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FWIW, I used a BTJ last winter on a new AGM battery bought the spring of 2018. I left the battery in the bike and the bike was left in the garage all winter, BTJ connected and plugged in. Last winter here in Michigan was cold, we had a couple weeks in a row right about 0 degs. This spring the battery was dead. Showing 11 volts at rest but would drop below 8 when trying to start the bike. It wouldn't take a charge either. I took the BTJ along with the battery to a local shop here that rebuilds and sells new and rebuilt batteries. My battery was tested and indeed it was junk. There was no indication on the BTJ or its packaging if it was designed for AGM batteries. It was either that or the inability to compensate for the cold that killed a new battery. This season my new battery is in the basement on a wood shelf and I will check the voltage every 2 to 3 weeks and charge it if necessary. No more BTJ for me. I don't know if it was battery style incompatibility or the cold but it wont happen again.

loudhvx wrote: After another 5 or 10 minutes, the voltage was down to 13.25v and the current was still -0.1mA. So clearly, in solid-green mode, there is no charging. If anything, there is a slight discharging.

That's a very nice test you ran. I like the way you science. And your results are exactly how I would expect it to work. Since the charge voltage is above the float voltage, when the charger switches from charge mode to float mode, the constant-voltage set-point drops. In this case 14.5v to about 13.1V. The current should drop to zero (the battery is higher than the output) and the voltage will slowly float down (chemical process distributing charge around inside). It should continue to drift down to the float charge set-point, and there I would also expect the current to stay very low, but not necessarily zero. If everything is working correctly, the float set-point should be close enough to the "full" voltage of the battery that the current should be roughly equal to the internal leakage current of the battery. So it's only replacing what is internally lost from the battery. This doesn't mean the charger is off or idle. It's just that the constant-voltage set-point of the charger is equal to the battery voltage, so no current flows.

loudhvx wrote: I predict it may take a day or two for the ambient voltage to drop back down to 12.9v.

If that is true, then my hypothesis would be proven incorrect. It'll be interesting to see what you find.

In any case, would you agree loudhvx that the battery tender Jr. is basically a reasonably well designed float charger that should be safe to use for extended periods of time, if maybe not forever?

During solid-red mode, the voltage on the battery would climb if I put the current sensing meter in line with the battery. Without it, the battery and BTJ output were at about 14.5v. With the current-sensing meter, the BTJ output was 14.9v and the battery was 14.5v. The difference dropped as the charging current dropped (as it must due to the constant resistance of the current sensing meter). This leads me to believe the BTJ has no target voltage during steady-charge mode, or at least not one that is within a practical range for our batteries. It acts as a current source, and not a voltage source. But again, these are very protracted tests, and any in-depth conclusion would be conjecture at best.

It seems that to reverse-engineer how it works will be thwarted by the device going into fault mode (flashing red). So you'd have to make some workarounds to get data while still having normal operation.

I agree that the BTJ is very convenient, at least for one battery. It's nice not to worry about accidentally leaving a charger connected. Before I had it, my maintenance was to use a car-battery float charger with a resistor in line, and the charger set to the lower setting. It would push somewhere between 10 and 100 mA. I've left that connected for several days without harm, but I wouldn't trust it for months and it definitely won't re-charge anything quickly (for that I would take out the resistor).

I always bring the batteries indoors for the winter. My buddy leaves his in the bike all winter without connecting any charger. Then in the spring his startup routine is to simply press the start button. He's got 7 years on his battery and has totally neglected it. I told him his battery would never last that way, but it has outlasted all of mine. So who knows?

I dislike batteries so much I have one bike dedicated to bump-starting.

Update: Now it's about 24 hours later. Solid-green. The battery measures 12.89v while connected. Right after disconnecting it measured 12.92v. After 10 minutes it measured 12.90v. Which is the same it measured after being connected for weeks.

This is my fully automatic Diehard charger (Craftsman?) . Been using it for many moons. It seems to work much like the BTJ, except it is too big to mount under a side cover. I had put it on my bike a few days ago when I started the bike after it had sat idle for a month or better. (battery was not low BTW) When It feels like it has flogged the battery enough, the current drops to zero and the green "full charge" LED activates.


I observed it going from charge to the green LED coming on (was putting my snow blower in the shed) , then I checked the voltage with the charger still hooked up. I measured 12.93 volts, and the battery was cool to the touch. (cool is a good thing) I then removed it. There is no need to leave a good battery in a constant state of charge. I have found that batteries in a constant state of charge (getting cooked) will soon NEED to be in a constant state of charge. IMHO


Not much riding going on in the lou today, but I did get out my snow blower.

Thanks all, for the in-depth monitoring. Interesting stuff. We've not done any of our own but, can speak to what's worked for us.

We have an old, original aluminum-case version of the Battery Tender. These days it's used mostly for the initial charge of a newly activated flooded wet battery.

We use an older version of this charger for routine maintenance:
www.amazon.com/Schauer-Battery-Charger-M...JJ35Y&qid=1576594596

Our older Schauer model SW121060 has 2/4/6A settings, unlike the newer unit that has 1/8/12A settings. Ours is always set to 2A. Depending on the bike, it's set to either wet or sealed AGM charging; once per month, per battery, for 24 ~ 48 hrs. or sooner if we happen to see the indicator for fully-charged. Our Taiwanese conventional flooded wet batteries (Z1's and CB-750 Nighthawks) typically last 3 ~ 6 yrs. Our only Taiwanese AGM battery (Honda ST1300A) is going on 6 yrs. & performs well.

We don't buy any batteries made in China any longer, as every Chinese battery we've tried has died prematurely. Taiwanese or Japanese batteries have always preformed very well for us.

We saw a suggestion long ago that a 2A dumb charger be connected to the lighting circuit of a garage door opener. Charging would occur each time the opener was cycled for as long as the light was energized.

We also have a small 2A "smart" charger purchased from Cycle Gear for $5.00 on Black Friday a few years ago that's dedicated to keeping the AGM battery in our 8Kw emergency generator topped up. Same once a month for 24/48 hrs. regime as the bike batteries. It's a tiny little thing, about the size of a thumb, and is claimed to be "smart; red for charge, green for "float", whatever that does . It does indeed keep the battery ready to crank in even the coldest weather. The current battery is going on 4 yrs. old. However, we discovered not long after buying this cheap charger that it applies a parasitic load on the battery when left attached to the battery but not powered on by wall voltage. Over about two weeks, it will drain the battery completely, damaging the battery in the process, because AGM batteries don't tolerate well to being deeply discharged. Disconnecting the charger from the battery prevents that. Maybe something to monitor if one has a cheap "smart" charger.

Interesting thread! Thanks to all who've offered technical insights.

Good Ridin'
slmjim & Z1BEBE

slimjim touched on something that should be pointed out to our members on the subject of winter time battery care. ..

Never leave a battery charger hooked up to a battery when the charger is not powered (AC power). The reverse leakage of the diodes & semiconductors can discharge the battery. Depending upon the internal circuitry, there may be multiple parasitic drains that can kill a battery in a relatively short period. I would consider the totally manual basic chargers the worst offenders with the usual transformer and bridge rectifier circuit drain, but this loss can vary,

Update: so after about 4 days of sitting unconnected, the battery is now at 12.80v. The basement is probably about 65 deg F.

Because I found the battery at 12.9v earlier, before any of these tests, while connected to the BTJ, it must kick into the flashing-green and top-off at some point (otherwise the battery would have been lower). Even though I never saw it happen, it must have at some point.