Batteries are like opinions, everybody's got one

[anonymous-user]

Car batteries are like opinions, everybody's got one - and more and more of them are fancy AGM batteries designed for start/stop systems etc. Meanwhile every dealership wants to sell their parts and service by saying you should only use OEM batteries and a new battery must be coded to the car at £££ + VAT.

Are car owners following this guidance or do you still buy a cheaper battery of the same type and fit it yourself? The first time I replaced an AGM was before I'd heard all this. Being fairly thorough, I checked the exact battery specification and bought an identical model. Simply removed the old one, fitted the new one and all has been fine. I'm now coming up to another AGM replacement and encountering ambitious dealer pricing (nearly £500 fitted) when similar batteries can be bought online for £160. Can't get the exact same model from the same manufacturer but a battery of identical specification is available.

I'd be interested to know if other people are replacing AGMs themselves and whether you're encountering any problems.

[anonymous-user]

My question is more around the technology and how the systems may be affected than just buying on price.

I'm wondering whether DIYers run into any difficulties with AGM battery replacements. From my own clicking around it sounds as though it's probably important to fit a new battery of identical specification (not just one that fits, or bigger capacity for luck) but that the "coding" part of things may be less critical.

There are suggestions online that it "depends on the car". Something about a difference between open systems and closed systems which I don't understand.

[anonymous-user]

Scrump said:

I have replaced two AGM batteries this winter. By coincidence both of them were on cars 6 years old.
Did both myself, batteries were not from the car manufacturer but were good quality ones with the same spec as OEM.
With both cars I maintained power to the vehicle using a spare battery, so had no problems.

Excellent, that's the way I'm planning to do the next one and have ordered some crocodile clips to make up a lead and keep things nice and easy maintaining power.

For mwstewart - the open/closed question is, I believe, about software of the electronic charge control systems rather than battery venting.

[anonymous-user]

Only difficulty encountered was how heavy it was to lift out an awkward corner on a F30 BMW. The torque is specified for each nut and bolt. With many online guides no longer free I would have to pay just to find out whether the battery terminal is 10nm or 11. Coding was straight forward. It was a two part process. Registration and then programming. Thankfully the app and adapter used were an investment. BMW wanted £458 for supply, fit and, coding. I did it for £300. Purchase of the battery, app, OBD II adaptor and, a torque wrench.

[anonymous-user]

Jakg said:

I always buy the largest battery that fits in the OE space,

Yes, that's the old-fashioned approach. The question here is what actually happens if you fit a new battery of identical specification and don't get it coded.

A number of people have said, "You should get it coded". Yes, we know that. The question is what, if anything, happens if you don't get it coded.

[anonymous-user]

KingofKong said:

The old battery wasn’t getting that level of charge, was charging slowly...

Again, yes. The clever electronickery must have a "learning" ability - otherwise it would start life charging a new battery at the new battery rate and never realise that over time the battery has got older, adjusting the rate of charge accordingly.

The internet suggests that if I fit a new battery (of identical specification) and don't have it coded then all that will happen may be it takes the charging system a while to learn that the new battery is better than the old one and can be fully charged. However, the outcome may depend on the particular make of car. (There are some reports of A/C and other accessories not working at full power until the learning process has been completed although I'm unclear whether those accessories will previously have been automatically restricted to protect the old, weak battery.)

My car knows it's got a weak battery because it sometimes puts up a "battery low" warning on the dash when the car is started. Since the battery is more than 8 years old it's fair to assume the warning system is correct. The main dealer wants nearly £500 to supply, fit and code a new battery. The local marque specialist offers the same for about £350. A new battery from a reputable online supplier comes in at £160 (plus delivery).

Unless significant risks are identified I'm going to DIY it and see what happens - after which I'll be happy to report the results, either good or bad!