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

[BabaYaga]

In a thread the other day

http://www.ybw.com/forums/showthread.php?514825-new-12v-installation&p=6687481#post6687481

I made a comment on what seems to be a fairly wide spread view on battery charging, namely that charging current must not be too high in relation to the total Ah capacity of the battery or battery bank being charged.
This view is supported by the recommendation that Trojan batteries make in their Users guide

https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf

where they say (p. 17, 18, 19) that charging current must be limited to 13 percent of Ah capacity for wet cells and gels and 20 percent for AGMs. (Capacity in Ah here defined as C20, which is most common).
I think this recommendation is very conservative and a bit surprising, even if I have read similar recommendations before.
The mainstream view, as I understand it, is that a battery will self regulate the current and only accept what it can 'swallow' – as long as the charging voltage is controlled (not allowed to rise above e.g. 14.4V or similar set point).
This perception goes well with the fact that alternators in common boat installations are likely to put out a current that by far exceeds 13 percent of Ah of the battery bank it charges.
In addition, makers of battery chargers also seem to have a much more relaxed attitude on the relation between maximum charging current and battery capacity. Victron says up to 25 percent of total Ah, Mastervolt says up to 50 percent and CTEK allows even higher.
So who is right here? Are Trojan batteries extra sensitive? (I always thought of them as capable of taking a lot of abuse...)
Opinions, views?

 

[halcyon]

Very complex thing, short answer ...... high charge rate results in lower charger level, due to incomplete conversion of active material.

Long answer gets into, is Switched Mode actually better than rectified transformer charger, which has a better charge profile, why that charge profile.

Brian

 

[A1Sailor]

Not really relevant to boating, but fyi my Electric Vehicle slows down its rapid DC charge once it gets to about 83% charge and stops accepting charge once it reaches 93%. I think this is to prolong battery life.
By Rapid I mean maximum 50kW. If I want to charge to 100% I need to switch to slower, 7kW AC charging. AC vs DC is possibly irrelevant, since the AC gets converted to DC - it will be the rate of charge vs remaining free capacity that matters. It is in nobody's interests to "fry" the battery. There is a certain amount of heat generated - in warm weather, which we do get in Scotland, a fan occasionally switches on below the bonnet during slow charging.
It is a Kia Soul EV with a "high voltage" battery capacity of 30kWh.

Edit: As an aside, the car also has a standard 12volt car battery which powers all the "ancillaries" - the same as an ICE car. It is kept charged by the high voltage system.

 

[William_H]

A1 sailor's story is of course related to lithium batteries so not really relevant. re lead acid I would be naturally leery of really high charge rates. However given most people have quite large battery banks and fairly modest charging capabilities and given that with correct (elevated) charge voltage actual current in is likely to be limited I don't see a concern. I suspect trogan's 13% of claimed AH rating of the battery would be a pretty good limit for any Lead acid battery.
ol'will (with no real answer at all)

 

[grumpy_o_g]

As Halcyon implies there's not a right answer - the current can be too low or too high but what is your aim? Do you want a quick charge rate but accept that you won't get such a high capacity? How full is your battery to start with? What a lot of people call "self-regulating" is actually an assumption that a charging system can only put out so much and obviously doesn't work too well if your charger can put out a few hundred amps. If it's just a 6 amp charger then it will charge comparatively quickly at first but the battery will indeed slow done the rate of charging naturally and will actually charge the battery to a higher level than a higher current would as Halcyon explained.

 

[Mistroma]

In a thread the other day

http://www.ybw.com/forums/showthread.php?514825-new-12v-installation&p=6687481#post6687481

I made a comment on what seems to be a fairly wide spread view on battery charging, namely that charging current must not be too high in relation to the total Ah capacity of the battery or battery bank being charged.
This view is supported by the recommendation that Trojan batteries make in their Users guide

https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf

where they say (p. 17, 18, 19) that charging current must be limited to 13 percent of Ah capacity for wet cells and gels and 20 percent for AGMs. (Capacity in Ah here defined as C20, which is most common).

Trojan don't say must be limited to 13%. I'm pretty certain that it is more along the lines of 10-13% is a guideline to maximise the life of these batteries. Similarly conservative for their AGMS, 20%, as you say. T105s are used a lot in floor cleaners and golf buggies where they get a good long overnight charge every day. Not much like a yacht when you want to grab as much as you can when it's available.

I expect that Trojan aren't bothered about that sort of thing and really do suggest the best charge regime for their normal use. They won't self-destruct if you charge at 20%. It's probably better to do that if it gets them back to 100% and 10% would only manage 85% every day.

Heat is another enemy of long battery life. Slower charge rates will keep battery temperature a bit lower. It seems reasonable for Trojan to suggest extended charging times which are acceptable to most of their core customers. It probably matches max. acceptance reasonably well for many boat batteries with usual charging kit anyway.

I think this recommendation is very conservative and a bit surprising, even if I have read similar recommendations before.
The mainstream view, as I understand it, is that a battery will self regulate the current and only accept what it can 'swallow' – as long as the charging voltage is controlled (not allowed to rise above e.g. 14.4V or similar set point).
This perception goes well with the fact that alternators in common boat installations are likely to put out a current that by far exceeds 13 percent of Ah of the battery bank it charges.
Probably not that common unless you have a really big hot rated alternator with a good regulator. I have 450Ah T105s with 80A alternator and that would be 18% of C20. Except it would fail pretty quickly if it managed to reach that figure. A more usual figure with my setup is 45A for a relatively short period and 60A is 13% at C20. Just a guess, I don't imagine too many people charge T105s at more than 15%. They'd probably be starting from 10% SOC anyway to get a high acceptance

I used to hear things like AGMs will accept up to 100% of quoted capacity and therefore ideal for yachts. I wonder how hot they get?

In addition, makers of battery chargers also seem to have a much more relaxed attitude on the relation between maximum charging current and battery capacity. Victron says up to 25 percent of total Ah, Mastervolt says up to 50 percent and CTEK allows even higher.

Perhaps shore power chargers quote 25% to allow for the fact that people want to arrive at a marina, use fridge, lights etc. whilst charging. It is useful to have some spare capacity so that batteries still get a decent charge overnight.

So who is right here? Are Trojan batteries extra sensitive? (I always thought of them as capable of taking a lot of abuse...)
Opinions, views?

 

[Lon nan Gruagach]

Very complex thing, short answer ...... high charge rate results in lower charger level, due to incomplete conversion of active material.

Long answer gets into, is Switched Mode actually better than rectified transformer charger, which has a better charge profile, why that charge profile.

Brian

Bit of a red herring there.
SMPS vs transformer does not really relate to regulation. Its the regulation that determines charge regime. Unless you mean an unregulated transformer supply, but thats not playing fair..

Heres a nice bit of light bed-time reading.. https://batteryuniversity.com/index.php/learn/article/charging_the_lead_acid_battery

First stage, constant current to 70% in 5 to 8 hours ties in with 10% of Ah rating...

and... https://batteryuniversity.com/learn/article/lead_based_batteries

 

[oldharry]

I was told that high rate charging a standard car battery can be done but there is risk of boiling and distortion of the lead plates. Lithium is a different matter as they can rapidly overheat and catch fire or explode. (see it done on You Tube). Extra care is needed with sealed lead acid batteries too as overcharging can cause them to burst, spraying hot acid everywhere!

 

[pvb]

Batteries effectively accept as much charge as they can, safely, as long as the charging voltage is correct. One aspect of a "correct" charging voltage is that it should be temperature compensated, so the charger should have a temperature sensor on the battery. Manufacturers specify the ideal bulk and float charging voltages. If these result in what some people consider to be too high a current, the only way to reduce the current is to lower the charging voltage.

 

[RichardS]

I rolled the extension lead out of the garage yesterday at lunchtime and put the battery of a car awaiting repair on charge. Bonnet left up, smart charger sitting on engine.

After it started to go dark I looked out of the window expecting to see the green "fully-charged" light ..... but it was still red. I was surprised that it was taking so long as I fully charged it only a month ago so I went out to see what was going on. I has set the charger to 12V Motorcycle rather than 12V Car. Doh!

Too few amps is as bad as too many. :ambivalence:

Richard

 

[halcyon]

Bit of a red herring there.
SMPS vs transformer does not really relate to regulation. Its the regulation that determines charge regime. Unless you mean an unregulated transformer supply, but thats not playing fair..

What is your concern ? I have made transformer chargers with more regulation / control than most SM chargers, it relates to using / optimising your charge curve profile.


Brian

 

[BabaYaga]

Thank you for your comments so far.

I don't mean to suggest that Trojan make their recommendation totally without any foundation. But I am curious to learn what is the nature of the harm done to batteries if the charging current exceeds the given limits and also how harmful it is – if at all.

It could be, as suggested, that the recommendation really is for having optimal charging conditions. But to me at least, giving such an exact and well defined interval as '10-13 percent of C20 capacity' (for flooded batteries) gives the impression that you had better not stray too far from there if you want to stay out of problems.
And they actually mention 13 percent as 'maximum current' in the Users guide (although they also say Contact Trojan Technical Support if charging time is limited).

Plevier suggests that a high current results i a lower level of charge – should this be understood as a result that can not be compensated by a proportionally longer absorption phase? Would be interesting if you could expand on this.

Heat build up is also mentioned as something that should be avoided, which makes sense to me. But is it related to current per se? I think I have heard ripple (AC leaking in) mentioned in this context, perhaps this is more relevant?

Some posters hints that in practice, with large battery banks, the charging current may not be much more than 13 percent (or 20) much of the time.
But consider for instance that Volvo Penta for the last 10 years or so has put a 115A alternator on all their small diesels. They claim it is capable of giving 100 A at normal cruising speed.
If so, a boat suitable for a D1-13 engine would have carry a battery bank of almost 800Ah in order not to over shoot the recommendation from Trojan.

 

[cpedw]

But consider for instance that Volvo Penta for the last 10 years or so has put a 115A alternator on all their small diesels. They claim it is capable of giving 100 A at normal cruising speed.
If so, a boat suitable for a D1-13 engine would have carry a battery bank of almost 800Ah in order not to over shoot the recommendation from Trojan.

The critical word there is "capable". The alternator will generate its output voltage and the batteries will absorb amps according to their SOC and the volts. Just because the alternator is capable of 100A doesn't mean it will deliver all those amps all the time.

 

[BabaYaga]

Just because the alternator is capable of 100A doesn't mean it will deliver all those amps all the time.

I fully agree.
But a boat of the size where a 13hp engine is suitable might well have a quite modest battery/battery bank.
Let's say a house battery of some 100 Ah, as an example.
When such a battery is well discharged and the engine is fired up and put to cruising speed, I imagine the charging current will be in the region of 100A, at least initially?
That would equal 100 percent of C20 capacity for a while, instead of the max 13 percent recommended.
If such a treatment indeed was harmful to the battery, one would think there ought to be some current limiting device that could be switched on on these powerful alternators.
But I have not heard of anything like that.

 

[pvb]

I fully agree.
But a boat of the size where a 13hp engine is suitable might well have a quite modest battery/battery bank.
Let's say a house battery of some 100 Ah, as an example.
When such a battery is well discharged and the engine is fired up and put to cruising speed, I imagine the charging current will be in the region of 100A, at least initially?
That would equal 100 percent of C20 capacity for a while, instead of the max 13 percent recommended.
If such a treatment indeed was harmful to the battery, one would think there ought to be some current limiting device that could be switched on on these powerful alternators.
But I have not heard of anything like that.

I wouldn't expect a 100Ah battery, even fairly depleted, to accept anything approaching 100A charging current for more than a few seconds, if at all.

 

[BabaYaga]

I wouldn't expect a 100Ah battery, even fairly depleted, to accept anything approaching 100A charging current for more than a few seconds, if at all.

On reflection, that might well be correct.

CTEK claims that their 25A marine charger (M300) is suitable for batteries from 40 Ah and upwards.
That equals some 62 percent of the Ah capacity, maybe a more realistic current level?
Still, a long way from 13 or 20 percent.

 

[pvb]

CTEK claims that their 25A marine charger (M300) is suitable for batteries from 40 Ah and upwards.

But that doesn't mean it'll push 25A into a 40Ah battery!

 

[lw395]

I wouldn't expect a 100Ah battery, even fairly depleted, to accept anything approaching 100A charging current for more than a few seconds, if at all.

It's unlikely in a yacht, because the alternator will have a finite output impedance, so you won't actually get 14.4V at the battery terminals. Even quite fat wiring has a significant effect at 100A.
Putting a kilowatt into the battery at moderate effiency will also generate some temperature rise.
The internal resistance of the battery is significant, and different between charge and discharge.

If you scale things down to a more manageable level, a 4Ah motorbike battery will take more than 4A quite easily. It doesn't have to be ever so flat.

I've measured some battery voltages while intermittently using a bow thruster, the terminal voltage tends to be noticealby low for some seconds after the end of a burst, suggesting the charge current is high enough to pull the volts down. And that's on a battery that was fully charged before being whacked with a few hundred amps of load for a few seconds.

'Accepting a charge' sounds a bit anthromorhpic.
You should avoid anthropmorphism with electrical components, they don't like it!

 

[pvb]

I've measured some battery voltages while intermittently using a bow thruster, the terminal voltage tends to be noticealby low for some seconds after the end of a burst, suggesting the charge current is high enough to pull the volts down. And that's on a battery that was fully charged before being whacked with a few hundred amps of load for a few seconds.

I don't understand what you've written. Are you saying that resistance in the charging wires is causing the lower voltage?

 

[AntarcticPilot]

Surely the charging amps are entirely controlled by the voltage and the internal resistance of the battery? Ohm's Law - E = RI! You can only control the amperage by changing the resistance of the circuit. But if you measure a voltage across the terminals of the battery, then the charging current is entirely controlled by the internal resitance of the battery,