Managing battery life with auxiliary/dual battery

[Clark Kent]

I recently read an article from Ctek on managing batteries in larger vehicles. See below. The context was 24v (2 x 12v) dual battery configurations.

There is an interesting point made about batteries in a dual configuration becoming electrically unbalanced because the first battery does more work, i.e. more loads, charging cycles and at higher Depth of Discharge (DOD) rates than the second battery.

This also applies to Grenadiers and Quartermasters equipped with the auxiliary battery option because all electrical loads are taken by the starter battery and the auxiliary (2nd, dual) battery supports the starter battery by pulsing a trickle charge to the starter battery via the battery combiner (Smartpass 120S) when the starter battery voltage is below the auxiliary battery voltage.

I raised an enquiry about this with Ctek on Monday and got a response today (Wednesday).
My question was about imbalance, and if this might affect the service life of the 2x EFB in some Ineos vehicles. I also asked if rotating batteries periodically would extend the life of both batteries.
Ctek's response was yes, we should expect the starter battery to fail earlier than the auxiliary battery because of the unbalanced loads, and yes it would be beneficial to swap the batteries around a couple of times each year to maximise the service life of both batteries. One battery is working quite hard and the other is not being worked hard enough.

A battery swap is not a difficult task but it's not made easy due to the under seat trim in 5 seat vehicles and the necessity to let the vehicle go to sleep first. Those inconveniences aside, a biennial swap around might be a worthwhile action.
There's a definite effort required to do this but if it means you're less likely to experience a starter battery fail in service (you'll be remote, on a Sunday, and it'll be raining), then perhaps it's worthwhile.

Food for thought as a preventative maintenance action

 

[DustCloud]

Or, when starter battery dies, move original aux to the starter battery position, and add a new battery in the vacated aux position?

 

[Clark Kent]

Or, when starter battery dies, move original aux to the starter battery position, and add a new battery in the vacated aux position?

Yes, absolutely you could do that. You could run without an aux battery installed if you had to rob it to replace the starter battery to get you home. I don't think the vehicle will notice that it is missing. Just isolate the smartpass at the input side. Even better, use a Noco style lithium jump starter provided the starter battery has not shorted.
The point is about preventative maintenance not reactive repair. It's not a straight forward task to pull the starter battery out on the side of the road because of the underseat trim.

 

[Itsdchz]

Or, when starter battery dies, move original aux to the starter battery position, and add a new battery in the vacated aux position?

My belief (recollection from numerous sources) is, it is highly recommended to swap both batteries at same time in dual battery setup.

 

[grnamin]

Or, when starter battery dies, move original aux to the starter battery position, and add a new battery in the vacated aux position?

Maybe instead of moving the aux battery, some advanced cabling with disable/enable switches will do the trick?

 

[Lars]

So my question would be: If you do switch batteries periodically, does the battery management system need to be reset or calibrated by a scan tool? That would mean an expensive trip to the dealer.

 

[Tazzieman]

If you get a portable battery tester ( in addition to the mandatory jump starter pack) you'll be able to check the health of the starter battery from time to time - and pre-empt "failure to thrive."
I'd rather just buy a new battery than go through the faff of swapping them around. YMMV!

 

[Clark Kent]

My belief (recollection from numerous sources) is, it is highly recommended to swap both batteries at same time in dual battery setup.

That's the practice in a series-wired system with dual batteries like a 24v truck because the batteries act as a single bank cells but in this system the batteries are electrically isolated by the combiner (Smartpass 120S). Interconnection is managed by the Smartpass so the aux gets a recharge from the starter battery when required, and the starter gets a pulse charge from the aux if needed.

 

[Clark Kent]

So my question would be: If you do switch batteries periodically, does the battery management system need to be reset or calibrated by a scan tool? That would mean an expensive trip to the dealer.

Good question. I believe this is the case with BMW vehicles that run an AGM battery. The new battery has to be registered with the BMS.
A few Grenadier owners have had their battery replaced under warranty but no one has specifically mentioned a BMS update.

 

[Clark Kent]

If you get a portable battery tester ( in addition to the mandatory jump starter pack) you'll be able to check the health of the starter battery from time to time - and pre-empt "failure to thrive."
I'd rather just buy a new battery than go through the faff of swapping them around. YMMV!

Absolutely. The point you're dismissing is this action might extend the life of both batteries. Solvable with cash.

I see it a bit like doing a 5-way tyre rotation instead of a 4-way. If you add the spare tyre into the rotation then all tyres last longer because one tyre gets rested while it's on the back door. Unfortunately that's also not straightforward on our vehicles due to the TPMS registration

I did say this is just food for thought. I'm not too sure anyone has thought much about the effect of load imbalance over time, hence the share.

I took the underseat trim out when I got my vehicle so my faffing is less than your faffing unless you've taken to the trim with your axe.

 

[Clark Kent]

Maybe instead of moving the aux battery, some advanced cabling with disable/enable switches will do the trick?

You go first. Post lots of photos.

 

[Znarfgh]

Good question. I believe this is the case with BMW vehicles that run an AGM battery. The new battery has to be registered with the BMS.
A few Grenadier owners have had their battery replaced under warranty but no one has specifically mentioned a BMS update.

I can't see the BMS being smart enough to distinguish one battery from another? An AGM battery has no smarts like a LiFePO4. The BMS will see voltage only. SOC and any other smart functions are just a mathematical calculation based on voltage.

 

[Tazzieman]

Absolutely. The point you're dismissing is this action might extend the life of both batteries. Solvable with cash.

At what point do you end up replacing a battery?

 

[Znarfgh]

At what point do you end up replacing a battery?

When it goes flat and won't charge again?

 

[Clark Kent]

At what point do you end up replacing a battery?

When it's knackered. But if rotation has the benefit of load levelling this should be later in its service life.

 

[Clark Kent]

When it's knackered. But if rotation has the benefit of load levelling this should be later in its service life.

Ctek's article appeared in an aftermarket automotive industry publication. Of course their end game is to sell more Ctek analysers and chargers to workshops, so they in turn can sell more services and batteries to retail customers.

Of relevance to the Grenadier and Quartermaster community is this idea that the starter battery is working harder than the auxiliary battery because the batteries are isolated from each other electrically by the battery combiner (Smartpass 120S or a stripped down version). Functionally they have different load profiles and will therefore age differently. This might be an actual thing or I might be getting waterboarded by Ctek's Kool aid. In any case I'm a man of leisure (still not ready to say I'm retired) and I have time to ponder these things.

Early Grenadier owners will have accrued two years of life on their original battery/ies. I'm 20 months into mine. I'll take a guess that is maybe 40% of the expected service life for a wet cell battery with stop-start duties, perhaps a bit higher for diesel engines and colder climates. If Ctek are correct, the difference should be measurable by a comparison load test of both batteries. Ctek did say 'slight' to cover themselves.

I'll see if I can arrange some testing on my batteries. If it's a thing then we're more informed to do something, or nothing.

We don't normally consider this aspect in a conventional starter/main and house/2nd battery configuration because the 2nd battery is typically AGM or lithium and not interchangeable.