Lead acid battery overvoltage

[mattko]

Hi all,

Does anybody have any information on what is an acceptable level of overvoltage for a lead acid battery?
I'm talking:
-ESD pulse discharge - up to 16 kV
-Surge testing - up to 6 kV, 8/20us pulse
We did some testing with a LC-filter to limit the voltage/current on the battery, but I would like to know more to be sure we are on the safe side. I read that these things can explode and throw acid everywhere if the buildup of oxygen and hydrogen ignites.   

I haven't found anything in the datasheets or from the manufacturer.

Any ideas?

[tszaboo]

Yes, it can generate gasses, and explode, if you leave it connected to a charger that overcharges it.
The same way, if you connect the same charger to dump energy into just bottle of water, it can do nasty things.
It is not going to explode on a ESD event or even on a typical surge event (I'm not talking about direct lightning strike into the battery terminal). An ESD event carries tiny amount of energy. Otherwise, how could a simple SOT23 diode dissipate all that energy. Surges are the same way. It has enough energy to bypass that few nanometer of gate material in a transistor, and the broken transistor will get some extra (lot more) energy from the power supply of the circuit.

[Stray Electron]

   How/WHY are you generating 16kv discharge pulses?   And what's with the 6kv 8/20uS pulses?   And why are you using a LC filter?  LC filters are mean to deal with AC power not DC power.  In a DC circuit an LC filter isn't going to significantly limit the voltage or the current going to or from a battery.

  Nothing that you said makes sense in a battery circuit.

[Miyuki]

Lead-acid battery tend to have very low internal resistance for pulses
And gases are released just during long overcharging
Short pulse will be just absorbed
You can add some big film capacitor and MOV if it will be high current pulses, but for ESD is battery just dead short

[mag_therm]

If it is an acceptance test, you could try to get a dispensation on the LA battery.

If there is a mix of air and hydrogen in the cells , a spark might make it detonate.
I saw a battery explode like that one time from an arc on the terminal.

(In my day, at least,) overpotential tests had provision for a list of exemptions, where the parties agree to either disconnection or short circuit.

[bdunham7]

What size/type of battery?  Perhaps a model number? 

[mattko]

I have four of these ones in series: http://nppbatteries.com/static/NP12-20Ah.pdf

LC filter does limit the pulse significantly. If you choose a big inductance and capacitance the cutoff frequency is very low. So the DC current will pass, but any pulses will get much lower.

I had also some varistors next to the batteries and a circuit breaker, to protect from potential short circuits.

[bdunham7]

I have four of these ones in series: http://nppbatteries.com/static/NP12-20Ah.pdf

LC filter does limit the pulse significantly. If you choose a big inductance and capacitance the cutoff frequency is very low. So the DC current will pass, but any pulses will get much lower.

I had also some varistors next to the batteries and a circuit breaker, to protect from potential short circuits.

I'm not sure what problem you are trying to solve or whose concerns you are addressing, but one thing LA batteries are good at is absorbing surges.  Your four batteries in series probably have combined internal resistance of less than 0.1 ohms.  If you are trying to figure out what happens in a surge condition, specifying the voltage without the source impedance is not helpful, you need to know the peak current that might be available.  The ESD test is a non-event for even a small SLA like these.  As for the 6kV surge, if the source impedance is 12 ohms and you ignore inductance, the peak current would be a little less than 500 amps.  500 amps and 0.1R means that you would have a 50V increase in battery voltage for 8 or 20 microseconds.  Those batteries won't even flinch.  The batteries will absorb surge currents much more effectively than anything else you are proposing to put in the circuit.

[mag_therm]

The 8/20 pulse with rise time of 8 us will have harmonic content like 32 kHz.

DFRA == (Digital Frequency Response Analysis)

There is a Battery Council International paper "DFRA/DFIS – A New Technology for Battery Instrumentation and Testing"

where a 12 volt automotive battery was tested. ( Paragraph 3 page 3)

The battery  was capacitive at low frequency  and passed through series resonance at 250 Hz then became inductive.

The whole paper (undated but fairly recent I think)  is quite interesting

[T3sl4co1l]

Compare the charge of those pulses, to the charge stored in the battery.

You'll find it's utterly negligible, at least unless the 8/20 is scaled to simulate direct lightning strike... in which case you have bigger problems than mere acid spatter.

In addition to the usual, slow, battery behavior, batteries also have quite large capacitance at shorter time scales, due to the combination of electrode area and electrolyte dielectric constant.  Ionic surface layers store a fair amount of charge by themselves.

Also, for high enough currents, the chemistry won't matter very much as the sheer voltage drop across internal resistances (mostly the electrolyte) dominates; in this regime, the battery doesn't act so much as an absorber, as a resistor.

Large peak currents probably have implications for disturbing (stirring up or mechanically shocking) the battery structure itself, but other than that, they're very much just big lumps of metal and other bulk chemical stuff that doesn't give a rat's ass about tiny electrical stimulus.

   How/WHY are you generating 16kv discharge pulses?   And what's with the 6kv 8/20uS pulses?   And why are you using a LC filter?  LC filters are mean to deal with AC power not DC power.  In a DC circuit an LC filter isn't going to significantly limit the voltage or the current going to or from a battery.

  Nothing that you said makes sense in a battery circuit.

16kV easily arises from plain old carpet.  8/20us pulses are typically used to simulate induced lightning transients.

ESD is trivially handled by a battery as the charge is minuscule (16kV * 330pF = 5.3uC = 1.47 nAh), but more importantly, the connecting wires will have significantly more impedance (stray inductance or whatever), being that the frequency content peaks in the 10s to 100s MHz range.

I don't know what surge would be doing there.  Surge is most often a mains power test, and a battery has absolutely no business connecting to mains.  Surge can also appear on long wires (mains wiring just happens to be the most common example), but I don't know where you'd be running 12V or so over very long distances (e.g., facility wiring, alarm systems, telco, automation and control..).

Tim

[mag_therm]

 8/20 pulse with rise time of 8 us, the battery should look like an inductor.

Apart from confirmation of that in the paper I referenced in post 8,
I was once involved in filter design of the no-break 800/1600 amp scr rectifiers for telephone exchanges  that were in parallel with 48V  battery banks.
In the last stage of the filters ( two or 3 sections), the battery was simulated  by an inductor, so the last filter stage was just a L L divider.
Those filters were subjected to testing by A weighted  psophometer
 (say that word after 2 beers!)

[Zero999]

   How/WHY are you generating 16kv discharge pulses?   And what's with the 6kv 8/20uS pulses?   And why are you using a LC filter?  LC filters are mean to deal with AC power not DC power.  In a DC circuit an LC filter isn't going to significantly limit the voltage or the current going to or from a battery.

  Nothing that you said makes sense in a battery circuit.

LC filters are often used for short pulses, which have a significant AC component and are often treated as AC.

The battery will be fine when zapped with typical ESD pulses.

[TheMG]

You would need a heck of a nasty transient to overcome the internal resistance of the battery enough to cause an arc between the tops of the plates (which shouldn't be exposed in a flooded lead-acid battery, but are exposed in an AGM battery).

This is not something I've ever heard of happening before. Usually if a lead-acid battery blows up it's from an external spark during charging while the battery is venting hydrogen/oxygen, an inter-cell link within the battery going open under load/charge, or some loose metal bits causing a short across the tops of the plates within a cell. Even then, these are relatively uncommon occurrences and affect mostly flooded batteries.

[trobbins]

Mattko, hopefully it only took the first one or two responses to alert you that what you have queried makes no sense at all so far.  You really do need to open up about what you are doing and why. and give as much detail as you can possibly link to.

[mattko]

Thanks for your answers.

What we are doing is surge testing (indirect lightning strike - 8/20us) for outdoor telecom equipment (-48VDC). Direct lightning strike would be 10/350us pulse with a much higher current. Surge generator has a 2 Ohm or 12 Ohm output impedance. We need enough current to power the circuit so we chose the LA batteries. You can also easily damage a power supply with a surge or even ESD events if you don't use filters.

I'm not sure if comparing charges that are stored in the battery to the surge charges is the right approach.
I was hoping there is more information about this topic, but we will still use LC filter + varistors and hide the batteries behind some metal, so that we have several layers of protection. I don't want to be sprayed by acid in any case

[tautech]

Thanks for your answers.

What we are doing is surge testing (indirect lightning strike - 8/20us) for outdoor telecom equipment (-48VDC). Direct lightning strike would be 10/350us pulse with a much higher current. Surge generator has a 2 Ohm or 12 Ohm output impedance. We need enough current to power the circuit so we chose the LA batteries. You can also easily damage a power supply with a surge or even ESD events if you don't use filters.

I'm not sure if comparing charges that are stored in the battery to the surge charges is the right approach.
I was hoping there is more information about this topic, but we will still use LC filter + varistors and hide the batteries behind some metal, so that we have several layers of protection. I don't want to be sprayed by acid in any case

First you need protect against lightning strikes with a lightning rod and with a remote earth field.
The cabinet and all associated equipment needs be earth bonded and fed to another earth well remote from the lightning rod earth. A few meters apart is sufficient however the lightning rod earth needs be 2 pegs 5 meters apart while the cabinet earth bonding earth needs only one peg.
The batteries and supply to inverters and such should be isolated from the cabinet and as such the insulation of cabling will be enough to protect the whole installation.
The 'telecom' equipment can look after itself if installed to manufacturers instructions although additional protection can be gained from using data cabling with an additional bond wire and RJ45 plugs with metal shells.

The above describes how the below installation was done using 4*12V 165AH SLA's in a series/parallel configuration for a 24V system. 3*295W solar panels provide more than enough power to keep this remote installation fully powered 24/365.

[trobbins]

Does that mean you are using a battery to generate a high DC voltage, and then switching that source DC voltage on to an intermediate capacitor, and then the charged capacitor is switched to a transient shaping network that is connected to DUT ?  The shaping network would be to some telco standard, and you would have tested it's performance to the standard.  That type of test generator scheme disconnects the battery from the DUT, and can be floating, and suitably insulated from anything else - which wouldn't then be a concern (?)

[mag_therm]

OK, so you are just using the LA batteries to power the surge generators and have added
LC and varistor between batteries and surge gen.
Is that right? If so it sound OK to me.

[mattko]

OK, so you are just using the LA batteries to power the surge generators and have added
LC and varistor between batteries and surge gen.
Is that right? If so it sound OK to me.

No. Surge generator is powered from 240 VAC mains. Output of the surge generator is connected to DUT DC power lines. DUT is powered by 4x series 12V batteries. LC filter + varistors are between the batteries and the point where surge generator output connects to the DC power lines. DUT needs has a high current consumption (especially during the surge event), so we need high current for that.

[mag_therm]

So the input to L is directly receiving the 8/20 pulse?
If so L needs to be physically able to withstand the 6 kV and not flash over.

[trobbins]

As per comment of L being able to withstand the surge across it, the layout of the C and MOV's should be such that the battery connects to the C terminals directly, to mitigate any influence of layout and cabling.  So I can't see any other concerns with respect to the battery.