SI Board Whoes - have I bought the right batts

[atw323]

Hi Chaps,

I`m getting confused ( it`s not hard) I have a 1990 325i cab, the temp guage rises up to red hot in about 30 secs with my engine running so it`s not the engine, the tacho occasionally gets the jitters and the econometer sometimes works erratically and the service lights occasionally flicker on whilst the engine is running.
Logical conclusion is that the SI board on kaput, so I pulled it out and it has the later lithium batteries not the old rechargable Ni Cads, I understand the Lithium ones are usually reliable.
I have assumed the lithium ones are also rechargable like the Ni Cads and after much searching I couldn`t find any rechargable lithium batteries and so went for rechargable Ni Cads of the correct size to replace the Lithiums. ( which I`ve ordered for Ԛ£10 and not fitted)
I find it hard to grasp that the two exisiting lithium batteries have run for 16 yrs without replacement however have I c**ked up and should I have gone for the non-rechargable lithiums instead.

P.S. I`m new to E30 ownership and I`m loving it!

[dsio]

It takes a cheap set of non rechargable AAs and a 0.10c battery holder to do it really. A set of standard alkalines last around 3 years. I have a cheapo set of NiMh AAs in my battery holder, which is soldered to the SI board, comes out the back of the cluster, and ends up mounted under the steering column. So any time I want to change the batteries, I just have to take the under steering column cover off, and change them over.

Also have a switch on the pack, so if I need to reset the SI lights I just flip the switch.

It is really not the rocket science that it is made out to be.

[Brianmoooore]

The fundamental difference between the old boards with the Ni Cds, and the later boards with the lithiums, is that the early board runs from the NiCds all the time , which are charged by the car, and in the later board, the batteries are only there to maintain memory when the car battery is disconnected, or the board is removed from the car. There is no charger circuit on the later board.
Replacement boards of the later type have a switch in the form of a jumper link to prevent battery drain before the board is used. Factory fitted later boards don't have this jumper, although there is a place for it on the PCB.
The problems caused by the batteries in the early board are caused not so much by the batteries losing their storage capacity, but by the fact they become short circuits, and load down the power rails on the PCB.
Can't see the point of bodging on external batteries for the early board, as replacing the dud NiCds with a couple of NiMH batteries should keep the board going for the remaining life of most E30s.

With your board, I would only fit the correct lithium batteries, but stick a voltmeter across the originals first to see if they are still giving their 3V output. I doubt that your problems lie with the batteries, because, as I said earlier in this post, they don't actually do anything under normal working conditions.

[Blackie_No1]

The Haynes manual say's that faulty vatteries on the SI board can give faulty tach/temp readings and the inabillity to turn of the service lights

[dsio]

If the batteries go short circuit, they cause a current drain, and as a result, the tacho and temp goes funny, if they go open cirtuit, the lights stay on, or flash, but the tacho and temp are ok. Unless batteries have done damage that is.

The thing with lithiums is they are extremely heat sensitive. Lithiums that are at 10C will last a much longer time than lithiums that spend their lives at 50C for example, and the higher the temp goes, the worse they get. Thats why you dont leave laptops in cars, because the battery will be destroyed.

That is also why I would be inclined to use a pair of heavy duty AA alkalines in a changable battery holder than new lithiums. But hey, its up to you.