Charging car battery?

Jul 18, 2017
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In a modern car, it is safe or even wise to charge the battery with it still connected? The battery is under the driver's seat so not easy to remove however there are connection points for the battery under the bonnet. Also the Jeep has two batteries in the same compartment and they are inter connected, one being the normal size stop start 12v 70AH battery and the other a much smaller stop start 12v 14ah battery. Both are AGM, I think the smaller one aids the Stop Start? On switch on the battery reading is about 12v.

Car starts okay, but does some weird things at times which is an indication that there may be an issue with the battery. The current normal size battery is a Bosch S5 A08 70AH battery CCA 760A. When doing a search for a battery all the batteries that are suitable show as being 80AH with 800 CCA?

The Jeep is only 6 years old and I have had it for 18 months so it seems unlikely that the AGM battery would have been changed prior to my purchase, but I cannot see a date of manufacture due to the battery being difficult to access.
 
Nov 6, 2005
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The vehicle-specific handbook is the best source of information.

My car has a hidden battery too, under the front passenger seat - but it has jump-start / charging posts in the engine compartment and these are used for charging without any issues.
 
Jul 18, 2017
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The vehicle-specific handbook is the best source of information.
If that were only true. I hauled out the Owner's handbook and could not find any reference to the AH or CCA specifications of the battery. I am on my own! LOL!

As a matter of interest if the AH and CCA are higher than current battery, would that cause any issues?
 
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If that were only true. I hauled out the Owner's handbook and could not find any reference to the AH or CCA specifications of the battery. I am on my own! LOL!

As a matter of interest if the AH and CCA are higher than current battery, would that cause any issues?
In my opinion, the AH and CCA provided are the minimum. Going higher is not a problem. So long as the physical size fits and it has the correct type of terminal orientated correctly.

My Volvo had two batteries like your Jeep. I was told that the smaller battery maintained the lights, fan, and other ancillary load so they didn't dim when the main battery started the engine.

Regarding charging from the engine compartment. I am aware that many cars can be safely charged in this way. But I have also heard that with some cars, doing this can cause damage to the electronics. (sorry, no help there then).

John
 
Jul 18, 2017
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Thanks John I suspected that a higher specifications within reason would not be an issue. I am not sure how the batteries are inter connected and whether the smaller battery is dragging down the bigger one. The chain shops seem to be totally useless!
 

JTQ

May 7, 2005
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As the Jeep is your tow car then it will have a tow electrics socket, with quite probably a permanent direct connection to the car's battery.
That is what I have for decade used to charge various tow cars over winter, cars where the batteries have been under seats or in the bonnet where access once in my garage would be a mega struggle.

I use a very moderate charge rate, either 3.6 or 5 Amps ( CTEK) smart charging. In my case with various vehicles I have never screwed up the electronics.
Currently I have a 13 pin plug uniquely wired into a CTEK charger DC plug, did see an expensive version offered on Ebay once but had easily made my own long before.

One of these
LINK eyes cut off and connected to a 13 pin plug, so the CTEKs can be plugged into that or plugged into their standard clips. Or if it works connect the adaptor onto the battery (via fuse) if that presents its plug in a readily accessible position.
 
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Jul 18, 2017
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I had a look at the battery to try and determine the age of the battery, but are not having much luck. Perhaps someone else can read the code? Bosch battery.jpg
 
Jun 20, 2005
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What voltage is the multimeter showing using the terminals under the bonnet?
Like Roger my VW has the battery under the seat . The specific easy access terminals under the bonnet have been specifically designed for jump starting or remote charging.
I imagine Jeep did the same as VW?

Battery size is no different to the caravan Leisure battery. Your choice subject a minimumAH. No doubt Jeep specify the minimum. There can be no maximum other than the physical space available for the battery(ies).
Anyone with a disability buggy will always strive to get the biggest AH available for extended performance duration.
 
Jun 16, 2020
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Thanks John I suspected that a higher specifications within reason would not be an issue. I am not sure how the batteries are inter connected and whether the smaller battery is dragging down the bigger one. The chain shops seem to be totally useless!
I suspect the batteries are interconnected by complex electronics. If they were simply connected in parallel you would get light dimming and radio interruption when the engine restarts. My MG only has one battery and does not suffer the above problems, presumably because they have tackled that problem in a diffent manner.

Try asking Halfords to drop test both batteries, They could do this by the under-bonnet terminals assuming these go directly to the big battery, and the smaller battery could be done separately.

If you are on a Jeep forum you should get more accurate assistance.



John
 
Oct 19, 2023
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I'm no expert but from what I've read the smart chargers with 'desulphation' mode can damage car electronics if they are used to charge the battery whilst connected. They work by pulsing high voltage to the battery to break down the sulphate deposits on the plates, but electronic components designed to work at 12v don't like the high voltages. I would imagine that a bog standard charger with a 15v maximum would be OK (car alternators kick out around 15v).
 
Nov 6, 2005
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I'm no expert but from what I've read the smart chargers with 'desulphation' mode can damage car electronics if they are used to charge the battery whilst connected. They work by pulsing high voltage to the battery to break down the sulphate deposits on the plates, but electronic components designed to work at 12v don't like the high voltages. I would imagine that a bog standard charger with a 15v maximum would be OK (car alternators kick out around 15v).
It certainly won't damage all cars but I can believe that it might damage some - the problem is knowing which models might be affected.

It doesn't affect my VW Touareg as I've used the CTEK charger on it several times without any issues.
 
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It certainly won't damage all cars but I can believe that it might damage some - the problem is knowing which models might be affected.

It doesn't affect my VW Touareg as I've used the CTEK charger on it several times without any issues.
Quite. The manual for my car states :
'Never charge a battery still installed in the vehicle unless a battery charger unit approved by Mercedes-Benz is being used. A battery charger unit specially adapted for Mercedes-Benz vehicles and tested and approved by Mercedes-Benz is available as an accessory.'
The battery charger available from Mercedes looks remarkably similar to one of the CTEK chargers, but with Mercedes written on the side and a much higher price tag.
 
Jun 20, 2005
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Quite. The manual for my car states :
'Never charge a battery still installed in the vehicle unless a battery charger unit approved by Mercedes-Benz is being used. A battery charger unit specially adapted for Mercedes-Benz vehicles and tested and approved by Mercedes-Benz is available as an accessory.'
The battery charger available from Mercedes looks remarkably similar to one of the CTEK chargers, but with Mercedes written on the side and a much higher price tag.
I’d love to know what “Specially adapted “ means
 
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Jul 18, 2017
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What voltage is the multimeter showing using the terminals under the bonnet?
Like Roger my VW has the battery under the seat . The specific easy access terminals under the bonnet have been specifically designed for jump starting or remote charging.
I imagine Jeep did the same as VW?

Battery size is no different to the caravan Leisure battery. Your choice subject a minimumAH. No doubt Jeep specify the minimum. There can be no maximum other than the physical space available for the battery(ies).
Anyone with a disability buggy will always strive to get the biggest AH available for extended performance duration.
On a multi meter the reading under the bonnet is exactly 12v, but I am assuming that is the combined reading of both batteries. If the aux battery is on its way out, I suppose that there is a possibility that it is dragging down the voltage on the bigger 12v battery.

The aux battery looks to be the original battery, but I am unsure which is why I would like to try and read the Battery code on the main Bosch battery code which is B22A081103J7.
Try asking Halfords to drop test both batteries, They could do this by the under-bonnet terminals assuming these go directly to the big battery, and the smaller battery could be done separately. If you are on a Jeep forum you should get more accurate assistance.
After my last very recent experience with Halfords I am admittedly too scared to go anywhere near them. They did not even know how to remove the battery. They wanted to remove the whole seat when there is no necessity plus despite using my reg number to identify the battery the battery was unsuitable for the vehicle. Not sure how they can do a drop test if you have two batteries linked together.

Unfortunately on UK Jeep forums very few have late model Jeeps with the 2 batteries so info on batteries is a bit sparse. US Jeep forums focus mainly on petrol vehicles.

My thinking is to disconnect the aux battery, then measure both batteries again. I think that the Jeep will still start etc without the aux battery?
 

JTQ

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12 volts, unless via some voltage regulation, but directly on the battery(batteries) suggests less than 50% "State of charge"!
Risky if they could do a cold start at that.
 
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12 volts, unless via some voltage regulation, but directly on the battery(batteries) suggests less than 50% "State of charge"!
Risky if they could do a cold start at that.
There is no issue starting the vehicle, but it does weird things i.e. switch on seat heating and it won't switch off, auto rain doesn't work all the time, auto lights are on when no necessity. Obviously not all the time, but occasionally and not all at the same time.
 

JTQ

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There is no issue starting the vehicle,
Then I suspect your measured "exactly 12 volts", is unlikely to be the true voltage at the battery's posts.
It is a very low-level, state of charge, for starter technology build LA batteries, as said standing below 50%.
 
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Then I suspect your measured "exactly 12 volts", is unlikely to be the true voltage at the battery's posts.
It is a very low-level, state of charge, for starter technology build LA batteries, as said standing below 50%.
It is exactly the same on the actual battery terminals which is why I want to find out which battery needs changing. It is an AGM battery as per the picture I posted.
 

JTQ

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It is exactly the same on the actual battery terminals which is why I want to find out which battery needs changing. It is an AGM battery as per the picture I posted.
Being an AGM, does not improve the picture of its Sate of charge at exactly 12 volts, in practice the opposite, to well below 50%.

The only proviso, I am assuming this 12 volts is effectively being measured "off load"?
 
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Being an AGM, does not improve the picture of its Sate of charge at exactly 12 volts, in practice the opposite, to well below 50%.

The only proviso, I am assuming this 12 volts is effectively being measured "off load"?
With car switched on and not running it shows about 12.1v. Unfortunately I am unsure how to test it under load?
 
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An accurate voltage reading will not be obtained whilst a battery is connected to the car battery terminals as there are several components using power even when the car ignition is in the off position.

For a more accurate voltage reading of the battery, the battery should be disconnected from the battery terminals and 'rested' (say 3 hours for want of a better guess, as this is variable depending on various circumstances) then take a voltage reading.
 
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An accurate voltage reading will not be obtained whilst a battery is connected to the car battery terminals as there are several components using power even when the car ignition is in the off position.

For a more accurate voltage reading of the battery, the battery should be disconnected from the battery terminals and 'rested' (say 3 hours for want of a better guess, as this is variable depending on various circumstances) then take a voltage reading.
I assumed that this would be the best way to check the battery, but accessing the battery to remove the negatives is a bit awkward for me due to the battery being under the seat. I will try and make a plan.
 
Nov 16, 2015
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With car switched on and not running it shows about 12.1v. Unfortunately I am unsure how to test it under load?
On my Santa Fe two years ago, I had slow starts a couple of times, measured the battery voltage across the terminals, whilst fItted to the car. A low reading about 12.3v. Battery wash years old, so bought a new battery. Fitted the new battery, OMG, new battery voltage, 12.3 volts.
So must be parasitic draw with all the background electronics.
Old battery sits in the garage at 12.7 volts as a standby battery.
 
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I assumed that this would be the best way to check the battery, but accessing the battery to remove the negatives is a bit awkward for me due to the battery being under the seat. I will try and make a plan.
As folk have said on forums before take it to your dealer. And ask to be there whilst doing checks.
 
Jul 18, 2017
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As folk have said on forums before take it to your dealer. And ask to be there whilst doing checks.
Our trusted Jeep dealer is 70 miles away and to change the battery the cost is in excess of £500! The cost of just doing a check is not cheap unless you give them the go ahead to do the change. :cry:
 

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