What current would a 12 V lead-acid battery draw from a 24 V charger

[Jonathan212]

I know this is not ok but just for a few seconds or minutes and for the purposes of "boiling" the acid a little to make it even, break some sulfate deposits off the plates and recover a young battery that was neglected for a few months, what current would a 12 V / 480 A / 85 Ah lead-acid car battery draw if an attempt was made to charge it with a 24 V charger capable of 20 A, maybe with a 10 Ohm resistor in series?

 

[berkeman]

Thread closed for Moderation temporarily...

 

[anorlunda]

Please don't boil the acid, or I'll be forced to close this thread as dangerous.

You are talking about a so-called equalizing charge to desulfate the plates. The procedure is common and many modern chargers have automated equalizing cycles.

It is not a matter of lots of current, but more than a 2 A trickle charge. As that happens the battery voltage increases past the nominal 14.2 V that we usually use for charging. I had my charger programmed to bring the battery to 15 V and hold it there for 30 minutes, and to do that once every 30 days. That maintains the battery.

If you are trying to recover a highly sulfated battery adjust the current dynamically to hold 15 V, but maintain that for up to 24 hours.

Old fashioned battery shops in the days before OSHA, used to do much more aggressively. But their employees often lost extremities, eyes and other bad stuff . I'm not going to recommend that in 2019. Use this advice.

A power supply can also reverse sulfation. Set the charge voltage [10%] above the recommended level, adjust the current limiting to the lowest practical value and observe the battery voltage. A totally sulfated lead acid may draw very little current at first and as the sulfation layer dissolves, the current will gradually increase. Elevating the temperature and placing the battery on an ultrasound vibrator may also help in the process. If the battery does not accept a charge after 24 hours, restoration is unlikely. (See BU-804b: Sulfation and How to Prevent it.)

If all you have is a constant voltage 24V supply, I don't think you can do it safely.

 

[anorlunda]

To avoid more posts in dangerous territory, this thread will remain closed.

 

[anorlunda]

@Guineafowl has some newer information, that sounds useful and safe. I'm reopening this thread so that he can post it.

 

[Guineafowl]

These people have done all the hard work for you:

https://www.ctek.com/products/vehicle/mxs-5-0

This device has a “recondition” mode that boils the electrolyte in a controlled manner to eliminate stratification. It also pulses AC to desulphate the plates somewhat. All in a relatively cheap package that is safe and CE marked. It can be left unattended and will trickle charge indefinitely to maintain a battery.

I have one of these, and it managed to recover a failing, 16 year-old 120 Ah battery, which went on for another four years. Highly recommended.

Edit: I’m not sure if the desulphation is AC or just pulsed DC. Seems to work, anyway.

 

[Jonathan212]

Doesn't look like it boils the electrolyte in much of a controlled manner. It just applies 15.8 V. No temperature feedback.

 

[Rive]

Doesn't look like it boils

Because it doesn't. There is no acceptable working condition for a lead-acid battery when the electrolyte boils, anywhere. However, when too much voltage/current is applied during charge, or the battery got overcharged some gas bubbles are produced at low temperature. It is generally a negative sign, and not a goal.
During regeneration the factory specifications got violated intentionally and some 'boiling' of this kind might happen. But it is still not a good thing and still not real boiling.

 

[Jonathan212]

I think by boiling they mean circulating by heating, not reaching the boiling point. But it seems possible to reach the boiling point unintentionally if there is no feedback.

 

[Rive]

The point is not the temperature, and especially not the 'boiling'. @anorlunda and @Guineafowl had given you very good references to follow. Just forget the 'boiling' and concentrate on the conditions of 'de-sulfating' and 'regenerative' or 'reconditioning' charge.
And please don't try these with any makeshift equipment.

 

[Jonathan212]

The point is exactly temperature when risk of explosion closes the thread. The device offered has a risk of explosion, especially with batteries lacking some electrolyte like this one.

 

[Jonathan212]

What I was really asking was what the V-I curve of a lead-acid battery is beyond 12 V.

 

[CWatters]

That can depend on the state of charge. Its possible for the internal resistance to fall as the battery charges. Even discharged the resistance is low which is why connecting a 12V battery to a 24V source is potentially dangerous (for the battery, source/charger and wire used to connect them).

 

[Jonathan212]

For a fully charged battery, below 12.2 V the slope dV/dI is small (V = E - Ir) and above E the slope gets high (V = E - IR) or something?

 

[Guineafowl]

What I was really asking was what the V-I curve of a lead-acid battery is beyond 12 V.

As I say, these people have done all the hard work for you (see graphs below). Now, you could run the programmes below with a bench power supply, but pressing a button and letting a microcontroller do the work is a lot easier.

Regarding ‘boiling’ - this is a loose term referring to the off-gassing of hydrogen, oxygen, hydrogen sulphide and others when a battery is being charged, especially with an elevated voltage. It doesn’t mean heating the electrolyte to boiling point in this context. You’ll hear and sometimes see, if the battery has a clear case, bubbles fizzing up from the plates. If the vent(s) block, the battery can overpressure and burst, showering you with acid. The H and O are produced at an ideal ratio for explosive combustion. Any spark could trigger what is effectively an acid bomb.

Edit: Another advantage to the Ctek unit is that the leads are dead until a battery is detected, reducing the chance of a spark. Reverse polarity is also protected against.

I’ve seen this happen to someone who was being careless with jump leads - the battery exploded almost in his face. Thankfully he wasn’t leaning over the battery at the time, but there was acid everywhere. Since then, I’ve always made sure to connect jump leads properly, with the final connection made to chassis ground away from the battery.

 

[anorlunda]

What I was really asking was what the V-I curve of a lead-acid battery is beyond 12 V.

As others said, a static V-I curve is too simple. At constant V, the I changes dramatically as the battery desulfates. The real equation is not Ohm's Law, but rather Peukert's Law. But that won't help you.

Another advantage to the Ctek unit is that the leads are dead until a battery is detected, reducing the chance of a spark. Reverse polarity is also protected against.

It also senses battery temperature, thus providing closed loop protection against overheating.

The more I hear about this Ctek, the more I like it. Intelligence can not only deliver function, but also levels of safety that we never had before. Imagine what fun it was for the EE assigned to design and program that device.

From their web site, I see that Ctek offers a whole family of specialized chargers. I'll wager they are 90% alike, with 10% special.

 

[Jonathan212]

Seen a guy comment somewhere online that he had bad experiences with expensive multi-stage chargers and much better luck recovering batteries with cheap elementary desulfators and he was not selling anything. Can't find the link now.

"Static V-I curve is too simple" is like saying "diode characteristic is too simple" therefore no design should ever look at it in datasheets. Of course it is different with different conditions like most things, but you've got to start somewhere and add parameters as you go along. Peukert's Law is about discharging, not charging.

 

[anorlunda]

Peukert's Law works both ways.

The real answer is math. it is not an algebraic equation you need but a time domain differential equation, meaning one with differentials and integrals.

I don't know about your math background, so I didn't say that explicitly.

 

[Guineafowl]

From their web site, I see that Ctek offers a whole family of specialized chargers. I'll wager they are 90% alike, with 10% special.

Just like multimeters - you can never find one that does everything you want, so you have to buy several...

 

[Guineafowl]

Seen a guy comment somewhere online that he had bad experiences with expensive multi-stage chargers and much better luck recovering batteries with cheap elementary desulfators and he was not selling anything. Can't find the link now.

I’ll wager you’re looking for reasons not to buy an intelligent charger.

If you must DIY, a bench power supply would be the next best thing - I have a simple Lavolta 30V 5A model that gets used for all sorts of things. The CV and CC functions are ideal for testing circuits, charging batteries, injecting current to find a short, etc. Your 24V supply is likely to be too crude for recovering batteries.

 

[Jonathan212]

Not at all, that charger is super attractive and affordable, I'm considering buying it. Just got a feeling a specialized desulfator-only device would have better chances, and I've got 2 chargers already, and several laptop supplies too.

 

[rbelli1]

(for the battery, source/charger and wire used to connect them).

And the people and property around them. Then the EMT and firemen.

BoB

 

[Hebbe]

I have two Ctek with recond button. They make a good job, especially if the life span on the battery is perhaps at 50 % or newer... I have also tested several reconditioners (8 different), they should also be used right in time, a newer battery the better!
This one has specially made measurable effects, e.g. on a 9-year old 135 Ah Tudor Heavy Pro battery:
https://www.amazon.com/dp/B00IQ5X91I/?tag=pfamazon01-20