BMS and battery tenders - myth or fact?

[donkmeister]

Whenever I see guides on fitting battery tenders, they invariably insist on fitting the ground connector to the chassis ground instead of directly to the battery. The reason being "because battery management will get upset and confused and overcharge the battery when the car is running".

However, lead acid is a 160 year-old, well understood technology and battery voltage is the metric to determine when a battery needs topping up. Does the BMS genuinely sit there metering the electrons in and out to maintain some sort of battery balance sheet? Or is it actually more like a battery tender with the decision to start and finish charging based entirely on battery voltage?

So is the "you must connect to chassis ground or you will upset the BMS" instruction the electrical equivalent of "don't let your tank run low or you'll get crap in the injectors"? Or is this really a thing?

[Scrump]

I have wondered the same thing.
On older cars there is no BMS so no worries, on newer cars I just connect to the chassis earth rather than the battery terminal as it is no more difficult so why take the risk.

[Panamax]

I've no idea, but several manufacturers sell battery tenders (usually C-tek) branded with their own name and which just plug into a 12v outlet socket. Example on this link,
https://www.youtube.com/watch?v=9OcgbiqVzsU

[donkmeister]

Ah, thanks. I'm actually installing a Magcode connection to the outside of my dad's car, hence me coming across the tales of BMS needing chassis ground on modern Nissans such as his.

He drives so rarely that he's killing batteries. I considered a fag lighter connector but personal experience is that charge cables through door openings eventually get knackered, and this way he can see at a glance from the window if he's plugged it in. So a little more work now for less aggro in future.

Might look at the possibility of putting a charging LED next to the connector for even easier visibility...