EarthX tells us that the internal circuitry of their batteries can only protect up to 60 vols:

<snip>

The reason for the over voltage protection is for your safety. This feature is required and done on all certified aircraft as standard. Over voltage protection is required if your aircraft has the ability to produce over 60V because the BMS can only protect up to 60V. If it goes above this, then the battery can go into thermal runaway. If you had a crowbar over voltage protection circuit, this would protect from this scenario. If your regulator and your over voltage crowbar protection fail, then you need to manually turn your alternator off. 

<snip>

Most certified aircraft have alternators with external regulators. Many experimental aircraft have alternators with internal regulators. The folks on the Aeroelectrics forum point out that an internally regulated alternator can fail in such a way that interrupting the wire that comes from the ignition will not turn the alternator off:

<snip>

Some internally regulated alternators have an ignition switch terminal that controls the alternator. But that input does not make and break the field circuit. All it does is command the internal regulator to shut off the field. A failed internal regulator could ignore that command and continue to supply excessive field current. The external voltage regulator has easily accessible wires and circuit protection. It is relatively easy to tap into the externally regulated alternator field circuit to add a switch or automatic over-voltage protection. The internally regulated alternator gets its field current from within the alternator. The externally regulated alternator gets its field current from the aircraft main power bus.

<snip>

As Jan suggests, it would be good to know "everything" about your alternator. Unfortunately, that's probably not possible. Specifically it would be good to know if the alternator is capable of producing more than 60 volts and it would be good to know if the alternator can fail in such a way that interrupting the wire from the ignition switch will not turn it off. Good luck finding that information.

It is also true that "if it ain't there it can't fail" but you have an electrically dependent airplane so your electrical system better be fault tolerant. In this case you would be adding a relay and a voltage sensitive microswitch, either of which could fail (along with the crimps and connections you make to install them). You need to be certain that if anything fails, your system will tolerate it, meaning that your engine will keep turning. You also need to have a plan for recognizing and dealing with any failures.

I think there are arguments both for and against installing OV protection. Given the placement of the lithium batteries inside the cabin, I personally would favor having OV protection. On the other hand, if the batteries were not in the cabin, I would tend to lean against installing OV protection.

If you are going to install OV protection and you have an internally regulated alternator, it would be better to use a relay to interrupt the alternator output, rather than the wire from the ignition switch.

Just 2 cents, nothing more.

Do not install anything in the alternator circuit  of the sophisticated ND alternator brand. 

You can be assured the designers of the internally regulated unit are smarter than you.   Because some idiot - I was one for a short time - believed the Chinese copies of the alternator where of the same quality, destroyed the EarthX faith in alternators, is not the same as the Japanese branded units not having the guts already built in to protect you.  

In 30years of using these, there has NEVER been an issue, of any kind, with the Japanese built ND alternators or their sophisticated voltage regulators.   

Just to make the point, in case some don't know, the difference with the Viking engine is the internally regulated alternator.

My Lycoming engine has an external voltage regulator that do have occasional failure problems. For this reason, OVP is important for an EarhtX battery if you have an external regulator.