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Author Topic:   Battery Charging: Production of Gas

jimh posted 12-03-2009 08:31 AM ET (US)
Assume a lead-acid battery is being charged and the terminal voltage of the battery is limited to 14.3-volts. Will the electrolyte of the battery release any gas?
jimh posted 12-03-2009 08:56 AM ET (US)
A corollary question: when charging, what is the minimum battery terminal voltage which will produce a gas discharge from the electrolyte?
[Note--I deleted a sidebar discussion which dragged this topic into a new area. I deleted the sidebar because I thought it was only confusing and really had no relevance to the discussion---jimh]
K Albus posted 12-03-2009 09:58 AM ET (US)
A quick Google search reveals numerous websites which indicate that a lead acid battery will start to emit hydrogen gas when individual cell voltage passes 2.37 volts.
See, e.g.: http://www.batteryuniversity.com/partone-13.htm ("At 2.37V, most lead-acid batteries start to gas, causing loss of electrolyte and possible temperature increases.")
2.37 volts x 6 cells = 14.22 volts. So it appears that it would be possible for some gas to be emitted with terminal voltage limited to 14.3 volts.
jimh posted 12-03-2009 10:19 PM ET (US)
Kevin--Thanks for you reply on the topic of my inquiry.
I have also found other references that indicate that a charging voltage of 14.2-volts is a desirable maximum long-term charging voltage because is prevents the electrolyte from being driven into a chemical reaction where gas is released.
I have been charging a battery with a precision float charger which limits its voltage to 13.3-volts, and the charging has been going on for about a year. It is important to limit the charging voltage because this particular battery is a sealed battery, or, more properly, a valve regulated lead acid battery (VRLA). In a VRLA battery there is a general sealing of the internal battery from the atmosphere, but, should there be a build up of pressure, a safety valve opens to release it.
If gas is released from the electrolyte, and vented to the atmosphere, the level of the electrolyte in the battery is lowered. In a sealed battery there is no easy way to add electrolyte, so it becomes important to prevent gassing.
The VRLA batteries are also called "maintenance-free" batteries because you do not need to periodically add water to them to compensate for evaporation. However, this also prevents you from adding water should the battery need to have its electrolyte level replenished from losses due to gassing.
The 13.3-volt charger has apparently been working well because the battery it has been holding on float charge for a year seems to be doing well and hasn't appeared to have gassed out any electrolyte.

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Author	Topic: Battery Charging: Production of Gas
jimh	posted 12-03-2009 08:31 AM ET (US) Assume a lead-acid battery is being charged and the terminal voltage of the battery is limited to 14.3-volts. Will the electrolyte of the battery release any gas?
jimh	posted 12-03-2009 08:56 AM ET (US) A corollary question: when charging, what is the minimum battery terminal voltage which will produce a gas discharge from the electrolyte? [Note--I deleted a sidebar discussion which dragged this topic into a new area. I deleted the sidebar because I thought it was only confusing and really had no relevance to the discussion---jimh]
K Albus	posted 12-03-2009 09:58 AM ET (US) A quick Google search reveals numerous websites which indicate that a lead acid battery will start to emit hydrogen gas when individual cell voltage passes 2.37 volts. See, e.g.: http://www.batteryuniversity.com/partone-13.htm ("At 2.37V, most lead-acid batteries start to gas, causing loss of electrolyte and possible temperature increases.") 2.37 volts x 6 cells = 14.22 volts. So it appears that it would be possible for some gas to be emitted with terminal voltage limited to 14.3 volts.
jimh	posted 12-03-2009 10:19 PM ET (US) Kevin--Thanks for you reply on the topic of my inquiry. I have also found other references that indicate that a charging voltage of 14.2-volts is a desirable maximum long-term charging voltage because is prevents the electrolyte from being driven into a chemical reaction where gas is released. I have been charging a battery with a precision float charger which limits its voltage to 13.3-volts, and the charging has been going on for about a year. It is important to limit the charging voltage because this particular battery is a sealed battery, or, more properly, a valve regulated lead acid battery (VRLA). In a VRLA battery there is a general sealing of the internal battery from the atmosphere, but, should there be a build up of pressure, a safety valve opens to release it. If gas is released from the electrolyte, and vented to the atmosphere, the level of the electrolyte in the battery is lowered. In a sealed battery there is no easy way to add electrolyte, so it becomes important to prevent gassing. The VRLA batteries are also called "maintenance-free" batteries because you do not need to periodically add water to them to compensate for evaporation. However, this also prevents you from adding water should the battery need to have its electrolyte level replenished from losses due to gassing. The 13.3-volt charger has apparently been working well because the battery it has been holding on float charge for a year seems to be doing well and hasn't appeared to have gassed out any electrolyte.