What does this tell me about my batteries.

[Keeping Up]

While cruising over the last few days I've noticed something about my batteries while charging, as seen on my BM2 monitor.

 

Firstly, at all times the charging current in Amps is almost exactly equal to the number of Ampere-hours that need to be replenished. So if the counted and displayed figure is 90 Ah of discharge the batteries will be taking 90 amps.

 

Secondly every hour the charging current in Amps is almost exactly one third of what it was an hour before. So in the above example if I look at the meter after an hour it will be showing 30 amps of charge (and 30 Ah of charge still required); an hour later it will be 10 amps and an hour after that it will be 3.3 Amps and so on.

 

I'm sure this should be telling me something useful about the state of health of my batteries (2 year old bank of 440 Ah) but my brain just won't tell me what, apart from the fact that I am at all times an hour away from fully charging my batteries.

 

Further info; they're Varta batteries with a very low tail current of around 0.2 amps the set, and charging is from a Beta 43 with the "soft" regulation of the Iskra alternator.

[WotEver]

Well, firstly, 90A charge for 90Ah DoD is pretty much 'standard'. The rule of thumb is that a battery will accept a charge current approximately equal to the discharge depth, just like yours are doing.

 

Secondly, you're again seeing the 'standard' charge curve, where the charge current drops more and more slowly but never reaches zero. That's why the 'best' way to determine when to stop charging is when the charge current hasn't dropped over an hour. Forget what that exact figure may be (1%, 2%, 4% of capacity), just stop when it stops reducing.

 

Tony

[Arthur Brown]

You are finding a typical charge process! It arrives asymptotically at fully charged.

[Sea Dog]

Beautiful figures! Enjoy them while you can as others will be along in due course.

[cuthound]

Well, firstly, 90A charge for 90Ah DoD is pretty much 'standard'. The rule of thumb is that a battery will accept a charge current approximately equal to the discharge depth, just like yours are doing.

Secondly, you're again seeing the 'standard' charge curve, where the charge current drops more and more slowly but never reaches zero. That's why the 'best' way to determine when to stop charging is when the charge current hasn't dropped over an hour. Forget what that exact figure may be (1%, 2%, 4% of capacity), just stop when it stops reducing.

Tony

Fully agree with this post.

 

Tail current is battery condition dependent. On a brand new battery it may be around 0.25%, on an end of life battery, where plenty of active material from the plates has fallen to the bottom of the cell, allowing a small current to track through it, it may be 5% or more.

 

As it takes forever to fully charge a battery (actually I'm not sure that is ever possible as all batteries have a small self discharge current, so at best you would match that), "fully charged" is a compromise between getting close to fully charged and the cost of running the engine or generator.

[Keeping Up]

The thing is, I'm sure the charge current didn't track the DoD precisely when they were new, they charged at a much higher rate at first (typically twice as many amps as the DoD, up to the 175 amps capability of the alternator) until perhaps 80% charged when the current rapidly fell away to a figure much less than the DoD which made it hard to push the last bit of charge into them. Right now it tracks to within a couple of percent all the way from 120 amps down to 1 amp.

 

Also, is the figure of a fall to precisely one third every hour good or bad? Is it mathematically consistent with charging at a rate equal to the DoD or not? I think I left my maths brain at home last week and it is refusing to perform an integration on the exponential function for me right now.

 

That's why I'm asking what it could tell me about my batteries.

[Keeping Up]

After a great deal of head-scratching and a few pages of scribbled mathematical notes, plus a consultation with Chris W of this forum over a pint of beer, it would appear that IF the charge current is always numerically equal to the Depth of Discharge THEN the current will always be reducing by a factor of e (2.718) per hour.

 

Not exactly a factor of 3 but near enough, and this is a mathematical relationship that has no dependence on size or condition of the batteries. Thus although the fact that the batteries are taking a current equal to the DoD may tell me something about my batteries (or about the alternator) the extra fact about the constant reduction factor per hour gives me no additional information whatsoever.

[dmr]

After a great deal of head-scratching and a few pages of scribbled mathematical notes, plus a consultation with Chris W of this forum over a pint of beer, it would appear that IF the charge current is always numerically equal to the Depth of Discharge THEN the current will always be reducing by a factor of e (2.718) per hour.

 

Not exactly a factor of 3 but near enough, and this is a mathematical relationship that has no dependence on size or condition of the batteries. Thus although the fact that the batteries are taking a current equal to the DoD may tell me something about my batteries (or about the alternator) the extra fact about the constant reduction factor per hour gives me no additional information whatsoever.

 

I will have to think about that, but its just too hot. e is a universal fundamental thingy but hours are just an arbitrary man made measure of time.

If we measured battery capacity in amp-seconds would the current then fall by a factor of e every second? So, is the e Factor per hour just a co-incidence.

 

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

[Keeping Up]

I will have to think about that, but its just too hot. e is a universal fundamental thingy but hours are just an arbitrary man made measure of time.

If we measured battery capacity in amp-seconds would the current then fall by a factor of e every second? So, is the e Factor per hour just a co-incidence.

 

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

If we measured the capacity of a battery in Amp-seconds then a DoD of 100 amp-hours would be expressed as 360000 amp seconds. If then somehow you did manage to force a current of 360000 amps into the batteries then yes I think that the current would decrease by a factor of e every second. As long as the units are constant I believe that the maths will be consistent.