[RE-wrenches] battery venting; Corrosion

[John DeBoever]

Wrenches,

Thank you for your patience (as I was busy all day).  I appreciate the valid issues raised and good perspectives provided so far by the Wrenches. I will try my best to clarify the hydrogen production issues as follow.


·         All lead acid batteries, flooded and VRLA require ventilation. Flooded lead acid generate much more hydrogen by design and require watering maintenance. VRLA AGM, per design, have up to 99% hydrogen recombination, so production of hydrogen is much smaller, yet still release hydrogen and require ventilation to keep the hydrogen content between 1% to 2%. Sealed enclosure is no go in term of safety, as 4% hydrogen content is highly explosive.

·         Here are the (approximation) ranges for Hydrogen gassing rates of Trojan deep-cycle flooded batteries for:
ü  Float Charging (2.20 VPC @ 25oC)                     1.0 to 2.5     cc/hr/Ah/cell
ü  Absorption Charging (2,47VPC @ 25oC)         2.5 to 4.5     cc/hr/Ah/cell
ü  Equalize Charging (2.58VPC @ 25oC)               4.5 to 14.0   cc/hr/Ah/cell

Please be advised that a range is given since Hydrogen production vary with the battery voltage during the said charge mode duration, quality of charge algorithm to tapper the current vs. voltage and total duration, and, vary over the life of the battery, ageing of the battery and application specifics. To be on safe side, I recommend to consider the higher approximate figure for conservative Hydrogen production value. Please note these are given at 77F (25C). Higher temperature will generate higher current levels and hopefully a temperature compensation feature of the voltage settings will take place to mitigate the impact.
Example: Trojan Signature T-105: 225 Ah @ 6V @ C/20: 225 Ah x 3 cells x 4.5 cc/hr/Ah/cell = 3,040 cc/hr during the absorption charge, @ 25C. During equalization @ 25C, production of hydrogen is intense: 225 x 3 x 14 =  9,450 cc/hr.  Use these values as inputs for further calculations for ventilation requirements per applicable standards.


·         The "float charging" mentioned above is kind of confusing to many. Indeed, there should be no mention of gassing during "float" charge mode, since we are below gassing voltage. It is mentioned for info here although not typical for deep-cycle products: the Hydrogen production is only a concern for UPS type of applications, where the battery stays at float charge for several days /months. Float charge mode is actually meant in "float" applications to mitigate the self-discharge of the battery due to its natural self-discharge. Indeed, even the very small float charge current build up hydrogen over time. Note: VRLA technology will generate less Hydrogen than flooded since the required float current will be less for VRLA technology (0.2 to 0.5% of C/20). Sorry for the confusion to cyclic applications minded Wrenches.



·         Here are the (approximation) range for Hydrogen gassing rates of Trojan VRLA AGM batteries for:

ü  Float Charging (2.25 VPC @25oC)                     0.01 to 0.019     cc/hr/Ah/cell

ü  Absorption Charging (2.45VPC@ 25oC)         0.019 to 0.025    cc/hr/Ah/cell

ü  Boost Charging (2.39 VPC @25oC)                    0.013 to 0.015   cc/hr/Ah/cell
Please be advised the "boost" charging is only acceptable on VRLA AGM, not VRLA GEL. It has the same role as the regular equalization event: to equalize the cells, although for a much lower voltage and very limited time. The "boost" charging is often not activated as a safety precaution on poor charge algorithms.

Please keep in mind that the above are approximations, as, when lead acid batteries issues are involved, "it depends" is the first words you hear from battery OEM's.  Hope this is helps.

Good evening,

John


From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Larry Crutcher, Starlight Solar Power Systems
Sent: Friday, January 06, 2012 6:20 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] battery venting; Corrosion

Hi John,

I am curious about what you saying "float or equalization". Here's my understanding: If the battery is able to accept all current presented, the electrochemical process, SO4 combining with H released through decomposition, is balanced and with little excess gas released. When the electrolyte becomes saturated there is no more SO4 to recombine and gassing is raised significantly, the volume increasing relative to voltage.  Thus, equalization charging produces the highest volume of H and O gas. However, when the voltage is lowered to float, the current is at the lowest state, about 0.2 amps/100ah capacity, decomposition at that point is practically nil.

Am I missing something or did you mean to say during the acceptance and equalization mode?

Larry Crutcher
Starlight Solar Power Systems


On Jan 6, 2012, at 9:00 AM, John DeBoever wrote:


Kent,

Thank you for pointing at the units. Please note that:
*         You are looking for cc/hr/Ah/cell rates at float and equalization voltages.
*         Gassing is a function of charging current (FLA: an ampere-hour of overcharge, i.e. during float or equalization mode, results in about 0.33 cc of H2O total gassing per cell)
*         For constant voltage (float or equalization) the current is a function of temperature and stage of battery life (plus any failure modes that begin to occur toward end-of-life)

I'll work up a response after a series of meetings.

John





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