Hey Ben, I didn't know you had it in a garage, I thought you had it parked 'outside' .. eg. like exposed to weather and public.
If in a garage and you have power there, then a mains trickle charger is fine.
Like Drivesafe said, a one hour drive is not enough to bring the battery back up to a really healthy state, and after a week or so(maybe two) it will do the same thing and lose charge again.
Just a cheap trickle charger would be fine to keep it maintained, and when connected to the main battery, and main battery in good charge, the redarc will do it's job and then connect the aux battery at some point.
So with brothers(Td5), what I do(only for now) is set up the solar panel(folding camp type) outside out back. Not a lot of sun there during winter due to angle of sun, but even just two hours at about 1amp keep the main battery at a minimum of about 12.0 volts. Over some nicer days(more sun) maybe 12.2v or so. Even when it's only been cloudy, no sun for a few days, this has kept the main battery at 12.0v or above. The panel I have is about 200w or so, and on a cloudy day I've measured about 0.5 amps through to the battery, about 3amps when sun does come out.
That alone isn't enough to keep the battery above 13.2v(I think) where the redarc cuts the aux battery into the charge loop tho.
So I've driven the brothers TD5 a couple of days(maybe 1 hr max) and has helped the main battery with better charge, and now when the solar charges the main, it get to 13.2v quicker/easier and the aux also gets a charge too now.
So, lets say you get a trickle charger(I think about $50-ish) it's probably not enough to charge the main quickly at first, but after a few days(depends on the condition of the main!!) eventually it will. So if you do this, you may notice that for a while at first, the redarc will connect the aux, then shut down again. This will be because the aux is drawing more current too, and the trickle charger isn't enough(at first). Once the main is fine and the aux also then gets a decent charge, it will eventually settle and then it will connect both batteries and stay on .. ie. both batteries will get a good charge with the trickle charger.
With the cig lighter low down on the dash in a D1, it is (standard) connected to accessories, not direct to battery. But that doesn't mean that someone hasn't rewired it, so you may have constant power there. I thought about that on mine too, but it's also nice to have accessories power like that .. I use it for my tyre pressure monitor(not good to have them always on).
So for a constant power plug(and USB ports) I got a three gang plug pack and bolted it to the dash in front of the passenger.
D800E_DSD_1790.jpg
Very easy to do, I think 19mm holesaw required tho. I have that one wired to the Aux. And the redarc(most of them as I understand) will work in reverse(mine does). So instead of charging the main first, I could plug into that one(aux battery) and once up to good health will then connect the main battery too.
Also bit of a tip: you said that the aux held 12.5v but main dropped down to 10.2v. First, my understanding of batteries, I think it will be time to replace main soon. That voltage is not good for long term. Maybe drivesafe can confirm.
But, with 12.5 at the aux, if you had used the override facility of the redarc, you could have started without the need for jumper leads.
On the redarc unit there is a blue wire connected to nothing(just loose). If you have a switch connected to that wire, going to one lead of a switch, and the other lead of the switch going to the aux terminal post(with a brown wire) on the redarc unit .. and then hit the switch to be ON, it will connect the aux to the main battery, push button jump starting. A momentary switch is probably better as the switch for this, only reason is that you don't forget it ON, and therefore making the isolator useless(in that it doesn't isolate when needed too).
If you ever need help with it all one day, post COVID period, feel free to come over, or invite me down to help.
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