Can a high charging current be harmful, even though the charging voltage is correct?

[BabaYaga]

Thanks again for your replies.

I took this description from GHA in post #24

in what's commonly known as the bulk phase on most boats the lead acid batts will take whatever current is available from the charger then the current they will accept will tail off as the voltage rises up to the absorption limit controlled by the charger.

to mean that during the bulk phase, given that the maximum current of the charging source is high enough, the current going into the batteries will start to decline even BEFORE the voltage has climbed up to the set point, after which the absorption begins.

However, reading the comments from lw395 and to some extent yourmomm suggest this might not be how things work.

A charger set to 50A bulk charging will turn off its 'bulk charging LED' as soon as the current falls below 50A because the voltage limit has been reached.
That 's what 'bulk charging' is.
The battery charger is operating as a source with a current limit and a voltage limit. Say 50A and 14.4V. If the load is drawing 50A, the volts will be below 14.4 and the current limit light will be on. If the current drops, the volts will rise to 14.4 and the voltage limit light will be on.

This seems to be saying that the current dropping below the maximum output of the source, on one hand, and the voltage reaching the set point, on the other hand, will always be one simultaneous event? In which case any reduction of current would signify that absorption has begun, if I understand correctly?
I'm not sure if I agree here, it would be interesting to hear other views on this.
I seem to remember, when charging by alternator, that the current indeed tapers off before the voltage has reached the regulator set point. But I guess there might be other reasons for this declining charging current, than diminishing acceptance of the batteries?
BTW, we seem to have opposing views on how regulation works. My understanding was always that the regulator reads voltage and adjusts (lowers) the current so that the voltage set point is not over shot.
But perhaps just an egg/chicken argument...

I've always understood that you'll get less charge accepted, if you charge at a higher rate (you can see this, whilst charging, as the charger will often switch to absorption phase on 50A, even though it remains in bulk charging phase if switched back to lower charging currents).

To me, this implies that the higher current output forces the voltage to rise to the absorption level, which triggers regulation to start cutting back current.
However, e.g. 45A in absorption is still more than, say, 30A in bulk.
Interesting that you seem to be able to switch between different current levels, which charger make is this?

But now I'm also wondering if charging at higher current repeatedly permanently lowers the amount of charge a battery can accept? Or whether, if you just revert to 2A charging, it will just revert to accepting the same charge 2A charge, it always allowed?

I wonder too.

 

[GHA]

Thanks again for your replies.

I took this description from GHA in post #24



to mean that during the bulk phase, given that the maximum current of the charging source is high enough, the current going into the batteries will start to decline even BEFORE the voltage has climbed up to the set point, after which the absorption begins.

However, reading the comments from lw395 and to some extent yourmomm suggest this might not be how things work.



This seems to be saying that the current dropping below the maximum output of the source, on one hand, and the voltage reaching the set point, on the other hand, will always be one simultaneous event? In which case any reduction of current would signify that absorption has begun, if I understand correctly?

This image doesn't show it very well as it was such a short discharge, but with my T105's the amount of current they will accept starts tailing off round about 14.4V. (Temperature might well have some effect). This is nothing to do with the current source , all the charger does is not go over whatever absorption voltage is set, the batteries decide what charge they will accept. 20A charger, 2 x T105's. Similar is seen with 120A alternator. (Which was a waste of money going bigger, the batteries have never accepted more than about 50A, though they never get too low over night. ).

Interesting link here about how little is gained with a bigger charger from 50%. For one set of AGMs anyway.

https://marinehowto.com/how-fast-can-an-agm-battery-be-charged/

EDIT - Thinking more about this I don't understand that, the amps are there so why wouldn't they push the voltage up to absorption? Seems too much for voltage drop. Dunno, the numbers should be accurate, voltage straight from batt terminals to within a few mV & current from shunt to easily within 0.5A, prob more like 0.1A. Does seem odd though....

 

[pvb]

This 'charge accepted' malarkey is really a bit of wooly language.
Where is the charge that is rejected?

This is nothing to do with the current source , all the charger does is not go over whatever absorption voltage is set, the batteries decide what charge they will accept.

There seems to be a difference of opinion here....

 

[lw395]

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Interesting link here about how little is gained with a bigger charger from 50%. For one set of AGMs anyway.

https://marinehowto.com/how-fast-can-an-agm-battery-be-charged/
......

That is quite a good link, it shows some real world data for charging a battery from about 50% SOC.
One of the scenarios is charging at 0.4C.
42A into a 105Ah battery.
It took about 20 minutes to reach 14.4V and become voltage limited.
That is some quite serious shoving in of current, and suggests that a 100A alternator would do similar for a bank of say 250Ah, if the alternator actually gave 14.4V and 100A simultaneously. And the wiring was really
low resistance.
People do not generally see this happening in yachts, because the alternator has output impedance and the wiring has significant resistance.

Towards the end of the document there is an assertion that high initial charge rates are good for the longevity of the battery.

 

[alahol2]

That article is very opinionated in places. I can't argue with his figures but I do have a problem with his language...

When a company tells you that “bulk” is a voltage limited stage of charging this is PURE MARKETING BOVINE DUNG. Simply put, bulk=constant-current charging not constant-voltage.

BULK – Bulk Charging is the constant current stage of charging where the charge source is limited only by what it can deliver in current. Bulk charging is not a voltage limited stage of charging despite many companies bastardizing the term bulk for apparent marketing purposes.

Bulk charging may well be constant current and that current may well be limited by the ability of the charger but how can you say it isn't "voltage limited" when the very thing that ends bulk charging is the voltage reaching (in his case) 14.3V?
The charger, by its own parameters, cannot continue constant current charging beyond 14.3V. It cannot apply more than 14.3V.

Or am I being totally dumb? (by no means beyond the realms of possibility)

 

[Lon nan Gruagach]

That article is very opinionated in places. I can't argue with his figures but I do have a problem with his language...


Bulk charging may well be constant current and that current may well be limited by the ability of the charger but how can you say it isn't "voltage limited" when the very thing that ends bulk charging is the voltage reaching (in his case) 14.3V?
The charger, by its own parameters, cannot continue constant current charging beyond 14.3V. It cannot apply more than 14.3V.

Or am I being totally dumb? (by no means beyond the realms of possibility)

Not dumb.
I think what most of this thread is, is a little knowledge is a dangerous thing. Except that what we really have is a whole range from little (not none) to really quite a lot of knowledge.
There are, as always, very specific technical terms that are being somewhat fudged. Not least because the technical terms are in fact theoretical only and are impossible in the real world.
Things like current and voltage sources cant really exist, but are used, here, with gay abandon.

Consider when you say that the bulk charging phase IS voltage limited since it ends when a voltage is reached... Time has snuck in there and fudged it. The voltage is only reached at a given state of charge (provided charge current is within limits) and that takes time. Voltage and current limiters that are time dependant need a whole new parameter added to their specification.

So, an ideal charger will have its bulk phase limited by the state of charge. of all the ways to measure state of charge, cell voltage is the most straight forward.

So, back to OP
Can you damage a battery with excess current if you stick to the voltage guidelines?
"Have you stopped beating your wife yet?" The question is somewhat irrelevant, or incomplete since it does not address the fact that the guidelines, during bulk phase, only refer to a indicator that signals the end of that phase.

What, I think, hasnt been covered is can you apply such current that, even considering internal and cable/ connector resistance, damage occurs without exceeding recommended charging voltage limits?
And the answer is "it depends". it depends on plate geometry, it depends on internal resistance. And almost certainly you wont manage it with an alternator and standard wiring and connections.


I did read recently how pulsed rejuvenating chargers, while they might revert sulphate build up (on the -ve plate) they almost certainly oxidize the +ve plate. So, you could get a bit more life out of a dead battery, but only at the expense of killing it (slowly) another way by over charging.

 

[lw395]

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...
I did read recently how pulsed rejuvenating chargers, while they might revert sulphate build up (on the -ve plate) they almost certainly oxidize the +ve plate. So, you could get a bit more life out of a dead battery, but only at the expense of killing it (slowly) another way by over charging.

My experience of a desulphator circuit is that it gets some useful function back into a battery that's otherwise ready for the recycling bin. I tried it on some car batteries, it improved their performance as 'house' batteries for lights in my lockup. But they were still weak in CCA and the useful Ah was probably improved from rubbish to poor. But it kept the lights going in my shed for a couple of years on batteries other people had thrown away.
The 'junkyard' characters in the world of 'off grid living' find they are useful.
Probably not relevant for yachties who can afford new batteries better than they can afford to carry twice as many half dead ones?

 

[lw395]

That article is very opinionated in places. I can't argue with his figures but I do have a problem with his language...

...

I agree.
You could describe the bulk charging phase as ' a voltage limited current source', or you could describe it as 'a power supply running at its current limit'.

I think real battery experts might tell us that 'bulk' charging is slightly chemically different from 'absorption', the difference being the efficiency of converting current in to charge stored that can be got out again.
Bulk charging is more efficient, absorption produces more gas. There isn't a fixed crossover point, there's a huge grey area in the middle. The battery charger industry has simplified this to the early/late stages being constant current/voltage. I am not a battery expert, but I do know the chemistry is more complex and subtle than I need to care about.

 

[Mistroma]

GHA's link did make interesting reading, I always like to see experimental evidence and details of the test procedure.

OP was primarily asking about Trojan T105s and I don't think the statement about high charge rates being beneficial is likely to be relevant to T105s. It was describing starved electrolyte mat, thin plates and high purity lead. Not a lot like the relatively thick paste plates in a flooded cell.

My own thoughts have always been along these lines:

1) A low charge rate is probably beneficial as T105 paste plates are relatively thick.
It takes a finite amount of time to achieve a state of equilibrium and there will be a gradient during charging. My assumption (it is just an assumption) is that a high current would be more likely to damage the plates than a low current.

2) I should aim to get back to 100% as often as possible (every day would be ideal)
Not getting back to 100% every day will reduce battery life but charging at a current above Trojan's specs. would also cause damage. It's going to be a balance and never perfect.

Trojan's 10-13% figure for 450Ah would mean I can aim for about 64A when charging. I'm always running a fridge and other kit during the day and 58.5A + 5A is a reasonable guess.

The reality on my boat is that I can get 40A from the mains charger (via generator) plus 20A peak at best from Solar. Similar output from the alternator instead of the generator. It only becomes a problem when I run the batteries down to 50%. Normal use during most of the summer is to find the batteries at ~85% each morning and usually get them back to 100% the same day using only solar.

Trojan's "conservative" figures might be relevant if I was charging a golf buggy. I imagine their design and preferred charging regime will work very well and give good lifespan. No need to charge as quickly as possible as golf buggies are on charge for a long time every night. I suspect that many sailing boats will need to worry too much about exceeding 13% regularly.