Charger instability at jump point?

A Harbor Freight 63350 4A battery charger is being used today as a battery minder by attaching it to the jump point in our 2007 Prius. Something odd is happening. The charger says that it is at 14.6V and the battery indicator is moving up and down (which is what that model does to indicate that it is still active). A clamp ammeter placed on either lead shows a very rapid cycle in the amount of current. It rises to a little over 1A and then drops back down to near zero and then back up again. The whole cycle takes around 1 second. It is set to AGM mode, which I would hope would disable any "desulfation" mode. This is with a UT210E meter and I doubt that these measurements are at all accurate because that meter takes longer to settle than the time each value is shown. There is no time varying magnetic field like that other than from the wire, with the clamp closed next to, but not around, the wire it reads stably as zero (once it is normalized in that position). An old fashioned analog voltmeter placed in parallel with the charger has a stable needle. The analog meter is pretty quick and the needle should have been vibrating if the observed cycle time was correct. Unless maybe the voltage change associated with this current change is too small to see.

I have used this charger before with this same battery - out of the car. There the clamp meter showed a constant current. So either it is now in some charging mode I didn't catch when it was on the battery or it behaves differently when attached to the car at the jump point.

An issue was also observed with a Battery Tender Junior attached to the car at the jump point. It ran for a long time but didn't actually charge the battery. Yet that one too worked fine on the battery out of the car.

I wonder if the brake capacitor box, which I think is in parallel with the battery, is throwing these chargers for a loop? They are expecting a battery after all, not any substantial capacitance. I have seen similar issues before elsewhere - a BIO-RAD model 200 power supply used to charge a Honda Civic Hybrid IMA pack was unstable until a small drop resistor was placed in series with that pack. In that case too the current/voltage were unstable on a short time scale. But the same power supply was rock solid with a purely resistive load.

Or I suppose the charger could just be of dubious quality (HF, right?) and is acting up or failing. Maybe later I will pull the other car into the driveway and verify that it is stable when attached to that battery (standard flooded,not AGM.)

 

Well, it isn't the brake capacitor box, it does this with that plugged in or not.

This seems to be how this charger behaves:

0. Charger detached, after headlights on for 20s, all doors closed, battery reads 12.60V (roughly 80% SOC on an AGM). Note, this battery has only around 12.5 Ah capacity and relatively high resistance.
1. Attach charger to jump point in car.
2. Charging current goes to a steady 1.0A (measured in clamp mode with UT210E). The voltage was 13.4V or 13.6V. I think if the voltage was substantially lower it would have gone as high as 4.0A, which was what was observed previously when the battery was out of the car.
3. After a while, current drops to close to ~0A (less than the UT210E can measure accurately in 2A mode). (The display said 14.something volts, which is pretty odd - no current with a ~2V voltage difference with the resting battery voltage.)
4. It enters the oscillating current mode as described above. The voltage displayed varies with time, wandering around slowly in the low 14V range. Using the UT210E as a voltmeter a .02V variation on the same time scale as the current variation is observed. In this mode the voltage measured on the battery or on the post is about .2V below that shown on the charger display. If the charger display voltage gets near a boundary (like 13.5/13.6) the display voltage can oscillate between those values at the same rate and at the same time as shown on the separate voltmeter. But again, higher by about .2V.

I wonder what that time varying current waveform is? My kingdom for an oscilloscope!

At this point it is pretty clear that the "charger voltage" displayed is an internal voltage and not the voltage at its clamps. It is acting like the charger has a voltage stage followed by a variable resistance stage. It displays the voltage of the former. The latter can apparently modulate current slowly (for the steady 1A and ~0 rates) and rapidly (for the time varying rate).

I will leave the charger attached until the sun goes down and see if it comes up much from 12.60V. This rapid current variation mode may be its maintenance mode, in which case I bet it is still at pretty much the same value after 3 more hours. (A 20% SOC change on this battery would be about 2.5Ah, but it isn't giving even a steady 1A, so I doubt it will get there in 3 hours.)

Edit: So after 6 hours on the charger, then 20s with the headlights on while the car is off, it reads 12.66V at the jump post with the car off and just the background (20mA) . That seems to be as high as this battery will go, voltage wise, once the surface charge is off with the background ~20mA load. (That is a trivial load, it really shouldn't measurably affect the voltage.) But the HF charger seems not to think it is fully charged because it doesn't turn off the background light on the display and set the battery emblem to full and solid. Could be that the HF charger is broken though, and I don't have a known good Prius battery to test it on. My 12V battery is known bad - only about 1/4 of full capacity in Ah.

Edit2: Also my wife accidentally turned off the power strip at one point so I had to restart it. It was apparently more or less fully charged when this happened. Watched the current with the clamp on continuously and it did 1A briefly, 0A briefly, then those two states again, then it went into the rapidly varying current state and stayed there until the sun went down and I turned it off. (It was in the driveway.) It is a pity this charger isn't a bit more verbose about what it is doing at any given time.

 

Checked the auxiliary battery 15 hours later, the car not having been used, and it was still reading 12.66V. So that's good.

It is annoying how long one must wait on this car after touching it in any way before an accurate measure of the auxiliary battery voltage can be made. Just opening the door with the fob, to access the hood release, means the car will be twitching electrically for several minutes (not sure what number "several" is exactly, but here patience is a virtue), which throws off a measurement of the "resting" battery voltage. I initially measured a little too early this morning and it was at 12.53V, but it was slowly rising, so I let it sit for a couple more minutes and by then it had stabilized at 12.66V.

 

This HF charger apparently has an issue if it starts when the battery is too close to 100% SOC, and especially if it has surface charge and starts at a voltage which is more than what it thinks 100% SOC should be. When that happens it goes into that odd oscillating current mode after the 1A phase and the 0A phase. On the other hand, if the headlights are turned on "long enough" (tested this on a standard battery on another car and the AGM in the Prius) and drives the voltage down far enough, instead it starts charging with somewhere between 1A and 4A, steady. When this happens it will eventually reach what it thinks is full charge, and then it sets the battery symbol to "all bars set", turns off the back light, and shows the current voltage. That is as described in the manual for "done". The measured current in this mode is indistinguishable from zero amps with the UT210E meter. Specifically, this "done" or "maintenance" state is not the odd varying current state. The steady voltage shown in the "done" state was about .7V above the voltage measured on the clamps with a separate meter, which is better than the .2V difference seen in some states.

This high voltage startup issue could be a problem in a couple of scenarios. If the voltage to the charger is interrupted for any reason while it is in maintenance mode, it will go into the odd state described in the 3rd post in this thread. (That state is not described in the HF "manual", it may not be a normal operating state.) This did happen the other day, when my wife accidentally turned off the power strip, and I turned it back on a few minutes later, expecting it would start back up normally. That was wrong. Also if the car has just been driven, and the charger is hooked up and turned on, then it will almost certainly see a surface charge voltage over 13V, and it will immediately go into the odd state. One of the vehicles tested had sat all night, and my son had to reposition it (Tacoma truck, not that it matters) so that the garage door could be opened. It was only on maybe 30s, but that was enough to bump the battery voltage up into the trouble zone for this charger. 30s with the headlights on (~19A) was enough to drop it to an acceptable start voltage. Seems counter productive to have to discharge a battery in order to charge it!

The HF charger also has a brief startup phase when it is first turned on where the backlight is green and it shows what is probably the battery voltage. This startup phase only lasts a few seconds though, and the green backlight is hard to see in sunlight.

Since I only have the one charger of this model I cannot say if they all have these issues or not. This charger is inexpensive, and it can be used to charge a battery, but these complications were really unexpected.

 

Bumping this thread for one more comment about this HF (Viking) 4A charger. It says that it is a 4A charger, but the highest steady charging current I have seen (with a clamp ammeter) it provide for an extended period is just under 2A. In retrospect, I'm not sure I recall ever seeing it above that current. Used it this weekend to charge up our 1998 Accord, with a standard flooded lead acid battery. It went up to 1.95A charging current after its initial "probing the battery" period, and that's as high as I saw it go during the entire charging period. I only checked it hourly though, and it could have had some 4A peaks in there. (Probably not.) That battery was at 12.1 V when it started, and 12.61V (in its quickly varying mode, not the "done" mode) when it was detached. So no reason I can think of for it not to go to 4A, at least when charging from 50% SOC to 80% or so. This old car has a fraction of the number of computer and electronic systems that the Prius does, so less to throw the "smart" part of the charger for a loop. That said, the battery was attached to the car at the time, so it wasn't a straight battery load for the charger. Maybe it will get up to 4A and stay there for a while with a detached battery, but having to pull the terminals off to charge the battery at the full advertised rate is pretty inconvenient.

Or it could be this unit is defective and really can only manage 2A, but some other instance of the same model would really do 4A reliably. It is from Harbor Freight, after all.

 

Mystery solved, I think.

Here:

Harbor Freight car battery chargers any good? - Badcaps

it says:

One review had blown capacitors, another said it's actually 12V 2A and 6V 4A so around 25W. The "4A" label is misleading.

That is consistent with my observations. The "manual" has specifications that say:

Charge settings: 4A, 6V/12V

which would seem to indicate it should do 4A at 12V. But it does not, or at least this one doesn't.

 

Well, it turns out that it can put out nearly 4A, but not in a very useful way.

My son has been sick for a couple of weeks, and his wife is off with her family, so his F-350 Super Duty has been sitting in our driveway for 2 weeks, immobile. Seemed like it might be a good idea to check up on it. Opened the door and popped the hood, and it was only reading 12.3V and there was over 2A of current measured on the positive lead of either battery. This thing has two 68Ah batteries wired in parallel (more or less, there is a battery management system of some sort), and 12.3V is below 50% SOC open circuit. It was turned on and idled for 10 minutes, then we had to go somewhere. Apparently the computers on this truck take forever to shut off, so the next day it was unlocked and the hood opened, and after two hours it was checked again. This time it read 12.48V and 25mA on the positive lead (of either battery). Depending on which table one consults this is somewhere between 50% and 60% SOC. So the pair of batteries is down at least 54 Ah combined. Seems like a situation where maximum current could be applied by a small charger, no?

OK, background set. Attached the positive lead to the positive post of the passenger side battery, and the ground to the ground post on the fender. The charger quickly moved up to 13.8V and 3.9A out the positive lead. So it can do (almost) 4A. But within a few minutes it had fallen to 2.9A and 90 minutes later it was at 1.9A. (All of this with a DC clamp ammeter.) Even if it had stayed at 4A there would still be 48 Ah of capacity left to charge at this point.

Anyway, this is the 4th vehicle we have used this charger on, and the only one where it exceeded 2A current at any point, also the only one with two 12V batteries in parallel. So I will stick with my view that it isn't a 4A charger in any meaningful sense.