According to Fig. 3.3A in BS 7671 to be sure of taking out a 200 A BS 88 gG fuse in 60 s you'll need about 650 amps. To take it in 10 s - a more reasonable time for cable protection - you'll need 1 kA. Remember that these are actual, rather than prospective, fault currents and in an ELV circuit like this the resistance of the fuse might be enough to make the difference between the two appreciable.
This all sounds like hard work to me. Why the 12 V system for kilowatt power levels? Why not go up to 24 or 48 volts and make life a lot easier in regard to cable sizes and overcurrent protection. Inverters are available for those voltages and you could just tap the battery if there are any 12 V small power DC loads (fused at say 30 A).
I think that's way off. Do you have tables? Even MCBs are nowhere near that discriminatory or quick.
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i'm begining to wish i was like most motorhomers, no idea where the battery lives let alone how it's connected, when the tv dies the batteries are flat.... oh and buy a new set every year :)
i've gone for true deep cycle batteries as according to my usage patterns, i should get 7 to 10 years life out of them, quoted 1300 cycles by the manufacturers to 50% dod, but the new bank is 50% larger, so i should never have to run them that low again (it's the damn fridge freezer that does it, need 80Ah a day to run the f***er, as long as i get sunlight i'm fine, the
370 watts of solar puts back more than that), could go 2 -3 days of very overcast weather when the old batts were new, but after 50 cycles that dropped to a day of no good sun and the next day was genny time, or the low voltage alarms would be going off in the night each time the fridge compressor starts up)
Anyway, i've ordered the cable, crimper, copper lug terminals, copper bar stock to make the bus bars, brass bolts, shake proof washers and so on, and have the batteries secured under the bed,
the cable is 50 sq mm, so should be more than enough for my 1 meter lengths i need from the terminals to the bus bars, one bus bar will be a double bar, with a shunt for an ammeter linking the 2, so batteries to one bar, loads to the other bar, not sure yet if to put the shunt in the pos or neg busbar, seems measuring the negative bar is the most common way the boaty people do it.
just need to work out what size fuses to put on the battery terminals, and if i should fuse each pos terminal... i.e. the series ones, and the paralell ones, or just fuse the paralell terminals.
the cables will be routed safely where they can not chaff on anything, they will basicaly be going from the terminals on the battery tops, allong the top in groups of 3... pos and neg bundles, through the wooden battery enclosure, and to the bus bars, no metal in sight of the cables.
From the pos busbar, the main load centre cable goes through a 70 amp DC breaker, the inverter cable will go through a fuse, size undetermined yet,
the cables from the bus bars to the inverter will be routed alongside the battery box, through the plastic of the inverter housing box (it's a battery box with an outside access door, sold for caravans, inside it is my inverter and mains charger and shore power connection) then the cables go straight to the inverters terminals, the inverter is fused internaly straight after the input terminals (its a wenchi 1500 watt pure sine wave inverter) connected to the inverters terminals is the battery chargers DC connections, it's only a tiddely 20 amp charger (sterling 4 stage charger)
The inverter and charger are in their on box, which is vented via a couple of cents in the door, both items have variable speed cooling fans that seem able to do their job fine, i fitted a fat to one of the vents to ensure airflow, running when ever the inverter or charger was on, but it didnt make any differance to the duty cycle of the internal fans, so i dont use it anymore)
The batteries will be in a sealed plywood box, varnished internaly, top rear will be a vent through the side of the van, vent is about 10 inches long by
5 high, has a mesh to keep out insects and a downwards pointing cover to keep rain out.
i will fit a couple of 100mm brushless fans in the vent, and have them running when ever the battery voltage is over 13.8 volts, i'll run them at a low voltage and mount them on silicone to make em as quiet as possible, as i dont want them to wake me up every time the solar panels start charging in the morning,
The batteries will be fitted with a centeral watering system at a later date, just a manual system, connect the squeeze bulb to the connector and pump in the deionised water untill the cell caps all shut off, but for now i will keep an eye on the water levels manualy, the cell caps have the quick release caps on ATM, push the 2 levers and all 3 caps are unlocked and lift off as one unit,
Anyhoo, all i need to really do is work out the fuse ratings, and if to fuse all pos terminals, or just the paralell ones, and i really want a remote indicator to show a blown fuse.
i only suggested the 200 amp fuses as that was the first figure i thought of,
max current draw in my system is just over 200 amps, so i figured if each battery was fused at 200 amps, then if one cell goes down and pops a fuse, the other batteries will still provide the current, if i fused them too low then trying to pull 200 amps with one set of batteries out of circuit could pop the rest of the fuses, then i'd be fecking about trying to figure out which fuse poped first, not to mention having to replace all of them, and they aint 5p cheapo fuses like in a mains plug.
At the end of the day, the fuses are there to protect the cables, the appliances have their own fuses to protect themselves, the possibility of a cell shorting out would i try and pull a fairly large current, and thus be similar to a cable short,
Maybe I've missed it, but to have any idea of what fuse to spec we'd need to know what your maximum currents are for how long, eg 1kW for 1 minute and 200w continuous. Then you simply look at a fuse chart to see which fuse rating is the lowest that wont blow with those loads. As Fredxx pointed out, you can get fancy fuses with much tidier specs than wire fuses, the latter have a pretty big overcurrent margin, ie a big gap between rated current and will blow current. As far as specifying fuses go, I think you can forget about startup surges, the fuses you're looking at will have plenty of thermal inertia.
Eh? Assuming some load (only a few mA required) you get the full battery voltage across an open fuse, it's an infinite resistor!
What ever you put across the fuse has to be current limited so an LED and suitable series R is fine. When the fuse is good it shorts out the R and LED so the LED doesn't light.
Ah, because we are talking about several packs in parallel here (so it would therefore only be the difference between the offline battery and the others).
Another reason not to run batteries in parallel then. ;-)
How do you propose to get full battery voltage across the blown fuse? Lets take a typical scenario, one fuses pops, all batteries have some charge in them, so the o/c fuse sees anything from nothing to maybe a few volts under some circumstances.
Comparators are dead cheap, ones that go the +ve rail on the input would do this job properly no problem. Simply set them to sense more than 0.1v difference either polarity across the fuse.
a big heavy chunk of copper bar, 30mm wide, 10mm thick, 300mm long, will make 3 x 100mm long bars, 1 for earth, 2 for pos terminals, battery connections will go to one bar, the other will be next to it and linked with a shunt feeding a panel meter, reading upto 199.9 amps. then from the shunted bar will come the load connections.
got me 50 sqmm welding cable, it's got 2mm thick insulation, designed to be dragged over sharp objects in the course of industrial welding, so should just about stand up to the task of being connected to batteries, cable tied together and then to fixed objects at regular intervals i rekon.
got all me terminals, and the crimper,
built the battery box, sealed it, put in a large vent out the side of the van at the highest level, i get a good airflow over the top of the batteries, but will add a small brushless fan to help draw the hydrogen out when the batteries are on boost and absorbtion charge stages.
will be fusing the inverter's main lead at 150 amps, not fusing the battery connections as something i didnt tell you lot was i have an emergancy start solenoid fitted, this links the house batteries to the starter battery at the push of a dash button, to enable me to start the engine if the engine battery is dead, self jump starting if you please.
so all the battery cable runs will be routed where they can not come to harm, as i will be checking the battery water levels regulary untill i fit a centeral waterin system, if any cells go faulty i will spot em by the lower level.
now to get cracking and get power back to this van,
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