Hello all, I'm new to this forum. I think I can answer this question since being part of this Nissan Leaf forum. The answer is yes, of course it can be done much the same way for Karma as it is for Leaf, although you did not define the term "reasonable cost". I don't own Karma so cannot comment on installation details, but what's involved is installing additional charger (or two) in parallel to the stock one and enable them whenever stock charger is active. These boosters will inject current along with stock charger into the battery, currents (and so power) just sums up. AS I understand Karma has 3.3kW stock charger - same as '11 and '12 Leaf. So to half charging time you just get another 3.3kW booster charger and to cut charging time to 1/2 of what you have now, you install two boosters. One can get power from the same J1772 inlet you have for 6.6kW total. Two boosters is pushing it - inlet contacts are not rated for such current (48A total), they will get hot and will fail prematurely.
You need to use isolated chargers that can be controlled either over CAN or analog way - either way is OK, but if you go CAN mode you will need to tap into Karma's CAN bus traffic, decipher messages being sent to/from stock charger, and make your own controller sending proper CAN commands to booster charger(s).
The reason I'm writing all this is because I got email from I think Karma owner who asked what it would take to retrofit Karma with the same charger people add to a Leaf. Anyone will tell you that one of best charger choices is of course BRUSA NLG513 OEM charger used in many production EVs, it's also very popular in Europe.
Now, it's time for disclaimer - my company sells and supports these chargers in the USA and Canada, and of course would benefit from selling them for projects like this (though it is tiny volume compared to OEM consumption), but I'm not marketing them or write as supplier - I'm writing as happy user who upgraded OEM way my '2011 Leaf with 3 units (10kW booster) and now usability of my Leaf that charges now 0-80% in about 90 minutes from any 240 VAC outlet (without EVSE) is greatly increased. This upgrade is kind of extreme though, most people would settle for one booster, very few - for two.
If you curious, here are couple of photos of what it looks like in my case:
Here is simple site I put together describing the process what I've done:
I did not touch CAN stuff - it works beautifully without it.
Same thing I'm sure can be done for Karma, but as I mentioned, most people would be happy to just double charging rate with only one unit. I don't do this type of work for living, and all the info how it's done is open source, so it can be applied to Karma or pretty much any OEM EV. BTW, there is group effort on Leaf forum to get a batch of NLG5 water cooled chargers to keep the cost down:
I don't know how much Karma fans talk to Leaf fans, but it many of you would like to explore this path, it would make sense to join Leaf group and benefit from larger quantity discount. Frankly, I don't know what the status of Leaf owners' group is,
I don't follow it.
Anyway, I just felt qualified enough to comment that from technical point of view upgrading Karma's 3.3kW charger with BRUSA NLG513 would also be trivial task, mostly mechanical work - brackets, cabling, etc. Speaking from personal experience.
With enough demand if someone wants to subject his Karma to the upgrade I can assist with this. Simple kit can be developed (as Leaf enthusiasts have done).
Have fun with your cool vehicles, I just thought you all would like to know about all this. Welcome to ask me any questions.