

So the OP could use a rather small LFP (climate depending) in his concept and life expectancy might be far better than a second AGM?

 

[Mach-Lee]

This is overkill IMO. Just replace the 12V every 3 years. Likelihood of failure is very low if you simply do that.

 

[Maquis]

Not necessarily.
Some of us Powerboost F150 owners replaced our auxiliary AGM battery with a cheap LFP. (~$60)

The auxiliary battery is under the passenger rear seat and is a small 7AH AGM that is in parallel to the H7 AGM under the hood. The purpose for the auxiliary battery is that Ford isolates/protects (battery isolator) the electronics onboard the truck from the High amperage draw of a ICE starting event. So for the brief moments that the traditional bendix/flywheel starter is engaged, the starter and underhood AGM are electrically isolated from the rest of the low voltage bus and the 40+ modules are powered by the little 7ah AGM.

Swapping to the LFP, which is 25AH for similar size to the AGM, we've found the higher resting voltage of LFP actually propping up the combined resting voltage of the two batteries in parallel.

So the OP could use a rather small LFP (climate depending) in his concept and life expectancy might be far better than a second AGM?

So his idea will work great as long as he does something completely different than what he proposed? Got it!

 

[Snakebitten]

So his idea will work great as long as he does something completely different than what he proposed? Got it!

I was just suggesting the secondary battery being LFP rather than another AGM.

 

[superdave80]

I think the simplest solution (which Ford should have put in) would be to have a 12V charge point inside the charge door area. It would allow you to apply 12v to the car if the battery dies, attach a trickle charger when it's needed (without having to leave the hood cracked open), and allow easy access to attach a power supply when doing certain updates.

I still laugh that Ford thought they could just seal up the 12V because '12V battery failures won't happen anymore'.

 

[MG101]

Sorry, hijacking.

Seen a lot of threads worrying about dead 12v batteries lately so I have to ask. Are the little frunk popper jumper wires not a reliable solution?

...carry on...

 

[markboris]

I think the simplest solution (which Ford should have put in) would be to have a 12V charge point inside the charge door area. It would allow you to apply 12v to the car if the battery dies, attach a trickle charger when it's needed (without having to leave the hood cracked open), and allow easy access to attach a power supply when doing certain updates.

I still laugh that Ford thought they could just seal up the 12V because '12V battery failures won't happen anymore'.

While I have always replaced batteries in my vehicles every three years (also as suggested by Lee) I have access to the 12V battery to trickle charge in case I need to. I do a lot of mods on my cars and sometimes don’t want to or can’t disconnect the battery while I’m working on them so I plug in my 5A CTEK charger so as not to run the battery down too low.

https://www.macheforum.com/site/threads/12v-battery-access.29646/page-2#post-674869

 

[Maquis]

I was just suggesting the secondary battery being LFP rather than another AGM.

I think that works in the trucks because the aux battery has its own BMS. I have my doubts about it being feasible in the MME, but I’d like for him to try it and prove one way or the other.

 

[Maquis]

Sorry, hijacking.

Seen a lot of threads worrying about dead 12v batteries lately so I have to ask. Are the little frunk popper jumper wires not a reliable solution?

...carry on...

Some people have had the battery fail in a state where it doesn’t have enough power to unlock the car, but still has enough voltage to inhibit the frunk jump.

 

[astrorob]

in another thread i had the idea to just interrupt the power to the frunk latch, so that if you find yourself in a 12V brownout condition, you can just disconnect the power to the latch and apply 12V to the contacts in the bumper. IMO this is a lot easier as you don't have to worry about switching all the current that the 12V battery supplies, just the amount that the frunk latch would draw.

problem is i'm sure it's easier said than done to find all the wiring and unwrap it, cut it, install the switch, etc. which would actually need to be visible from the bumper port. mach-lee thought this was insane as well, so yeah, probably just changing the battery every 3 years is the way to go.

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