Toadster

the E55 has been sitting for a bit so I put a trickle charger on the main battery - it's 12.5V sitting and 13.8-13.9V charging...

can you trickle charge the AUX battery? they're not AGS so just checking...

green1212

sure you can. i do it all the time when the car do not get driven much. but over the years, i found out that the aux battery is MAINLY for when the main battery is weak or getting weak and the aux battery is there for the sbc brake system. the main purpose of the aux battery is for backup electrical power for the sbc brakes if/when the main battery is failed or failing. most times no matter how long the car had sat without being run, when i placed a trickle charger on the aux battery, it would not take much to be fully charged. even checking the aux battery after weeks on end, it would still be right at 12.6 to 12.7. sorry for the LONG explanation, but yes you can place a trickle charger on the aux battery but as long as the battery is in good shape, it hardly ever really needed it.

mbmg

I do it as well if the car will not be driven for a while as well as trickle charging the main battery (2 chargers). I've had no issues do it.

eFifty5AMG

Thanks for answering; had the same question. Where do you guys ground your chargers?

Dublinoh

I have always placed the leads on the two poles of the battery, wrong?

mbmg

Directly to both poles of both batteries.

hayseed

iTrader: (2)

I put a battery tender quick connection dropping out behind the back bumper for main battery, I need to do the same for the front so I don't have to keep removing the cabin air filter -- one day when I have some time

mbmg

I ran the front connector right above it in between the hood weatherstrip and hood. When I want to connect it I just reach in by the windshield and pull it up enough from the weatherstrip to connect it.

tlambert

Got a pic of where you ran it out?

C32owner

The aux battery I think has more duties than supporting just the main battery and sbc brakes. It is functionings the convenience features/functions.

rapidoxidation

I don't believe that's true. The only cars that have the aux battery are the ones that have SBC brakes.
If I'm not mistaken, the small battery is solely for emergency power for the brake hydraulic pump.

Maybe someone who knows better than I will chime in.

Toadster

hooked up the charger for a few days - battery issue is gone now

yay!

StarvingArtist

Aux battery is short term back up in case of main electrical system power failure. It's there because we have electric brakes. Convenience features will be disabled when the main battery drops below a set voltage. Somewhere around 11 volts IIRC.

Shadow5501

The aux battery is AGM. It's a common size in the power sports world - motorcycles, atv etc. nothing exotic about it at all. My facelift w211 cars without abs did not have it, only SBC cars do.

Toadster

Battery Tender to the rescue best $50 spent!

JoeJErnst

The reason I knew my aux battery needed replacing was because I kept getting the message about "convenience features" being disabled (or something to that effect). Once I replaced the aux battery I've never seen it again. I have never replaced the alternator, regulator, or main battery, so I assume the aux battery is actually used for more than just an SBC backup.

bbirdwell

The auxiliary battery is monitored by the Battery Control Module (BCM). If the health of that battery is bad, the BCM will signal for the message to be displayed on the instrument cluster.
Additionally, if the BCM determines the load capacity of the main battery is low, it will connect the auxiliary battery to the main system while simultaneously disabling convenience features in two stages depending upon voltage.

whoover

You know the OEM battery is marked "AGM. Replace with same technology only!" Is this a conspiracy? I guess MB must own some AGM factories.
https://dyw03owr7vs3n.cloudfront.net...8a.?1544113524

I'm all for saving a few bucks when it makes sense, but ignoring warnings about the battery technology specified for my car is not such a case in my opinion.

AgSilver

For those who are unfamiliar with the circuitry, the aux battery does more than just provide backup power to the SBC. Prior to engaging the starter, the auto-start circuit takes the ECU off the main buss and powers it from the aux battery, thus providing a constant voltage and preventing low voltage issues and spikes. Once the engine starts the the ECU is returned to the main buss.

Also, the aux battery only charges when the engine is running. A Battery Tender connected to the main battery will have no effect on the aux battery as it is isolated from the main buss.

5spd97

AGM stands for Absorbed Glass Mat. It is a type of lead acid battery where the acid is absorbed in a fibreglass mat rather than rather than being a more typical wet cell. It does not have battery caps and cannot be serviced meaning that you can't add water. AGM is not a brand and pretty much every major battery manufacturer makes AGM as well as wet cell lead acid batteries.

Fountain35

AGM batteries are also required in vehicles where the battery is in the cabin. Fumes I believe.

akoustic

So when you trickle charge your aux battery should you disconnect all of the terminal connections first? Or is it okay to trickle charge with the connections intact? Just curious as I don’t want any negative impact to the battery control module or the main battery… thanks folks!

Benzo63AMG

can I hook up 2 battery tenders?

Fountain35

I would just trickle the large battery in the trunk.

kevm14

I thought AGM was sealed, too, but it turns out the cells hold water just like a normal wet plate style lead acid. The caps are right on top.

In fact one of the most common failure modes appears to be the water evaporates and the capacity diminishes. It is very possible to restore function by adding water and I guess it can take a few months for the chemistry to catch up with the mats. I am currently performing this experiment on my 11 year old aux battery on my S550 (it's a group 49 so quite large like the W211 main battery).