My 01 CTD has 2 batteries and both of them are weak. Each one will start the engine when it is warm, but only one will start the engine cold. If both of them are hooked up, they will start the engine cold. My question is this, is it ok to replace the weaker of the 2 batteries now or do I need to replace both of them at the same time?

Will a weak battery drain a new battery? Could this be what is happening now? I mean do I have a good battery and a bad battery and the bad one is draining the good one? Or do I more than likely have 2 bad batteries?

 

you need to replace both at the same time, the weak one will only cause the strong one to give out quicker.