Will a 24V 16A relay work?

[Das_DIY]

I have an inverter powered by a car battery that I use when 220V power goes off (see block 1 below). I use a battery charger to charge up the battery before I can re-use it (see block 2 below). Id like to connect these devices so that the battery is charged when 220V power is available. (A mains power switch is needed for the charger thats not shown here). A relay is required to switch from the "charge" position to supply the inverter when mains power is lost.

Will a 16A 24V relay work for this? These relays are typically found in vehicle applications. Or am I missing something? Any recommendations are welcome....

 

[telectrix]

so your relaywould be energised when mains power is available, thus charging the battery. mains power drops off and relay de-enerises to switch the battery to the inverter. however, you need a relay with a 220V A.C. coil. a 24V relay will not do. also your battery current with inverter at full load would be 1000/12 = 83.33 A. you'd be lucky to get an hour's run before the battery was drained.

 

[Lucien Nunes]

If you want to switch the 12V feed to a 1kW inverter with a relay, it will need to have contacts rated for about 1000 X 1.2 / 12 = 100A continuously. This is a larger relay than normally found in automotive applications. Don't use a plug-in cube style relay no matter what current rating it claims. You want something with big bolted terminals. Also, inverters usually have large capacitors across their inputs which cause inrush when the circuit is closed. This tends to weld relay contacts in the closed position, unless they are amply rated for the purpose.

However, the relay might not be needed. Many inverters have a remote-enable / control switch function, that allows control via a small switch carrying a fraction of an amp. The battery charger can remain connected permanently to the battery, although if it includes a voltage indicator that remains on all the time, it might create a small drain on the battery when the mains is off, although likely much less than the inverter idle current.

you need a relay with a 220V A.C. coil.

If a relay is going to be used in the inverter feed, and as I say I think it's best avoided, the main 100A contact would have to be closed when the mains is off. You will have a job finding a 100A N/C mains relay for a sensible price, so the way I would do it would be to use a small wall-wart power supply to operate a small N/C relay to hold its contacts open, which controls a large 12V N/O relay fed from the battery.

Advantages of the wall-wart approach are that it provides separation so you don't have to deal with mains and 12V in the same enclosure, and it provides a short hold-up time so that the relay doesn't 'glitch' the inverter on a mains transient.

 

[Rockingit]

Ermm...... just buy a caravan kit??

 

[Das_DIY]

so your relaywould be energised when mains power is available, thus charging the battery. mains power drops off and relay de-enerises to switch the battery to the inverter. however, you need a relay with a 220V A.C. coil. a 24V relay will not do. also your battery current with inverter at full load would be 1000/12 = 83.33 A. you'd be lucky to get an hour's run before the battery was drained.

thankyou for the response. I was hoping to use the 12V from the charger to power the relay coil and feed the battery. it may be a better option to go for a toggle switch?

 

[Das_DIY]

Ermm...... just buy a caravan kit??

im using what I have. Trying not to invest further. Tx

 

[Das_DIY]

If you want to switch the 12V feed to a 1kW inverter with a relay, it will need to have contacts rated for about 1000 X 1.2 / 12 = 100A continuously. This is a larger relay than normally found in automotive applications. Don't use a plug-in cube style relay no matter what current rating it claims. You want something with big bolted terminals. Also, inverters usually have large capacitors across their inputs which cause inrush when the circuit is closed. This tends to weld relay contacts in the closed position, unless they are amply rated for the purpose.

However, the relay might not be needed. Many inverters have a remote-enable / control switch function, that allows control via a small switch carrying a fraction of an amp. The battery charger can remain connected permanently to the battery, although if it includes a voltage indicator that remains on all the time, it might create a small drain on the battery when the mains is off, although likely much less than the inverter idle current.




If a relay is going to be used in the inverter feed, and as I say I think it's best avoided, the main 100A contact would have to be closed when the mains is off. You will have a job finding a 100A N/C mains relay for a sensible price, so the way I would do it would be to use a small wall-wart power supply to operate a small N/C relay to hold its contacts open, which controls a large 12V N/O relay fed from the battery.

Advantages of the wall-wart approach are that it provides separation so you don't have to deal with mains and 12V in the same enclosure, and it provides a short hold-up time so that the relay doesn't 'glitch' the inverter on a mains transient.

thanks for the reply. Definitely an option I need to give some thought to

 

[Lucien Nunes]

I was hoping to use the 12V from the charger to power the relay coil

But the battery would energise the coil when the charger mainis fails, so the relay would remain on permanently. You can't isolate the coil from the battery with a diode because the charger would not see the correct battery voltage. The loss of mains has to be detected separately.

 

[Das_DIY]

so your relaywould be energised when mains power is available, thus charging the battery. mains power drops off and relay de-enerises to switch the battery to the inverter. however, you need a relay with a 220V A.C. coil. a 24V relay will not do. also your battery current with inverter at full load would be 1000/12 = 83.33 A. you'd be lucky to get an hour's run before the battery was drained.

ts for the response. Seeing that the battery charger produces a 12V output, I was aiming to use this to energise the relay and charge the battery, (thats how I got to a relay spec of 16A, 24V). when 12V is dropped, the battery provides input to the inverter

 

[Das_DIY]

But the battery would energise the coil when the charger mainis fails, so the relay would remain on permanently. You can't isolate the coil from the battery with a diode because the charger would not see the correct battery voltage. The loss of mains has to be detected separately.

i was thinking of something like this

 

[Lucien Nunes]

Yeah but... a relay with a N/C contact good for 100A+ is not a cheap off-the-shelf item.

 

[telectrix]

3 pole contactor??? use 2 poles, split the load??? problem could be (energised in normal circumstances.

 

[Das_DIY]

Thanks All for your responses and suggestions!!

 

[Lucien Nunes]

Tel has a point, you can parallel poles to gain extra current rating, although current sharing isn't guaranteed and if one contact goes high resistance it puts more load onto the others. This sort of think would do the job but it's not cheap:

40A 230V 4 Pole 4 NC Contactor

Ideal for heating, motor control and lighting loads

www.cef.co.uk

Hence my preference for a single-pole, high current N/O DC relay energised off the battery when a small N/C mains relay releases. Although that has its own drawback; taking the relay current from the battery could measurably reduce the standby time depending on the standby load of the inverter.

But I think the relay is a red herring. I would find any possible way to avoid using it.

 

[pc1966]

Tel has a point, you can parallel poles to gain extra current rating, although current sharing isn't guaranteed and if one contact goes high resistance it puts more load onto the others.

One trick is to have all of the cable tripled up, etc, as well from source to load so in effect they form "load sharing" resistors.

But it only works well if the cable R values are a few times worst-case contact resistance, so longer the cable (within reason) the better.