Glass Matte and Gel-Cell Sealed Lead Acid Batteries

Overview

Sealed glass matte or gelled (gel-cell) lead acid battery types are frequently used in wheelchairs and scooters because liquid (wet or flooded) cell lead acid batteries (like car batteries) can leak acid because they are vented (not sealed). Unfortunately lead acid batteries are easily damaged if they are left in a deep discharge condition, or if they are overcharged, so it is important to charge them up immediately after use and to use a quality charger designed for lead acid batteries.

12 Volt glass matte and gel-cell batteries are made up of six cells inside a battery, each of which is nominally about 2.15 Volts for a total of about 12.9 Volts charged, with no electrical load. Glass matte batteries use a very thick paste (thick like candle wax or toothpaste) of sulphuric acid electrolyte between the positive plates (lead dioxide) and negative plates (sponge lead) of the battery, with a porous insulator between plates so they don't short out. Gel-cell batteries are similar except the electrolyte is like a jelly (like Jello) and can therefore be damaged by freezing. The plates in a glass matte or gel-cell are relatively thin compared to wet cell lead acid batteries, so glass matte or gel-cells discharge/charge a bit faster than wet cells. Glass matte or gel-cells have no fluid movement within the battery like wet lead acid and are therefore no-maintenance because they cannot be topped up with electrolyte. For this reason, it is very important to have a high quality charger that doesn't boil off the electrolyte (called gassing), as this would dry out the electrolyte and cause the battery to fail. After several years of useful life, all rechargeable batteries should not be disposed of properly, contact you municipality for the location of a hazardous waste site. Do not dispose of lead acid batteries in regular garbage which goes in a landfill site.

Charge/Discharge Rates

Glass matte, gel-cell and wet lead acid batteries have a high internal impedance (resistance), so they will stay in a charged condition longer than nicad batteries before they self discharge, though glass matte and gell-cell lead acid batteries don't self discharge as quickly as wet lead acid batteries. Unfortunately this high internal impedance also means that lead acid batteries can't put out as much current (amperage) over a short period of time as a similarly sized nicad battery, plus glass matte or gel-cells can take longer to charge than nicad batteries.

A rough rule of thumbis that a glass matte or gel-cell lead acid battery can be charged at an average of up to 40% or more of it's Amp/hour rating up to the 80% charge level, then charge more slowly from 80-100% (i.e. take a minimum of 2.5 - 3.12 hours to charge a fully discharged glass matte or gel-cell battery using an intelligent charger). The charge pattern isn't linear and a lead acid battery will accept more of a charge when it is nearly empty (sometimes at a 60 - 70% rate) and the charger must gradually taper off that current charge or pulse charge as the battery starts to fill up (over 80% full) as well as limit the voltage, or it will overcharge causing the battery to get warm and gas, which damages the battery. A slightly warm battery during heavy discharge or charging is ok, but a really hot glass matte or gel-cell battery indicates that damage is being done to the battery. The maximum safe temperature to the battery is 125 degree F (52 degrees C).

Quality Charger And Regulator

Because Glass matte or gel-cell batteries are so easily damaged by overcharging, the charger must intelligently limit the voltage level and taper or pulse the current level when topping off of the charge. Voltage levels on a lead acid battery during charge should typically be regulated to be between 13.6 and 13.8 Volts for the float level, sometimes going as high as 14.2 Volts (maximum 14.4 Volts) for pulsing or end of bulk charge stage. Once a gel-cell is almost charged, the "float" level can drop to 13.2 - 13.6 Volts and can be trickle charged or pulse charged with very little current for several hours or days using a quality intelligent charger, without damage. Using a proper intelligent charger, glass matte or gel-cell batteries do not give off corrosive or explosive gases.

It is rare that a 12 Volt automatic CAR battery charger can do the correct job of charging glass matte or gell-cel lead acid batteries in the 6-14 Amp/hour size. Some can overheat and destroy the battery. 2/12/75 Amp fully automatic car battery chargers are poorly designed for small batteries (6 Amp/hour), overcharging and destroying the lead acid battery when set on the 12 Amp setting, despite it's claim of "automatic shut off when battery is fully charged". The problem with these models are that they only output in the 5-12 Amp range (doesn't taper current down to zero Amps), which can be too much current for small batteries. Remember that 40% of the Amp/hour rating is the ideal textbook maximum a glass matte or gel-cell battery should take, so a 6 Amp/hour glass matte or gel-cell should never be fed more than 2.4 Amps from the charger and a 14 Amp/hour glass matte or gel-cell should never be fed more than 5.6 Amps. Contact your local dealer of wheelchairs and scooters to get the correct charger for your needs.

Deep Discharge

As a general rule of thumb, the deeper you discharge a lead acid battery and the longer you leave it in that discharged condition, the more it will shorten the useful life cycles of the battery. Often it is a good idea not to discharge a lead acid battery beyond it's 50% charge level, but if you do deep discharge it beyond 50%, it is all the more important to get it charged up immediately, because sulphation can start to occur when a lead acid battery is left in a discharged condition (especially liquid lead acid) and this will dramatically reduce the useful life of the battery. An example of not discharging a glass matte or gel-cell beyond 50% would be to only use a 100 Watt 12 Volt light continuously for a maximum of 22 minutes on a 16Amp/hour battery before you put it back on the charger. It is not uncommon to be able to get 4-7 years of life out of a lead acid battery if it is properly taken care of and depending on the number and depth of the discharge/charge cycles. Glass matte or gel-cell lead acid batteries are less susceptible to sulphation and, for this reason, are more suited for deep discharging, which is common in video, marine, golf cart and handicap applications.

Battery Capacities

Liquid lead acid car batteries are rated in "Cold Cranking Amps", and rated in "reserve capacity" . Unfortunately those ratings are meaningless for some other applications such as for wheelchairs and scooters, so glass matte or gel-cell batteries are also rated in "Amp/hours", which is the number of Amps times hours at nominal 12 Volts at 80 degrees F (26.7 degrees C) that the battery can continuously deliver measured over a 20 hour period and going no lower than 10.5 Volts. For example, a 20 Amp/hour gel-cell battery should be able to deliver 1 Amp at 12 Volts (12 Watts) continuously for 20 hours before it is completely discharged. The rating is over 20 hours because glass matte and gel-cells have a high internal resistance. The quicker you discharge the lead acid battery, the less total Amp/hours you will get out of it. For example if you discharged this 20 Amp/hour battery at 2 Amps (24 Watts) you would only get 16.8 Amp/hours or 8.4 hours (84% of rated capacity). Discharged at 4 Amps (48 Watts) you would only get 14 Amp/hours or 3.5 hours (70% of rated capacity). Discharged at 10 Amps (120 Watts) you would only get 10.8 Amp/hours or about 1 hour (54% of rated capacity) before the battery was fully discharged or reached it's minimally acceptable level of 10.5 Volts.

Using A New Battery

There is a misconception that a battery must be fully discharged and charged 4 - 5 times during initial periods. This is not true. A deep cycle battery does not require a deep discharge at any time in its service life. In fact, for best results, it is recommended to shallow or moderately discharge the first 5 to 10 cycles.

Batteries and "Memory"

Lead-acid batteries do not develop a memory. This is peculiar with Ni-Cd batteries used in cordless phones and computers. Lead-acid batteries have the ability to cycle to various amounts of depth of discharge anytime during their service life without a memory developing inside the battery. So ignore this aspect.

Are They Still Any Good?

Batteries that are inactive for long periods don't act normal on the first discharge. They need a discharge followed by a vigorous charge to start accepting current normally. It may take more than one discharge and recharge cycle to make the batteries work as expected. If you've done this, and the batteries still refuse to charge and discharge properly, they are probably ready for the recycle bin.

The Overnight Test

Use the following table to determine the battery's state-of-charge. The best way to measure the state-of-charge is to check the specific gravity in each cell with a hydrometer. But this is not possible with a sealed battery. So the possible procedure is to measure the battery's voltage with a good quality digital voltmeter with an accuracy of 0.5% or better.

Capacity Testing

The best way to determine the health of a battery is a full blown capacity test. As previously mentioned, this involves charging the battery fully, and then placing a load on the battery which is about 5% of the expected capacity.

We suggest a capacity test at least once a year.

At the end of a full night's charge, using an accurate digital multimeter, check the voltage on each battery is not beyond 14.0V while the charger is still plugged in. If above this then it can be almost guaranteed the batteries are being over-charged and their life shortened (the acid "dries out").

The difference in the voltage of each battery should not be more than 0.25V between them. If more, this indicates a difference in the health of the batteries. It would not be considered a waste of time consulting the battery manufacturer for their charging specs, which varies by altitude and temperature.

Unplug the charger and allow the battery to stand unused for a half hour. The battery voltage should not fall below 13.2V. Then place a load on the battery (using a load tester) for about 30 seconds and if the voltage starts to drop away then the plates cannot deliver the current.

A simpler test could be to use the scooter itself as the load if one can accept the disadvantage of the scooter moving. If the handlebar meter dips badly as the scooter pulls away, then this is indicative of the battery starting to give up the ghost. This shows it is no longer capable of supplying the current required. Even though the actual electrolyte may have all the capacity (the stated 38 to 44Ah) the plates in the battery are starting to deplete - although do pay attention to probable dirty contacts that can show similar symptoms.

If the current can be delivered, leave the battery for a resting period of 24-hours and measure the voltage across the terminals with a good digital voltmeter. If the batteries aren't holding 12.8 volts (for gel cells), then they are in poor health.