Charging external battery

Keep us informed as to how it works out
Loads of different solutions

I assume your trolling battery system is 12Vdc
Have a similar setup for external batteries from the rigs 550 watt array
The outboard motor might be of assistance - My Yamaha outboards can supply as much as 25 amps external charging current, a few switches to isolate when needed and all is good. Additionally installed a 100watt portable solar panel that can sit for hours / days on the boat to top off the batteries if / when needed.

Keep that trolling motor alive and even better catch some fish while enjoying life

Configuration completed with house batt's as source. Should be good for charging house batt's from TV if/when needed. Now testing:

I tried solar direct on a pair of T-1275s which were at 90+% SOC at the time from my 230w panel (37 Voc) aimed south-westerly at the late afternoon sun. Sort of like the OP's situation.

As expected the amps were a bit less than panel Isc of 8.3a, ISTR I got about 7a due to the sun being down some. Battery voltage climbed steadily but not sharply from 14s to 15s but the rate of climb slowed as voltage got higher till it stalled out at under 16v, forget exact number.

I think this was from a combination of the panel output declining slowly in amps as the sun got lower plus the natural stalling out you get with any charger that reaches what limit it has to get the battery voltage any higher.

Anyway, nothing broke and I got the batts to a higher SOC than they would have reached that day if left on the controller.

( That controller was on Float at that high an SOC going in, and its adjustable Float voltage only goes to 14.5v, so that's why I tried the solar direct trick. It will go to 15.5 in Absorb, but once it gets to Float you can't use the Absorb until batt voltage drops to 13.2ish. Here is where it would be handy to have a controller with an adjustable voltage that goes to 16v whenever you want it to)

At 18-20 volts, that's 5+ amps. These batteries are designed to take 5 amps charging. But it never said anything about 20 volts. I think that was a little bit much.

NinerBikes wrote:
18Ah is pretty small for 100 watts RJ...

MEXICOWANDERER wrote:
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?

Mex ... did you mean a "real" DC (12 volt) to DC (12 volt) battery charger like one of these?:

http://www.sterling-power.com/products-battbatt.htm

18Ah is pretty small for 100 watts RJ...

I remember when I got my first 100 watt panel,.....controller on a slow boatfrom china, I connected the panel directly to an 8 yr old 18 ah SLA AGM battery that still functioned, but not well. I left for only a few minutes, came back to a really stinking acid smell. My advice, don't do it on a single battery !

If you only have a "few" hours of sunlight left, I'd follow Mex's advice and just wire the solar panel direct to the trolling motor batteries, no controller involved. A few hours is unlikely to even come close to topping off the batteries, if you use them again the next day, first thing in the morning.

KJINTF wrote:

Have not seen a Helitrope for several years now (as I remember they were AM Solar's controller of choice a few years back) not sure if it requires a battery connection before an array connection - check it out before disconnecting your house battery from the controller.

Being a cheap old fard, I've been waiting over a decade for that obsolete controller to die so I could upgrade. 8>) I did move it to cut down the original (1999) distance from the batteries by 4/5.

Yes, the controller needs to sense a batt before the array. I have the switching to do that.

Bend

Now that I understand better your situation (guess I should have read your signature)
Directly connecting the Array to the battery could be a solution to getting your battery charged a bit quicker - I was a bit concerned if you were running a high voltage array (as I do) with a direct connection to the battery. My rigs got 550watts at 40Vdc which can/will destroy a battery in no time if directly connected to a small battery.

As RJ said a jumper cable to the trolling battery from the house battery will indeed help to get it topped up after a day of fishing.

Have not seen a Helitrope for several years now (as I remember they were AM Solar's controller of choice a few years back) not sure if it requires a battery connection before an array connection - check it out before disconnecting your house battery from the controller.