Moving battery to trunk, any suggestions?
 

I want to move the battery in my brother's W116 300SD to the trunk. I have already figured out how to route the positive wire, but I need some suggestions of how exactly to do the other parts.

1. What gauge wire should I be using to go from the front of the car to the trunk? I am planning on to put an amp and subwoofer in the trunk to balance out the sound system (bigger alternator is on the way). Would 4awg be fine or should I go for 2awg?

2. What should I ground to? A hitch is bolted to the bottom of the spare tire hole (not technical term), would that be fine to bolt to?

I don't understand. If you aren't racing, you shouldn't have much of a reason to move the battery to the rear. If you are racing, you shouldn't be putting in amps and subwoofers. What's going on?

It frees up a lot of room in the engine bay and helps weight distribution

(And batteries in the trunk are cool)

The bracket for the vent for the battery tray was extremely loose and rusty (ground goes to this bracket, ground was poor) so we removed it, started cleaning it up but its pretty knackered up and needs to be sanded, painted, the bolts for the battery tray drilled out, new nuts welded on, then weld it back in (which will be done eventually. We need the car ready by Saturday and I don't have much time between now and then to do all of that work. I can't weld it, my friend has a welder and the expertise and can't do it until later, and running the wires will be easier.

I also see benefits of moving the battery such as no more battery acid leaking into the engine bay, which killed the window washer pump ($70) and reservoir ($60). The battery will be in a sealed box to make sure there is no acid leaking into the trunk. Supposedly there is extended battery life because there is less heat in the trunk, these batteries are quite expensive so its nice to have them last longer.

90 - 01 R129 SL's and 95 - 00 C class W202 have the bat in trunk, 96 - ? W210 E class have the bat under the rear seat.

I'd have to measure the cable dia but it looks to be AWG " 0 "

On the ground side, run a cable from bat to body. Then run a cable from starter mounting bolt to body. AWG " 2 " should be fine for the ground since it is a short run.

The engine to body cable is critical, if left off other smaller grounds will be overloaded. Sometimes items like speedo cables , shift cables , bearings become a ground path.

It's some kind of disorder I think. :D If you think putting the battery in the trunk is uncalled for, there's someone putting it under the back seat. Sure, it's gonna be unique. Bragging rights!

Instead of moving the battery, put an additional battery in the trunk. Run a 0 gauge wire from the front battery positive to the rear batt positive, and don't forget to put an inline fuse.

You can make your own ground, just find a good chunk of steel and sand it.

You will add an additional 700 or so cranking amps, and the rear battery will act somewhat as a capacitor to your amp.

I might do that later when I can get this bracket back in and have some cash, I would put an AGM battery in the engine bay (Sam's has a Group 49 AGM battery for $165).

Get 20 feet of cable. I know folks say it can be done with less, but not so IMO. Hop on S T D dot com and I did it. Now, I am going for under the back seat. It can be done, but it's pricey. Only benefit that I can see is more space in engine bay, simpler design in some regards but more issues on the other end. Get a quick release from Anderson. It's nice to un-do the battery with a click. When you route the cable through the back, but a jacket over the cable, e.g., plastic tubing or something similar where it runs thru by the fuel tank. Maybe overkill, but at least you'll have peace of mind with any vibration.

What gauge wire did you use?

EDIT: Never mind, I found it. I think I will go with the 0 AWG since I can't get 1 AWG at Lowes. I still may try to fix this bracket because my friend just told me he is open Thursday to work on it, I can get the rest of the work done by then, and its cheaper to weld than spend $40+ for wire and $20+ for a box.

Lowes usually sells home building wire. This is not what you want to use as the thick strands make it difficult to work with. And, home wire is usually aluminum so you need to go up a gauge to equal copper.

Truck parts stores have the wire you want for battery use or you can use welding cable.

I am not sure that is correct. All wire I have purchased for my home (20-50 amp circuits) has been copper. Do they switch over to Aluminum at those really large sizes?

The 2/0 wire has an aluminum option. Nothing they sell at Home Depot Is ok for this application. You want to go to a stereo shop, truck parts house, or welding supply. You want to find 0 gauge copper wire that is the thinnest strand possible.

I've done several of these in race cars. For the S-10 I bought 25' of #2 welding cable from the welding store that I use. I don't remember the price but it wasn't TOO bad.

I got 25' because past experience has shown that it's best to run the ground back up to one of the starter mounting bolts. I know it seems like everything SHOULD ground thru the frame and sometimes does but I've had too many problems with electrical gremlins when I don't do the Big Ground. Also be sure to ground all components together - body to engine, engine to frame, and if you do the extra ground, battery to engine. You want NO resistance anywhere in the ground circuit.

Dan

You can probably pull some parts from a late model W107, the battery for those cars is in the trunk IIRC.

Pelican Technical Article - Mercedes 450SL R107 - Battery Replacement

http://www.pelicanparts.com/techarti...mall/pic04.JPG

Yea just thought about this, forgot how hard even 6awg wire from lowes is to work with. I don't know anywhere local that has wire like that, no welding or truck parts places besides dealerships but there are some stereo shops. I think I am going to upgrade the grounds in all of my cars anyway so not a bad idea to find it even if I don't move the battery

you will get ZERO additional cranking amps... but you will get additional capacity. cranking amps is a function of the battery design, and it is NOT accumulative. what this will give you is longer attempts to crank the car before it dies... better voltage under load, and faster starts, IF the starter was overloading the battery.

94+ v8 Eclass has the battery in the rear also...

I think greazzer did this on his 123, and is also the guy moving his battery to under the seat!
Check his sleeper threads, im pretty sure he included some pics of what he did exactly.

Cant imagine the exercise would be that different for a 116. Definitely post some pics of what you do, i also need to do this on my 114 as well

To clarify, I should have said home wire above # 4 or so is usually aluminum when purchased at a big box home center. Home entrance cable is nearly always aluminum.

You can get copper in larger sizes at a electrical supply house but it still will be thick strand and hard to work.

If you drive the car backwards all the time the battery will have effectively relocated itself to the trunk, of your now rear engine front wheel drive Mercedes. OK now that would be cool, and you could be the first!

Two batteries in parallel will give more cranking amps, the affect is the same as a physically larger battery.

If a battery has a poor Peukert number is won't like rapid discharge ( cranking ) but will do better over a long draw. Still, it would have to be impossibly bad for a a pair zero gain in cranking amps.

Great thread! What a novel idea, relocating the battery to the trunk!

Please, please don't move the battery to under the rear seat. If you must, make sure it has the proper vents on the battery and car to get the generated gases out side the car. If the gas builds up around the battery, then you get an arch from a bad connection anywhere around the battery, you could get a very violent explosion. I have seen this happen in a Buick and it wasn't nice. If anyone had been sitting in the seat, they would have been hurt/burned very badly. It happened when the driver tried to restart the car after being run for a long time. He had stopped to get fuel. He only received a bad head ache and was very lucky. Car was totaled.
The same thing could happen with the battery in the trunk. Now remember where the fuel tank is in a Mercedes.
Venting of the battery is the important part here. Some batterys have the proper vent hose connections and can be used safely.

Next safety issue is in using 2 batterys, one in the front, and one in the rear. Please put large notes on the batterys saying that there are 2 batterys. Why? If someone goes to work on the car, say the starter or alternator and they unhook the front battery but the rear (unknown) battery is still hooked up, it could cause them great harm.

Please don't dismiss the above and say "Oh, that can't happen." I worked on electrical problem for 28 years professionally and have seen both of the problem happen on more than one occasion.

Paul

If the explosion danger don't harm you quickly, breathing the fumes will harm you slowly in a not properly vented system. It's one thing if done by car manufacturers where stringent safety requirements must be met. Not a good idea unless it's a race car

Before the alarm bell is rang regarding the potential fire ball of death ... take a peek at some newer technology batteries. Relocation under the back seat, for example, involves NO lead acid batteries. They DO NOT fit. They are too big. EarthX, et cet., and the newer technlogy batteries are non-toxic, they don't ooze toxic fumes, or any other of the nasty stuff which LEAD ACID batteries do. Actually, they can be mounted or oriented in any direction, even upside down (some of them, but not all manufactureres).

In regards to a trunk mounted battery, there is ample room on the passenger side rear cargo "pocket" area (or the driver side too). There is NO need to lose your spare tire. No one mentioned this, but the spare tire in a W123 is actually part of the safety features for a rear end collision. The only confusion about a trunk mounted battery is a forum member who claimed and jumped up and down it won't fit. Well, a Group 49 Battery does in fact fit. I had a battery box oversized to permit dense foam all the way around. and it still fit perfectly in the cargo pocket area.

As for all of the anticipated problems, I discovered nor had any of them.

Battery Box Relocation

I had the battery in my trunk, neatly and nicely tucked away in a homemade battery box for well over an entire year. As for charging, no issues. I ran a bunch of electrical odds and ends for my WVO system and still kept the OEM 55 AMP altenator and never a charging problem, never a starting problem, et cet. My car got totaled and that ended that until I got a new MB. The only thing I would do different is the following:

1. Thicker cable, next size up
2. Make sure you try out some Anderson connectors
3. Get 20' feet of cable and don't sweat being short
3. Think about under the back seat. Very pricey compared to a lead battery, but it neatens up the area. I initially said no way, but I was 100% wrong on that. I had to search outside of traditional batteris. The folks at Odyssey, EarthX, and a few others really know batteries, their safety issues and considerations, et cet.

Does this mod HAVE to be done ? Of course not. It's not necessary at all. But, if you are doing mod's which require turf be cleared out, then it's an option.

Also moving the battery to the trunk would make it much less of a pain to take out from the bay, which takes much fidgeting and annoyingness.

I would do the same if there werent giant puddles of water forming in my trunk after it rains.

Yes, access is simpler and more easier. Just like moving the oil filter housing ... easier for clumbsy folks like me ! :)

It's not a new idea. For drag racing in the sixties we used a marine battery box and mounted it as far to the right rear as possible to help traction. We used cable
Barely he y enough so as to save weight. For street purposes however, use HEAVY cable
For such a long run. No lighter than 0 I would think.

you old Greazzers ... :D

I have decided not to bother with the battery relocation idea for now but may try a few things in the future. Once I looked them up I realized I have plenty of Anderson Connectors from winches, they really are nice to have. My idea instead of moving the main battery to the trunk is either one of the sealed batteries under the seat or in the trunk or maybe even a bank of capacitors under the rear seat (some people use them as a supplement to a battery). Sam's sells an AGM Group 49 battery for $167, might be the best bet for a battery in a sealed environment.

Got it welded up last night, the battery is staying in the front for now. Hoped the ground was what was causing other issues in the car like the tach, fuel gauge, and antenna, but nothing changed. I checked the antenna that had been working when we bought it, voltage is going to it and the housing is hot. The PO had the mast replaced a few months age but now it seems the motor isn't working.

New V6 Passats have the battery in the trunk. fWIW there is a huge fuse (1200 amp?) on the battery cable.


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You're in the wrong car to consider weight distribution. Save your money for something more useful.

noone else seems concerned with this, but the best reason to do this mod in my book, is to clean out the engine bay.