what could it be? i am stumped
 

I have what I believe to be an electrical or battery problem happening. I am running 2 optima blue tops. One is older and won't hold a charge, so I usually start on the same battery then switch to all. Lately right after I turn the motor off, its like the good battery has gone completely dead. No turning over, nothing. I usually just wait a couple of minutes and after some frusteration it seems to come back with enough volts to start. Any ideas what may be going on? I am totally stumped.

I'm no electrician or battery expert, and someone correct me if I'm wrong, but won't that bad battery drain that good battery faster? Maybe instead of switching to "all" you should just experiment with the good battery and keep it on that switch. Start it on the good battery and keep it there while you're out. Once you've shut it down, see if you can start it right off the bat or still have to wait a few minutes. Also, running your switch on "all" will put more strain on your alternator, probably even more so if you've got a battery that doesn't charge. I'm sure others will chime in who have way more knowledge then me.

Moon

have you tried pulling the batteries out and haveing this tested?

x 2 Get them tested.

Try and take them to a Sears Auto or somewhere where they can load test them. You want to see how the battery is responding under this condidtion.

That is the only well to tell if you need a new battery.

If you have 2 batteries and switch them in parallel with each other the voltage between the two will equal out. So you don’t want to be paralleling a 10V and 12V battery…especially if one of them is not holding a charge (probably a bad cell). The good battery should in a sence recharge itself after sitting, which allows you to start the motor. Just run on the one battery and don’t switch to ALL until you get the bad one replaced.

you have an old battery that won't hold a charge and you are stumped ?

I am stumped as to why that would make my other battery act the way it is since im not starting on the weak battery. Thanks for the helpful responses. Gonna buy a new battery tonight.

Ryan, when you have batteries in parallel (switched to all), they will only charge to the battery of the lowest voltage. So, if you have a bad battery that, for example, is only putting out 10v, that's all you're gonna be able to charge the batteries to.

Add to that that the bad battery and good battery will try to equalize and you can see where the problems can come from.

As suggested just run the good battery and see if your problem goes away. If it does, you have eliminated the problem. If it doesn't, keep looking.

You could also find that your good battery has a shorter life since the bad battery is keeping it from getting a full charge.

The bad battery is robbing the good battery, simple as that.

Take them both out and have them checked. If the good battery is still 100% then you can get away with changing out just the one. However, if it has been compromised even a little you should change them both. This is if you intend to run them in parallel.

your batteries
 

replace the bad optima, or replace both at the same time.

Replaced the bad battery and the problem seems to have gone away. I didn't have time to get it tested etc as I am going on a trip tomorrow with the boat. Thanks for the help everyone