Battery Switching 4

[leonccv]

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.

 

[dpc]

Do a little research on "diode auctioneering". Use of diodes will allow either battery to supply power without having one battery charge the other.

This is commonly done for dc power supplies.

 

[roydm]

As DPC says this is common practice. I like to use a potted bridge rectofier for this, one supply Positive connected to each AC terminal and the load on Positive terminal. The 2 diodes connected to Negative terminal are redundant but the potted bridge allows bolting directly to a panel (heat sink). If you use 2 diodes then you have to make sure the heat sink doesn't ground out.

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

 

[sreid]

I would think with some CMOS comparators, some CMOS logic (4000 series)and a couple of N channel FETs (almost no power when static), you could rig up a circuit.

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.

 

[MacGyverS2000]

They have simple chips that accomplish this, less than a dollar. As dpc said, Google diode auctioning or diode ORing chips.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[sreid]

I understand battery isolation using diodes but for the OPs request, don't the diodes simply parallel the batteries and cause both of them to be drained?

 

[VEBill]

Depending on the battery, the open circuit voltage is likely to float upwards once disconnected. If the supervisor circuit tries to switch to the highest voltage, it might start chasing its own tail. So the algorithm should probably just hard switch to Battery 2 once Battery 1 reaches the lower-limit trigger voltage.

 

[roydm]

VE1BBL,
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

 

[leonccv]

"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."

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.

 

[VEBill]

The main problem I see with using a normal relay is that it may require several mA and thus eat into the battery life.

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?

 

[roydm]

leonccv,
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

 

[peebee]

Around here these things are usually called "best battery systems".

 

[MacGyverS2000]

I think this design might prove useful to you:
FET-OR Circuit Switches Between Main and Backup Supplies

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Compositepro]

I had to look-up "diode auctioneering". That is not really what is needed here. That places diodes on all supplies and simply parallels the power supplies but, if one supply fails it will not affect the other.

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.

 

[roydm]

Yes I think we ruled out diodes several posts ago
Roy

 

[Compositepro]

My last point was that no circuit will solve the problem. The answer is that the battery must have more than enough capacity to last to the next battery change

 

[roydm]

Compositepro,I agree with you both batteries could be flat one after the other or in parallel. I think you need some way to warn the technician after battery A is discharged. Perhaps this could be as simple as a flashing LED with the caption ?Please phone ######.
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

 

[itsmoked]

How about:

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