Battery Tender Hookup Quetion

[jmatchco]

I recently hooked up a battery tender to my '24 EB FX4 truck, to accommodate our infrequent use and short trips. I followed the advice of several posts here to hook the positive to the battery positive and the ground to a chassis ground, much as you would for jumping a car. Things seem to be working fine (although the charge time seems a little longer than my old Nissan with less advanced electronics).
But now I wonder why not just hook up to the negative post directly?
Other posts imply this is to avoid "confusing" the battery sense piece (resistor/current sensor?) by the negative post and the battery control module, but why would that matter? I assume the battery control module can't really do anything with the engine off. So what if some charge snuck in around the sensor overnight? Anybody tried this?

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[Decayed]

What does the instruction manual for the tender say to do? Do that.

 

[Delhi Falcon]

I used battery tenders for my classic cars. I connect the negative to the battery post as the directions indicate. One good reason to not use the battery post when jumping a dead battery is to prevent an ark from igniting the battery gases.

If connecting for over a week, I use a timer and only charge for one hour each day. It's good to let the battery drain some rather than a constant charging.

 

[The Real Maverick]

You can attach to both battery posts.

If you can manage without squishing the sensor (if you have small / gentle alligator clips) clip to the chassis side of the sensor. The side away from the battery.

These trucks are never asleep.
Unless they are in deep sleep.
So 50/50.

 

[jmatchco]

Thanks, all. I tracked down the manual, per Decayed's suggestion, and it recommends the chassis connection, too. It seems to be to avoid arcs/sparks, just as in jumping. They don't discuss monitoring circuits. Looks like I'll leave well-enough alone.

 

[Nemo]

Yes, the point of not connecting directly to battery ground, is so that the system sees the battery being charged and updates the battery state of charge accordingly.

 

[jmatchco]

But how does the system see the charge when it's off? And what if it misses the secret charge? Won't it adapt anyway?

 

[RJC]

Reset the BMS , with ignition on flash high beam five times, press and release brake pedal three times. The battery light in the cluster will flash and then go solid, BMS reset.

 

[jmatchco]

No, I'm sorry for the confusion. I don't need to reset anything.
My real question: Why does the BMS know or care about any battery charging when the engine is off? Does it even know it happened??

 

[Darnon]

No, I'm sorry for the confusion. I don't need to reset anything.
My real question: Why does the BMS know or care about any battery charging when the engine is off? Does it even know it happened??

There's a number of modules that are still awake even if the vehicle isn't running. The BMS specifically tracking power used (or charged) while parked so it knows how many amp-hours the charging system needs to replenish early next start and then it can lower charge load for efficiency.

 

[todd92]

More important than where you connect the negative is to get an AGM capable charger. A typical Battery Tender brand isn't.

 

[jmatchco]

There's a number of modules that are still awake even if the vehicle isn't running. The BMS specifically tracking power used (or charged) while parked so it knows how many amp-hours the charging system needs to replenish early next start and then it can lower charge load for efficiency.

Thanks, that makes the most sense. I'm still surprised it doesn't control charge based more on state of charge from voltage readings than on trying to integrate current in and out.

 

[jmatchco]

More important than where you connect the negative is to get an AGM capable charger. A typical Battery Tender brand isn't.

OK, this is lasting much longer than I meant. But why do you say that about Battery Tender. Their box/specs say they handle AGM batteries.

 

[Jeff D.]

OK, this is lasting much longer than I meant. But why do you say that about Battery Tender. Their box/specs say they handle AGM batteries.

I was curious about Todd92's message, also? I've 4 Battery Tenders (brand name) and all are rated for AGM batteries. Unless "battery tender" was referring to a general type device, and not the particular brand?

Kinda like my wife refers to anyone who's snowmobiling as them "Ski-Dooing".

 

[todd92]

AGM batteries have a different optimum charging and final voltage than a conventional lead acid batteries. Battery Tender brand chargers are dumb devices with one fixed charging profile. NOCO or CTEK chargers let you pick the battery type.

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