Battery Explosion: What's Causing It?

[charanth182]

Battery "Explosion"

Hi Everyone,

My knowledge base is very limited when it come to electrical. I'm dealing with a simple charging system that has had problems.

We have a system ran by 2 x 12V 5Ah sealed lead acid batteries in series. They are to be charged with a Schumacher CR2. Of about ten units so far produced there has been two which the batteries have "exploded" (see attached).

Many of the sources I've looked through mention that the battery may be overcharged, but isn't that what the CR2 would be designed to prevent? I've been in contact with the battery company (interstate) and trying to get in contact with Schumacher. I'm confident that with the knowledge base on this forum that someone unbiased would be able to point me in the right direction. Thank you!

 

[jim hardy]

Have you checked the temperature inside that box during charge?

At first glance, 1.5 amps into a 5ah battery seems okay.

Here's two articles with some basic information

http://www.powerstream.com/SLA.htm
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

 

[mjhilger]

In addition to Jim's suggestion; are you charging the batteries isolated from powering your system? If the current draw from the charger is such that the charger believes that the battery is still in a heavy charge mode; the charging voltage might be high (spec sheet shows 30V + possible) which would overcharge/fry the electrolyte. It seems that the charger you are using is intended to charge the battery isolated from any other circuits.

 

[charanth182]

Thank you both for your responses!

Have you checked the temperature inside that box during charge?

I can tell you that when this happened the box was warm enough to melt the batteries together. I've got a charger going right now and will be monitoring the temperature during a normal charge. We'll see if the the box is causing them to overheat.

Here's two articles with some basic information

http://www.powerstream.com/SLA.htm
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Thank you Jim for those articles as well as a lead if it turns out the charger is the problem.

In addition to Jim's suggestion; are you charging the batteries isolated from powering your system? If the current draw from the charger is such that the charger believes that the battery is still in a heavy charge mode; the charging voltage might be high (spec sheet shows 30V + possible) which would overcharge/fry the electrolyte. It seems that the charger you are using is intended to charge the battery isolated from any other circuits.

The charging of the batteries is completely isolated from the powering of the system. The battery case is removed from the system and hooked to the charger using the same connection: The charging circuit contains only the charger, a fuse, and the batteries. Mjhilger Is there any reason you might know of that the charger would still think it is in a high charge mode?

 

[jim hardy]

http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

says don't charge them above 120 degF, and i really don't know why.
Anyhow - two thoughts to discuss at the water cooler::
1. put your charger in good thermal contact with battery case . The Unitrode charger IC's i have used adjust voltage per temperature of the IC and it's the designer's job to locate the IC accordingly near the battery.

2. A thermal cutout inside the battery case on the charge line.

Observe the different slope of volts vs temperature for standby vs cyclic charging here...
http://www.powerstream.com/SLA.htm.
I'd open that charger and see what IC it's based on, read that datasheet carefully.

Hopefully you won't find a doggone microcontroller in there.

 

[dlgoff]

http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

says don't charge them above 120 degF, and i really don't know why.

It probably has to do with ...

As a lead-acid battery charge nears completion, H2 gas is liberated at the negative plate, and O2 gas is liberated at the positive plate. This action occurs 'cause the current required to charge the battery exceeds the current necessary to reduce the remaining amount of lead sulfate on the plates. The excess current ionizes the water in the electrolyte. H2 being less dense than air, leaks out of the battery. The hydrogen gas that batteries make when charging is very explosive. Thus, H2 is emitted only when the charging current exceeds the current causing electrolysis. BUT, Caution is must while doing home-experiments here... Also If temperature exceeds, H2 and O2 gases expand and BOOM. Maybe that's why a battery shouldn't near 50 C.

http://physics.stackexchange.com/questions/39558/what-is-the-volume-of-gases-liberated-when-a-battery-is-charged

Bold by me.

 

[meBigGuy]

I think dlgoff hit it. Lead Acid batteries need to be charged in a well ventilated area due to H2 and O2 gas production. I don't think Lead Acid batteries ever explode for other battery chemistry reasons. Can you link to a battery data sheet?

 

[charanth182]

Hopefully you won't find a doggone microcontroller in there.

I take it you don't like micocontrollers :) how come?

http://physics.stackexchange.com/questions/39558/what-is-the-volume-of-gases-liberated-when-a-battery-is-charged

Thank you for the additional information. When i talked to the manufacturer of the battery about venting they stated that since this battery was sealed it didn't need to vent. I'm sure the reaction still occurs but if it doesn't vent it would explain why the battery expanded like a balloon.

I think dlgoff hit it. Lead Acid batteries need to be charged in a well ventilated area due to H2 and O2 gas production. I don't think Lead Acid batteries ever explode for other battery chemistry reasons. Can you link to a battery data sheet?

Absolutely the data sheet can be found here.

 

[mjhilger]

I think the others have provided the answers you seek. Under normal circumstances, as the battery (gel cell) becomes more charged, the current falls off and the charger (if smart - electronics controlled) adjusts the voltage downward so as not to boil the electrolyte and present the gassing. Cool down your enclosure, time limit or modulate the charger, or monitor the temp and modulate the charger. Beyond that there should be no reason the charger should continue to charge at a high current (high voltage which induces the higher current).

 

[jim hardy]

the charger (if smart - electronics controlled) adjusts the voltage downward so as not to boil the electrolyte and present the gassing.

Observe once again
http://www.powerstream.com/SLA.htm
the gassing voltage drops to 2.3 volts per cell at 50C, which is well below the normal charging voltage at 25C. i only presume it continues to drop as temperature rises. So the smart charger needs to know the temperature of the battery , for an overheated battery looks to it just like an undercharged one, and obedient little robot that it is it'll keep on trying to bring voltage up.

I take it you don't like micocontrollers :) how come?

At surface level, because they render a product irrepairable.

I had to give up on Sunbeam electric blankets after having three of them that reduce their heat output as the room cools down. They are absolutely useless on cold nights.
The old fashioned mechanical thermostat type had a small heating element inside the controller so that as the room cools it raises the duty cycle . They are very reliable, i only opened one out of curiosity to see how it works.
Opening the new ones showed only a microcontroller and what i think to be a thermistor. I am absolutely convinced that either some programmer reversed his temperature correction algorithm,
or it's a ntc thermistor that overranges the adc when room gets cool, flipping MSB which delivers a negative number to firmware reversing temperature correction.
Found in a thrift shop one from 1950's with old fashioned mechanical thermostat. Works great. The occasional soft "click" that i hear in the night both comforts me and reminds me i am no longer at the mercy of some half baked software alogrithm, as i relish the warmth .

On a more fundamental level, i dislike the doggone things because they enable mediocrity. But that's a discussion for a philosophy forum. Not to mention I've strayed off topic.

Back on thread- - i do like the analog lead acid battery charger IC's from Unitrode because it's clear from the datasheet what criteria they use to swap between charging states. What can you find out about a microcontroller with only a proprietary part number on it?

old jim

 

[AlephZero]

I think dlgoff hit it. Lead Acid batteries need to be charged in a well ventilated area due to H2 and O2 gas production.

For "sealed" or "no-mainteance" lead acid batteries, H2 and O2 production when the battery is overcharged or charged too fast is bad news, because it permanently removes water from the electrolyte and will gradually degrade the performance of the battery.

This is not a problem with "wet" lead acid batteries that need to be regularly topped up with distilled water, but those are not very common now - if only because accidentally spilling sulphuric acid from the battery is not a nice thing to do!