ånønÿmøu§ said:
You can use a 12V relay with both normaly open and normaly closed contacts.
Just wire the coil to the primary voltage source. So, when the voltage goes away,
the relay switchs from the "NO" to the "NC" contacts.
I like'm when there simple!
Basically a relay which is normally kept operated by the AC supply. When the
AC supply fails relay releases and its contacts supply the 12 volts.
Considerations are that the relay must be suitable for continuous use, its
contacts must be able to switch and carry the 12 volt current load (also
check to see if the 12 volts includes any inductive/capacitive reactance,
because this increases the chances of excessive sparking and burning of the
contacts) and also that if you get power hits/surges and/or brief
interruptions the relay will tend to release and then re-operate quickly
each time which may be undesirable? So then you might want to look into time
delay relay circuits that will 'hold on' for a few seconds?
Words of caution about keeping batteries on what I will call 'Continuous
Charge'.
You can either ruin a battery by continuously overcharging or keep it in
such a state of undercharge that in either case that when required it is not
capable of performing!
The voltage of say, a typical (six cell lead acid) car battery is
'nominally' 12 volts.
Once charged such a battery, at normal temperatures, will maintain 12 volts
(that's 2.0 volts per cell) until the point at which the battery is almost
fully discharged; at which time the voltage, depending on the load drawn
from it, will quickly 'tail off'. (In other words she's empty!)
Using voltage to try and tell the state of charge of a battery is not a good
indicator. If some time after the battery has come off charge the voltage is
12 volts that may indicate it is somewhere between fully charged and
approaching discharge.
The 'charging voltage' of a '12 volt' battery is about 14 volts. (that's
about 2.3 to 2.4 volts per each of the six cells).
Once the battery is fully charged pumping any further input into it becomes
heat and can, if prolonged, damage the battery; I have seen where a battery
has been just about 'boiled dry' and was ruined.
Also a fully charged battery left standing will gradually self discharge,
due to 'local chemical action' and after a few months can become useless.
People sometimes store away their recreation batteries in a discharged state
and next season find they are useless!
Once a battery is fully charged and 'standing by' ready for use many
administrations carefully 'float' their batteries, depending very slightly
upon the type of lead acid cells, at between 2.15 and 2.20 volts, per cell.
A typical figure for the Bell Telephone System, for over 50 years, for
example, was 2.17 volts per cell; chosen to offset local chemical action
without overcharging. In those large installations battery acid specific
gravity was normally checked on a regular basis, due to the large investment
and the critical nature of the use. e.g. 911, Ambulance services, Fire Dept.
phone systems etc. However these days car batteries are now basically sealed
and rarely tested that way.
For our typical car type battery that's 6 x 2.17 = 13.02 or approximately 13
volts . 'Floating'.
That one or two volts may not seem like much of a difference; much of our
equipment is quite capable of working fine over quite a range of "12 volt
voltages". However 14/12 = 1.17 (i.e. 17% over voltage!).
I don't think our tube type 117 line voltage radios would take kindly to say
117 x 1.17 = 136.5 volts!
But when it comes to the long term care, feeding and reasonable life (also
costs of replacement!) of a rechargeable chemical lead acid storage battery,
those voltages are critical to the battery being ready when it is finally
needed.
The history is that at one time large battery installations were used on the
charge-discharge method; so there were often two batteries while one was
being used and discharged down to say 30% to 40% of capacity, the other was
being recharged by an engine generator, windmill or whatever. later circuits
were designed to be able to charge the battery and also use it at the same
time without damaging the equipment using it with the too high charging
voltage.
These days we have much better voltage regulators and equipments that
regulate their own voltages internally using zener diodes and the like.
As a suggestion; just don't hook up a battery charger to the battery and
leave on day and night. Either use a more sophisticated charger, such as
those incorporated with the internal battery into a UPS (Uninterrupted Power
Supply used for computers), or have a manual regime of 'topping up' the
battery on say a weekly basis for an hour or two.
We have a UPS which we run in conjunction with some old truck batteries.
However after some years the batteries, which had many road miles on them
before they were given to us, have now died due to age. Also I'm not sure
that the UPS charger has the capability of fully recharging them following a
prolonged power failure and or the UPS has been switched off for a while.
Hope suggestions help?
