Battery charging question

[Bob V]

I carry a pair of Optima Deep Cycle batteries with me when I cruise to power my electric dinghy motor (or the TV) depending on supply/demand. These two batteries are hooked up in parallel. My house bank consists of three 4-D batteries that are used with the switch in the "all" position to minimize draw down. I also carry an Optima Starting battery just in case I have a problem that draws down my power overnight beyond what is needed to start the engine.

When I am on the hook my recharging comes from 260 watts of solar panels that supply an excess of power during the day. When I need to recharge the dinghy batteries I plug in a 120 volt charger to the inverter. This works ok but I wonder if there isn't a better and simpler way to get some of that excess charge from the house batteries into the Optimas.

It seems like it doesn't make a lot of sense to use an inverter to make 120 v from 12 v and then a battery charger to make 12 v from 120 v. I know there are 12 volt chargers on the market but I had something even simpler in mind. My plan is to use a 12 volt outlet of the heavy duty variety which is already at my transom to power up an electric downrigger and supplied with rather heavy wire of a short run since the downrigger is a big draw when the electric motor is lifting a 12 lb ball up from the depths. This line is hooked to the battery buss directly with an inline fuse. I would need another male plug for the outlet and about 6 feet of heavy wire (#10?) with a large pair of aligator clips on the end. With the dinghy at the transom while we are at anchor I could plug it right in without removing the batteries from the dinghy.

What I would be doing essentially is hooking up a small pair of partially discharged batteries (in parallel) to a large trio of fully charged batteries that are receiving a charge from the solar panels at the time. There is a charge controller in the circuit between panels and house batteries.

Is there anything wrong with this plan assuming that I do not plan to leave the Optimas connected this way long term and they will receive proper charging form my 120 v smart charger when I am at a marina or back at the home port.

 

[sailingdog]

The main problem with doing this is that the house batteries don't have enough voltage to properly charge the Optimas. Why not use the solar panels, via an echo charger or duo charge unit to charge the Optimas, essentially treating them as a second bank. However, one issue you may have is what charging profile you're using. If the house batteries are wet-cell or gel, the charging profile may be wrong for the AGM Optima batteries.

To handle the wiring and make it simple to connect/disconnect the Optima batteries, use a trolling motor outlet.

 

[Anchor Down]

Agreed: charging directly from the panels is the most elegant. Maybe set up so that one of the panels' output can be diverted to charge the Optimas directly, then disconnected from that circut back to throwing its output at the house bank? Or both, temporarily? That would just involve a switch downstream of the panels, plus whatever monitoring you want.

If you're going to be on board to monitor it manually, something simple like that would work and avoid the efficiency losses of DC/AC/DC conversions/inversions.

 

[Bob V]

The problem with using the panels

is that to do that I have to switch off the charge to the house batteries. My charge controller only has one output which is now going to the three 4-Ds hooked up in parallel. The panels see that as one big battery. I probably should have bought the more expensive charge controller or the echo charger as Sailingdog suggested.

As inefficient as the 12v/120v/12v setup that I presently use is, I still have over 13v at the battery terminals on a sunny day when the 120v charger is plugged in. I want to put as much back into the house bank as possible during the daylight hours on those occassions when I want to stay put for a few days.

I think if I want to invest in the equipment to do it right, another solar panel that I clamp to the stern pulpit and a second controller is the way to go. I could stow the third panel whenever I was underway and then I could even keep the boat plugged into the third panel back at the homeport dock and just retire the 120v charger altogether. Now that would be elegant and of course expensive.

Thanks for the feedback guys. I was thinking that my low budget plan left something to be desired. Alternately I could use the oars more often and the tv less. That would have benefits for my battery budget, my body and my mind as well. It would also leave a big surplus to use my anchor light which I have not been doing when in a designated anchorage.

Or maybe I could put a Honda generator on the swim step and crank up the volume on the tv to cover the noise when I'm watching the "Tonight Show" or "Saturday Night Live" like some of my "neighbors" do.