Battery Tender Jr.

Bought one of these maybe 3 years ago and it seemed to work well on my battery after I brought it in for the winter. But it maintained it at 13.03 and slowly started to use up the water. After adding water a couple of times I figured it was more trouble keeping a eye on this and went back to using my 1 amp charger on it at the beginning of the month for 5 hours again. This has worked well for 39 years and counting.

Fast forward to now and I figured why not use it on the car battery out of the Mustang I just got and put away for the winter. Hoping you don't mind if I ask this (with this being a bike only forum) but do you guys think the 13.03 volt it maintains at after charging from approx 12.76 fully charged will be ok and not too high or wondering if I should just charge them both once a month. Both are in doors of course. It discharged from 12.76 to 12.46 in just 3 weeks so am trying to decide what would be the best thing to do.......... Its a Duralast Gold 160.00 2 year old battery and want to try and keep it strong for as long as I can.

It'll probably be fine. Is the battery tender a smart charger or dumb charger? a dumb charger keeps pumping electrons regardless of the state of charge. When I couldn't drive my Ford Escape for a while, I used my CTek smart charger to maintain the battery. I removed the battery from the car for convenience in charging.

13.03 seems too high .

I have the Battery Tender Junior model 021-0123 and the ambient voltage on the battery is 12.9v.

I believe it goes through a cycle, so maybe you happened to measure it just after a charge cycle while the battery was higher than ambient but was on its way back to ambient.

I've used it to maintain a car battery and it seemed to work fine.

KZB2 650 wrote: it maintained it at 13.03

Several sources I've found say that float voltage should be about 2.25V per cell, which works out to 13.5V. Of course, different lead-acid battery types have slightly different values, and it's also temperature dependent.

en.wikipedia.org/wiki/Float_voltage

One thing to note is that i don't think the "Jr" is temperature compensated. So if you have it in an unheated space, and it's well below "room temperature", the float voltage might be too high. Something to consider.

But everything I can find suggests that 13.03V is a perfectly normal float value.

I suspect that the battery is just aging. I don't think there is anything you can really do about that. Look at "Figure 3: Capacity loss on standby"

batteryuniversity.com/learn/article/char...he_lead_acid_battery

If you are worried about float-charging resulting in over charging, the only other option is to leave the battery disconnected and let it self discharge. But you need to make sure to recharge it periodically because if it gets too low that will also cause damage to the battery.

www.batterytender.com/Float-Charging

I have 3 of the "battery tender Jr." chargers. I pull the batteries out of my bikes for the winter, and keep them on the trickle chargers in my basement. My oldest battery was 6 years old and still working great when I sold that bike this summer.

Thanks guys
13.03 is for sure where it stays after the charge (checked 7-8 times over 2 days) and is slightly warm so it stays working. The manual says it keeps it at 100% to 103% fully charged so am thinking I have one that's closer to 103%. Was too much for my bike battery like I said was burning up the water slowly. Not sure maybe I'll just charge em both at the 1st of each month and not worry about it.

i can tell you a "tender" tends to be invisible after a bit and can likely overcook your battery like it did mine on my gpz while i was reconditioning the top end. i like the idea of sporadic use (no more than a day a week)

Float charging is active charging. After charging the battery will slowly drop down to a plateau somewhere between 12.5 and 12.9 depending on temp and other factors. Then will very slowly drop from there over a period of weeks or months.

A smart tender should shut off until the battery falls below a set threshold or after a set amount of time. I'm not sure what the BTJ uses. It would be easy to investigate, but I haven't since it seems to work on my motorcycle and car batteries.

I also am in the habit of leaving it unconnected for longer periods just in case.

Sounds like someone's battery tender has a problem. For about the past 6 years I've been using a Deltran Junior Battery Charger to keep the battery in my Cadillac Allante charged. I leave it hooked up all the time except when I drive the car, and I drive it less than once per week. I have never had any problems with over charging or anything else. The battery stays charged and the car starts right up. Of course I unhook the charger before starting the car, and I hook it up with the car shut off. Anyway, that's been my experience and I would not hesitate to recommend that charger to anyone. Ed

Hey Ed and DOHC if you guys could get a chance could you check and see what you get for a reading after a few days or longer. Thanks.

KZB2 650 wrote: Hey Ed and DOHC if you guys could get a chance could you check and see what you get for a reading after a few days or longer. Thanks.

I checked the two batteries I have charging. The first, using the newer "Battery Tender High Efficiency Junior", read 13.04 V. The second, a standard "Battery Tender Junior", read 13.09 V. Both of these had green lights, which I believe is float mode.

Based on everything I've read, leaving the Battery Tender Jr. connected forever should keep a battery healthy. That assumes three things. The battery started out healthy, the temperature in roughly "room temp" (the "Jr" is not temperature compensated), and the battery has enough water.

Regarding enough water, I've sworn off unsealed batteries. I've had too many bikes and cars where the paint was eaten off by acid vapors. So all my batteries are sealed AGM. Leaving an unsealed battery on a float charger may make the water evaporate faster if you have an open vent, like the traditional motorcycle batteries with the drain tube. I don't know. But the article I linked to said it's critical to keep the plates flooded. So that would be something to consider.

Regarding starting with a healthy battery, when I went to check the voltage today I noticed that the battery from my son's power-wheels car was only at around 12.5V while on the charger. The charger was still flashing red after more than day. The battery was also slightly warm to the touch. This battery is clearly bad, and I'll have to get a new one. It probably has a failed cell. Since the voltage never reached the float threshold, the charger kept putting out the full charge current (which is still pretty small). But since the battery is bad, it was not storing any of that energy but was instead turning it into heat, which was probably doing more damage to the battery. This is an example of a continuous charger being a bad thing.

As loudhvx said, it's fine to leave them off the charger entirely for some period of time. I do this often as I have more batteries than chargers. You just need to charge it back up before it gets too low. That article I linked to says "Recharge every 6 months while in storage; AGM every 6–12 months." But if you observe the voltage dropping faster, you should charge it.

Also, if you do only connect to the charger occasionally, you don't have to use your 1 amp charger to bring it back up to full. The tender will bring it up, it will just take longer. But then you could leave it on the tender for a 2-3 days before disconnecting it, and not have to worry about coming back to it within 5 hours.

loudhvx wrote: Float charging is active charging. After charging the battery will slowly drop down to a plateau somewhere between 12.5 and 12.9 depending on temp and other factors. Then will very slowly drop from there over a period of weeks or months.

A smart tender should shut off until the battery falls below a set threshold or after a set amount of time.

Based on the articles I linked to, that's not how lead acid float chargers typically work. Lithium batteries for sure shut off after the float period, but lead acid charger seem to typically actively maintain the float voltage forever. That's definitely how the typical computer UPS works. And apparently how the Battery Tender Jr. works, based on their FAQ.

Thanks DOHC for the time you spent on this and the exc info. Sounds like its working like it was intended to.

Yes, old "dumb" float chargers stay in float mode forever. That is fine for short periods like hours or days, but eventually they will dry out a battery. They have to compromise between being able to re-charge a battery in a reasonable amount of time, and providing a slow enough charge that they don't immediately cook a battery. That's why smaller batteries need smaller float chargers. The other option is to remember to disconnect it sooner if the battery is smaller. But people forget and eventually they will leave a battery on a "dumb" float charger for months.

The battery tender junior, as I understand it, is a "smart" charger. It may enter a quasi-float charge mode, but will eventually have to shut that down or drop the output significantly if it is to be left connected for extended periods like weeks or months. The fact that I measure 12.9v on the battery tells me it's not charging. Even small amounts of current will get a good battery over 13v.

What they are calling float mode is probably some mode where the battery is topped off periodically, then allowed to rest. So for most of their "float" time, it's actually doing nothing. It's more of a semantic argument, over the definition of "float".

I did some quick tests on an fully charged, good battery.

When the LED is red, the BTJ is outputting roughly about 400mA steadily, initially. It may have been higher since this value drops steadily while the LED is red. The voltage on the battery was somewhere near 14.4v and climbing. It takes a while for a battery voltage to climb so even though the current was dropping, the voltage was climbing. The LED was only red for about 5 minutes or less.

Then the LED was flashing green. The current was at 150mA and the battery voltage was about 14.5v. During the flashing green, the current would drop to 0mA every four seconds or so, for about half a second. This lasted for about 5 to 10 minutes. The voltage stayed pretty solid at 14.5v and the current stayed at near 150mA with 0mA pulses.

Then the LED was solid green. The current dropped to -0.1mA. and the battery voltage quickly dropped to about 13.5v.
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. I predict it may take a day or two for the ambient voltage to drop back down to 12.9v. I predict when it drops below a certain threshold, it will go back into flashing green mode. I've never seen it happen, so it must be brief and infrequent. I walk past the charger everyday on my way in and out of the house.

For these tests, I had to use 2 meters because any dis-connection would cause the charge cycle to start over. Also, meters measure current by measuring a voltage drop on a known resistance. This means the battery tender's output voltage is not exactly the same as the battery voltage. There is a drop associated with the current-measuring meter which is in series with the BTJ and the battery. Occasionally I would short the meter leads to the battery so that the BTJ output was at the same voltage as the battery. The voltage on the battery always stayed the same. This means the BTJ is not actually asserting a voltage onto the battery. It is acting as a current-source and the voltages end up where they end up.

These are just very rough tests to get an idea of whats happening.

BTW, it may be late to mention this, but it not uncommon for different meters to disagree by .1v or more when measuring roughly 13v. I don't think I have never come across two meters that measured the same battery to within .05v even same make and model. This is frustrating when trying to design regulators... if you can't trust your measurements.