Battery condition

Could anyone please help me to understand the figures that get reported on the battery conditions? 

say - 85% and 13.3V

Just what do these numbers tell us?

Hogmeister wrote:Could anyone please help me to understand the figures that get reported on the battery conditions? 

say - 85% and 13.3V

Just what do these numbers tell us?

I would have thought it meant that the current battery voltage is 13.3 volts and the controller estimates that it has 85% of its capacity left.

Sorry, I don't have a controller that gives figures like this, so I'm just guessing - but I'm struggling to think what else it could mean...

 I agree. But what does it mean in relation to the health of the battery?  That's my puzzle. 

Hogmeister wrote:But what does it mean in relation to the health of the battery?  That's my puzzle.

Ah I see what you mean - can you infer anything about the overall condition of the battery, as opposed to its current state of charge?

I'm fairly sure that, for your average motorhome electrical controller, the answer will be "no". The algorithms built-in are almost certainly too simple - just a table giving the relationship between terminal voltage and remaining capacity.

It's far more complicated to measure "health" because there are several different factors that come into play as a battery ages - for example loss of electrolyte by evaporation, sulphate formation on the plate surfaces and physical disintegration of the plates (there are probably more that I haven't thought of). Each of these will manifest itself in a different way, so modelling that in a simple controller would be difficult.

There have been threads here before that show the general relationship between voltage and capacity for an "average" battery in good condition (search for posts by Brodco and Peter, as I'm sure one of those two has put up this information in the recent past).

But to measure "health" properly, you would need a fairly complex test procedure, starting with the battery terminal voltage when fully charged and then looking at the discharge characteristics into a standard load, corrected for any changes in ambient temperature during the test. I've no doubt that lots of research has been done on this, because companies like BT have in the past maintained huge banks of lead/acid cells to power their exchanges, and they must have had some empirical basis to their maintenance/replacement programmes - but maybe none have put their research into the public domain.

Thanks for that. What really puzzles me is that sometimes I see say 14.4v and 83% and a few days later 13.1v and 97%. This is what really confuses me. These readings are when no power is being drawn other than the background devices. I would have expected the two readings to be reasonably in line.

Hogmeister wrote:Thanks for that. What really puzzles me is that sometimes I see say 14.4v and 83% and a few days later 13.1v and 97%. This is what really confuses me.  These readings are when no power is being drawn other than the background devices.   I would have expected the two readings to be reasonably in line.

It looks to me as though both those readings were taken while the van's charger was connected and charging the battery. 14.4V is the charging voltage when the battery is first connected to an "intelligent" battery charger.  After the battery has been on charge for some time (and assuming that is is not clapped out) its own voltage will nave risen to around 12.7V and the voltage applied by the charger will fall to around 13V - 13.5V to complete charging gently.  If a battery is charged at over 14V for more than a couple of hours the electrolyte will start to boil and steam will come out of the breather vent(s), albeit failrly gently. If the battery is "sealed" then that water loss from the electrolyte cannot be replaced and the effect is to reduce the amount of the plates that is sitting in the electrolyte and therefore the ongoing capacity of the battery.  If the battery is not sealed then the lost water can be replaced by topping up to just above the level of the plates.  But even with a battery that can be topped up excessive charging voltages (or too long at 14.4V) can cause irreparable damage as the plates will get hot and may distort causing internal shorting.  That is why sometimes a battery will fail because one cell has shorted or partially shorted, preventing it from taking a charge or giving out current.

So, my guess is that the readings that you took were soon after the charger had been switched on (14.4V) and then, as you say, 2 days later (13.1V) when the battery was virtually fully charged.  To reliably use that type of indicator of battery condition the charger needs to have been off for at least 10 minutes.  Then you might see 12.7V - 95%, 12.2V - 50%, 12.0V - 25%. If a 5A load was attached these voltages would all fall by about 0.2V.  A better way to measure the health of a battery (if it is not sealed) is with a hydrometer. The electrolyte of a fully charged battery should have a specific gravity of 1.265, at 50% that will have fallen to 1.19 and at 25% to 1.155.  If one cell has an SG markedly lower than the others then that cell is failing/has failed.  Don't measure the SG just after topping up - it is likely to be 1.00!

Hope that helps. (Don't take the figures that I have quoted as accurate - they are ball park)

Thanks again.  I will do some tests. What you say makes sense now. 
The crucial question now - is it safe to leave the unit in storage on a 'hook up' ?

As suggested above, some charging systems have quite a complex charging routine, which may look nonsensical until you know how it works.

For example, in its most recent variant, the Sargent EC328 system causes the battery charger to ~not~ charge the battery for a period (several minutes IIRC) after the charge function is turned on. This allows it to check the initial battery state, so that it doesn't try to "bulk charge" an already charged battery and cause it to gas. It will then switch to maintenance mode if fully charged, and so will continue to sit at around 13.7V, or it will jump to around 14.4V if it needs the charge topped up.

You don't say what make/model of system you have. If it's a recent one with a multi-stage charger then I would have no problems leaving it on charge all the time. I do with mine, though I swap the charge output between leisure and vehicle battery every week or so if it's sitting unused.

Hogmeister wrote:Thanks for that. What really puzzles me is that sometimes I see say 14.4v and 83% and a few days later 13.1v and 97%. This is what really confuses me.  These readings are when no power is being drawn other than the background devices.   I would have expected the two readings to be reasonably in line.

Hi , I would say your battery is ok.
What you are getting is a snapshop of your battery condition at the time you read it.
Much like taking your blood pressure.
If you really want to make any sense you can do electrolyte test as suggested. Usually difficult and maybe messy,take precautions.
There is also a test with a load on the battery that a garage could do.
What i would do is check the readings every day, at the same time ,assuming you are not using any load on the battery.
As you say you have a parasitic load on the battery, so readings will change slightly as you have found out.
Hope this helps .To be honest i would not worry about these readings.
There are much bigger puzzles in life.
regards
mike