it's not always a simple matter of electrolyte level in the battery that can cause premature failure, although it isn't a maintenance free battery nor Mazda USA explicitly says is a maintenance battery in the shop manual, but is implied so based on the many pages dedicated to the "w/i-eloop" battery pages in the USDM 3's and 6's shop manuals ... based on the Japanese article I've read on Kowa Seiki's website (full of sales pitches, because they're selling the product), they've seen high failure rates of the idling start-stop (aka EFB) batteries at dealers all over Japan, not just Mazda, although Kowa Seiki is officially Mazda's tool supplier. The article claims that in collaboration with CTEK from Sweden, they determined that the battery failures are caused by acid stratification and they have released a special CTEK charger that can remove this acid stratification. They claim that after some dealers started using this $1200 CTEK charger, their warranty claims on the Panasonic or Yuasa batteries have dropped greatly and they've seen the battery life go from just a little past a year to 3 years average. Acid stratification causes specific gravity issues, and thus, false state of charge is detected by the i-eloop/i-stop system's current detectors. This false state of charge detection, can cause under or over charging issues with the battery, leading to cell damage. The acid stratification is exacerbated because the constant deep cycling caused by heavy battery usage from idle start-stop use. And when the battery is undercharged, cell plate sulfation results, which causes the battery's capacity to be reduced, and eventually cause failure. Half of this stuff isn't mentioned in the shop manual, which is pretty interesting.
That $1200 JDM only CTEK charger has claimed advanced "ISS battery refresh" function that's not in the cheaper JDM models.... whatever, I can't afford a $1200 battery charger (if I ever come across a used one for a good price, I'll buy it for sure).
What I could afford was Yuasa's own charger that has its own "ISS battery refresh" function, for a fraction of the price from Japan. It seems to behave the same way that Kowa Seiki claims their own private labeled CTEK charger does... recharge the battery at low pulsating current, to desulfate and recharge, then do a high voltage (17-18 volt), high current charge to "stir" the electrolyte. "Stir" it does... after a few hours of running the charger with the battery caps off (you must do that, in accordance with Mazda's shop manual or run great safety risk), the electrolyte is "boiling" until the cycle ends after about 8 hours on my very healthy Q-85 battery. This was one of the few affordable chargers in Japan that is fully compatible with a EFB at the time I got this early last year. Nothing in the US DIY/consumer market was fully EFB compatible until CTEK USA released their MXS 5.0 charger mid last year. There's plenty of choices from Europe as they've been ahead of us in putting out cars with start-stop features, but importing a charger from Europe meant having to get a step up transformer to run the charger, which is a pain in the ass and additional fire hazard, despite the fact that I have plenty of step up transformers.
These batteries are funny beasts... standard smart chargers and testers falsely detect the state of charge because they seem to have a very different surface charge behavior than regular batteries, so you can't fully use any regular charger with these batteries. You could use a fully manual charger, and time the charge, after using a hydrometer to check the specific gravity to calculate a state of charge (all stated how to do so in the shop manual), but that's a huge messy pain in the ass. I instead, chose to buy a battery tester that can handle EFB, which will properly give me an accurate state of charge and state of health readout digitally, and of course also the special smart charger, because this is easier and faster to do a quick do over. In any case, you must use these batteries with start-stop systems or with Mazda's i-eloop system because it has deep cycle characteristics from the denser cells, but it also can accept high current charges more easily without cell damage like a standard battery does... hence why standard batteries can't last long in cars with these systems
Anyhow, for many of you, get a European/220 volt CTEK charger that can handle EFB, with the "recondition" mode... no point getting the charger without it. For the few of us in the US or Canada that have a 3 or 6 with i-eloop, get the CTEK MXS 5.0 or import the Yuasa charger from Japan like I did, which is designed for 100 volts but works fine on 120 volts... the Yuasa doesn't really have a float charging function from what I've seen, but it does have a similar "refresh" function as the expensive $1200 JDM CTEK... but the non-JDM CTEK only has a "recondition" mode, rather than the "refresh" mode... it is still probably a better buy as it's easier and cheaper to get anyway, which makes it a more practical long term use as you can use it as a float charger on any car, including ones with start-stop systems, when you are parking the car for an extended period of time (winter hibernation or vacation, for example)...
what's the difference between the "refresh" and "recondition" modes? "refresh" does a slow ramp up to high voltage, high current charge after fully recharging the battery to force out gassing of the battery which forces the electrolyte to aggressively "stir", eliminating acid stratification completely.... this is a one step process on the Yuasa charger, but on the $1200 JDM CTEK, you must do a standard recharge or "recondition" first, before switching to "refresh" mode... "recondition" mode on the CTEK does a desulfate, then recharge, before slowly ramping to a slightly higher voltage (15.8 volts) low current charge to force out gassing of the battery to "stir" the electrolyte to remove acid stratification... on a standard battery, the "recondition" mode works fine, but on an EFB, it will either remove stratification incompletely or will take a longer time to do so, hence why the higher voltage and higher current profiles are used on the $1200 JDM CTEK and the affordable Yuasa chargers... both of these chargers will completely remove stratification on an EFB and fairly quickly... so if you want to have an EFB compatible float charger, and also have a true "refresh" mode, without breaking the bank, buying both chargers is the only way at this time, unfortunately
I run the refresh function once a year. But if your battery sees heavy use, twice a year is probably better... be sure to top off the level first with distilled water before running the charger, or you'll damage the battery!
