Tail current

[Col_T]

Many threads talk about tail current being the most accurate way of determining when a battery is fully charged e.g. charge until tail current falls to a specified percentage of the original battery capacity and doesn't fall for a further time period. 2% and three-quarters of an hour seem to be favourites.

We know that battery capacity falls as said battery becomes more sulphated, so it would reasonable that the tail current value also falls. Example, using the above, I would expect to charge a new 110 ah battery until tail current didn't fall below 2 amps (2%) for 45 minutes, and charge a 110 ah battery that now had 82 ah capacity (75% of original) to 1.5 amps for 45 minutes.

Is this correct, or have I misunderstood something??

[Tony Brooks]

No idea but remember that the self discharge rate of batteries tends to increase as they age so maybe it sort of self compensates. In any case as the battery never gets fully charged and we do not really know what that is I suspect that you are trying to be over exact. It's the failure to fall that is probably more important than the exact figure. The 1%, 2% etc. is given more as a guide.

[Col_T]

Thanks for this, Tony, especially as your "failure to fall" comment is what I kind of expected.

In all honesty, "trying to be over exact" seems to be one of my dominant traits, or so my wife tells me!!

[dmr]

I think there are several variables and so even if tail current does fall with Sulphation (and I am not convinced that it does) I don't think this could be reliably measured.

Tail current certainly increases as batteries as age.

Comparing a Smartgage reading against an amp-hour reading of some sort looks to be the easiest method of monitoring capacity and sulphation.

...........Dave

[WotEver]

59 minutes ago, Col_T said:

Is this correct, or have I misunderstood something?

I'm not really sure what it is you're asking in the OP?  

Tail current doesn't decrease as a battery ages from sulphation, it increases. Charging current is dictated by the battery's internal resistance which increases as the relative density of the electrolyte increases.  The stronger the acidic content of the electrolyte the lower the current. This is why the charging current slowly reduces as a battery charges. A sulphated battery has a weaker electrolyte because many ions are tied up in sulphate crystals, hence it will never get down to the 2%-ish figure that it used to achieve. 

[Gareth E]

Now that solar is doing its thing the meter on my MPPT controller tells me that 'fully charged' equates a tail current of 0.4A (single 110AH battery). This is much lower than the suggested 1 to 2%.

During winter I charged with a generator most days, then for several hours every few days, until the tail current reached 1A. Do my current results suggest that I was under charging my battery during the winter? Would you say that down to 1A is a reasonable compromise between generator running time and battery longevity, or should I run the genie longer, get closer to 0.4A?

[WotEver]

37 minutes ago, Gareth E said:

Now that solar is doing its thing the meter on my MPPT controller tells me that 'fully charged' equates a tail current of 0.4A (single 110AH battery). This is much lower than the suggested 1 to 2%.

During winter I charged with a generator most days, then for several hours every few days, until the tail current reached 1A. Do my current results suggest that I was under charging my battery during the winter? Would you say that down to 1A is a reasonable compromise between generator running time and battery longevity, or should I run the genie longer, get closer to 0.4A?

Yes, it's all compromise. There's no such thing as 100%, you only ever get to 99.99x% charged. It's not worth the fuel to chase those last few 0.00001% so we generally say to stop when tail current hasn't reduced over 45 mins to an hour. That generally coincides with approximately 2%. 

[cuthound]

What is the voltage when you are measuring yout 0.4A? The charge source should still be in absortion mode (14.1 to 14.8 volts for a 12 volt battery,  depending on battery type), not float mode when measuring tail current.

[WotEver]

9 minutes ago, cuthound said:

What is the voltage when you are measuring yout 0.4A? The charge source should still be in absortion mode (14.1 to 14.8 volts for a 12 volt battery,  depending on battery type), not float mode when measuring tail current.

A very good point

[Richard10002]

3 hours ago, WotEver said:

A very good point

 

I read somewhere, (Smartguage site??), that these 2% tail current measurements are based on a charging voltage of 14.4V.

 

I generally absorbtion charge at 14.8V but, when checking tail current, I change briefly to the 14.4V absorbtion setting.

[WotEver]

Again, the figure (2%, 4%, 1%, whatever) is irrelevant. It's when the current has stopped decreasing that you stop charging. 

It will be somewhere around 2% at 14.4V, maybe around 3% at 14.8V but the percentage isn't particularly relevant as long as it's low. 

If the tail current is closer to 10% then either your batteries are sulphated or at least one cell has a soft short. 

[MtB]

Doesn't matter how long I leave my batteries on charge, the tail current never stops creeping downwards. 

[WotEver]

3 minutes ago, Mike the Boilerman said:

Doesn't matter how long I leave my batteries on charge, the tail current never stops creeping downwards. 

Over a 45 minute period?  If so then keep charging...

[MtB]

I have to stop after 12 hours...

[cuthound]

2 minutes ago, Mike the Boilerman said:

I have to stop after 12 hours...

Ask Bizzard if he has perfected his lunar panels yet 

Edited April 14, 2017 by cuthound
Spillung

[rusty69]

3 minutes ago, cuthound said:

Ask Bizzard if he has perfected his lunar panels yet 

Loonie panels more likely 

Edited April 14, 2017 by rusty69

[WotEver]

26 minutes ago, Mike the Boilerman said:

I have to stop after 12 hours...

Well there's your problem!  You need longer days.

You could move to Venus and have 116 times as long (although the nights could get tedious). Mars is no good, it's about the same as here, and Saturn is a waste of space - only 10 hour days. 

[nicknorman]

2 hours ago, Richard10002 said:

 

I read somewhere, (Smartguage site??), that these 2% tail current measurements are based on a charging voltage of 14.4V.

 

I generally absorbtion charge at 14.8V but, when checking tail current, I change briefly to the 14.4V absorbtion setting.

It doesn't make a great difference and anyway depends on the recommend charging voltage. If our batteries are fully charged, sitting at 14.4v (engine alternator) and then I crank the voltage up to 14.8v (Combi) the current increases a bit for a short time, before falling back pretty much to what it was. 2% is not fully charged, although it might be an appropriate time to stop charging from a genny. 0.2% is pretty close to fully charged, but it will take many more hours to get there.

[Col_T]

Thanks for all the replies, and apologies for not responding sooner.

I know from other threads that battery capacity falls as it sulphated and, because I hadn't understood things properly, I guessed that 'fully charged' tail current would fall as the actual capacity of a battery falls. As I've learned from this thread, the 'fully charged' tail current will actually increase, and should be measured when the battery charger is in absorption mode rather than trickle charge, which is something else I didn't know.

Many thanks to all for the contributions.

[Gareth E]

Another thought on this: If tail current tends to rise as batteries sulphate can it be assumed that if the tail current is low, 0.4% in my case, that the batteries are healthy?

[WotEver]

2 hours ago, Gareth E said:

Another thought on this: If tail current tends to rise as batteries sulphate can it be assumed that if the tail current is low, 0.4% in my case, that the batteries are healthy?

I'd say yes

As long as you're measuring that current at absorption voltage.