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Power Management
This method is not really Charge”. The charger output changes
recommended for charging SLA from constant current to constant
batteries as it can often shorten voltage and now monitors the charge
battery service life due to poor current. When the charge current
control of the final fully charged drops to 0.05C Amps, which is 0.2
voltage. However, because of the Amps for a 4Ah battery, the battery
simplicity of the circuit and will have recovered approximately
subsequent low cost, taper current 70-80% of its charge. At this point
charging is often used to charge a the output voltage should be reduced
number of series connected batteries to 13.65V (2.275V/cell) - this is the
that are subject to cyclic use. When “Low Absorption Charge”. The
using this method it is recommended remaining 20-30% of the charge is
that the charging time is either carried out at this lower voltage in
limited or that a charging cut-off order to prevent over-charge. The
circuit is incorporated to prevent charger will stay in this mode until
overcharge. the battery is fully charged.
The final stage of the charge cycle
Two Stage Constant Voltage is the “Float Charge”; this can be
Charging: This method is a done by accurately maintaining the
recommended for charging SLA
batteries in a short period of time
Figure 2: Suggested Charge Profile
low absorption voltage level.
All the above charge voltages are
and then maintaining them in a fully based on an ambient temperature of
charged float (or standby) condition. between 20˚C to 25˚C. For the best
to prevent overcharge. Conversely as lower trace show the charge current. performance these voltages will need
Each of the above has its advantages temperature falls, the change voltage The first part of the multi-stage to be temperature compensated by
and disadvantages, but using a should be increased to avoid charge cycle is constant current approximately 4mV/˚C/Cell (reduced
simple charger design may not be undercharge. mode “Bulk Charge”; this should be at higher temperatures and increased
cost effective in the long term. Using a combination of the limited to a maximum of 0.25C at lower temperatures). There are
Checking battery condition and constant current charging and two Amps (a quarter of the battery limits to the battery operating
replacing batteries with lost capacity stage constant voltage charging capacity). For example, if the temperature and SLA battery life is
is very costly and environmentally techniques and also by monitoring capacity of the SLA battery being greatly reduced at higher ambient
unfriendly. So designing a charger to the battery terminal voltage and charged is 4Ah, then the constant temperatures. For more information
maximise the life of the SLA battery temperature a multi-stage charge current should be limited to 1 Amp. on this you will need to refer to the
is very important. profile can be implemented to reduce During this stage, the battery battery manufacturer’s datasheet.
Another important factor that has stress on the battery while giving the terminal voltage is monitored until
to be considered when charging an shortest possible charge time. the terminal voltage reaches 14.4V
SLA battery is temperature. As the Figure 2 shows the multi-stage (2.40V/cell). Silvertel | www.silvertel.com
temperature rises, electrochemical charge profile for a 6-cell battery that Once the terminal voltage reaches Tony Morgan is Senior Applications
activity in a battery increases, so the will achieve this. The upper trace 14.4V; the charge cycle moves on to Engineer at Silvertel
charging voltage should be reduced shows the charge voltage and the the second stage “High Absorption
www.cieonline.co.uk Components in Electronics September 2009 29
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