Guys, I'm on the horns of a rather large dilemma here. Please bear in mind that this is going to be a rather long post as I explain what exactly I'm on about. OK, we had a CD (Civil Defence) exercise here last weekend and so I'm told now (Thursday) the batteries in the Fire Station next door that run the radio equipment, didn't cut the mustard (so to speak), in fact they wimped out after half an hour of discharge. The Fire Chief tells me he would like the whole system ripped out and replaced, as it's been there since the mid 1960's and it's only been used once a year since then. The batteries are all sulphated to bits and it's a wonder that they haven't caused a Hazardous Chemicals incident. It's a whole bunch of 1.2V lead-acid cells hooked up in Series-Parallel to give 14V at the transcievers end. They were purchased second-hand from the old Telecom exchange at the time. Now I would like to start from scratch here and install a new system altogether, here are some ideas I had:
The whole system as I see it would be 12VDC, I need the Control circuit to still be operational under Mains failure, so that isolation can still take place.
Radio gear will be supplied at 13.8VDC as is normal.
Battery system will be made to automatically switch over upon Mains Failure, to ensure radio gear keeps working.
I am also loking to interface some sort of a solar recharge feature into this system, so that there is no need to manually recharge the battery banks up, this is the reason IMO why we are at the stage where we are now.
We need at least 4-500Ah (maybe even more) of capacity in the cell banks, when the original system was installed we only had a single transciever (the Main Station radio),now we have Inter-service (Ambulance, SAR, etc) and a link to the local Amateur Band Repeater.
I'm pretty much up with AC switching systems, but DC is not something that is first hand to me. What sort of batteries should we be buying for this application?, that will not be too worried by sitting for a while un-used, but still fully charged. I was thinking of using two 3pole AC Contactors (one for -ve and the other for the +ve of the Batteries), with a 12Volt DC coil in each contactor, would this be enough contact area for switching DC?. I've already worked through interfacing a couple of PV panels into the system. Now the question of protection, what sort of fusing is best on a system like this one, considering that it is DC?. Any help I can get with this, wiil be gratefully recieved, I've just hit a stumbling block in my head over this one.
Trumpy, You might consider a bit of lateral thinking as an alternative option. A new 500Ah + set of deep-draw lead-acids is going to cost NZ$4000-5000, and in a few years the Service will be looking for replacements. I'd look at a generator backup. A small Honda '1000i' or similar (Yamaha do one) would be less then NZ$1000, giving 230v 900Wac continuous and 12v 100Wdc. Automatic idle-control on board, clean ac with an inverter on board helping with noise on Tx/Rx application. You can parallel two of these to double output and give redundancy in the system. Manual start of course, but auto hardly justified in a manned station. You might consider small sets of NiMH (solar?) batteries sufficient for say 10 minutes of uninterupted communication, because there might be a co-relation between an outage and the need to get on air, ( ie plane strike on pylon ). Advantages in cost, life expectancy, maintenance, wiring & switching etc., time on air limited only by fuel supply. You'd have to keep 'em locked up though, highly desirable items!! Alan
I would second the move from pure DC backup over to generator backup. Then use UPSes to keep things running during time between the outage and manually starting the generator. Also, consider your generator fuel sources. Having a 2nd generator that uses a different fuel type can be a plus in a national crisis. NG is convenient unless the disaster knocks it out (that can happen) or demand overtakes it (thousands of homeowners in Florida are installing NG generators and I fear the gas distribution will be swamped in local areas after the next hurricane event). Diesel has storage life issues and prices fluctuate with crude prices. Propane stores longer, but is more hazardous to handle and transfer. Gasoline should only be the last resort. And don't put all your eggs in one basket.
Honda do an LPG kit for those 'i' series generators, (range units are 1kw ,2kw ,3kw nominal, no diesel as the engines are small 50-150cc 4 cycle). All 'i' series are linkable in parallel with no sync problems, and very low idle fuel-consumptions. Honda's small engines are very good - we just dragged the mower out last week to start cropping the lawns, it being spring here- it started first pull after 5 months in hibernation. (As an aside, tip, I always winterise our mower by just pulling the motor over to tdc. This seals the cylinder off & shuts the valves. I agree with pdh on the multi-fuel thing, but your CFO will probably baulk at the LPG safety issues in the Station, (which won't arise with a liquid fuel as the tenders will be in there too.)- plus all the added capital and running costs. It's all taxpayers' money and he'll have to fit this into his budget. In a major disaster situation, say, God forbid, a big trans-Pacific tsunami event hitting NZ, gasoline (or is it petrol in NZ?) would be priority issue to you anyway, and as a last resort you can always raid the CFO's car with a hosepipe. Sorry boss- it's a bloody emergency! Alan
Wood work but can't!
Re: Emergency Supply Batteries Problem#129594 05/06/0501:46 AM05/06/0501:46 AM
OK, So I take on board the comments so far. This system has to be instant switch-over, which is why I'm using 12V contactors to do the job. When the 230V supply drops out (via a 230/12 Transformer) the LV supply will come in automatically. I'm not looking to say your advice is bad or anything, thanks a lot, but radio transcievers are very finnicky things when it comes to being supplied from anything else apart from a battery. Generators, make them do all sorts of nasty things, considering that one of our sets draws 22A on transmit. Especially this new Digital stuff, it hates any ripple at all!.
Re: Emergency Supply Batteries Problem#129595 05/06/0507:10 AM05/06/0507:10 AM
You will find that many switches, contactors, and circuit breakers are not rated for DC use, or have lower interruption ratings for DC use.
I think that you should be able to buy 500Ah of 12V PbAC for under $1000 USD.
One questions is: why bother _switching_ you DC supply at all? The equipment is designed to operate at 13.8V for a reason: this is the float voltage for a 6 cell lead acid battery. The equipment could be operated _continuously_ on your lead-acid battery bus, with your mains supply acting to keep the batteries charged. The battery and charger would act to filter the input AC supply, so you could probably tolerate using a generator.
Your equipment would need to be able to tolerate the 14.4V or so required to recharge the battery after a discharge, but this is probably the case, and you would have to find a charging system that can deal with correctly with charging a battery connected to a load. You will also probably require banked batteries and isolation switches so that you can change out batteries without taking the entire system down.
Thanks for your input Jon, I've had a good look around here and had a talk to the Chief and gave him some prices and we are going to go with a Generator unit. It's too much mucking around with batteries and so forth if I go the other route. And yes Alan, it WILL be attached to some steel sunk into a concrete pad, with a removeable cage over the top of it. If it doesn't get pinched, it will either be vandalised or have the fuel siphoned out of it otherwise. No-one's immune from that sort of behaviour here, not even the Fire Brigade.
Trumpy, a possibly silly idea here. If there's any money left after the generator, maybe one of those portable 12v power systems that get advertised on late night tv? Not ideal by a long shot, but they are rated 30 or so amps I believe, with maybe 24 A/H capacity at 12v. If for some reason the generator failed to start, it would be good to be able to just swap one plug into a different socket and have the system back up again.