cove3
Posts: 276
Joined: Tue Jul 22, 2014 12:36 pm
Location: White Plains, NY

Charging the 12V battery if car is not driven much

Sun Mar 08, 2015 8:58 am

After replacing my BMW 528E and 525I 12V batteries every few years, I was told by BMW to use a trickle charger to the cigar lighter if a car is not driven much, which was the case my being retired. The battery life is reduced if they are undercharged from lack of driving. The batteries draw some current constantly even if the car isn't used, and I found myself charging them from dead with a charger..

I'm wondering about the e-golf battery when the car isn't driven much. With the snow I haven't used it for over a month:
1. The manual says the battery will drain if not driven for long periods, the doors are unlocked, or lights left on
2. It appears the 12V battery is charged from an alternator while driving
3. The battery capacity is small as it's not needed for starting like an ice, so the safety margin is less
4. The manual says only the dealer should charge it and that the dealer has to reset things when changing a battery
5. There doesn't seem to be an indicator for the 12V battery state of charge

This leaves 3 questions:
1. Is it really necessary for the dealer to charge the battery.
2. How to tell if it's in the danger zone of discharge?
3. Is it safe to use a trickle charger through the cigar lighter as with my ICE to to be on the safe side?

Ron
Last edited by cove3 on Sun Mar 08, 2015 2:07 pm, edited 1 time in total.
2015 e-Golf Limestone Gray metallic
1988 BMW 528e, 1992 BMW 525i
Prev: BMW 2002, 3.0cs, 2800cs, XKE V-12, Beatle
Clipper Creek 40A

tigger19687
Posts: 46
Joined: Fri Aug 08, 2014 9:38 am

Re: Charging the 12V battery if car is not driven much

Sun Mar 08, 2015 1:26 pm

There must be something wrong if you are going through 12v batteries like candy !?

If you are going to leave a car unused for more then or about a month I would disconnect it.

My trucks don't go through batteries unless I leave a light on or don't drive it for 4 months straight.

Not sure if you have Electronics in the car (navigator etc) that are not turned off ?
Or, like my son, who leaves things plugged in the lighter of the truck.

cove3
Posts: 276
Joined: Tue Jul 22, 2014 12:36 pm
Location: White Plains, NY

Re: Charging the 12V battery if car is not driven much

Mon Mar 09, 2015 6:52 am

Not like candy, but every 2 years or so. Disconnecting the battery isn't practical, as I use the cars for the grocery and short trips, but these trips aren't enough to charge the battery up. I was periodically finding them dead and charging them in the garage. So BMW told me the battery life was shortened by being at a low state of charge over time. That's the reason trickle chargers are sold.....to keep the battery topped up.

The question is, is there any way to tell the state of the 12V battery, and can/should you use a trickle charger with an electric car or is there some circuitry which would advise against it.

I know the battery is draining. Responding to car net, a red light flashes every few seconds to show the car door is locked, etc.

Ron
2015 e-Golf Limestone Gray metallic
1988 BMW 528e, 1992 BMW 525i
Prev: BMW 2002, 3.0cs, 2800cs, XKE V-12, Beatle
Clipper Creek 40A

rjnerd
Posts: 43
Joined: Wed Jan 07, 2015 10:19 pm
Location: Boston suburb
Contact: Website

Re: Charging the 12V battery if car is not driven much

Tue Mar 10, 2015 3:47 pm

First, the e-golf has much lower demands on its "house" battery. No starter to turn, so it doesn't take much to get to "ready", just enough to pull in the contactors in the HV pack. It is the case that all modern cars have more idle drain than in years past, if nothing else, powering the various receivers for the key fob, and the car-net system. (in the old days, key off meant the only thing drawing power was the clock)

The house battery is charged by a dc->dc converter from the hv pack, no alternator to worry about. It should be fine with a battery tender, but not thru the lighter socket, as those are usually disconnected when the car is. You can get a tender with a quick disconnect cable long enough to reach thru the grille slot, making it easier to reach.

Your short trips should be sufficient to keep it topped up. especially since the converter is a more intelligent charging system than your old alternator, and isn't dependent on engine speed to get peak charging output. Your old car at idle may or may not have had enough output to cover the load of ignition and fuel injection, back in the day when cars had ammeters rather than check engine lights, it would take a bit of revving to get the thing to the positive side. As soon as the contactor clicks in, the house battery will be charged to full as soon as is reasonable.

One thing to consider, if the battery does lose it, make sure its replacement is "deep cycle" rated. (simple way to tell, it will be a lot heavier than the one it replaced). It will be much more tolerant of sitting around.

cove3
Posts: 276
Joined: Tue Jul 22, 2014 12:36 pm
Location: White Plains, NY

Re: Charging the 12V battery if car is not driven much

Tue Mar 10, 2015 7:47 pm

Thanks rj. Very helpful

1. While it's true there's no start load as with an ICE, this also allowed VW to use a small battery so concerns still exist
2. There's nothing in the manual as to how long the car can be sitting idle before the 12V battery drains below a safe thresh hold. 3 weeks, 2 months?
3. Nor is there any easy way to test the capacity or state of charge to know whether to use a trickle charger.
4. Does the 12V battery get charged when you charge the main battery? If so, and the main battery is at 50% for storage puroses, I could charge it to 80% in order to top up the 12V battery
5. It appears the 12V socket requires the ignition to be on, but in any event the manual says not to put a charger, solar or any power input into the socket
6. Looking in the engine compartment, there don't seem to be any openings in the grill to run a trickle charger through, so the hood would have to be up
7. If the 12V battery is charged by the car main battery, couldn't I just turn on the electric motor for a couple of hours?
8. I called my dealer service and they said the 12V system is just like an ICE and is independent of the main battery system, and I could charge it with either a trickle charger or battery charger without loosening any battery connections

Ron
2015 e-Golf Limestone Gray metallic
1988 BMW 528e, 1992 BMW 525i
Prev: BMW 2002, 3.0cs, 2800cs, XKE V-12, Beatle
Clipper Creek 40A

NeilBlanchard
Posts: 298
Joined: Sat Feb 21, 2015 12:19 pm

Re: Charging the 12V battery if car is not driven much

Wed Mar 11, 2015 9:56 am

The 12V battery is charged with a DC-to-DC converter, so it should last as long as any regular battery.

In my experience, 12V lead acid batteries last at least 5 years, and as long as 7 years. If it only lasts 2 years, then there is something else going on.

cove3
Posts: 276
Joined: Tue Jul 22, 2014 12:36 pm
Location: White Plains, NY

Re: Charging the 12V battery if car is not driven much

Wed Mar 11, 2015 10:34 am

<<<12V lead acid batteries last at least 5 years, and as long as 7 years. If it only lasts 2 years, then there is something else going on.>>

That's probably right if kept in a reasonable state of charge, but since retirement I was using my cars so little that the batteries were not charged up much, and coupled with normal constant drain, meant the batteries were sitting in a low state of charge for months on end. BMW service told me that's why the batteries were failing after 2 years, and that I should keep them on a trickle charger to extend their life.

The main egolf battery is the same per them manual. It should not be allowed to go below 10% or so for an extended preriod or you will shorten the capacity and life

Ron
2015 e-Golf Limestone Gray metallic
1988 BMW 528e, 1992 BMW 525i
Prev: BMW 2002, 3.0cs, 2800cs, XKE V-12, Beatle
Clipper Creek 40A

cove3
Posts: 276
Joined: Tue Jul 22, 2014 12:36 pm
Location: White Plains, NY

Re: Charging the 12V battery if car is not driven much

Thu Mar 12, 2015 9:02 am

The e-golf service rep said the 12V battery is charged by an alternator and not by the traction battery, either during charging or while driving the car. The owners manual also references an alternator on Page 15 and 301

He said to use a trickle charger if the car is not driven for several months, but not through the 12V convenience outlet. This doesn't work anyway if the motor isn't activated and if it is, the manual says not to put any kind of power like a trickle charger or solar into the 12V outlet.

Ron
Last edited by cove3 on Thu Mar 12, 2015 11:25 am, edited 2 times in total.
2015 e-Golf Limestone Gray metallic
1988 BMW 528e, 1992 BMW 525i
Prev: BMW 2002, 3.0cs, 2800cs, XKE V-12, Beatle
Clipper Creek 40A

NeilBlanchard
Posts: 298
Joined: Sat Feb 21, 2015 12:19 pm

Re: Charging the 12V battery if car is not driven much

Thu Mar 12, 2015 10:30 am

What alternator would that be?

Hint: there is no alternator in the e-Golf.

It has to be a DC-to-DC converter, powered by the main high voltage battery pack.

cove3
Posts: 276
Joined: Tue Jul 22, 2014 12:36 pm
Location: White Plains, NY

Re: Charging the 12V battery if car is not driven much

Thu Mar 12, 2015 11:32 am

See page 15 and 300 of the owners manual. There are 2 possibilities:

1. The manual is wrong. This is possible as the manual does have some references which can only apply to an ICE car
2. The service rep is wrong, although I thought I was talking to the man who went to e-golf school

If it is charged from the traction battery, it would seem the 12V should be charged while charging the traction battery or when the electric motor is activated, but the rep said the 12V is only charged by driving the car

Ron
Last edited by cove3 on Fri Mar 13, 2015 10:08 am, edited 1 time in total.
2015 e-Golf Limestone Gray metallic
1988 BMW 528e, 1992 BMW 525i
Prev: BMW 2002, 3.0cs, 2800cs, XKE V-12, Beatle
Clipper Creek 40A

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