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In my field-operating device I use a simple PWM step-down to charge a 6V 3.9Ah lead-acid battery from a 5W solar cell with a voltage of 7.2V. Unfortunately the DC regulator got damaged today and the battery is charged at 8.6V, that is 1.1V above the limits specified by the battery manufacturer. At the sunny days the charging time is approx. 8h. I have no access to the device, I cannot fix it right now.

My question is: what can happen to the battery, can it explode, set on fire?

I use three 1.3Ah batteries connected in parallel.

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This information can be found at various lead acid battery manufacturers, such as Power Sonic. Here is an excerpt from their FAQ:

Q: Does overcharging damage batteries?

A: As a result of too high a charge voltage excessive current will flow into the battery, after the battery has reached full charge. This will cause decomposition of the water in the electrolyte and premature aging. At high rates of overcharge a battery will progressively heat up. As it gets hotter it will accept more current, heating up even further. This is called thermal runaway and it can destroy a battery in as little as a few hours. For further information about charging please refer to our Technical Manual pages 12-19. To access our Technical Manual please go to the Literature - SLA Batteries section of our website

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    \$\begingroup\$ Destroying the battery is not my biggest problem in this very moment. What I am more afraid of is getting the battery exploded and started a fire (even though closed in polycarbonate box with a vent hole). Is that likely to happen? \$\endgroup\$ – tml Sep 17 '14 at 15:53
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    \$\begingroup\$ I haven't had the misfortune (yet!) of overcharging my SLA's to the point they've exploded. For example I connected a 12V charger to a 6V battery once, and it got extremely hot before I noticed and disconnected it. I would say it's possible they could start a fire, but I can't say how likely it is. \$\endgroup\$ – JYelton Sep 17 '14 at 15:58
  • \$\begingroup\$ Well here's some confusion for you: youtube.com/watch?v=AIT4BYJ2jCg and youtube.com/watch?v=d_TnsHu2u4c . I bet thermal runaway would lead to melting of the polycarbonate enclosure. But generally, they don't burn chemically (like Lithium Ion do!) Explode, yes... \$\endgroup\$ – rdtsc May 15 '15 at 5:17
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A doubt 5 watts of heat is enough to even get hot or explode the battery unless is was poorly vented such as in a sealed box.

What happens is the sulphuric acid electrolyte (H2SO4) liberates Hydrogen easiest from excess energy wasted and if there is a spark with H2 in a container it can be dangerous as 4% H2 plus any amount of oxygen is an explosive condition with a tiny spark. But with vented H2 it dissipates in the atmosphere quickly and rises (lightest gas) out of reach, so is not a problem. But in large car batteries under a hood with enclosed gas and an spark from a loose jumper cable, it could detonate. (big boom)

Most likely the battery has now dried its electrolyte and if after a long time the battery is worn out.

FYI Sealed gel cell batteries MUST have a vent . In external cases it must have an added Teflon plug or "H2 vent" to allow it to H2 leak in case of over charging as Teflon allows tiny hydrogen gas to escape but prevents moisture from getting into a sealed box.

In an open room, it is no sweat for such a tiny battery. If you had a small room full of big batteries , I would be cautious about venting.

You should be able to hear bubbling of H2 in a lead acid battery being created but watch out for ESD static discharges in an enclosed space. H2 gets easier to trigger above the lower explosive limit of 4%. Hydrogen is odorless, colorless and tasteless, so unless you can hear it, you cant see or smell it , but you may smell the sulfur-oxide gas which is often added to H2 tanks or from the sulphuric acid.

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    \$\begingroup\$ I also think that 5W divided over 3 batteries is unlikely to cause significant thermal effects. The extra hydrogen generated is also likely to be rather limited at the available solar charging available. Eventually the batteries will dry out and fail though. \$\endgroup\$ – KalleMP Jan 7 '17 at 19:53
  • \$\begingroup\$ 5W is sufficient excitation to eject H2, so the question is how gas-tight the enclosure is. SLA's are vented for this reason. \$\endgroup\$ – Sunnyskyguy EE75 Jan 7 '17 at 21:57
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I know this is an old question but since I stumbled onto it while investigating the effects of overcharging a Lead Acid battery I'll contribute my findings for whom it may be of interest.

As described in this Battery charging basics Article from PowerStream. Overcharging a battery causes hydrogen gas to be released.

Sealed lead acid batteries can recycle the generated gasses as long as they are being overcharged at less than C/3. However, leaving the battery to be overcharged even at C/10 will corrode the plates if left on for weeks at a time.

The amount of hydrogen gas generated is an important factor to take into account , especially in your case where your battery is in an almost closed box only with a vent hole for ventilation. As hydrogen is lighter than air it will move upwards, so if your ventilation hole is facing upwards or downwards will have an effect on the gas concentration accumulated within your box. The Lower Explosive Limit for Hydrogen gas is 4% which means that Explosion is a possibility above this concentration.

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The answer to this question can be found on Canbat Batteries' FAQ page.

  • Overcharging is simply when a charger overcharges a battery. Overcharging is the number one factor to braking a battery as the battery’s performance will decrease. This is why it is important to buy a smart charger that automatically shuts off once the battery is full.

  • Smart chargers come in different capacities, such as 1Ah, 3Ah, 5Ah and 20Ah. Depending on the size of your battery, your charger will need to have the appropriate specifications.

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    \$\begingroup\$ This answer does not seem to be specific to lead-acid batteries and only generically mentions what overcharging does. The "smart charger" isn't necessarily the desired device for trickle-change applications and this situation was one of a failed charger, anyways. \$\endgroup\$ – user2943160 Nov 27 '16 at 2:48

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