battery change over switch
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solar pete
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- Location: United Kingdom
battery change over switch
Can anyone recomend decent battery change over switch. I have 2 banks of ten. 12 volt 97 ahr. ex telecom batteries. Last one I bought (£15) Plasmo ?? or similar sounding. melted when using 1000 watt microwave. via Power master 1800 watt pure sine invertor. I don`t want to spend a fortune but up to £50 is ok. also would it be wise to reduce battery bank size to 5 ?. Ie 4 banks of 5
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adam2
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If the microwave oven has an input of 1,000 watts, that will be very roughly 100 amps at 12 volts.
If the 1,000 watts is the output, into the food being cooked, then the supply current could be about 150 amps.
A 1,800 watt inverter if fully loaded will draw about 180 amps continously and perhaps twice that very briefly.
This suggests that any changover switch needs to be rated at at least 200 amps, continous.
Such articles do exist but are relatively expensive and I doubt that you will get a decent one for £50.
Alternatives would include two switches, one for each battery.
Mains rated switches designed for 3 phase at 100 or 125 amps are readily available, they are intended for mounting in an enclosure similar to a domestic consumer unit.
Use all three poles of such a switch in parralel to roughly double the current rating.
Under most conditions it is probably best to use both battery banks together, as this will be more more efficient.
The use of so many relatively small batteries is non optimum, though worth it if you got them very cheap.
If you need to purchase a replacement battery bank, then fewer larger units would be better.
If the 1,000 watts is the output, into the food being cooked, then the supply current could be about 150 amps.
A 1,800 watt inverter if fully loaded will draw about 180 amps continously and perhaps twice that very briefly.
This suggests that any changover switch needs to be rated at at least 200 amps, continous.
Such articles do exist but are relatively expensive and I doubt that you will get a decent one for £50.
Alternatives would include two switches, one for each battery.
Mains rated switches designed for 3 phase at 100 or 125 amps are readily available, they are intended for mounting in an enclosure similar to a domestic consumer unit.
Use all three poles of such a switch in parralel to roughly double the current rating.
Under most conditions it is probably best to use both battery banks together, as this will be more more efficient.
The use of so many relatively small batteries is non optimum, though worth it if you got them very cheap.
If you need to purchase a replacement battery bank, then fewer larger units would be better.
Last edited by adam2 on 23 Nov 2009, 11:18, edited 1 time in total.
"Installers and owners of emergency diesels must assume that they will have to run for a week or more"
What about auto relays. I'm sure you can get one or two very hefty relays (for the starter motor?) and use these controlled by a small switch (this is what I do on my system, though it's much smaller). Although this increase your power consumption by a watt or so.
On my system I have the inverter on a relay switch which I control remotely by some 4 core phone wire.
On my system I have the inverter on a relay switch which I control remotely by some 4 core phone wire.
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adam2
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- Location: North Somerset, twinned with Atlantis
Most relays for vehicle use are only rated at from 20 to 40 amps and therefore not suited for this application.Junkie wrote:What about auto relays. I'm sure you can get one or two very hefty relays (for the starter motor?) and use these controlled by a small switch (this is what I do on my system, though it's much smaller). Although this increase your power consumption by a watt or so.
On my system I have the inverter on a relay switch which I control remotely by some 4 core phone wire.
The relays intended to work the starter motor are rated at several hundred amps and could perhaps be used but are far from ideal.
When the car battery is cranking the engine, the voltage can drop substantialy, and this is allowed for in the design of the starter relay.
If the coil of such a relay is energised continually at 12/13 volts it would be liable to overheat.
Also the coil of such a relay can use several watts which is an undesirable drain.
If it will operate reliably, a 24 volt nominal starter relay can be used on a 12 volt system, it will use much less power and be most unlikely to overheat.
"Installers and owners of emergency diesels must assume that they will have to run for a week or more"
Just a note of caution: There is a world of difference between a device designed to handle 100A DC running across it's contacts and a device designed to switch 100A DC...
Your standard "boat-type" rotary 100A 'battery selector' (1/2/both/off), or anything else without sprung contacts, is not designed to make or break heavy currents. Using AC rated components (designed to switch 100A AC load on and off) to switch heavy DC loads can also result in some unplanned welding.
The AC load crosses zero 50 times a second, so the switch contacts make and drop the arc (spark) loads of times as it makes/breaks... the DC gives you one long contact-eating arc. Fzzzzzzzzzp!
That said, pro-tek rate one of their 125A 3-pole rotary (Ker-LUNK!) switches for 80A DC (albeit with a little prompting). It's about £140, tho...
@pete - Assuming you weren't switching under load but the thing melted anyway, if you want a manual one, it's time to hit the bay with £60 (250A)... or cheaper at £30 (175A) (but you gets what you pays for, eh?).
The '24V coil starter solenoid' idea works, btw - find a commercial vehicle breaker locally, or findapart etc... but if you're that pikey, why not make your own from copper rivets, coach bolts and plywood?
Your standard "boat-type" rotary 100A 'battery selector' (1/2/both/off), or anything else without sprung contacts, is not designed to make or break heavy currents. Using AC rated components (designed to switch 100A AC load on and off) to switch heavy DC loads can also result in some unplanned welding.
The AC load crosses zero 50 times a second, so the switch contacts make and drop the arc (spark) loads of times as it makes/breaks... the DC gives you one long contact-eating arc. Fzzzzzzzzzp!
That said, pro-tek rate one of their 125A 3-pole rotary (Ker-LUNK!) switches for 80A DC (albeit with a little prompting). It's about £140, tho...
@pete - Assuming you weren't switching under load but the thing melted anyway, if you want a manual one, it's time to hit the bay with £60 (250A)... or cheaper at £30 (175A) (but you gets what you pays for, eh?).
The '24V coil starter solenoid' idea works, btw - find a commercial vehicle breaker locally, or findapart etc... but if you're that pikey, why not make your own from copper rivets, coach bolts and plywood?
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adam2
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The switching of DC is indeed more challenging than AC for the same voltage, since as posted above it is liable to form a continous arc.
A useful "rule of thumb" is that switches, relays, contactors and the like if marked "AC only" can in fact be used on DC, but only at about 10% of the voltage rating.
For example most domestic light switches are marked "6 amps, 250 volts AC only" I would consider such a switch to be suitable for DC, but only up to about 25 volts.
A lot of large industrial switches are rated for AC at up to 480 or 550 volts, these I would use on a 48/50 volt system.
Broadly similar arguments apply to relays and contactors.
A useful "rule of thumb" is that switches, relays, contactors and the like if marked "AC only" can in fact be used on DC, but only at about 10% of the voltage rating.
For example most domestic light switches are marked "6 amps, 250 volts AC only" I would consider such a switch to be suitable for DC, but only up to about 25 volts.
A lot of large industrial switches are rated for AC at up to 480 or 550 volts, these I would use on a 48/50 volt system.
Broadly similar arguments apply to relays and contactors.
"Installers and owners of emergency diesels must assume that they will have to run for a week or more"