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Designing a Battery Buffered System

Things are less complicated if there is an unstable mains grid available that does its job every now and then. In that case, all that is needed is a decent automatic charger that is capable of fully charging a battery of sufficient size. A switched mode charger with a wide range voltage input and sophisticated charging characteristics is desirable. This will help protect against the grid, which may provide varying voltages. Cheap chargers that feature a simple transformer may never charge your battery at all if the voltage of the grid is too low. A simple charger designed for 230 Volts AC will provide little to no charging current when operated at 200 Volts or lower. No matter how long it operates, it will never achieve a full charge. On the other hand, it will burn out if the voltage is a little higher than expected -or it will simply ruin the batteries after a while. An AC voltage stabilizer that prevents your charger from burning out due excessive high voltage may be a really good idea in many situations.

A battery buffered system looks like this: Suppose our device draws 7 Watts at 12 Volts. We need the service 24 hours a day -so the device will draw:

Figure 7.7: The complete battery buffered system.
168 Wh = 24h * 7 W

At 12 Volt the current in ampere would be:

14 Ah = 168 Wh / 12 Volt

Now, lets assume that occasionally we get a situation where the grid fails for one week.

98 Ah = 14 Ah/day * 7 days
1176 Wh = 98 Ah * 12 Volt

If we allow our battery to get discharged from 100% to 30% charge, thus consuming 70% of the capacity, we need a storage capacity of:

140 Ah = 98 Ah / 0.7

A truck battery is available with this size.

Usually power comes back for 5 hours a day, thus the system will run 19 hours on battery.

133 Wh = 19h * 7 Watt

Charging and discharging a battery is never 100% efficient. There will always be energy loss in the battery, so we have to charge with more energy than we get. Charging/discharging efficiency usually is about 75%.

177.4 Wh = 133 Wh / 0.75

We want to charge aggressively and achieve a full charge within 5 hours. Considering charging efficiency:

166 Wh = 148 Wh / 0.75

Converting to Ah:

14.8 Ah = 177.4 Wh / 12 Volt

Considering charging time:

2.96 A = 14.8 Ah / 5h

While we are charging the access point/router still draws power. 7 Watts equals 0.6 Ampere at 12 Volts:

3.56 A = 2.96 A + 0.6 A

We should consider that the charging process slows down near the end of the charge period. It would be better to have a higher initial charging current than calculated to achieve a 100% charge. A charging time of 5 hours is quite short, so a IUIa-charger with 8 Amperes or more is a good investment.

Even a cheap truck battery should last for 5 years, given that the electrolyte is checked frequently. Don't forget to use a low voltage disconnect circuit. It is not a mistake to oversize such a system to some degree. No matter how well designed the system is, the battery component will wear out and need replacement. In general, it is more cost effective to oversize the power source rather than batteries.

Last Update: 2007-01-25