Battery charging, not a problem, but trying to understand what is happening.

[EricMark]

I use smart chargers, and the basic design is each time it hits a set voltage the charge rate drops, so starts at 3.8 amp, then 3 amp, then 0.8 amp and finally 0.1 amp, however if the voltage drops below 12.8 volt on the 0.1 amp charge rate it will return to 0.8 amp charge rate, some times it has returned to 0.8 amp, but normally if left even at 0.1 amp the voltage will slowly raise, so once at 13 volt I considered fully charged, and charger moved to next battery, the Honda Jazz and Kia Sorento batteries were both put on charge at the same time, I use the Honda engine compartment to protect the two extension leads from weather as not long enough to reach Kia with one lead.

So Honda has charged up and at 0.1 amp reached 13.8 volt and was removed from charge, but the Kia is alternating between the 0.8 and 0.1 charge rate. the patten after finishing 3 amp rate, and the pattern some days latter, now I can understand why the time at 0.8 amp is shorter, as it gets closer to fully charged then I expect the time at 0.8 amp to reduce, but why is the voltage dropping faster?

It is a 95 Ah battery still connected to vehicle, and it had been allowed to self discharge to under 12 volt, Did not try to start, knew would not be using for some time, so wanted to top up before winter set in, it was taken for a run around a month ago and I have no reason to suspect any fault. It is purely interest, there is no known problem, without having an energy monitor I would not even know what was happening, and would have taken off charge, as an auto electrician I did not have the ability to monitor charge rates like this, it is pure interest, not a problem, but why is the voltage decay time decreasing?

 

[EricMark]

Nearly 2 weeks on charge now, I have 12 Ah, 40 Ah, 75 Ah and two 85 Ah batteries both on and off cars, and with the same charger they go through the four charge rates and end up with a 0.1 amp charge rate over 12.8 volt between 13.2 and 13.8 within a few days, as did the 95 Ah on the Kia last time charged, but this time it continues to swap 0.1 to 0.8 and back again showing regular spikes on the energy monitor. Around 14 spikes an hour, average shows at 3.5 watt, where same battery charger on the 85 Ah shows average of 2 watt and voltage raises above 12.8 volt with 0.1 amp charge.

 

[Simon47]

For the first part, different cars will have different key-off loads - all modern cars have some key-off load to keep security systems running (even if it's only a receiver for remote locking). The higher the key off load, the faster the battery voltage will drop back and trigger teh charger to step up the charge current for a short time.
Also, different batteries will have different self discharge rates, self discharge causing the same effect. Typically, the larger the battery, the higher the self discharge rate (in terms of internal leakage) for a given construction - so "smart" chargers typically only work for a range of battery sizes, or can be set to one of several profiles. Use one on too small a battery and the charge rates will be too high, too large a battery and it might not ever reach the threshold for dropping down into maintenance mode.

 

[EricMark]

I have put two chargers on the same battery so charge rate now 0.2 amp, and still not holding, and it jumps to 0.9 amp 4 times an hour.

What I am trying to work out is the battery reaching end of life, or has it got some sulphation and just needs a little longer.

 

[static zap]

One of my end of life batteries ,took charge from a 3A charger , but got /stayed warm as it self discharged .. Current never properly tailed off,
stuff from plates all over bottom of battery--connection path.
(Was the older chemistry -but still looked good terminal volts with a BIG load applied just after charge)

(due to how they work - 2 charger does not guarantee more current -unless can actually monitor)

 

[EricMark]

I am monitoring, OK input to chargers not output, opened back door and slammed closed again just in case, and it seems it may have been some boot light on.

 

[Simon47]

Ah yes, interior light left on will drain a battery in a surprisingly short time - as I've found out by experience

 

[EricMark]

Needed charger for other cars so removed, then used it to go to doctors, sure left hand inertia light was not on when I drove down, but while waiting for wife noticed it was on, so put back on charge just to see if this time it does not cycle, but some 5 hours and exactly the same patten as last time.