Adding another battery

[cyrilslugg]

What does the team think?
Under pressure from SWMBO, I've just bought a new battery for the car. The old one is tired but not dead, although it will fade away if left unused for a month or so. Since I've got quite a lot of instrumentation and an autohelm on my boat, I thought I'd increase the umph and connect this 'spare' in parallel to my existing battery (both 55AH). A stupid/good idea? Anything I should be careful about?

 

[alan17]

You are likely to find that the tired battery will drag down your other battery/batteriers. Conclusion BAD idea

 

[jeremyshaw]

I agree with Alan17. Also are both batteries of the same type? The car one will almost certainly be a cranker whereas the one on the boat may be deep cycle. If the boat one is a cranker then it's probably all you need. If it is deep cycle then adding a tired car cranker will just make it cranky too!

 

[cyrilslugg]

Thanks guys. Point taken.
I'll keep as an emergency standby/ballast!

 

[William_H]

Yes fit the spare battery if that is easy. Put switching in so that you have either main or spare battery on but not both. It is probably OK to charge in parallel but discharge one at a time. olewill