Second battery under seat DEAD

[Electrician]

Hi.

I opted for the factory fit second aux battery under the passenger seat in my 2024 T6.1 van.

I fitted an inverter from it for charging laptop and power tools, and occasionally use it for larger loads such as drills and breakers, or even a kettle.

It has worked great for almost 12 months.

Recently the inverter now alarms and shows battery low at 12.2v ish, and I’ve seen it drop to 11v. I’ve checked charging voltage at 13.8v with engine running, so it is meant to be charging.

I’m assuming because the battery isn’t being used to a great extent, it’s died?

Wondering if there’s a better type of battery to use than the lead acid there?

Spoke to one chap and he’s said to try charging the battery with a charger to give it life, but isn’t that just the same as what the alternator is doing daily?

Any advice would be great!! I’m an electrician, but batteries and the best option isn’t my line.

Thanks.

 

[burtonshawv]

I'd say that's probably warranty claim. No battery should die in only 12 months unless it has been fully drained for the duration

 

[roadtripper]

If you have the factory fit AGM battery and you've been regularly running loads as heavy as a domestic kettle off it via an inverter then most likely it's been damaged by being regularly discharged below 50%

Have you had times when the battery has been below 12v at rest?

Add some pictures of your current setup and details of your inverter and folks will be better able to advise.

Long term a LiFePo based leisure battery system is superior in every way to an AGM based one. There is an upfront cost but it's getting lower all the time and general advice is to consider it.

 

[Salty Spuds]

I had a very similar experience with a factory second battery in my previous T5 work van. Tool charging/kettle (low wattage camping type)/microwave etc. I mullered the battery after 12 months & VW didn't want to know as I had "abused it".
Another problem that I discovered, was that due the the battery having a simple split charge relay rather than any kind of "smart charging", was that if I ran a heavy load e.g.kettle, with the engine running, the power would be drawn from the alternator via the split charge relay, rather than the battery. This would cause the factory cable between the vehicle battery & the split charge relay to over heat & eventually blow the 90A fuse. This is because the split charge relay offers no current limiting capabilities, it's a simple direct connection to the alternator, which can supply more current at a higher voltage than the 2nd battery, ergo the inverter will draw from the highest source..the alternator.
I eventually changed the 2nd battery & fitted a Sterling power B1230 B2B charger between the split charge relay & the 2nd battery, no problems going forward, apart from I never ran the big loads on the inverter without the engine running, this allowed the B1230 to supplement the 2nd battery. In a nutshell, the factory 2nd battery is a smallish lead acid, intended for light loads, it was never intended for running power hungry kit & if you choose to do so, you need to be prepared to change the battery on a regular basis, OR go the whole hog & do a lithium upgrade.

 

[Electrician]

I’ve not used the battery to excess. If using large loads, engine on. Possibly once or twice a month. Other than that, most weeks, to charge a laptop or tool batteries. Nothing heavy. Was hoping for better life.

 

[roadtripper]

Are you using something like a BM2 to monitor the charge level of the second battery?

As @Salty Spuds mentions with the basic split charge relay and smart alternator it's only getting to about 80% charge anyway and if you use the 50% discharge rule you only have about 30% usable working capacity, and I'll bet it's the standard factory fit 75Ah AGM. So you have a "usable" 23Ah.

A 100w laptop charger will draw approximately 10A on the 12v side accounting for inverter losses so that's about 2.5 hours of run time.

If you do a lot of short runs but spend a lot of time on site with inverter running chargers (or idling) possibly you never replaced the charge some days.

If you switch to LiFePo then you can use a substantial amount of the capacity, 80% is the generally advised level for long life but occasional dips over are not as damaging as they are for lead acid. You'll also need a DC-DC charger but that means you'll get 100% charge. With a 100Ah battery that gives 80Ah usable, so about 4 times as much in a similar physical space, you can see why most folks make the switch.

If you do the good news is you have the heavy duty cable already under the seat and a charge signal to the split relay. Replace the relay with a Victron Orion XS and the battery with a Fogstar 105Ah LiFePo, for example, and you have a tidy auxiliary electrical system with good monitoring and control.

The life of a leisure AGM is usually around 2-3 years. 1 year is unfortunate but not hugely remarkable with inverter use so I suspect any conversation with VW is going to rely heavily on the goodwill and relationship you have with your dealer. It might be worth a try as costs little and you'd get another year to plan a longer term upgrade.

 

[t0mb0]

It's very much dependent on how hard a life it's had. I've got a 2016(!) vintage 75Ah AGM factory leisure battery still under the passenger seat and it's fine. Granted, it's undoubtedly not got the charge capacity it had when new but for 8 years of use, including a few multi-week off-grid road trips, it's not doing badly. The difference is that the load is moderate (no inverter, just the usual lights/fridge/etc) and it's hooked up to a DC-DC and solar panel to keep it topped up. It has been badly run-down a couple of times probably on rare occasions I've accidentally left it on whilst sitting on the driveway in winter but it doesn't seem to be as fragile as the some of the stories suggest.

 

[Stay Frosty]

What size inverter bearing in mind my 2 kw Renogy will happily drain towards 200A from my seatbase Roamer, 100A of battery per kWh out of the inverter is a reasonable rule of thumb.