This is a tough one, trying to get an answer.

Customer buys a new BMW 535i sedan for his vacation home in Coronado. He is away from the car (read that, the car does not get driven or charged) for 6 months at a time.

His old MBZ, he just used a basic battery disconnect on the + terminal when he was away.

Unfortunately, you can't do that anymore because there are so many electronic modules on the vehicle that have to have a bit of current going to them.

If you disconnect them entirely, when you do reconnect the battery the dash lights up with warning lights like a Christmas tree and you have to take the car to a dealer to have the system reset.

So the customer does not want to have to go see a dealer every time he goes on vacation, that won't work.

Oh, and he can't depend on a trickle charger because the car is in a community garage and no outlet is available.


Ideas?

Some sort of solar solution?

Sorry, I've got nothing...

Some kind of trunk-mounted auxiliary battery that could be removed by a neighbor and recharged every couple of months?

Hire someone to drive the car once a month or so, whoever is the maid at the vacation home? :-s

Have him fly me out there once a month for a weekend, and I'll take the car out and charge the battery for him.

So he's away from his VACATION HOME and NEW BMWi Sedan in Coronado for 6 months at a time. Geez....Ya'd think the guy wouldn't worry what the frick the cost of a new battery and computer reset was. But that's me. (BTW, battery $50-$70, reset $0. Every 6 months... )

Just tell the dufus to buy a cheaper car so he CAN afford a new battery every now and then.

230Mike's idea: http://www.batterystuff.com/solar-chargers/

interesting system could be put together with that stuff

if f14 can't make it one of those weekends he gets flown out, I'll put my hat in...wish I had the problem of even having a 535, let alone using only at my vaca home....ps. don't have the vaca home in CO either

The vehicle is in an indoor (underground) parking lot. Condo highrise on the water...

What if you hooked up a deep-cycle battery to the existing vehicle battery? Would it keep the standard battery charged and deplete its own charge before the regular battery died? Sort of a double-your-battery-life solution? A no maintenance gel/mat battery could be thrown in the trunk to function that way...

BMW batteries are in the trunk, BTW.

Dayam, maybe I could even use a Yandina battery combiner on it...

Maintenance free batteries still need vented!!! I don't think that's a good idea. Find out what the draw rate of the BMW is sitting. Do the calculations to see how much of the battery will drain over 6 months.

troop,

What do you stand to get out of this? A beer max! This guy is a cheap-arse! Unless he is a close personal friend, dust him. He ain't worth your time.

If you connect two batteries in parallel, they would discharge at the same "voltage rate". Even if the AH rating was different, the two batteries would discharge the same percentage.

Venting is not an issue - because a battery only vents during overcharge.

To tell if you have sufficient battery, you would need to calculate the load, apply Peukert's Law, and determine the required AH rating to say no lower than 30~40% of capacity. Since this would presumably be a very low load, my guess is that the natural battery's internal discharge rate would exceed the load (unless you went to an AGM).

My first thought would be a solar recharger. Unless the garage is pitch black - it might provide some charge. You would have to experiment with that idea though.

But I have a much better idea.

Start the "TroopLewis rich @#&*%# valet service"; find a secure, fenced in spot in your dealership's lot, or a secure garage with power, and provide a $29.95 $49.95 $79.95 per month valet service for car storage.

Maybe an optional value added "walk the car" to keep the fluids in suspension so the internal engine components don't rust (drive it to work once per week). Hey, might be bogus, but who cares?

Give him a car wash when he pulls the car out, and I bet you would find a customer.

A lot. It's an important deal to me.

Draw is 35ma in sleep mode (vehicle goes into sleep mode about 1-1/2 hours after you walk away from it). According to my service dept, that will kill the battery in about 3 weeks.

No Shorepower connection, eh?

Say a 60AH battery (I don't know for sure what size it is), at 60% (for a 40% reserve), is 36AH; / 0.035A (35mA) = 1,028 hours, or about 42 days.

AH means Amp-Hour;
or mathematically:

Amps * Hours * a Ratio derived from Peukert's Law (always less than 1).

But this would hold mostly true for a deep cycle. I am not sure how a standard automotive starting battery would hold up.

You can also discount Peukert's Law, since you have such a low discharge rate. Peukert's basically sez that the battery will supply a higher AH rating at a lower lower AH drain.

For instance, one typical AH rating method for a battery is calculated at 20 hours. So a one Amp drain at 20 Hours would be a 20AH battery. But if you put a 20 Amp drain on the battery, it would not last one hour, but something less. That is Peukert's Law.

This is due to internal heating and inefficiencies present from the high discharge rate - the capacity actually goes down.

But if the rate is at 20 Hours or 2,000 Hours, the rate of discharge difference is nil in the sense of heating, so you can probably discount it.

The other issue, if not an AGM battery is that there will be some internal discharge that occurs naturally. This will also reduce the length of time, especially at the low load you are estimating.

I see your only option is to add a big arse battery in the trunk temporarly, like a pair of Trojan 6V batteries, maybe 200AH. Might be overkill, but it might be the only option.

But then, someone is going to have to install the batteries, charge them initially, and maintain them in the "off season". Seems a lot of work. The dude's golf clubs ain't gonna fit in the trunk with those Trojans, so you will have to remove them when he gets the car. The cost of this solution, both in initial expense and time required to keep the batteries in good condition when in off-car storage, and storage for those batteries "off-car" logistically is going to take a lot of effort.

I still think the "Valet Service" is the best idea. You or your dealership might have an income potential for other customers as well...

It basically comes down to either providing a source of power (sitting outside with a solar charger), or a source of power in the darkened garage. If you go the extra battery in the trunk route - there is a lot of logistical problems with that solution, as I mentioned, the reverse problem of "off-car" storage of the batteries, and maintaing them in good condition (trickle charge) when they are not needed. Where is he going to store those until the next time he needs them.

But put it into perspective. I pay $1,700 for winter storage for my boat - and that doesn't even include a source of power. A valet service would be cheaper than that. Good thing he doesn't own a boat.

This has got to be one of the most insensitive replies here on BABC that I've read in a while. Good grief!

Troop, I did some research for you because I was curious as to which lights appear on the dash after getting the vehicle going again. I know that German engineers like to make sure that a system is operating correctly before clearing a code, and sometimes a road test is all that is needed for the computers to give the "all clear".

Unfortunately, this is not the case with your 535, as the radio and nav system will have to not only be reset with OEM equipment, but the tech has to be thoroughly familiar with the system to do it, as the nav system is ultra-complex (uses fiber optics too).

I can see no harm in adding more batteries in the trunk. Fill 'er up!

I am stunned that BMW designed a car that would kill the battery in just 3 weeks! That isn't very long AT ALL.
At 1 dead battery every three weeks, he would need 9 batteries in the trunk. Of course they would fit, but they would be awfully expensive. If he unhooked the battery (in the exact way that BMW recommends) what would it cost to have the warning lights reset upon the rehook in 6 months? If it's less than 9 batteries, that is the way to go.