Tuesday, April 23, 2013

How to calculate ventilation requirements for a battery room ?

The following is an example of a calculation of ventilation requirements for a battery room with lead acid batteries.  Lead acid batteries release hydrogen during charging as well as on float (trickle charge after the full charge).  Hydrogen concentration above 5% is not a safe situation.  A spark can create explosion when the hydrogen concentration is high.  The following procedure explains ventilation requirements for a battery room to keep the hydrogen concentration below 1%.

This following calculation will show you the ventilation requirements during normal float conditions.

During abnormal conditions higher ventilation may be required depending upon the condition.

Amount of hydrogen release during normal float condition for a flooded battery is given as

VHy (cubic feet per hour) = 0.0148 x FC (where FC is float current in Amps)

If the battery room has N number of cells then the total volume of hydrogen released per hour

VTHy  = VHy x N (cubic feet/hour)

Let us assume the free volume of the battery room is Vr (cubic feet)

(free volume is the actual volume of the room minus the volume taken by objects in the room including batteries)

Time required to reach 1% concentration of hydrogen t = Vr/100 VTHy.

In other words complete air exchange is required in every t hours is enough ventilation to keep the hydrogen concentration below 1%.

Higher charge voltage, temperature, thermal runaway, overcharge condition will significantly affect the hydrogen generation rate of a lead acid battery.  Care should be taken to accommodate all the possible abnormal conditions.

If you have any questions feel free to contact us at (877)394-3941.

Thursday, April 18, 2013

Difference between flooded and VRLA batteries

Lead acid batteries are divided into two major technologies, namely Flooded and Valve Regulated.  Valve regulated lead acid batteries are also called VRLA or SLA (Sealed Lead Acid) batteries.  

Flooded Lead Acid Batteries:

  • The battery electrodes are immersed in the electrolyte (Sulfuric acid).
  • When the cell or battery is punctured acid will leak.
  • Lower float voltage compared to VRLA (SLA) batteries. 2.17 to 2.22V per cell.
  • Float voltage changes depending upon the acid concentration used.
  • Water loss with time occurs, and hence it is required to water as part of the maintenance.
  • Usually bulkier than SLA batteries.
  • Cycle life is lower than SLA batteries since the electrode material will shed with cycle life.
  • In general the capacity will reduce with time except in certain designs (Eg., Round Cell batteries).
  • Shipping is expensive because of large volume of free acid.
  • Normally not allowed in commercial air carriers.
  • Old/matured technology and well understood.
  • Higher ventilation is required in the battery room.
  • Spill containment is required around the battery stand as per the Uniform Fire Code (UFC).
  • Thermal runaway is not a concern.
  • Requires monthly maintenance in large standby applications.

Valve Regulated Lead Acid Batteries (VRLA/SLA): 

  • The battery electrolytes are held in glass mat also called separator.
  • When the cell is punctured there should not be any acid leak. Some manufacturers use excess acid and could have small acid leaks when the cells are new.
  • Float voltage is higher than flooded batteries (2.25V/cell plus).
  • No need to add water since the water loss is significantly lower than flooded batteries.
  • Takes lower volume compared to flooded batteries.
  • The electrodes are warped with separators and held tight.  The electrode shedding is generally reduced and hence the rate of capacity decay with cycle is also reduced compared to flooded batteries.
  • Higher cycle life compared to lead acid batteries.
  • Lower amount of hydrogen is release during normal operation and hence generally lower ventilation is needed compared to that of similar size flooded battery.
  • Thermal runaway is a concern in VRLA batteries.
  • Maintenance requirement is low compared to flooded batteries.
  • In general Spill containment is not needed - However, in UFC there is no differentiation between flooded vs VRLA batteries and depending upon the local fire marshals you may have to put spill containment for valve regulated batteries.
  • Thermal run away is a concern.  
  • More and more flooded installations are being with VRLA batteries.
If you have any question regarding this feel free to contact us.

Battery Consulting

Friday, April 5, 2013

Hydrogen evolution rate from lead acid battery

Do not charge any lead acid battery in air tight container.

All lead acid batteries release hydrogen during operation.  In flooded lead acid batteries the hydrogen evolution is normally much higher than that of (Valve Regulated Lead Acid) VRLA/ (Sealed Lead Acid) SLA cells of the same size.  Excess accumulation of hydrogen can lead to explosive conditions.

In VRLA/SLA batteries most percentage of hydrogen is generated are recombined to form water and hence only lower percentage is released.

Volume of hydrogen release may be approximated using the following formula for flooded lead acid batteries, after the fully charged condition.

Volume of hydrogen released (cubic feet/hour) = Vh = 0.015 x FC

Where FC is Float Current in Amps.

During charging majority of the current will go towards charging the battery.  Amount of current going towrds hydrogen generation will depend on many factors like, temperature, battery design, available current etc.,

For VRLA batteries under normal operating conditions at 77F depending upon the battery design 90 to 99% of the hydrogen is recombined.  Check with the manufacturer for recombination efficiency of a particular battery.

For calculating the amount of hydrogen released can be calculated by using the recombination efficiency.

If you have any question regarding this feel free to contact us.

Battery Consulting

Wednesday, April 3, 2013

What information I need before I try to buy wheelchair scooter batteries online ?

There are number of companies selling batteries online and also in store.  The prices of these batteries vary a lot depending.  Sometimes for the same product the prices can be very different depending upon the profit expectations, operational efficiency, cost structure etc.,

There are number of variations among the batteries.  Voltage, capacity, technology, terminals and dimension.

 By not paying attention to many of these facts might result in getting the wrong batteries, costing more money in shipping and more importantly time and inconvenience.

Before you try to find the right battery it is good to know what you have first.

Step 1:

      Find out the make and model of the wheelchair/scooter you have.  Sometimes the model number may be difficult to find.

Step 2.

    Find out the battery you currently have.  Make and model number of the batteries will be good.

Step 3.

   Find out the terminal type - This is one of the main reason that the replacement batteries are often returned back.  In general there are three different type of terminals.

FP/L1/L4 type - Flag Post terminals- the Flat terminals stick up with hole and with at nut and bolt.

RT type - Recessed Terminals - the terminals are flushed to the surface of the battery with a female inserted thread.

Z1 type - Round Terminals with a hole sticking up the battery. (Usually for batteries with capacities higher than 40Ah) with nut and bolt.

If you have all the above information then you may not need the dimension of the battery.  If you have a battery that is not popular then it is good to get the dimension also.

You can call one of the quality battery supplier to order your batteries.

Battery Consulting

If you need additional help feel free to e-mail us.

Wednesday, March 27, 2013

How long can I leave the wheelchair batteries without charging?

All Lead acid batteries get slowly discharged on open circuit called self discharge.  During discharge the battery voltage will slowly decrease.  When the voltage reaches below certain voltage it can permanently damage the battery and may not be able to recharge. 

When the battery is fully charged and left with out recharge for about six months at 77F, can be recharged back. 

Higher temperatures accelerate the discharge rate. This means when a Lead acid battery is left on open circuit at higher temperature have to be recharged sooner than 6 months. 

When the batteries are not fully charged the voltage is already low the batteries have to be recharged soon.  Never leave a fully discharge lead acid battery with out recharge for a long time. 

If you are leaving on a long trip, fully charge the batteries before you leave.  Don't forget to unplug the batteries from the charger.

Over charging the lead acid batteries can cause the batteries to loose water from the electrolyte   Loosing water can reduce the life of the batteries. Unplug the charger after the battery is fully charged.  Avoid leaving the batteries on charge for extended period of time.  If you still have questions feel free to contact us at (877)394)3941.

Battery Consulting

Thursday, December 1, 2011

Can I leave my Wheelchair scooter in my garage during winter?

During winter time many people may not use their wheelchair much and preferred to leave it in the garage.  The the question is what happens to the batteries.  Most of the SLA batteries use sulfuric acid as electrolytes with a specific gravity of 1.300.  The freezing point of this sulfuric acid used in SLA batteries is (-71 C or -95F).  The lowest temperature ever recorded in US was in Alaska at it was (-62C or - 80F) on January 23, 1971.

Looking at the above data the electrolyte will not freeze during winter if you leave the wheelchair/scooter in your garage in USA.  However, some of the plastic parts in the wheelchair/scooter might get brittle and one need to be careful while handing plastic parts at cold temperatures.

Battery Consulting