Charging external battery

[Bend]

What are the pro/cons/concerns?

I want to charge my trolling (marine semi-deep cycle) battery (80 AH) using solar in the afternoon after its been discharged 30-50%. House batt SOC would be 90% or above. Would just adding a pigtail/jumper cable to my house batts work? Solar max is about 17 amps, trolling batt would external to the house batt box but near.

[Bend]

Thanks all.

I currently have my solar bypassing the master battery switch. If I include the solar in the master switch, I could dedicate the solar charging system to the external battery via pigtail when the master battery switch is off. Therefore, not affect the house batts. I would lose the refrigerator, small inverter and parasites for those few hours. 1 quart of ice would compensate in the refrigerator. Or, I could add an additional switch to temporarily cut solar to the house batts but still power the pigtail. Might confuse the Trimetric either way.

Time to buy a digital amp meter to check the ways.

[MEXICOWANDERER]

Afternoon does not mean crack of dawn. 5-7 amps for 3 hours is 21 amp hours max. This is hypothetical balderdash and has nothing to do with reality.

If the panel is switched and dedicated, UNCONTROLLED directly from the panel to the battery the battery will have a chance to enjoy a modest snack.

My opinion is based upon actually doing this and not on theory. Try four feet of eight gauge jumpers and 50 amp solid copper Mueller clamps.

A better idea is to charge with a DC to DC converter. Huh? Yeah, Huh! it's going to rob some kWh out of the main house bank but hey what are friends for?

[time2roll]

At 90% charge is your existing voltage still at 14.2 or higher? If so it will work well. If you are down at 13.6 or lower then it may not amount to much.

Could use an inverter and 10 to 20 amp portable charger but you will lose close to half the energy you pull off the house/solar.

Or if you go boating every other day then the boat battery can finish the next day. Or a second boat battery to have one on solar and one in the boat to swap each day. Price of free solar just keeps climbing 😉

[KJINTF]

X2 on RJ's comment

I do it all the time - similiar but have an extension cable RV<>Boat - Two GC 6Vdc batteries for trolling with the Ipilot enabled Terrova - been doing it for many years while camping and fishing - way better solution that firing up the generator

Works great - My PV array gets me in excess of 40 amps when in direct sun. Yes sure the 75ft 8awg extension cable from the RV to the boat has a voltage drop but it really does not matter - done with fishing for the day plug her in and enjoy a cold beverage - Good to go another day - I additionally added a switchable system to charge the trolling batteries from the Yamaha F90 too bad the F90 only puts out 25amps and uses almost 9 amps just to run the engine

Enjoy the fishing

PS - Never bothered to check the charge current best guess the batteries get a good 10 to 15 amps for a short time when really discharged - The voltmeter in the boat reads 14.4V after a short 30 minutes or so

[RJsfishin]

One telling it will charge slow, another telling it mite charge too fast. First of all, it Won't charge too fast, trust me !

In reality it will work just fine,....or just as fine as my house batts charging off the alternator when the chassis batt is full charged.
I've done it lots a time when I spend the morning fishing. Paralleling to the house batts is the way to do it. I always used jumper cables, and it charged quite well.

[wa8yxm]

Use a clamp on DC ammeter the first time you try it.. This way you can tell if you are charging the marine battery too swiftly (more than 24 amps) if you are then you need to put a series resistor in it, you need a good Positive Coifiecent Resistor so that as the current drops and the resistor cools the resistance goes down.. say a couple of 12 volt 100 watt light bulbs in parallel.

[MEXICOWANDERER]

It's going to charge s-l-o-w

Expect a 4-8 ampere charge rate to start off with then from that point it gets sadder.