batteries in parrallel

[papamike]

i have one deep cycle battery in my boat now deep cycle 3 yrs old, i buying another one this week im wondering if its ok to wire them in parrallel or run one down and switch to the other? both are the same size but different brands they are used to run tm only, maybe spotlights come bowfishing season. any advice would be appreaciated, thanks

 

[Georgesalmon]

Re: batteries in parrallel

You can do either if both are good batteries. I guess I'd put them in parallel.

 

[papamike]

Re: batteries in parrallel

thanks, i wasnt sure if a new fresh battery would somehow affect the old one and make it burn out faster. i had a freind put a new battery in his rv with an old battery and the newone melted, he must have hooked them up wrong. thanks again.

 

[Georgesalmon]

Re: batteries in parrallel

I'm think'n that could happen if his old battery had a shorted cell. The more charged battery will charge the lower battery until they are both at the same potential. Once they even out they will act like just one bigger battery.

 

[Grandad]

Re: batteries in parrallel

I don't think I'd parallel them. There's no advantage that I can see. You still have the same stored energy whether they're working one at a time or twinned together. I wouldn't risk a good battery if the other decided not to play nice in the sand box. Bite the bullet and buy a switch that will allow you to switch between batteries and yet will still allow a parallel switch position should both batteries get exhausted. (OFF - 1 - 2 - BOTH) Any voltage imbalance between batteries in parallel will add a current flow between the batteries. When one fails completely from, for example shorted plates, the other will be called on to supply "short circuit" current levels and will deplete quickly. I like knowing that with a switch isolating my batteries, I've always got a fresh reserve if there's an engine problem or a battery failure. I rotate their use each time I'm out, sometimes every couple of hours, just for reassurance. - Grandad

 

[texasatm2011]

Re: batteries in parrallel

I have a similar situation except that I want to use both batteries for cranking. I have a '77 Johnson 140hp motor that is a little temperamental at times.

I usually carry an extra battery but it is a pain to have to switch them out when I'm bobbing up and down in the middle of the lake!

I would like to install a switch like you suggested. With this set-up can I switch between battery 1 and battery 2 while the engine is running or does it have to be off?

Thanks for the help!

 

[low277]

Re: batteries in parrallel

I don't think I'd parallel them. There's no advantage that I can see. You still have the same stored energy whether they're working one at a time or twinned together. I wouldn't risk a good battery if the other decided not to play nice in the sand box. Bite the bullet and buy a switch that will allow you to switch between batteries and yet will still allow a parallel switch position should both batteries get exhausted. (OFF - 1 - 2 - BOTH) Any voltage imbalance between batteries in parallel will add a current flow between the batteries. When one fails completely from, for example shorted plates, the other will be called on to supply "short circuit" current levels and will deplete quickly. I like knowing that with a switch isolating my batteries, I've always got a fresh reserve if there's an engine problem or a battery failure. I rotate their use each time I'm out, sometimes every couple of hours, just for reassurance. - Grandad

Good advise! +1

 

[Auger01]

Re: batteries in parrallel

Batteries should be of the same make and age if they are to be left paralled for any period of time. Batteries of different ages could have different rates of self discharge.

If you have a 1-2-both-off switch, then it wouldn't be a problem as the batteries would be seperated with the switch off.

 

[UncleWillie]

Re: batteries in parrallel

I would like to install a switch like you suggested. With this set-up can I switch between battery 1 and battery 2 while the engine is running or does it have to be OFF? ...

You can switch between batteries 1 and 2 but ALWAYS switch 1 > Both > 2, and NEVER 1 > OFF > 2.
Turning OFF the Battery switch for even an instant with the alternator turning could produce fatal results to everything electrical. Some alternators will go out or regulation. You might get Zero volts, or you might get 100 Volts delivered to all your 12v equipment.