Alternator "charge" wire

[E34-520iSE]

Hi All! I've just installed a split-charged second battery under my rear seat to power a portable fridge(refrigerator in USA). I have installed the correct gauge cables, fuses and split charge relay, to work at a 30A max load. I need to find a wire that comes from the alternator "charging" pin. Is there one under the rear seat or a wire that I can splice into behind the dash? If so, what am I looking for? Anyone got a wiring diagram of the charge circuit?

Cheers!

Shaun

[E34-520iSE]

Hi! I found a really useful site full of info and wiring diagrams. My problem is almost solved - now I need to find the unloader relay - it's at the front SOMEWHERE! Have a look at http://bmwe34m5.com/faqs/?theme=1&le...nt_id_select=3

Cheers!

Shaun

[ukm5]

i have second battery installed in my boot as i have too many black boxes under my seat, phone etc - but i struggled with having my alternator charge both batteries at the same time, i had alsorts of issues, like it would only charge the back one or only charge the front one...however FYI i attached the alternator wire at the back of my speedo, i cant remember the wire though sorry
theres a link somewhere on here i think to the electrical circuit
i seemed to remember BMW did an uprated alternator for the 750il as it had a lot more electrics to power...maybe thats the way to go when running 2 batteries
from my knowledge the split charge relay is just a relay, it doesnt work out which battery needs charging and direct power more to one than the other so if both battaries are connected together with this relay perhaps the electronic guys can confirm this...if the batteries are not equal charge does one zap more than the other? also does one discharged battery pull charge from the other? etc...
i just never seem to get it to work rright so ive dissconnected the split charge and every now and again manually recharge the second battery, its only there incase my other goes flat, i can flick a switch and heypresto car will start again

[E34-520iSE]

Thanks for the reply. I've poured over a downloaded wiring diagram for about two hours today, to find which wire goes into the back of the dash to switch on the relay. I'm looking for a blue wire going to the dash cluster, pin 5, 14 or 16. I'll have a look tomorrow and find out exactly which one it is. The second battery now under my seat is fused to draw a maximum of 30A. My reasoning is that it can't draw large currents away from the main battery without blowing the 30A fuse. I will keep an eye on the charge of both batteries with my multimeter, just to make sure the main battery stays fully charged. I am toying with putting a bypass switch and the gubbins of an LED battery tester in the overhead console. This means I can charge the second battery manually when the red LED is lit. I have some nice round backlit switches here that I bought from Maplin to use in the overhead console too. Should look pretty when it's all installed!

Cheers,

Shaun

[ukm5]

no u miss understand, its never going to draw 30 amps even if its flat!
and also i think its just how you wrote it but you wont be able to check charge on each battery while its running as at that moment both batteries are connected together and will have a combined total becuase of the split charge relay connecting them together.

the issue is that if you have 2 batteries, and say one is run down, lets say the back one is run down, the alternator can only pump out so much power, so in theory the run down one will suck the power that way...so the power majority goes to the dead rear battery...however the front battery is used to start the car...but that loss isnt replaced because its busy charging the rear battery..allowing the front battery to run down, eventually u get to a point where the rear battery overtakes the front on charge level, now you have 2 semi charged batteries where the alternator needs to charge both at the same time, which it struggles to do unless u only run the car in daylight with no other lights etc draining the charge and drive for long distances.

that seems to be my problem i end up with neither battery propperly charged...which is why i suggested the uprated alternator which has more power output like on the 750il

i may be wrong on the reasons, but i do know it struggles to charge 2 batteries successfully on standard journeys, ie average less than 30 mins

[E34-520iSE]

I think I could have described my "overhead switch" idea a little better!! If I switch it off, this will break the "charging" signal from the alternator to the relay of the 2nd battery, isolating it completely. Then I can tell at a glance whether the 2nd battery is fully charged or not, using the LED's and gubbins from a cheap battery tester. If it's fully charged then it can stay out of the circuit. If the red LED is lit (battery flat) I can switch it back into the charging circuit. What do you think about that? It's great to discuss the problems now before I finish the install.Oh, and I never bought an overpriced relay for this project. I just used a standard 30A Bosch relay from an E30 - cost £0!

Cheers,

Shaun

[Javier]

the basic principle is that alternator has a limit. If you draw from the electric system more power that the alternator can produce, you will be discharging your batteries, one or both depending on the way you connect. You can have a hundred batteries in the car, they will be useless unless you PRODUCE with your alternator, enough electric power to run the car and its accessories.

Think on the batteries as a temporary deposit. you can draw the power when you need it, but if you do not replace it, you will be discharging them. The only benefit added with an extra battery, is the handling of peak loads, as long as long-term average load is less than alternator capacity.

Javier

[E34-520iSE]

The only benefit added with an extra battery, is the handling of peak loads, as long as long-term average load is less than alternator capacity.

Javier

That is my exact plan mate. I realise that I can't charge 2 batteries 100% of the time from an alternator that only generates 90A. My plan for the second battery is this:-
Make sure it is fully charged, by adding it into the circuit during daytime running when necessary. It is ONLY going to be used to power a portable fridge when the car is parked up and the engine is switched off. I am not expecting to draw large continuous currents from it. By having a state of charge display and an isolator switch in the car I can tell at a glance the state of the battery. I would think this battery will stay fully charged for 5 days a week because of light use. If I was going to demand a lot from it I would have used a blocking diode instead. The fridge will only be used occasionally, primarily to keep my baby son's bottles cool on trips to the park etc., as well as soft drinks for us.

Keep the ideas coming in guys, they're greatly appreciated!


Shaun

[ukm5]

The only benefit added with an extra battery, is the handling of peak loads, as long as long-term average load is less than alternator capacity.

Javier

i think u have misread why i and the other user want 2 batteries - i use it as a back up incase my main one goes flat, perhaps left the lights on etc... the other guy uses it to power a fridge without draining the main battery

[Paul in NZ]

i had a second battery too.It was dead flat,the previous owner denied the car even had a second(auxilary) battery.It was only a 25 ahr battery tho......I dont know what accesories fitted made the fitment of the auxillary battery standard,but i have had no battery charge issues.