Can I use this AGM Auxiliary Battery?

[TeddyRuxpin]

Hi,

Auxiliary battery fault on pre-facelift W212 without stop start. It’s the one under drivers dash, nothing to do with a boot battery (this car doesn’t have a boot aux battery).

Ordered a new one with code ending 4039 but received one with code ending 9504. Same size and voltage / ampere, but the new one is AGM.

You can still buy the battery with code ending 4039 so it’s not been replaced by 9504.

Attached is a screenshot of the old (on the right with blue logo) and the new.

Am I able to use the new AGM one on a pre facelift W212 or have I received the wrong part and cannot proceed without a matching number?

Thanks!

 

[rf065]

I'd have had it fitted already!

 

[TeddyRuxpin]

I did actually fit it, though I haven’t started the car yet. I’m charging the main battery on trickle charge as the car has been a bit unloved. Will give it a spin in a bit…

 

[Bobby Dazzler]

An AGM battery is a derivative of a traditional lead acid battery however the charging profile is different. Whilst the car will charge it, it won’t know it’s an AGM and will charge it using the profile of a traditional lead acid battery.

There’s a risk of overcharging which at best could shorten the life of the battery and at worst could cause it to swell, maybe even blow a safety valve or in extreme conditions might even explode albeit unlikely.

Personally I would switch it for the correct battery. If the correct battery isn’t available then it may be possible to use STAR to tell the car that an AGM battery has been fitted and charge it appropriately.

 

[190]

There’s a risk of overcharging which at best could shorten the life of the battery and at worst could cause it to swell, maybe even blow a safety valve or in extreme conditions might even explode albeit unlikely.

I don't know how the car goes about charging this particular battery but in the normal scheme of things my understanding was that an AGM battery tolerates a higher charging voltage than a normal wet cell so I would have assumed the AGM would be undercharged in this situation. It all depends on the voltage. What any valve regulated battery can't stand is an excessively high float charge as it will eventually gas off and vent once the safety valve pressure is reached.

Standard Gel batteries are the ones that are particularly sensitive to overcharging but hybrid Gel/AGM batteries are becoming more common and they are much more robust.

 

[ALFAitalia]

Yep.....I fiited an AGM battery in my last ALFA that I was talked into.....big mistake......they need a higher charging voltage than the car generally supplied. It worked OK for a while and always started but was never fully 100% charged without my charger......but it shortened the life of it and it failed after less than 2 years....which may or may not be down to low charge voltage. Swapped it under warranty for an ordinary one. Basically if there is not a stop start version in the range (there was not with the 159)....then it probably wont charge it adequality. My 212 uses an AGM.....but has no S/S.....but it was designed with AGM in mind so no issues.

 

[190]

.I fiited an AGM battery in my last ALFA that I was talked into.....big mistake......they need a higher charging voltage than the car generally supplied. It worked OK for a while and always started but was never fully 100% charged without my charger......but it shortened the life of it and it failed after less than 2 years....which may or may not be down to low charge voltage.

I've just bought a new AGM battery for my 46 year old bike. Not not sure AGM's even existed back then. I've tweaked the old electromechanical voltage regulator up 0.5 volts and that seems to work without detriment to battery life. The battery I'm replacing was also an AGM of sorts and it's lasted 11.5 years. Funny how AGM's go off a cliff all of a sudden. It's never failed to start and it didn't this morning but it cranked so slow for the first time I new something was wrong. Fully charged rested voltage has dropped to 10.7 volts so it's surprising it starts at all.

 

[rf065]

I've also replaced my bike battery with an AGM one, went for a Motobatt as the same size battery has increased capacity, I.e. cranking power, no problems with it charging.

 

[190]

It's worth examining why AGM's should be charged at a higher voltage than a standard wet cell.

In the normal course of battery discharge soft sulphate crystals build on on the plates. When the battery is fully charged again these soft sulphate crystals are removed. Should the battery not be fully recharged the soft sulphate crystals turn in to hard crystals which are much harder to remove and that is what we call a sulphated battery which will now have a shorter life.

If an AGM is substituted for a standard battery it will appear to work fine but being charged at a standard wet cell voltage it will never reach full charge and will therefore become sulphated and have a shorter life than it might otherwise. AGM's really should be charged at an appropriate voltage which is why cars are programmed with different charging profiles according to battery type.

 

[ALFAitalia]

..... which, I'm guessing, is exactly what happened to my ALFA battery.