Changed battery, now alternator won't charge...

[strategygoose]

I'm having a weird issue with my 2021 Polo AW - the old battery (original WAG) was knackered so thought it would be an easy enough job to replace it. I've changed it out for a 092 Bosch AGM (had a 027 WAG EFB+). Everything fitted fine, just needed to add a 2AWG cable to connect the positive battery terminal to the positive terminal block.

Coded in the new battery using OBDEleven (selecting "fleece" as the battery tech as noted on a few forums, changing the Ah rating and changing a digit of the serial number). Car started fine. Took it around the block and was fine, however then noticed the alternator isn't charging the battery. Voltage when running is indicated at 11.7v (headlights on). Cleared the fault codes generated with the battery being out of the car too.

Have reseated the battery terminals and checked if anything looks visually off - seems all fine. I had this same issue when I removed the original WAG battery (to charge it) - the alternator wouldn't charge the battery until I used a jump pack to start the car (as after two starts, it wouldn't actually start).

Have left it alone for tonight but is there anything worth looking at here? Could changing the battery from the 027 to 096 (newer has a larger capacity) be the issue?

[RUM4MO]

Sounds like being an issue with the alternator, changing battery type, size(capacity) and technology from EFB to AGM should not have changed anything, even if left as it was initially for a short time - okay letting the BMS know that the battery had been replaced and coding in the one is desirable long term.

If the charge warning light behaving like it did when "all was well" ?

Are the leads connected to the +VE post of the battery in good condition etc?

[strategygoose]

Cheers RUM4MO - wanted to make sure it was done as close as to how it would be done in a workshop so sorted the BMS before first start!

Charge light behaving exactly as before. Comes on then goes out alongside all the other warning lights. Diagnostics throws up no codes to do with charging status (just the voltage being 11.7-11.8v when started). Everything is seated correctly too.

Leads all look fine. The only thing I’ve thought of is giving the alternator positive connection lead a quick scrub with some sandpaper (and associated mating surface in the terminal block) to see if that may help as I had to remove it to get the old battery out.

Battery terminals are new as it’s a new battery, and considering the car starts perfectly, I doubt that the battery clamps are corroded/have a bad connection.

The quick drive round the block wasn’t super heavy (all 20mph roads here, barely got the engine above 3000rpm). Not sure if a longer drive with the engine working harder might help?

[RUM4MO]

I would have expected the charging to be "spot on" as soon as you started it after replacing the battery, ie within seconds.

For some reason, maybe to monitor charging behaviour on cars with smart charging/regenerative braking, I bought a cheap digital volt meter that plugs into the 12V power socket, that lets me know exactly what is going on - so now, when out in either of our cars I feel the need to have it and have it plugged in! Over kill, but there you are, doing that lets me see "strange" things like how how I drive can end up with the battery being harvested of spare charge and so give better MPG figure, and that of course leads to temp loss of auto stop, just how things have been designed, not a complaint, just a comment.

[strategygoose]

Update from this morning: alternator terminal cleaned but no difference! Though I’ve observed something a little bit interesting:

The charge voltage (with engine running) jumps about. Sometimes it’s at 13.5v (this is after the car has sat for about 15 seconds with headlights on to remove surface charge then started) but then fluctuates quite a lot. Lowest measured with the engine running was about 12.2v, highest 13.6v. I have a tracker in the car which records voltage so have a record to observe.

Start stop system works fine (got me at a junction, excellent).

Have got a multimeter that I stuck into the 12v port, pressing the accelerator doesn’t make the voltage jump either at the battery terminals or the 12v port, probably a 0.03v jump at 4000rpm.

Starting to think this may be an alternator issue. Will observe but thankfully the car is under warranty! Might need to get a properly sized battery as the 2AWG short lead mod to fit the new bigger one is very very obvious!

[iichel]

did you check the fuse? i think it would be good to change the battery type to AGM in your coding. otherwise the car will charge it like an EFB. that's not very ideal.

[strategygoose]

did you check the fuse? i think it would be good to change the battery type to AGM in your coding. otherwise the car will charge it like an EFB. that's not very ideal.

I wasn’t aware there is a fuse - fuse box diagrams show nothing for the alternator. In any case, the link between the alternator connection point in the battery terminal box (not sure of the exact name but the thing that sits above the battery) and the battery has continuity so it should be sending power into the battery.

On changing to AGM in coding, there are two options - “fleece” and “Binary - AGM”. I’ve changed it away from EFB to fleece. From what I could glean from other forums, VAG cars that come with an AGM from the factory use the “fleece” option. I thought that was odd as one literally has AGM in the name, bur apparently some translation quirks suggest fleece is correct. Though iichel, happy to be corrected if I’ve missed something!

[iichel]

fleece is indeed the correct one for AGM. That's good.

[RUM4MO]

Just another comment, where in the past you could press the accelerator to increase the alternator voltage, now, especially if you fully charged that new AGM before fitting it to your car, the charging system will not supply any more charge into that battery until the BMS considers that it needs it.

That is another thing that I witnessed after fitting that cheap DVM to the 12V power outlet, even with my old 2011 Audi S4, if I have not used it for a couple of weeks, I'll connect it up to a CTEK charger, after it has been charged fully up by that CTEK, if, a few hours later, I drive that car with the DVM connected, the battery voltage for maybe the first 2 miles will drop back as the BMS target is to have the battery at roughly say 75% charged so that it will remain in a state of charge that will always allow for an engine restart - but have enough spare "uncharged capacity" to absorb ALL or at least most of any otherwise wasted energy during over run or deceleration.

[strategygoose]

That is another thing that I witnessed after fitting that cheap DVM to the 12V power outlet, even with my old 2011 Audi S4, if I have not used it for a couple of weeks, I'll connect it up to a CTEK charger, after it has been charged fully up by that CTEK, if, a few hours later, I drive that car with the DVM connected, the battery voltage for maybe the first 2 miles will drop back as the BMS target is to have the battery at roughly say 75% charged so that it will remain in a state of charge that will always allow for an engine restart - but have enough spare "uncharged capacity" to absorb ALL or at least most of any otherwise wasted energy during over run or deceleration.

This is interesting - when I first got the battery, I thought I might as well hook it up to my Noco charger while I went around the house looking for my socket set. Took me about an hour but came back to the Noco on "maintain" mode (all four status lights lit with the green one slow blinking) and a voltage of about 12.9v. Fitted it, have started the car about four times on the day.

The open lead voltage on the battery sits around 12.6-12.7v according to the tracker. I've got the power readings and annotated them with what I was doing - it does seem the alternator kicks in randomly (14.7v for about 20 seconds at one point). It was a short trip (I stayed in the car as was in a loading bay...) but kept the engine going. If the battery isn't being massively discharged and the BCM wants to keep it at ~75%, that would make a lot of sense if it now knows its got an AGM battery installed.

Screenshot 2024-08-24 190655.png (532.67 KiB) Viewed 1818 times

Driving from London to Cambridge and then back tonight so will see what the power readings are during and after the trip. Got a jump pack just in case it all goes wrong!

[Johntheo1]

That's all pretty normal, my charging voltage jumps to ~ 14.8V almost immediately after every start but only for 15/20 secs or so, presumably to replace the ~0.25AH or so used in starting the car, it occasionally rises to 14.8V while driving (frm 13.3V) but allways stays at a constant 14.8V on overrun, the SOC is normally around 80% but did show 85% on last scan, below. Car/(EFB) battery, 5.5 years old, "fleece" shows in battery type on VCDS.

22:37:22 14/07/2024 Engine OFF
IDE01834 Battery voltage 12.351 V
IDE01836 Battery current -4.747 A
IDE01837 Temperature of battery sensor 24 °C
IDE01839 Battery charge level 85 %
IDE01841 Battery internal resistance 7.6 mOhm
IDE01842 Usable battery charge 31 Ah (59AH battery)
IDE01843 Battery voltage at rest 12.6 V

[strategygoose]

Thanks John - that's interesting about the SOC around 80-85%.

I've taken the car around (put 430mi on it over the last couple of days) - I think the alternator may have a problem. Looking at the speed vs power line graphs (I've tried to overlay the two but looks like a mess) - the points when I'm braking is where the voltage jumps to around 14.6v, which suggests the jumps are the regen braking. I remember the chap at the dealership mentioning the car does have regen braking, but thought that was only for hybrids and full electric cars, but if it does have that, the voltage makes sense.

Have put a battery tester on the new battery too, it's reading SOC as 57% at 12.34v (car unlocked, key in my hand, no lights on apart from the digital dash) and that the battery needs charging. The electronics are probably drawing a tiny bit of current, so to account for voltage drop, think the true voltage might be 12.5v.

Next step will likely be testing the voltage coming from the alternator (disconnect the alternator lead from the battery terminal block) when starting and running for a few mins. If it is the alternator, I suspect that what might've been the death of the old EFB+ battery (being too discharged). Originally thought it might've been its age (mfg 05/21) but considering your battery is 5.5 years (and even though this car was used in London traffic by its previous owner), it shouldn't have died that quickly methinks.

[Johntheo1]

The "regenerative" braking on mine is just coasting with no throttle pedal, it charges at 14.8V irrespective of the SOC so I would expect yours to be similar. I deliberately left the ignition key switched on this morning for 10 minutes so the DRLs (daylight running lights) were drawing 4.75A for that period, 0.8AH drawn from the battery but the SOC fell to 71%, the battery then charged at 14.8V during a 5 minute trip but quickly fell back to its normal minimum of 13.3V on the return trip So, if yours does not fall below 13.3V ish while idling and charges at 14.8V during all overrun periods then difficult to see a Alternator fault. You might purchase a DVM (digital volt meter) and plug it into the cabin power point, you can then glance at it now and then to give a very good picture of whats happening. Of course another good indicator of the battery condition is the stop/start, if this is working normally then the battery "must" be OK?.

Overall, the BMS seems to do a pretty good job in maintaining the battery condition, this Polo does very short runs, it has only 30548km-18981miles up in 5.5 years, less than 3,500 miles/annum.

[strategygoose]

Small (good, albeit confusing) update - I swapped out the battery terminal on the positive side so it fitted better. Tried using the jump pack to keep car settings (unsuccessfully, it turned off). To my surprise, car sits at 14.6v at idle and charges. For comparison, I was at 12.5v when idle before (literally five minutes before it fixed itself).

I'm not sure if using the jump pack did something, changing the terminal (I'd used it before and observed the same issue - went back to the original VW one thinking that might be the cause) or wiggling the wires for the BMS/start-stop (little module on the negative terminal) fixed it.

But for now, all is well. Will look forward to seeing if the issue reappears when I need to put an 027 battery in and remove my jumper wire when it's due its service in April (still under VW used warranty) but at that point, can make a point of it to the dealership if its a fault.

Thanks all - wish we had forum stickers available so if I was on the M25, M11 or M1, could give a friendly flash.

[grazuncle2]

seems like you did it all ok

I fitted a larger battery to but kept the electronics live whilst removing the old battery with a simple gadget in the OBD socket. Perhaps you did too?

this was my route https://forum.obdeleven.com/thread/1895 ... -obd11-app