Is there any danger when dealing with the12v Battery on a Hybrid Car?

[John77]

Hi I want to check the output of the 12v Battery on my 2017 Hybrid and maybe charge it. Is there any danger from the High Voltage Battery when testing or removing the 12v battery? I believe that there is a HV Battery isolation switch. Does the HV Battery need to be isolated when dealing with other non HV electrical components on the car?

 

[malcolm E53 AMG]

The 12V and HV 48V batteries run on totally separate circuits so no problem charging the 12V battery from the engine bay jump start points. The only interface the LV and HV circuits have is at the 48V/12V inverter which allows the 12V battery to be charged by 48V Starter/Generator.

When I replaced the 12V battery on my MHEV I maintained the 12V with supply by connecting a spare battery at the jump start points.

 

[Chrishazle]

IIRC Television recommended that as standard procedure changing a 12V battery on an MB - but just remember the 12V terminals you are disconnecting are therefore still live so do not let them touch anything while you change the 12V battery!

 

[John77]

Thank you for the replies. They are very helpful. Next question. So if I test the out put of the battery by connecting to the battery terminals. Am I getting a true reading of the 12v battery or do I need to disconnect it first?

 

[malcolm E53 AMG]

What’s the battery reading while connected? What problems are you having with the battery and what is the date stamp on it?

 

[EvanGrande]

On the true reading question, it depends where you're clipping on. On these hybrids the 12V battery itself usually lives in the boot, while the terminals up front are the jump start points, and those have the car's own charging system still in the loop. So if you measure at the engine bay posts with the car awake, you're reading system voltage, not the battery's resting state.

For a proper look at the battery on its own, let the car go to sleep with everything off for 15 minutes or so, then measure right at the battery posts in the boot. You don't need to fully disconnect it to charge it though, just hook up at the front jump points like the others said.

What readings are you getting, and what's actually prompted this, a slow start or a warning on the dash?

 

[John77]

On the true reading question, it depends where you're clipping on. On these hybrids the 12V battery itself usually lives in the boot, while the terminals up front are the jump start points, and those have the car's own charging system still in the loop. So if you measure at the engine bay posts with the car awake, you're reading system voltage, not the battery's resting state.

For a proper look at the battery on its own, let the car go to sleep with everything off for 15 minutes or so, then measure right at the battery posts in the boot. You don't need to fully disconnect it to charge it though, just hook up at the front jump points like the others said.

What readings are you getting, and what's actually prompted this, a slow start or a warning on the dash?

Thank you. I have seen the battery next to the HV Battery, C Class estate and will check the readings. The car apparently starts off the HV Battery, which is stupid as when it dies the car dies instead of still being able to start on the 12v and use petrol. The reason is my other post Torrential Rain faults on dash.

 

[John77]

The 12V and HV 48V batteries run on totally separate circuits so no problem charging the 12V battery from the engine bay jump start points. The only interface the LV and HV circuits have is at the 48V/12V inverter which allows the 12V battery to be charged by 48V Starter/Generator.

When I replaced the 12V battery on my MHEV I maintained the 12V with supply by connecting a spare battery at the jump start points.

My next quest that may seem silly, but I have a reason to ask. What is the jump start points in the engine bay for? I ask this because the car is apparently started off the HV battery, not the 12v system.

 

[malcolm E53 AMG]

My next quest that may seem silly, but I have a reason to ask. What is the jump start points in the engine bay for? I ask this because the car is apparently started off the HV battery, not the 12v system.

The jump start points aren’t really that in a MHEV so it’s a bit of a misnomer they are only there to charge the 12 battery. If the 48V hybrid battery develops a fault the car cannot be started and a it’s dealer only situation.

 

[John77]

The jump start points aren’t really that in a MHEV so it’s a bit of a misnomer they are only there to charge the 12 battery. If the 48V hybrid battery develops a fault the car cannot be started and a it’s dealer only situation.

Ok thank you

 

[John77]

Ok I received a message saying that my 12v battery is low and needs charging for 12 hours Via MB App. I have checked the output at the jump start point and it is 11.8v. I have put the battery onto charge and my charger shows the state in lights from 0-8 and it is at 3. I bought the car second hand, but the battery is probably the original 2017. The access to the battery on the hybrid is not easy to check the date. I will have to look.

 

[John77]

I think that it is time to change my battery lol (51 16). Now do I buy a MB Battery at £235 (See Picture) or the same Varta battery at £153? No need to answer.

 

[malcolm E53 AMG]

I will answer - Varta!

Whichever one you go for make sure you give it a top-up charge before fitting a lot of batteries sit around waiting to be sold and the retailer isn’t going to bother giving them a charge

 

[John77]

I will answer - Varta!

Whichever one you go for make sure you give it a top-up charge before fitting a lot of batteries sit around waiting to be sold and the retailer isn’t going to bother giving them a charge

Yes thank you. I don't get why people buy a OEM battery made by VARTA when you can get the same one £80 cheaper.

 

[mioba]

Yes thank you. I don't get why people buy a OEM battery made by VARTA when you can get the same one £80 cheaper.

Post back in ten years and tell how long the Varta lasted.
Experience.

 

[ALFAitalia]

According to my Varta rep when I was in the caravan game (we used Varta and Yuasa) they are the same as normal retail batteries......they did not make different batteries for car manufacturers apart from different labels. With one or two specialist applications aside underneath the sticker, the core construction and manufacturing processes are identical. I can't see any reason for him to lie as he could not even sell me a battery....our contract was only for leisure batteries. But if you have independent evidence to the contrary (so not from Varta or MB) then I stand to be corrected.

 

[John77]

Post back in ten years and tell how long the Varta lasted.
Experience.

I had a VARTA on a previous Mercedes that had been fitted by a previous owner and that was still churning out the power 12 years on.

 

[John77]

OK changed the battery, which watching you tube takes about 20 mins. I finished 2 hours later. The problem was getting the old one out. Yes removed the brace bracket, undid the crash bar, or whatever it is called, and at one end and put a wedge under it? The battery still would not lift and gets stuck between the lower coolant pipes behind the battery and the edge of the battery bay lip. In the end I Had to hit a hammer on a wooden pole at the bottom of the battery to get the battery to lift over the lip and turn so that you can get the positive clamp off. No matter how you put a wedge under the end of crash bar it will not lift enough to get the battery to move as it is held on the plastic safety cover that is over the positive clamp. If the battery was 2-3mm shorter it would be a breeze. It is not easy to get the new battery in as you have to put the positive clamp and cover on before the battery is flat. Then same method with the wood to get it to turn under the crash bar. The problem is that the crash bar runs under the EV battery and doesn't move much. Well all done now.

The next question is does the new battery have to be registered on the car computer or BMS and can this be done on a MB ICarsoft scanner?

 

[rf065]

Post back in ten years and tell how long the Varta lasted.
Experience.

My OEM Varta bit the dust after 4 years, the replacement Varta I fitted is still going strong 6 years later. That's on the C Class, the SLC was exactly the same.

 

[malcolm E53 AMG]

I’m sure there is a way to register on Xentry that a new battery has been fitted but I don’t think there is a battery management system like BMW use that adjusts the charging rate relative to the age of the battery. When I changed my 19 plate E53 12V battery I didn’t bother and everything runs fine.