Aux Battery: real life functionality

Yes, one does have to care about low temp. If your new battery has low temp charge cutoff you could leave the switch on. That said I have tested a couple of batteries that said they had low temp charge cutoff, but didn't. I put them in the freezer and they still allowed charge. Ymmv.

Yep I’m pretty sure that’s probably the main reason in put a switch for the Victron, and even though my new battery say it has low temp cutoff, “when tested by Will Prowse” it did seem to work like it should, but this way I can guarantee I won’t charge the battery till my van is warm. But there are folks on the interweb that say not to float or keep the lifepo4 batteries at 100% all the time? I’m no expert but this article seemed informative, but you know what they say about how much to believe about stuff you read?

Oh yeah it’s kinda a long read too.

Backing the max voltage off the cells more less eliminates the concern about keeping a battery topped off. I have two 100ah lifepo4 batteries that allow me to set cell voltage min and max.

It might be years before we see any real world data. I'm a big fan of solutions you can set and forget. Who wants to worry about their battery when they are enjoying their time off? Not me. I just want it to work.

My newest battery doesn't allow cell voltage adjustment, but it has internal heat, 150ah and fits in the stock location under the drivers seat.

Nice 👍🏼

pounce said:

@zeph what do you want to power with an aux battery? What are your power requirements?

This will help us make some recommendations.

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I guess I had the wrong idea. I thought the aux battery was powered by the alternator, but it sounds like that’s not the case.

I wanted to run a cooler and a kettle from it, but if the battery can’t be charged from the engine, I’m thinking maybe solar and 120V is the way to go.

@zeph

The factory aux battery is charged by the alternator. Same is true when we use a dc/DC charger to charge an aux battery that is not matching the stock agm starter battery. Many of us want to use lifepo4 battery chemistry because you get more ah in the same size and they can last many more years. My 150ah battery has 3 times the amount of usable watts than the stock agm. In order to mix battery types you need a dc/DC charger because lifepo4 wants 14.6v to charge.

If you mean a 12v kettle, no problem.

Charged by the altenator when the engine is running along with the starter battery. When the engine is off it is isolated from the main battery to keep it from being discharges with the auxiliary battery..

@pounce what 150ah battery did you install under the driver's seat and was it a perfect fit or did you need to create some type of custom bracket?

thefinnisher said:

@pounce what 150ah battery did you install under the driver's seat and was it a perfect fit or did you need to create some type of custom bracket?

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Perfect fit. Same form factor as the agm. It's a custom made unit. The larger US upfitter offers them as a package add-on now.

imacd said:

Charged by the altenator when the engine is running along with the starter battery. When the engine is off it is isolated from the main battery to keep it from being discharges with the auxiliary battery..

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But. Not exactly. We got some valuable insight from @Gottlieb not long ago that there is a time delay and voltage sensor trigger on the aux battery relay.


We are taking this thread in many directions so apologies to the op.

My recommendation is to use a dc/DC charger and bypass the relay for any serious aux battery usage. Not atypical to need power when stopped within the first 75 minutes. Also, I'm maybe not totally clear what could trigger the relay to close and reset the time when parked. Good thing it's not really an issue when you delete it.

So if I add an aux battery now, is the wiring already there? The wiring to charge it from the alternator, I mean. And how would you run stuff of the aux?

zeph said:

So if I add an aux battery now, is the wiring already there? The wiring to charge it from the alternator, I mean. And how would you run stuff of the aux?

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Sorry, if your van did not come with the aux battery under the driver's seat your van would not have the wiring or battery hold down bracket under the drivers seat.

Is the factory battery an AGM? I've seen that acronym thrown around here on the forum so I'm trying to learn, interesting. I had no idea there was something between regular lead-acid and lithium.

zeph said:

Is the factory battery an AGM? I've seen that acronym thrown around here on the forum so I'm trying to learn, interesting. I had no idea there was something between regular lead-acid and lithium.

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Yes. It's a type of lead acid battery.

Yup. Tiny 12v.

It is confusing, between different vans, options, ... and lack of clarity or wrong imagery from MB at times.

That little battery depicted is the typical MB backup battery, that will / should allow the vehicle to be powered up and be shifted into neutral so it can be towed, when the main ( starter ) battery is depleted too much. I say should, because I had a personal experience this summer with our SUV, where she had zip nada??

There can be battery alerts about battery replacement where people replace the main battery, still have the error as the tiny one is toast. MB hopefully have gotten better about specifying which in recent models.

Oh, and the tiny battery can be in wildly different locations depending on the car model, like: either side of the dash, passenger seat, trunk.