Battery Switching
Battery Switching
(OP)
My boss ask to do a project. I have a pay phone that is powered by a 12 volt battery but the company which owns the phone says the battery gets to a low voltage before they change it making the phone not work properly so the company ask us to make something to connect two independent batteries that when one is discharged the other kicks in to work so when the technician goes to check he changes one while the other one discharges until is not functional and the new one enter to work. I have been having certain ideas but none seem to work. Any Ideas? Basically i want to switch batteries.





RE: Battery Switching
This is commonly done for dc power supplies.
RE: Battery Switching
Another option would be to use battery A to hold in a relay in series with a pot and N/O contact. The pot is set so that the relay drops at < 12V
A changeover contact on the relay switches from battery A (energised) to battery B (de-energised). The technician would change out battery A then re-energise the relay by pushing in the armature or shorting out the pot and holding contact.
Roy
RE: Battery Switching
Both batteries high voltage, select battery A (turn on FET 1)
A battery low, select battery B( turn on FET 2)
B battery low, select battery A.
A and B batterys low,select both batteries.
RE: Battery Switching
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Battery Switching
RE: Battery Switching
RE: Battery Switching
Yes, that's what I suggested with relay in series with a pot & N/O contact. The remaining power in battery 1 could be used to flash a warning light.
Roy
RE: Battery Switching
A changeover contact on the relay switches from battery A (energised) to battery B (de-energised). The technician would change out battery A then re-energise the relay by pushing in the armature or shorting out the pot and holding contact."
Thanks roy, just one question. When the discharged battery(A1) is exchanged for a charged one wont the circuit start taking the current from the battery just installed battery(A2) or will the other battery(B) continue to discharge?
To eveyrone;
And the owner of the phone wants for the switching of the batteries to be automatic, he only wants the technician to go and change batteries no move switch or anything.
Thanks to everybody for their insight on my problem i will take all your recommendations on consideration; and more help is welcome.
RE: Battery Switching
The requirement as just defined is calling for a fairly intelligent supervisory circuit to symmetrically switch between two batteries. There would need to be some sort of indicator to show the tech which battery to change. But the system would remain on the 'present' battery until it is getting flat, then switch to the next battery.
It's looking more like an ultra low power microcontroller and some FETS (a la SREID). The indicator might be RED/GREEN LEDs with a switch to activate them (save power).
The thing is, the fresh battery will be half-flat by the time the tech arrives, so he can cut his trips almost in half by swapping both batteries (one at a time) during the same stop.
PS: Would a small solar panel help?
RE: Battery Switching
Here is a sketch showing the relay scheme I suggested. I agree with some of the other posters, a solid state device would draw less current from the battery.
Let's see those sketches.
Roy
RE: Battery Switching
RE: Battery Switching
FET-OR Circuit Switches Between Main and Backup Supplies
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Battery Switching
On the other hand, putting a diode or bridge rectifier on one batery but not the other will cause the battery without the diode to discharge first. The second battery will only start to discharge after the first one drops in output by 0.7v which should be close to discharged if it is not heavily loaded. But what is the advantage of having one battery discharge first? You are losing power in the diode.
When you put in one fresh battery how do you know how much charge is left in the second? If the second is almost gone you are back down to starting with one battery that does not have the capacity to last. The simple answer is that you need to parallel two batteries for more capacity and change both at the same time.
RE: Battery Switching
Roy
RE: Battery Switching
RE: Battery Switching
macgyvers2000 has an interesting circuit attached to his post buy I don't think it quite does it either.
My circuit might work but the relay is a drain on the battery.
Perhaps using a latching relay that only requires a pulse to change from A to B
C'mon guys & girls, there must be a simple solution.
Roy
RE: Battery Switching
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4204
Otherwise I'd just roll a micro on a small pc board controlling some MOSFETs to max the battery life, signal the charge state, and auto switch. Done right you add a solar charge controller to it too. It would also prevent battery damage from over discharging - a common lead acid malady.
Keith Cress
kcress - http://www.flaminsystems.com