Lithium Battery

Hi Matt, with lithium cranking batteries, you only need quite a small battery, but it must be up to the charging algorithms of the D4.

Your Subaru’s charging voltage is pretty well set at 14.4v and even the new Subaru’s with STOP/START, the alternator voltage is either 14.4v or at the battery’s voltage during a STOP/START event, but in this case, you will need a much bigger capacity battery, and there are other potential problems.

Reverse is the case with your D4. Most lithium’s will not tolerate much over 14.4v max, but some can take 14.7v.

As the D4 regularly operates at 14.v, you will need to make sure the brand of lithium battery will take 14.7v.

As for a dual battery isolator with lithium batteries. Because lithium batteries sit at a constant voltage during discharging, around 13.2v, a conventional VSR will not work.

I set up my first lithium auxiliary battery in a D4, about 6 years ago, and learnt by trial and error, what will work and won’t.

If the vehicle is NOT a STOP/START type, then a IGNITION controlled solenoid can be used.

With STOP/START featured vehicles, if your want both batteries to remain connected during a STOP/START event ( a good idea ), because the ignition remains on while the motor is stopped, then the Ignition controlled solenoid is fine.

If you need to sperate the batteries during a STOP/START event, ( or need to turn a DC/DC device off ) then with most new European vehicles, there is a FRIDGE circuit, that is only on while the motor is running ( all D4s have this feature ), and you can use this circuit to control a solenoid to separate the batteries when the ignition is turned off, or during a STOP/START event.

I went down this path a while back.

The overwhelming result - stick to AGM for now.
The technology isn’t mature enough to gain any benefit in something like a D4. (Apart from perhaps 20kg).

Cost Benefit - AGMs kick LiFePO4 systems arse.
An auxiliary LiFePO4 is different, there’s performance gains to be had. But at the moment Pb Crystal is more cost effective and has performance benefits.

Only yesterday I decided to do some battery maintenance and as a result I had to replace 2x boat batteries, 2x caravan batteries and I had to install 2x aux batteries in the new (Well new to us) 79 series ute because it didn't have enough power reserve with a single aux to run the 120l 12v freezer and this was quite an expensive exercise even though the replacement batteries were all AGM types.
To replace all these batteries with lithium types would have been hugely expensive at about 5x the cost and personally I just couldn't justify the expense because I can't see the lithium types being 5x as good as the AGM types.
The batteries in the boat and the van were both about 8 years old and were still working But at a lower capacity than their specs and this is the reason why I changed them out.