Miata Battery and Battery Tender Question

Hi Folks. I have not been in here for a long time, but HELP>>LOL. Since the OEM battery, in my 2000 Miata, with less than 44,000 original miles, I hav
e been using the Duralast battery from AutoZone. And since I drive it so ve ry little (Obviously with those miles), I keep a Battery Tender Jr. on it. Well now that it is cold here, with fall setting in, it appeared that this 4.5 yr. old battery was giving up the ghost, with the tender going into fla shing Red, then green and back and forth. I thought maybe it was because I had moved the tender to the floor from the lip of the trunk in Summer and t hat maybe the floor now (Concrete) was way too cold for it, so moved it bac k, and I thought that solved it, going back to solid green, but a couple of days later SAME thing, hence decided today new battery time. Now the guy t here, tested and said the battery tested good, what did I want to do. Well since I planned to replace it at 5 yrs. anyway, I still bought it and put i t in, figuring it was probably just low enough to make the tender do that t hing.
Well got home hooking the tender back up and the same thing, so now appeare d that this 4 yr, old tender went bad. Went down and got another Tender Jr. and by all appearances looks exactly the same, but is feather weight comp ared to my 4 year old one. Long winded, sorry, but now some ???s: 1) In the future how can I protect my wallet by knowing which one really needs repla cing?? and 2) Even though this battery is brand new and fully charged, this new tender was also red, probably would have gone green in maybe 15>30 min utes or so, but impatience me fired up the car, and it went green pretty qu ick and has stayed green just fine. But I did decide to re-route the extens ion cord, unplugging it, and upon re-plugging it, it went RED again. Fired up the car briefly again, being IMPATIENT. Now is it a bad idea to have the Tender on and connected at the same time the car is running??? I have gone over its' instructions with a fine tooth comb and it does not say??? Does this brand new Tender Jr. model just function slightly differently, taking a bit longer to go from Red to green.
TIA for ANY info :-) And by the way, since about noon has been fine and sol id green since the last engine turn on.
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On Thu, 05 Nov 2015 18:34:57 -0800, moeshefrankenstein_1 wrote:

I had one on my 1996 and after some years, it started cutting out the engine while I was driving. So I threw it away and did not get another. Instead I put in a lights-on buzzer.

See whether the battery starts the car with the tender disconnected?

Leon
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