Backup Battery charging rate limiter

[kosie_sparks]

Our complex with 44 units has an electric sliding gate, running off a built-in 24V battery (2 x 7Ah 12V) which is kept charged at a rate of 0,5A from the mains power supply. However, these batteries only lasts 4 - 6 hours in the event of a power failure. We would like a backup period of at least 12 hours, but the motor housing does not allow installing physically larger batteries. Since the gate battery charger also powers the gate motor control circuits, switches, sensors and remote control activating devices, it is not possible to just disconnect its own built-in batteries and to connect it to an external battery supply.

We also have 2 x 100Ah 12V batteries (also charged from the mains power supply) within a few meters from the gate, for emergency lighting at the gate, as well as backup for an electric security fence. We would like these batteries to also power the gate during extended power failures. Power failures often happens early evening, and the gate batteries are depleted at an awkward time at night, when the gate have to be unlocked and left in the open position until mains supply power is restored. During this period, the security in the complex is non-existent as anyone may then enter the premises.

Will the motor's charging circuit blow out if this larger 2 x 100Ah battery backup is charged at a rate of 1 to 1,5A when the mains power supply is restored? Is their some kind of limiting device that can be added at the small batteries, to prevent its own built-in charger and the rest of its electronics, from being overloaded or fried by the charger for the much larger batteries?

The supplier of the gate motor refuses to co-operate in helping us with advice on how to prevent the gate motor's re-charging electronics, to be overloaded by the main battery backup while it is being recharged. They have all sorts of stories of not disclosing their design to the public, or that it is not built for working in conjunction with a more powerful charging system etc. etc.

 

[Gary B]

What you can do is diode isolate the main batteries from the gate batteries. Now the gate charger won’t even see the large battery. The main backup system won’t supply power to the gate until the gate battery drops 0.7 volts below the main battery. The main charger will attempt to recharge the gate battery but, as long as both batteries use the same chemistry, that should not be an issue. Also, since the gate charger comes on at the same time and there is going to be resistance in the power cable from the main battery, even that should not be a significant issue.

 

[peufeu]

I suppose both the old and the new batteries are lead-acid, yes ?



The charger in your device probably charges the battery using a constant current until the voltages reaches a preset value. The voltage is the same for all 12V lead acid batteries, so no problem here. Since the charger outputs a constant current, the charge current will not change, whatever battery you use. If it is designed to charge a 7 Ah battery, and you give it a 100 Ah battery, the charge will be 14 times slower, but it should not burn. It will just charge much slower.

> Will the motor's charging circuit blow out if this larger 2 x 100Ah battery backup is charged at a rate >0,5A, typically 1 to 1,5A

Do you intend to change the charge current ? This would need hacking of the thing's electronics.

 

[MrAl]

Hi there,


If i understand this correctly, you have two systems already in place: one smaller system (smaller battery, smaller charger), and one larger system (bigger battery, bigger charger), they are both the same voltage (12v), and you want to tie them together so that when the smaller system battery runs down the larger system battery can still operate the equipment (gate) the smaller system usually powers.
If this is correct, then yes, diode isolation might be an option if the voltage drop doesnt affect the system too greatly. You may want to use two diodes, to logically "OR" the two power supplies to the gate motor electronics. That way the small system continues to charge with it's smaller charger and the larger system continues to charge with it's larger charger and there is never an attempt to charge either battery with the wrong charger. The diode drop will affect motor speed but it should only be a little bit different.
So each charger goes to its respective battery as usual (no change) but the power comes through a diode no matter which system is powering the gate motor and electronics. This means you have to be able to separate electrically the small charger and battery from the gate electronics and motor so that you can insert a diode and tie another diode to the gate electronics too.

Alternately, use a somewhat larger SPDT switch (instead of diodes) that can switch the power to the gate electronics from either the small battery or the big battery. This will however require manual switchover when the smaller battery runs down and maybe again after it is again fully charged.

 

[kosie_sparks]

Backup Battery Charging - Answers

Thanks to all for your quick answers! As I am only an occasional electronic hobbyist, I was not sure what to do, but you all basically confirmed my original proposal.

In the meantime I have also spend some time on the internet looking for answers, and my plan now is to fit large (15A) diodes on the positive side of each battery. These diodes will allow the 24V motor to draw a high current on startup, although the motor does have electronics to give it a slow start etc.

Plus a TransZorb (30A/30V) across each of the 2 sets of battery pairs (24V total each pair), plus a resettable fuse (100A) on the 24V+ line to the smaller batteries from both chargers. The fuse will protect the batteries from possible short circuit if the TransZorb fails, and the additional originally proposed diodes will ensure that neither charger will "see" the other charger.

I will still have to add a "suitable" switch to totally disconnect the large batter backup system from the small backup battery system. This switch has to operate automatically, by sensing the mains power. When the 220V AC mains fail, then this "suitable" switch (possibly via a transistor switch and relay) should make the connection between the 2 systems automatically, and also automatically disconnects when the mains power comes back on again.

Maybe all you clever guys can now also give me advice on how to construct a "suitable" electronic switch??? This will hopefully make the safety precautions on the system setup infinitely redundant, and totally free from manual interference. The complex custodian, is very senior in age (over 75) and technologically challenged. So let the mains power fail whenever it wants to, and the complex custodian does not have to lose sleep or follow "complex, written" instructions on what to do when it does fail!

 

[MrAl]

Hi again,

Oh yes, you are working with a 24v system, but the discussion is the same except the diode drops will affect the systems even less than at 12v nominal.

As long as you are going the full distance, might as well include a capacitor across the motor electronics to swamp any short transients during switch over times. Say 100uf at 50vdc should be fine, observing polarity.

So you want to add an automatic line sensing switch...that's fine too, but are you sure you want to switch out the larger system as soon as the mains line comes back up? The reason i ask is because this will put the smaller system in limbo again for the time it takes to charge the smaller 24v battery pack up again. During that wait time, the gate will not be operable. On the other hand, a built in time delay to allow the smaller system battery to charge up again might do the trick. You'll have to find out how long it takes to charge the smaller battery.
I guess this entire discussion also begs the question: Why not simply run everything off of the larger system and keep the smaller system for backup emergency?

Another issue is the true current requirements of the motor. Do you know how much current the motor normally draws? I ask because there are commonly available 40 amp diodes that only cost about 3 to 5 dollars each and they can take a real beating and can handle higher current surges. If i rem right the part number is 1N1188a or 1N1188r or similar. I talk about this because one common failure mode of a diode is to short out, and the consequence of this happening to only one diode means the larger system might start charging the smaller system battery, so we want to be extra careful about preventing that from happening so oversizing the diodes is a very good idea as well as make sure they have adequate heat sinking.

 

[Gary B]

What you are talking about is called a static transfer switch and tends to get pricy when you talk zero drop time. If a delay of 25ms. or so is acceptable, just use a DPDT relay across the mains. When the power drops, so does the relay causing the normal contacts (swinger to NC) to connect the two batteries. When the power comes back on the relay pulls in breaking the circuit.

 

[MrAl]

Hello,

Havent heard from kosie for a while now. We were talking in PM's for a while too but i picked up the flu and couldnt get back here for at least a week. Havent heard back from him/her since before that.

 

[kosie_sparks]

Timed and/or auto Switches

I did send some additional comments, but they are not shown on this thread. Maybe they went somewhere else!

I think the basics has been sorted out and explained to me sufficiently, of how to protect the different chargers and battery sets from interfering with each other.

The next concern is to install switches to sense mains power, which will then connect/disconnect the backup system from the rest of the circuit. To make it free from human or manual intervention, the switches also need to work via timers, which should be adjustable between 0 (or 1 !) to a max of 60 minutes.

My little knowledge of electronics tells me an electronic switch using a transistor or thyristor coupled with a 12V heavy current relay, could form the basis of such a switch. Sensing 220V AC, it switches the timer on (timer circuit powered by 12V DC and not 220V AC) , or the relay directly without any timer. Not sensing any mains power, timer will start its delay, whereafter it will switch off the relay contacts. The idea is to delay switching on the backup battery pack after mains failure, by 10 - 30 minutes, and also delay switching off the main battery backup, 30 - 60 minutes after mains power is restored.

Then the final safety measure will be a second 12V relay after the first one, without any timing. Its sole function is to protect the main backup battery from damage due to over discharging during a prolonged power failure, by disconnecting the mains backup battery from the 3 systems which it is backing up. Immediatiately when the battery voltage falls below a preset limit, it should switch off this second relay, and when the battery power is at a sufficiently high but not yet fully charged level, it should switch back on again.

Searching a bit on the internet, might provide me with suitable circuits, but will probably not tell me how to determine the values of each component. And it may also not be very suitable circuits for my requirements. So I need your experts to provide me with these 3 circuits with component values, please.

The 3 circuits will be a 1) mains sensing circuit including a relay working off 12V DC.
2) An adjustable timer circuit, 0 - 60 minutes, also powered from 12V DC, to go between the sensing circuit and relay in circuit 1 above.
3) A low and high voltage sensing circuit (for a 24V battery pack), to drive a 12V DC relay. The lower limit (I guess about 10,5 x 2 = 21V) switching off the relay contacts, and only switching it on again above a second upper limit (I guess at about 24V, which is still below theoretical max of 13,8 x 2 = 27,6V). Power to drive this circuit also 12V DC. This circuit should include 2 LED's to indicate whether the voltage is below or at the lower limit, or at or above the upper limit.

Our electronics shops will only re-open on 10 January due to the local holiday season, so I cannot even start with the diodes and resettable fuses now!

 

[Gary B]

The PFT can just be a DPDT relay rated for your mains voltage (either 120VAC or 240VAC).

Battery protection is normally related to battery voltage. A normal UPS battery will be disconnected when the battery voltage drops below 1.75VDC per cell. A 24 volt battery will have 12 cells so the drop out voltage would be 21volts. Float voltage will be 2.25VDC per cell or 27VDC. You can maintain a battery at the float voltage indefinitely. A 24 volt relay with a resister in series with the coil selected to drop at 21 volts will do as battery discharge protection.
Pick a 24 volt relay from whatever source you have available.
Measure the resistance of the coil.
Connect an ammeter in series with the coil to a variable voltage source.
Apply 24 volts and then lower it until the relay drops and measure the current.
The size of the required series resister is (21/measured current)-coil resistance. The power rating of the resister must be greater than the square of the current times ohms.

Fast recharge is bad for the battery so should be limited as much as possible. How often does the power fail? The recharge time should be limited to about half of the mean time to failure. If you can recharge the batteries fully at float voltage between failures, approximately 24 hours, you will extend battery life to the maximum possible. Otherwise, I would use a charge current sensor instead of a timer for fast charge. If you use a fast recharge voltage of 33V, when the current drops below 20 amps, I would go to float voltage. This will get you a quick initial charge when power comes back on that drops based upon battery state. That way you don’t risk damaging the batteries by over charging after a short power interruption.

Notice, no timers required.

 

[Diver300]

I guess that your 2 x 100Ah 12V batteries are in series.

I also guess that the gate is powered from the two small batteries even when the mains is on. In other words, the mains power supply for the gate isn't big enough to power the motors of the gate.

If both of those are the case, you could just disconnect the mains permanently from the gate. Run a wire with a 20A fuse at each end to parallel the two 24 V batteries. The whole lot will be charged perfectly well from the larger charger.

The fuses at each end of the wire are needed in case the wire gets shorted in the middle. Both ends need a fuse to stop the cable and batteries being damaged.

 

[kosie_sparks]

Relays vs Timers

Gary B,

Thanx for that info. Luckily I do have a good PSU and multimeter, so with your formulas and directions on what to do, I can play around and see how to make it work for me. A bit like showing me how to catch a fish, instead of giving me a fish! I like that, as it makes things more flexible and understandable for me.

However, I still need something to sense the mains power, as well as when the batteries are at a usable level. Since batteries do degrade over time, I think the 27V is a bit high for a permanent upper level. Also, the connection between the backup and the backed up items should be restored before the backup battery is fully charged, as there will still be a mains sensing switch after this one, to disconnect if mains is on, and connect when mains is off.
How do I go about setting an upper limit (for the backup battery voltage) on a relay? All the electronic ckts I have seen, works on a minimum level to switch on, while I need the opposite which is a max level to switch off. I.e. the contacts should close when batteries are at approx. 26V

 

[Gary B]

Gary B,

However, I still need something to sense the mains power, as well as when the batteries are at a usable level.

You seem to have missed my first point. Connect an AC relay rated to operate at your mains voltage across the line. While power is available, the relay is active. When power drops, so does the relay. Some commercial UPS systems use this method because it is so cheap and simple.

Lead/ peroxide, lead/antimony or lead/calcium batteries are fully charged at 2.20 to 2.25 volts per cell depending upon which chemistry is in use. A 24 volt battery has 12 cells so 12 x 2.25= 27 volt float voltage. The little bit extra depending upon chemistry is not going to produce a significant current. It is excessive heating from the charging current that causes damage to batteries, not voltage. I run a 48 volt stack at 56 volts float (2.33 vpc) since 2004. Since this is the backup power for the telephone system of a US Embassy, you have got to know that we are doing things the most reliable way. We do not use a higher recovery voltage after a failure. What we do is monitor the maximum current. This can be done sloppily with a relay and resister (Current gets too high and it pulls in connecting a current limiting resister in series. It is how the old automobile regulators worked) or more precisely with an electronic current monitor. However, unless you have a shorted cell, in which case you have to replace the battery anyway, the current will never get high enough to be dangerous if you never go above the float voltage.

 

[kosie_sparks]

Battery backup - Mains relay

Gary B,

Sorry,

I thought you meant that the rating of the relay contacts, should equal the mains power supply! I was assuming the relay worked off the battery pack, sensing mains supply, and using 24V to activate the contacts.

Tx,