So as I have read in my research on a replacement battery for our X5, it seems that maintenance charging the battery with an external charger is recommended for a few reasons. If I am understanding correctly, one being that the car itself is designed (maybe more accurately programmed?) NOT to fully charge the battery. And this gets compounded if the vehicle is not driven enough, or for long enough on each trip. As a result, I am now down the rabbit hole of research on a charger for this purpose. I understand that AGM batteries require different charge voltages than flooded lead acid, and that it is best to have a charger that has specifically programmed modes for AGM charging. Easy enough, no shortage of these available.
CTEK comes up over and over on the BMW forums, and BMW rebrands a CTEK charger to sell it for twice the price, so it seems an obvious choice. The
CTEK MS 5.0 seems to be the same charger
BMW rebrands, and the
CTEK MUS 4.3 also seems to have a popular following. Other than the ‘test’ functions of the MUS 4.3, the charge functions appear to be identical, both having 4.3 A Max outputs.
NOCO also has some chargers of interest, the one being most closely comparable seems to be the
NOCO Genius 5, which charges at a max rate of 5 A. There is also the
NOCO Genius 2D, that Charges at a max rate of 2 A. I like that both are weatherproof and designed to be ‘installed’ in the vehicle. The Genius 5 has most of the same features of the CTEK, but the 2D is a bit more basic.
The noted battery guru on Bimmerfest (yes, Doug) cautions repeatedly about not charging at more than 4 A. The CTEK he uses charges at a marginally higher rate, and the GENIUS 5 at again, marginally more. I’m not sure I understand where this warning comes from, as it is seems AGM batteries can accept charge rates much higher, and I have to assume the car itself can charge the battery at much higher rates when called for.
I would love to understand this better,
but from what I can find online, it seems like the general rule of thumb seems to be to charge at about 10% (.1C) of the rated Ah capacity (about 10.5A for a 105 Ah battery), and that you can charge AGM batteries as high as 30% (.3C) of the rated Ah capacity (31.5A for a 105 Ah battery) occasionally, but not recommended to do so regularly. So throwing out the max permissible and sticking with the typically recommended 10% of Ah guideline (.1C), it seems like up to 10 A would be safe on a regular basis. So is it something in the BMW electronics that risks damage at higher rates?
I have used Deltran Battery Tenders (1.25 A) for decades now on motorcycle batteries as well as an older classic vehicle that doesn’t get out much, with great success. But they don’t seem optimized for AGM, and like they are a step behind the CTEK and NOCO offerings. Maybe there are other charges out there I should be considering as well, I dunno..
I like the idea of mounting a NOCO under the hood, permanently hooked up, so I just have to plug it into an extension cord (our X5, sadly, is not garage kept), and I am leaning toward the NOCO Genius 5, but this isn’t a requirement. I just want to get the best tool for the job. So I’m all ears everyone..