angrypengu

One of the downsides of the inherent design of the 48V/12V system on the D5 is the lack of longevity of being able to run the electronics with the engine off. So those of you who want to listen to the B&O system for more than 20 minutes will see that the car turns all of the electronics off rather soon.

When the engine is off, the 48V battery keeps the 12V battery charged, and that's not a good thing to have a discharged 48V battery. As a result, the battery's management system essentially tells the entire vehicle to go into energy conservation mode.

So, I've been looking at battery chargers/tenders. Buried in the documentation of the D5's manual is a line that states that battery chargers must NOT exceed 14.8V, which is the max charging voltage of all AGM batteries, of which all of our D5s have a 68AH AGM 12V battery.

So, if you're looking at battery tenders, make sure they do NOT have a "desulfation" mode which applies 16.5V of voltage to the vehicle in order to resolve the sulfation buildup of all lead acid batteries. While this is good for most cars, it is NOT allowed on our cars. I'm just confirming this, but I do believe the official branded "Battery Tenders" do not have this feature. I was interested in the NOCO ones but they do have this feature (I've reached out to see if the regular charge mode will hit 16.5V like the "repair mode" does)

Just a PSA. The one thing we do NOT want our cars to ever be into is a state of empty charge. The two 12V and 48V batteries are linked. take a look at the below. The next question for me is what current do I want the tender to charge at? I've read 10% of the AH of the battery is a good starting point, so perhaps a 5A or 7A charger should be sufficient...



Edited to add, I decided to buy this:
Battery Tenders do not have the desulfation option which is exactly what we need for our cars

gk1

FWIW the last Audi charger I saw was just a rebranded CTEK which does indeed have a desulphation process and like you pointed out does pulse up to 15.8 but in my experiences last less than a second since my batteries are not in bad shape.
Unless maybe Audi came out with a new charger with the release of the A8 that does not have a desulphation process. If they did it probably won’t be available for years...lol.

angrypengu

Thanks for the insight. I did indeed see the Audi branded ones but I suspected that they were just rebranded. I also do not think that anyone at Audi actually took a look at the OEM battery charger to see if it would be a good fit for the MHEV nature of our vehicles. Glad I ignored them! The problem with the desulfation process is that any charger with one can enter a state which is not allowed by Audi (>14.8V), and short of sitting there with a multimeter, we wouldn't know!

--

On another note, as per the photo above, there is this talk about "Terminal 15" on/off. I believe (don't quote me) that after reading all of the documentation that Terminal 15 = state of vehicle such that all of the electronics are running. So, while the engine off, terminal 15 off = ACC is off. Terminal 15 on = pushing the start/stop button once (without the brake pedal depressed) such that all of the car's electronics are on - climate control, radio etc

This makes sense to me based on the photo. For both states while the engine is off, if terminal 15 is on, and the current by the battery charger isn't sufficient, then the 48V battery is not charged because we'd end up with an infinite loop. If the 48V battery charges the 12V battery all the time, and the battery charger's current isn't sufficient to overcome the demand of the 12V battery, then the last thing the DC/DC converter should be doing is transferring power from the 12V battery -> 48V battery. On the flip side if Terminal 15 is off, and thus the entire car/electronics is off, then with a battery charger connected, then the 12V battery gets charged and the DC DC converter then charges the 48V battery.

The worst thing that batteries can face is heat, and the greater the current that you apply to charge them = the hotter they get. Since battery tenders/chargers are not meant to do anything other than to keep the charge of batteries high (especially useful for those of us who don't drive long distances) there is no need to have a high current output charger. I could have probably stuck with 1.25A or a 3A charger, but since I bought a 5A charger, I'll live with that and I'll stick a thermometer probe to the 12V and 48V battery to see how hot they get. I suspect a lot of data collection is coming up :-)

Edit: Based on perusing Google, my guess above re: Terminal 15 is correct.
https://forums.vwvortex.com/showthre...1#post73483876

gk1

I wouldn't worry about the 5A rating of the battery tender. That is just the max it capable of doing for charging largely dead batteries, for your purposes I doubt it will hang out much at 5A if at all or risk overheating the battery.
The older models used to have a nice chart with them showing the specific charging states.
That newer model shows 3 primary charge modes:
The bulk mode (full charge, constant current, battery is 0% to 85% charged) - it could hit 5A in this mode but I would think only if your battery is very depleted <85% (I get the feeling this will never be your case )
The absorption mode (high constant voltage, battery is 85% to 100% charged) - here voltage is held constant at current decreases well below 5A
storage/float maintenance mode (low constant voltage, battery is 100% to 103% charged). - obviously just a trickle here...very low current

Similar example to older chart which used to be included with battery tenders.

angrypengu

Thanks! Your post was very helpful as it not only opened my eyes but I stumbled upon this while trying to get a better handle of what you posted: https://marinehowto.com/how-fast-can...ry-be-charged/

AGM batteries apparently should be charged at a rate of at a current of at least 0.2C or even better 0.4C (so 0.2*68AH or 0.4*68AH). So the min. charge current should be 13.6A. According to the article there, AGMs prefer a higher current rate of charge.

That said, Battery Tender, for all intents and purposes, only sells battery chargers up to 5A of current, so between this and the 1.25A I was looking at, I'm sure 5A will be just fine :-)

Relevant line from the article:

"For AGM batteries generally speaking the more charge current the better the longevity of the bank. It helps, not hinders, with overall longevity to charge at high rates. Higher charge rates in AGM batteries actually yields longer life not shorter life. Dave V. the lead engineer at Lifeline battery published a study supporting higher charge rates being good for AGM’s a long time ago but it still holds true today.. Odyssey battery, thin plate pure lead AGM batteries, wants to see a minimum of .4C in charge current and Lifeline wants to see a minimum of .2C as recommended charge current."

gk1

Also fwiw I have the CTEK MUS 4.3 and I have connected it with my Fluke meter attached and have never actually seen it do the high voltage desulphation it just tends to skip right over that step. I can understand though that you would not want to risk it given the specific voltage statements you outlined from the manual. Also you avoid the risk of hitting the recondition mode which would definitely be bad in your case.

angrypengu

Thank you. I was surprised that after placing a multimeter against the 12V leads in the hood that my lead acid battery was only 45% full. I have a permanent "voltimeter" hooked up while my vehicle is running and noticed that the DC/DC converter had stopped charging the battery continuously, (after startup, it would for about 15 minutes). I have no doubt everything is working fine, but as we know, the BSG charges the 48V battery when the car is running, which then charges the 12V battery. My recent lack of highway drives has probably caused both batteries to fall into a state of being only mildly full. (no pun intended).

So last night I hooked up the battery tender, and let it charge to only 80% full. My thought process here is the 12V battery gets charged in this scenario, and then charges the 48V battery (Terminal 15 off, engine off). As we know, after the tender is disconnected, the 48V battery charges the 12V battery. Thus, lead acid batteries (AGMs to be precise) like to be 100% full, but lithium ion batteries do NOT like to be in a full SOC (state of charge). So, my thought is, charge up the 12V battery to 80%, disconnect the tender, and let some of the electricity flow from the 48V ->12V battery and let the DC DC converter do its thing to figure out a happy equilibrium.

Now some of the above is based purely on logic and what I believe is common sense. I figure the last thing I should be doing is to let the battery tender get to 100%. I have no idea if the DC DC converter will 'stop' the charge of the 48V battery at some stage and to prevent it from getting to 100%. I tried attaching a multimeter to the 48V battery but it's impossible (sadly). I do have an OBDEleven on the way to see if it will give me that info.

Thoughts on my thinking above? Seem reasonable or would you do something different? Thanks!

gk1

I don't know near as much as you about the A8, but I'd like to think even with the 12V battery on a permanent battery tender and charged to 100% that the car's internal systems would maintain the 48V battery at the proper SoC. Otherwise what is the point. If your 12V starts to fail will the 48V struggle to keep it alive and then just fail too? I'd hope there would be some warning or something. Or vice-versa, if the 48V is failing will the 12V just obliterate itself and leave you stranded? I guess I'm asking if they are treated as one single battery system or do they have their own independent control modules to keep track of them? I'm sure you will find out when your OBDEleven arrives as I'm sure there is a wealth of electrical status information.
On a side note, I'm just curious, is there a particular reason you chose OBDEleven over VCDS?

angrypengu

Thanks for the questions! I've thought threw some of these and the others were great food for thought. While I cannot comment with full authority, I do have some guesses here.

The 12V AGM battery does not have its own battery management circuitry like all lead acid batteries. I would think the management of the charge of the 12V battery is done by the DC/DC converter, but I can confirm that the 48V battery does indeed have its own battery management module highlighted below (taken from the self study program for Audi techs)



There is no information that I have found from LG Chem in regards to this specific 13 pouch, 9.6AH battery for our D5s. However, LG Chem does have public facing information surrounding this particular design and LG's implementation of other MHEV batteries like ours will focus on limiting current output, as an example, when the battery is too hot. Since our batteries are passively cooled and lack the active cooling that Audi claims to have in some D5s, I would suspect this module is the only module that provides battery thermal management. The pouch nature of the 48V battery means it can tolerate higher temperatures than other designs which is good.

Funnily enough I reached out to a contact I have at MIT last night and I posed a very specific question. For batteries like ours, if one absolutely had to choose between temperature versus humidity, which one is more important? He confirmed that temperature is more of a killer of these batteries than humidity. Each pouch is obviously sealed and for humidity to get inside - well, we'd have other problems as lithium and water simply do not mix (kaboom anyone?). Thus, for all intents and purposes, keep the batteries cool, however you can. My specific question was asked because my intent is to throw a few icepacks in the trunk on a really hot day. I'm lowering temperature at the expense of raising humidity, hence that question.

Sorry for the tangent - going back to your question. Under normal vehicle operation with the engine on, the BSG always charges the 48V system first, and then from there it'll charge the 12V battery. So to your question in regards to a failing 12V battery, I would suspect that the very behaviour of a failing 12V battery wouldn't cause the 48V battery to die. Mainly, a failed 12V battery will simply refuse to accept a charge which is what leads to its demise. So lacking some sort of system, my gut (not backed by science) would say in that particular instance we should be OK regardless. Vice versa, if the 48V battery starts to die, the BSG will continue trying to charge that, and fail, and whatever energy it can get from the BSG will be transferred over to the 12V battery. I'm not particularly certain though that that would be a problem which is perhaps why Audi engineers designed the D5 such that the BSG charges the 48V battery first versus the (SQ7?) design previously where a 12V alternator charged the 12V battery and then charged the 48V battery. In our case, if the 48V battery dies or approaches death, it's not that big of a deal because indirectly the BSG will just be charging the 12V battery which we require. The 48V battery provides moderate boost for up to 5 seconds in certain conditions, and start-stop, which if both stopped working, our cars would still work. However, I do think you're right. Looking at my own D5, my 12V battery was less than half charged after I could see 14.8V on my vehicle readout for about 15 minutes after ignition. So the DC DC converter must have been given instructions to stop charging the 12V battery despite it not being full.

I think what is key here is the DC DC converter, and as these things become more mainstream hopefully I can find one at a junkyard somewhere to dissect. I think I've hit a wall here and will leave the rest to guessing and/or the Audi engineers.I would suspect that as the vehicle ages we'll know more. Ultimately, LG Chem does state that their MHEV batteries (in general, again, nothing for our Audis) are designed to last 10 years - but that is assuming a SOC of never more than 80% as one can double the lifespan of a li-ion battery by keeping it <80% versus it being charged to 100% all the time. I would hope that the Audi boffins programmed in some sort of logic to do just that.

VCDS vs OBD11: The answer to your question is going to bore you - unfortunately. I chose OBD11 because it's completely new to me. Back in the old days, vagcom was the way to go, and while I'm confident I can pick up VCDS again, even with the limited enthusiast version which can only run 3 VINs, OBD11 is still cheaper (and much more convenient). If I can't get the info I need, I'll just return it (locally sourced here so that should be easy). On the upside, OBD11 is something new for me to learn and if it does work well, it's a lot less painful to use! Plus, all the kids these days are raving on about it - so either they're right and this is a better mousetrap, or they just don't know how powerful VCDS is and they're missing out. I'm looking to finally bury the hatchet with the pseudo war against these two tools

gk1

Excellent. Yes indeed do tell your feelings of OBDEleven vs. VCDS. I am also from the old-school when vagcom was the only game in town.

I noticed in your picture on the left it says "Cell assembly comprising 13 battery cells" which makes sense to me 13 x 3.7v = 48V
However what do you think it means on the right where it says "Plug-in module with single battery cell"? Does that mean there is a 14th cell? A hot spare if one fails? Weird, I know you have pretty much extracted all available printed info about the battery but I am wondering what you think that means.