If you use your wheelchair daily, then you should charge it daily. You do not want to get caught out with low batteries! This is especially important if you use your chair or scooter outside your home. If you only use your chair or scooter periodically, then charge before you intend to use it and charge again after use. The ideal recharge time would be when your gauge or voltmeter shows that the battery is about 50%. But remember, if you do not use the battery and keep it on charge or perpetually leave it in a discharged condition, you will eventually damage its ability to meet your chair's/scooter's power needs.
In many respects, undercharging is as harmful as overcharging. Keeping a battery undercharged or continually undercharging allows the positive grids to corrode, lead sulphate to build up, and plates to shed, which can dramatically shorten life. Also, an undercharged battery must work harder than a fully charged battery, which contributes to shortened life as well. A constantly undercharged battery has a significantly reduced capacity because of the effects of acid stratification. It will easily be inadvertently over-discharged and eventually damaged.
A specific time is difficult to determine because recharging depends on so many variables:
- Depth of discharge
- Temperature
- Size and efficiency of the charger
- Age and condition of the battery
The initial charging current with a Discover battery should be 15% to 30% of the battery's C20-hour capacity rating.
It will take about 60% of the total charge time to bring a VRLA DRY CELL AGM or GEL battery from 0% SOC to 95% SOC. It will take the remaining 40% of the total charging time to put the last 10-20% of the charge back into the battery.
Charge is a quantity of electricity equal to the rate of flow (Amperes) multiplied by time (hours) and is usually expressed in Ampere-hours (Ah). Once the charger has been turned on for 1 to 2 minutes, the charge rate in amps will indicate the approximate charge time in hours. A battery with a 0% state of charge is defined as having been discharged to a point when the terminal voltage is equal to or less than 1.75 volts per cell (10.50 Volts for a 12-volt battery) measured under a steady load at the battery's 20-hour rate at 80°F. The 20-hour rate is the battery's capacity divided by 20 hours.
Typically, the total charge (capacity of the recharge) that must be returned to a VRLA DRY CELL AGM or GEL battery to achieve a 100% state of charge is from 104% to 112% of the charge removed. For comparison purposes, the returned charge for flooded electrolyte batteries must be between 115% and 130% of the charge removed.
NOTE: Variables such as the rate of charging current, ambient temperature during the charge cycle and the control of the voltage during the charging cycle will impact the ability of the battery to be properly replenished and the ongoing performance of the battery.
You will reduce the life of your battery by between 5% and 7% for every 1/10th of a volt above the recommended charge voltage. For example, suppose you continually charge your battery at a temperature-compensated value of 14.60 volts when it should be at 14.20 volts (4-tenths over). In that case, you can expect to reduce the life expectancy of your battery by 20% to 30%.
As mentioned, VRLA DRY CELL AGM and GEL batteries are voltage sensitive. All batteries are voltage sensitive. If you continually charge your VRLA DRY CELL AGM or GEL battery over the recommended voltage levels, you will shorten the life of your battery.
All lead-acid batteries give off hydrogen from the negative plate and oxygen from the positive plate during charging. VRLA DRY CELL AGM and GEL batteries have pressure-sensitive valves. Without the ability to retain pressure within the cells, hydrogen and oxygen would be lost to the atmosphere, eventually drying out the electrolyte and separators. Voltage is electrical pressure. Charge (ampere-hours) is a quantity of electricity. Current (amperes) is electrical flow (charging speed). A battery can only store a certain quantity of electricity. The closer it gets to fully charged, the slower it must be charged. Temperature also affects charging. A battery will accept charge at its ideal rate if the correct pressure (voltage) is used for the temperature. If too much pressure is used, the charge will be forced through the battery faster than it can be stored. Reactions other than the charging reaction occur to transport this current through the battery – mainly gassing. Hydrogen and oxygen are given off faster than the recombination reaction. This raises the pressure until the pressure relief valve opens. The gas lost cannot be replaced. Any VRLA DRY CELL AGM or GEL battery will dry out and fail prematurely if it experiences excessive overcharge.