Elektrik: zwei Batterien im 750i - Fragen

[Christian]

Heute erhielt ich nachstehende Anfrage. Vielleicht könnte sie jemand (am Besten direkt in englisch) beantworten:


Hello Christian!

I hope you are doing good. I found your contact information on 7er.com and I hope you could guide me to the right direction.

It seems my 750 iAL is not well known by local BMW dealers in Finland because I have not got a good explanation on how the batteries are connected and what they are for etc. I bought the car from BMW Niederlassung Munich two years ago. (I think it's manufactured in 09/2000 and I guess that means that it should be model year 2001....but it's missing something which should belong to BMW 750 iAL 2001 models and that missing feature is the remore control for the Webasto).

- Why two batteries? Is the smaller one for the Webasto only?

- Are they connected in parallel or are they "totally" independent on each other? The user manual says that there is a switch between them!

- (If they are independent) could I only charge the bigger one with a battery charger? Of course the question would remain how the smaller one would get charged? Perhaps by the generator when the engine is running?

Another thing is that my car is standing typically for 2...3 months at a time when I'm out of country and the batteries may become bad during that
time. I have considered to buy batteries which survive such conditions, like the ones manufactured with AGM technology (gas-recombinant absorbed glass mat). For instance, Optima is one I have thought of. However, the case may be that there are no AGM batteries which would mechanically fit into the available space (inside rear right fender)? Have you anything to say to this?

Best regards and looking forward to your advice,

Antti Suoniemi


Vielen Dank,
Chriss

[schneeflocke42]

hi antti,
I am no expert in electronics but when I asked the same question, my dealer gave an answer that made sense to me: because of the many additional gadgets in the 750, the 750 has two batteries being charged together, but several users - like the webasto - which could cause servere performance loss use / are connected to the smaller battery only. the switch protects that the main battery is touched at all (one way).
as the space for the batteries is very limited, I think you'd better recharge the batteries through the + and - pols at the engine (those for jump start) regularly. that will keep them in good shape...
cheers,
r.

Zitat von Christian

Heute erhielt ich nachstehende Anfrage. Vielleicht könnte sie jemand (am Besten direkt in englisch) beantworten:


Hello Christian!

I hope you are doing good. I found your contact information on 7er.com and I hope you could guide me to the right direction.

It seems my 750 iAL is not well known by local BMW dealers in Finland because I have not got a good explanation on how the batteries are connected and what they are for etc. I bought the car from BMW Niederlassung Munich two years ago. (I think it's manufactured in 09/2000 and I guess that means that it should be model year 2001....but it's missing something which should belong to BMW 750 iAL 2001 models and that missing feature is the remore control for the Webasto).

- Why two batteries? Is the smaller one for the Webasto only?

- Are they connected in parallel or are they "totally" independent on each other? The user manual says that there is a switch between them!

- (If they are independent) could I only charge the bigger one with a battery charger? Of course the question would remain how the smaller one would get charged? Perhaps by the generator when the engine is running?

Another thing is that my car is standing typically for 2...3 months at a time when I'm out of country and the batteries may become bad during that
time. I have considered to buy batteries which survive such conditions, like the ones manufactured with AGM technology (gas-recombinant absorbed glass mat). For instance, Optima is one I have thought of. However, the case may be that there are no AGM batteries which would mechanically fit into the available space (inside rear right fender)? Have you anything to say to this?

Best regards and looking forward to your advice,

Antti Suoniemi


Vielen Dank,
Chriss

[Sinclair]

I have a slight different view, please correct me, if anyone knows better:

0. car is not started, but only measurements are done
1. if I measure voltage across the motors' jump start poles, I get 12.8 V
2. if I measure at the cigarette plug inside, I get 12.1 V
3. if I start the car, after 3 sec the system voltage ramps up to 14 V
and the smaller board battery is charged, too.

I conclude:
a) at point 1), see above, I measure the larger starter battery
b) if the car is switched off, the two batteries are not connected via the switch, they are electrically separated.
c) the smaller board battery on-top (see point 2), is charged during normal running, together with the larger starter battery, using the automatic interconnection switch, then closed.
d) if one wants only, to recharge the smaller board battery (70Ah), then it is usefull, to connect the external charger directly to the bord battery. I use an additionally build-in 12V connector to that battery (where I otherwise connect e.g. a cooling box), the additional connector is 16A-fused for safety.
But be careful, to stop "constant current charging process", when 14.2 V are reached. I recognized a buzzer, when voltage is higher than charge end voltage ?
e) I'm not sure, if by charging the large starter battery (110Ah) using the start poles at the engine compartment, that the smaller board battery will be charged two, because the separation switch seems normally opened, when the car is not running.

[AnttiS]

Hi!

You may be right that the smaller battery is normally only charged while the engine is running. If we assume that the smaller one is connected to the cigarette lighter I would need to have two chargers to charge both batteries at the same time. And I would also need to have a cigarette lighter plug type of connector for my charger. I have now CTEK charger which should have all the necessary safety measures but I'm missing the right type of connector plug.
I have also noticed that the cigarette lighter is always connected to a battery (to a smaller one only?) i.e. even though the car key is in off position. I'm not sure if this is so in all cars?

Further, I would still like to hear thoughts of what type of batteries I could choose because my car is out of traffic for long periods and I would think that AGM batteries would work better in my case. However, it's not necessary easy to find mechanically right size batteries. I have heard that some battery manufacturers provide "fitting pads" to fit their batteries to various makes/models.

Brgds,

Antti

[RS744]

Hi Antti,

Zitat von Antti

- Are they connected in parallel or are they "totally" independent on each other? The user manual says that there is a switch between them!

True, there is a switch between the two batteries. The switch is activated as soon as you start charging (i.e. >13,5V).

Zitat von Antti

- (If they are independent) could I only charge the bigger one with a battery charger? Of course the question would remain how the smaller one would get charged?

You can charge both of them: according to the BMW TIS you should use the starting points at the engine, and the E38 750 has two different starting points under the hood:
the starting point (B+) at the front of the engine can be used for charging the starter battery (110Ah), and the other starting point (B+) at the left wheel house (rear of the engine) can be used for charging the other battery (55Ah).

Zitat von Antti

... how the smaller one would get charged? - Perhaps by the generator when the engine is running

Yes; as soon as the engine is running the generator is charging both batteries.

HTH. As I do not have two batteries I have no practical knowledge whether this works as described in the TIS.

Regards
RS744

[Sinclair]

for this job I can only guess two possibilities, I have not done this already:

1) ask BMWs inside technicians, who can assure, that charging at the starting poles in the engine compartment will charge both batteries at the same time (I'm not sure if this works !).

2) if 1) cannot be answered definitely, do this by yourself-technique:
a) take the CTEK charger
b) connect minus to chassis Ground
c) split-off its plus-output by two resistors of 2.2Ohm/5W (cement type),
- connect one resistor "output" to the plus of the start pole in the engine
compartment, which charges the large starter batterie
- connect the second resistor "output" to the plus of the cigarette lighter
which charges the smaller bord batterie
d) limit the voltage output of the CTEK to about 13.2 V
e) if you apply this method, one should be shure, that both batteries may be held at e.g. 13.2 V for your long "winter sleeping time", without gasing effects.

[Sinclair]

to RS744: interesting ! I have not recognized so far, that two plus poles are available in the engine compartment of the 750i.

to Antii:
if the two plus poles can be found, you have simpler wiring, using my proposal before, you need only access at the engine site, but I still recommend to use the two resistors for slight decoupling the simultaneous charging processes for both batteries from one CTEK charger.

[AnttiS]

Hi All!

It was also a new piece of information to me that there should be two plus poles in the engine compartment. This sounds logical but the manual does not mention about it. Unfortunately I'm unable to verify this for the next couple of months.

I would be happy if somebody out there would try to check this because I'm eager to know the final truth if any.

[Sinclair]

@Rs744
could you please identify the two plus poles, using your TIS ?
thanks...

[RS744]

Zitat von Sinclair

@Rs744
could you please identify the two plus poles, using your TIS ?
thanks...

Hi,
k.A.
Vorne auf dem Bild hast Du einen Batt.+ - Pol, und am Dom einen Batt - Pol. Mehr kann ich da nicht sehen. Im TIS (RA E38 6120) gibts kein Bild dazu, und ich habe keinen Fuffi.
Kann Dir da also nicht weiterhelfen beim Suchen. Siehst Du denn keine zwei Batt.+-Pole bei Dir?
Greets
Rs744