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BBC battery,


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#1
samplerdave

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One for the greybeards.

 

Many suggest that when replacing the CMOS battery on the BBC Master you should use three alkaline AA cells in series. Then add a diode and resistor in the line to prevent charging.

 

However, is there any reason one can't just use a single 4.3 Volt Lipo cell and retain the charging capability ?

 

Thanks,

 

Dave.


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#2
Kemasa

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Yes, unless it was designed to charge that specific type of battery, bad things can happen. The battery might be able to protect itself, but perhaps not.


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#3
samplerdave

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Thanks,

 

I'll look further into just which Lithium battery was used.

 

Dave.


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#4
samplerdave

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Just checked the battery to motherboard connector.

 

Black (neg) lead has resistance of 98 Ohm in both directions.

 

Red (pos) lead has 1.6 MegOhm one way, infinity in the other.

 

So it appears that it already has a resistor and diode fitted.

 

I'll take the easy option and just fit three AA Alkaline cells.

 

Dave.


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#5
Kemasa

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You need to make sure that it doesn't try to charge the non-rechargeable batteries. Your measurements doesn't ensure that.


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#6
samplerdave

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The previous measurements showed no current flow down the positive battery lead against the battery . I,e the diode was blocking flow.

 

However, the situation has changed.

 

I decided to measure the voltage at the motherboard socket .( the connector to the battery was disconnected and on the bench along with the battery carried.

 

Whilst I was getting the VOM ready, there was a loud bang and a cloud of smoke. from the power supply.

 

So, looks like I'm in for more work.

 

Now I have to find a circuit diagram for the Master.

 

Thanks again for the help so far.

 

Dave.

 

(This machine hadn't been turned on for around five years)


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