Make it a schotky diode - they only have about 0.3V volt drop rather than the 0.7V of a 1N5401 silicon diode. Rip one out of an old battery-backed psu..?
Include external battery for emergency power outages
- Thread starter bobthedob
- Start date
- Status
Sponsored Links
After thinking about that diode comment I have added a relay on the -ve lead from the external battery to the internal battery and powered it off the mains.
If the mains is on, then the relay is open and the external battery is not connected. If the mains fails then the relay will close and the external battery will run in parallel with the internal.
Might even add the diode as well!
If the mains is on, then the relay is open and the external battery is not connected. If the mains fails then the relay will close and the external battery will run in parallel with the internal.
Might even add the diode as well!
It is not that simple. Connecting the two batteries in parallel is OK only if they both have the same level of charge. What will happen is one battery will try to charge the other one and high currents will flow until both are "equally" charged. ( until both have exactly the same voltage )
You need to add two diodes. One for each battery to enable it to supply power to the panel but prevent power form the other battery flowing into the battery. The panel's battery charging circuit needs to connect to the terminal of the battery and the diode between the battery terminal and the power intake of the panel's printed circuit board. This may require modification to the printed circuit board.
If a new external battery ( full capacity ) is connected directly to an old internal battery ( knackered and reduced capacity ) the energy from the external battery may be enough to seriously overcharge the internal battery with the risk of overheating the battery and/or the production of an explosive mixture of hydrogen and oxygen gases. This happens because battery to battery there is no current limiting. The charging circuit in the panel is current limited to prevent a worn out battery being over heated by the charger.
Sponsored Links
A
Alarm
Agreed, hence my stance on the correct method as offered by his alarm company.
Electronics and systems design. My career was 19 years in communications and nine years industrial control equipment. Last 15 years self employed, trying to retire but would be bored if I did.
Blimey! All getting a bit complicated now!! 
If it was me, I think I'd get a simple battery-backed PSU (Elmdene 1301N?) and use it to power the Pet-f D/Ts (and bell?). Remember to common up the negatives and link the tamper switch in somewhere. But this is what the alarm company wanted to do in the first place...
This would be the easiest, simplest and most effective solution and it wouldn't look too bad. This should easily last 24 hours, especially with a 10AHr battery. After that, I'd be more worried about the contents of my freezer than upsetting the neighbours!!
I guess it's down to the OP to decide how much he wants to do this, how much he wants to pay and whether he wants to p155 0ff his alarm company.
If it was me, I think I'd get a simple battery-backed PSU (Elmdene 1301N?) and use it to power the Pet-f D/Ts (and bell?). Remember to common up the negatives and link the tamper switch in somewhere. But this is what the alarm company wanted to do in the first place...This would be the easiest, simplest and most effective solution and it wouldn't look too bad. This should easily last 24 hours, especially with a 10AHr battery. After that, I'd be more worried about the contents of my freezer than upsetting the neighbours!!
I guess it's down to the OP to decide how much he wants to do this, how much he wants to pay and whether he wants to p155 0ff his alarm company.
Early on in the thread I was thinking "diode"
You'd only need one. The external battery won't try and charge the internal one - the load will pull from both, initially more from the internal until it's charge has dropped a bit due to the volt drop in the diode from the external battery. Since you'd be leaving the second battery permanently connected, you'd not have the opportunity for the internal one to get so low that it could accept a charge from the second one.
Then I thought ...
If you are adding a charger for the larger external battery, why not just disable the built in charger and use a bigger battery with a separate charger ?
Or, you could limit the charge current from the internal charger by fitting a diode in parallel with a resistor. On mains fail, the diode will allow as much current as the system needs. On recharge, the resistor will limit charge current and allow the fitting of a larger battery - the battery will still charge, just at a lower rate.
You'd only need one. The external battery won't try and charge the internal one - the load will pull from both, initially more from the internal until it's charge has dropped a bit due to the volt drop in the diode from the external battery. Since you'd be leaving the second battery permanently connected, you'd not have the opportunity for the internal one to get so low that it could accept a charge from the second one.
Then I thought ...
If you are adding a charger for the larger external battery, why not just disable the built in charger and use a bigger battery with a separate charger ?
Or, you could limit the charge current from the internal charger by fitting a diode in parallel with a resistor. On mains fail, the diode will allow as much current as the system needs. On recharge, the resistor will limit charge current and allow the fitting of a larger battery - the battery will still charge, just at a lower rate.
A
Alarm
We`ll deal with this first.
The panel will not accept this as it has a monitoring system and deep discharge protection.
You missed this from the OP
And I repeat that connecting two batteries in parallel without adequate protection is dangerous. Even connecting two new and identical batteries in parallel is not recommended as failure of one cell will result in that battery (which drops by 2.2 volts) being un-controllably charged from the other.
A
Alarm
Totally agree.
A series diode AND resistor will stop the panel charging the external battery (diode) and limit the current coming out (resistor). If the internal battery is old and tired, but not too far gone, it should be at 12.5V. The new, fully charged external battery could be at 13.8V. Lose 0.3V across the diode and there's only 1V potential difference between the two batteries. A 1 ohm resistor will allow 1 amp at 1 volt which won't blow up the internal battery and will still be able to supply current to the panel.
So there you have it - a 1 ohm 1watt resistor in series with a diode and the external battery.
Now the next problem: the panel's low-battery-voltage detection circuit will be measuring the voltage on the wrong side of the resistor so it will cut out roughly 0.5 - 1 volt too early Maybe half an ohm 2W (or 2x1 ohm in parallel) would be better...
Power supply designers spend a lot of time sorting this stuff out - so we don't have to!
So there you have it - a 1 ohm 1watt resistor in series with a diode and the external battery.
Now the next problem: the panel's low-battery-voltage detection circuit will be measuring the voltage on the wrong side of the resistor so it will cut out roughly 0.5 - 1 volt too early
Maybe half an ohm 2W (or 2x1 ohm in parallel) would be better...Power supply designers spend a lot of time sorting this stuff out - so we don't have to!
A
Alarm
Your still missing the point, if the system is under contract it is possible ( and if not so what) it is on response.
It is not protecting the aux power and supply not tamper protecting the new equipment.
Not monitoring it, that could fail and when switched over cause more issues.
It is not protecting the aux power and supply not tamper protecting the new equipment.
Not monitoring it, that could fail and when switched over cause more issues.
A
Alarm
Not monitored.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
- Status
Sponsored Links
Similar threads
