Some very smart people, including EnerSys and others involved in battery system integration for many military clients, have verified the accuracy of Chris Gibson's invention, and expressed amazement and mystification at the fact that it only uses two wires.
If there is a coulomb-counter that comes close (without frequent recalibration) I want to know about it. Note it doesn't work (at all) with LFP, just lead. And Chris has always been very open about accuracy being reduced (but still better than everything else!) while charging.
This thread is a bit funny:
http://www.ybw.com/forums/showthread.php?73924-Smartbank-battery-management/page5 In the US, Balmar sells and supports it well.
See also "Maine Sail", Rod @ Compass Marine Services', detailed review, he started out very skeptical too.
"tested against a Vanner Battery Monitor (around £10'000 worth - designed specifically for testing batteries), a Link 10, BEP DCM600 and a Victron BMV. We tested it under various types of load, battery sizes, types etc. After 3-4 cycles (this is needed for the SmartGauge to learn about the battery - and the key to a shuntless design), we found that the SmartGauge was (overall) more accurate than the shunt based meters".
Personally, I think it's got a little neural-net learning that cause it to get **more** accurate over time - you do need to leave it hooked up - clever preprogrammed profiles, dis/charge cycle algorithms to compare its observations against internally and finally (this is key!) I believe it also measures internal cell resistance as that changes over time.
Chris and Merlin are of course playing things close to their chest. Best of luck cracking that nut, other ideas welcome.
And if anyone knows something even close for LFP do Please speak up!