School me on dual battery setup

[tonyjh63]

Ok, my boat is having a dual battery setup installed as I write this. Now, my plan is to crank the boat on both batteries and when we pull into a cove and turn on the music, switch to the #2 battery (deep cycle). Switch back to both batteries when cranking back up. My question is, since some folks use only the cranking battery to start the boat, does the second battery get charged even if the switch is just on the first battery? I would think not, but just wanna make sure that to charge both batteries, the switch needs to be set on "both". Do I have this correct? Thanks!

 

[sam am I]

When just running a simple "0ff/1/Both/2" switch

does the second battery get charged even if the switch is just on the first battery?

No

I would think not

You're correct

but just wanna make sure that to charge both batteries, the switch needs to be set on "both". Do I have this correct?

Yes to charge both at the same time and of course, if you choose, just to "2" to charge "#2 deep cycle" by itself......

 

[tonyjh63]

Thank you, sam i am!

 

[sam am I]

Quite welcome............Also, do realize if you discharge #2 too deeply (it happens) then start up on #1 and then switch to "both", if your alternator current capacity is exceeded by the #2 battery charge load, start battery #1 will begin to dis-charge back into #2 battery. This typically is where you'd want to make sure you switch solely to #2 to charge it and at the same time, to preserve the charge in #1 which is typically the point of using battery isolation (see below).

A good/easy/quik way to know #1 is discharging into #2 when switched into "both" from #1 just after'ish startup, is to monitor the volt gauge on your dash. When switched into "both", If the voltage meter drops to say less then 12.5VDC there is a good chance your fully'ish charged #1 is discharging into dead'ish #2. Simply switch over just into #2 until #2 is say up to 13.5VDC then switching to "both" will be best practice in those cases.

I run one of these in combination with a "off/1/both/2" switch and in this case I just described, it prevents this automagically as it goes into a priority management mode where it isolates/perseveres the full battery (#1) and routes the available current (however much, I only have 40 amp which, at times, does/can get exceeded even with lightly/moderately dis-charged house deep batts) where needed (#2 in this case) as opposed to other systems that have to always parallel (both) in order to get charge to #2.

I just leave my switch on start batt #1 and forget about the two deeps and how low they might be. I run a 3 battery setup (1-start, 2-deep w 40 amp alt) and it works very well for my preference/needs.