earth bars / busbars

[gazz]

i am hooking up 6 x 6 volt batteries in my motorhome to give a 12 volt 700AH battery bank, the series connections are sinple... lead from pos to neg on each pair of batteries,

for the paralell connections i am running equal length cables to bus bars, where all the loads and charging inputs will go also, this is to ensure the batteries always get charged and discharged equally.

so i need some idealy 6 way 300 amp busbars,

i can buy them from marine shops, but they cost about 45 quid each, so you can guess why i'm not keen to go that route,

anyone able to point me to an alternative produce... things like earth bars in industrial switch gear or something,

they need to have M8 studs for the connections, as my battery cables will have crimped on copper tube lugs, and be rated for at least 300 amps, and as i'm working with DC currents, they need to be idealy copper, low resistance is very importiant,

[R]

[NT]

Anything wrong with half inch copper pipe? Flatten and drill.

NT

[gazz]

Anything wrong with half inch copper pipe? Flatten and drill.

------

well, the jobbies i was looking at in marine stores consist of an inch wide by about 3/8 of an inch of copper for the bar,

i may end up making some up, if i knew of a place to buy little bits of copper bar in nottingham that is, or i could get some off ebay, but then i'm waiting for the damn post....

on ebay i can buy a 300mm long bit of copper, 25mm wide, 10mm thick for a tenner,

then i'd need copper M8 bolts or studs, those seem hard to find, i have taps and dies... so i guess some 8mm copper rod and i can make me own studs up, some copper nuts, those are easier to find (mine are gold so they wont do... fnar fnar) but they are used for exhaust manifolds all the time,

then some insulating material, a couple of double socket blanking plates would do me,

[Dave Liquorice]

Why?

Aren't the nuts and bolts just to provide the clamping force between the conductors? Not really paying any part in the current flow.

I'd have thought copper a bit soft and ductile to get any decent clamping for any length of time.

I though they where brass, some what harder than just copper.

[Andy Dingley]

Stop right there. You _REALLY_ do not want to be building parallel battery banks like this. Not at that current, not unless you're an expert.

If it's low current, just use Schottky diodes (as used for wind turbine or solar generation) and it's easy - up to the current rating of your diodes.

If you're not using diodes, then any asymmetry in the voltage from each battery will cause one to discharge through the other. Which would be bad.

[gazz]

the bods at US Batteries where i bought the batteries from saw no problems doing what i've done, it's done in thousands of motorhomes and boats all the time.

i have 6 x 6 volt 232 amp hour batteries, 3 series connections giving me the

12 volts i need, then the 3 paralell connections giving me the capacity i need (well double the connections if you are counting pos and neg)

the reason i need the bus bars is i am running equal length 50sqmm cables from each battery to the load point, so the battery furthest away from the bus bar is 1 meter, the closest is 30 centimeters, but they all get 1 meter long cables from the terminals to the bus bars.

then the busbars connect the loads and charging sources, this is apparantly the best way to ensure all batteries discharge and charge equally,

200 amps is the absolute max load, with the inverter running the centeral vacuum system which pulls 1500 watts... so 150 amps from the batteries, and all the other DC loads on at once, which is very very very unlikely to ever happen,

likewise if all charging sources were on at once, then theres 140 amps there, but that is something that will never happen, as the mains charger is cut out when the alternators on the engine start charging,

2 of my charge sources provide equalizing charges, the solar one automaticaly every 30 days, the mains one if i initiate it, so that should hopefully take care of keeping the voltages in each battery the same as the next one,

[gazz]

cos i was being a tit, and wanted to make something with as near to zero volt drop as possible, not worth the hassle really.

yup, i was thinking of the ammount of contact the bolt and nut has being almost as much as the copper bar,

dunno, i used copper nuts on my last vans manifold, granted they were twice the length of steel nuts, prolly for the reason you mention about strength.

[Andy Burns]

The currents circulating *within* the battery bank that Andy warns about will tend to overheat and shorten the life of the batteries, someone selling batteries might have the scruples to point that out.

Where are you considering inserting fuses within the bank?

[gazz]

fuses, fuses are for wusses :)

i was going to fuse each positive terminal at 200 amps, (but that may be too high???) using mega fuses, since they have 8mm mounting holes, i could bolt them directly to the battery studs which are M8,

however, the 200 amp current draw is the absolute max id draw, 150 of that being the inverter, which will never run more than 5 minutes at max output anyway, takes us 5 mins to do the hoovering, dont use the microwave (90 amps) for things other than 3 to 4 minute ready meals, hairdryer is again a

5 minute load.

washing machine pulls 30 amps on full speed spin, and again is not on for long, and the DC loads, biggest in one go would be the fridge being on (9 amps) heater starting up (10 amps) laptop on charge (4 amps) tv and sat box on (5 amps) and a few lights on... they are all cold cathodes or fluorescents, no more than 4 amps of lights on at a time, oh, and if someone were in the shower, that'd be 4 to 10 amps for the water pump depending on the speed it's running at (variable flow pump), but we shower as you should when you have a limited supply of water.... water on, get wet, water off, soap up, shampoo hair, water on, rinse, water off, repeat for poncey conditioners if needed,

so 41 amps would be the highest load on a night, and the heater only pulls

10 amps when starting up, once running it's 3 amps, fridge runs for 14 minutes every 35 mins,

i would have liked to have used a set of 700 Ah 2 volt batteries, but they were all way too tall to fit where my battery bank lives, not to mention too expensive, so the 6 volters seemed the best compromise,

[Mike Tomlinson]

In article , R writes

I've used the 12-way version of those. Although intended for DIN rail mounting, they have holes in the 'feet' allowing them to be screwed to a surface.

[T i m]

So you aren't going to use fuses now then?

So you are worried about sleeping on said battery without crimping the terminals but (as others have also mentioned) you are happy to sleep over same with no fuses (or diodes) with no worry about a faulty cell creating circulating currents within the system?

No two batteries have the *exact* same characteristics (internal resistance and capacity to name two) [1] and there is even less chance of them staying that way as they age. Therefore what you might find is

3 x 12V 'battery' in parallel may not last as long as 3 x 12V 'battery' used in turn. I appreciate that may no be easy in your RV but a point to note none the less (considering the cost of the things).

Whilst I read you are trying to do your best to keep things 'balanced' (equal length conductors / busbars etc) I feel the chemistry of the system will be far more of an issue in the real world than the wiring.

T i m (who has just disconnected and taken 1/2 tonne of UPS battery for recycling).

[1] Even matched racing sets never stayed 'matched'.

[T i m]

If he's not going to bother with fuses / diodes on each leg (and if he is has them bolted at the battery end) why doesn't he just bolt all the tails together directly to the load cable? If he wants the connections safe then bring them into an empty junction / terminal box of some sort?

Cheaper, simpler, quicker and less resistance?

T i m

[Fredxx]

Of all the battery types and chmistry I would have thought lead acid would be the best to parallel. Their charge and discharge characteristics would ensure that no bank would be overcharged, ie constant voltage charging, and on the discharge side, as long as you stop at a sensible voltage, no harm would come to the weaker depleted bank. Whilst I accept one bank may discharge/charge in preference, it is only a slight preference and as long as the banks are of the deep discharge variety I feel it is a sensible economic solution.

If they are kept in close contact with each other, then at least the effect of temperature can also be minimised.

When a battery set is being discharged or is part discharged (ie < 13V), the only circulating currents would be leakage. For any bank accepting even the smallest charge you'd need a terminal voltage exceeding 13.2V.

[T i m]

But if (say) one bank ages to a point where it's capacity is reduced compared with either of the other banks then won't it be being 'overcharged' when the other banks are still being charged? ie, the weak band could want to be in the absorption / maintenance phase whereas the other bank(s) could still be in the bulk phase. The lower the capacity the more that bank becomes overcharged and there begins a viscous circle?

Granted *if* you can stop before the weakest bank got below 100% discharge (~ 10.5V). However, if that bank is running at 80% capacity of the other two, when they are at 80% discharge won't the weak battery will be at 100%?

Fairy muff. I think my (and others) concern was what would happen in the event of a fault (re no fuses / diodes).

Understood.

Within *that* group of cells yes. But in a 'battery' such as this (especially with a parallel component) there is a chance of issues simply *because* of their parallel configuration.

Understood. It's not the basic charge / discharge functions that concern me but what happens at the extremes (of charge and discharge).

If a cell fails in a purely series configuration it's pretty easy to detect (in both charge [1] and discharge phases) as I'm sure we all know when we go to start the car that started perfectly well yesterday and is now dead (but will still light the lights). ;-)

Cheers, T i m

[1] Many std auto chargers having built in timers to avoid over charging a partly failed battery. Not sure it would so easily detect a failed cell in a parallel configuration?

[The Natural Philosopher]

Unnecessary. The charge/discharge voltage characteristics of the batteries make it all come out right.

Mostly because lead acid need to be significantly above the voltage they can deliver, to charge.

Its used widely..

[The Natural Philosopher]

..dont in reality exist.

[Andy Dingley]

Yes, and with either a fuse (which suggests you don't need a big busbar) or else a contactor that's monitoring for current flow between cells and will isolate the link if an appreciable current does start to flow.

If you just bash this together with copper strip then it will work fine - _until_ one cell or battery decides that it's time to die, at which point the other battery discharges through it. There's then a likelihood of an overheated battery and boiled acid, or even (a remoter possibility) an overheated conductor and a risk of fire.

[Fredxx]

If you apply a constant voltage charger to a set of parallel banks it is impossible to over charge a battery. Voltage is key.

The discharge characteristics may be different from battery to battery. But on the whole the internal resistance of a weaker battery may well keep them in step. I'd be just as worried about temperature differences as I believe the ptotential difference of the cathode and anode materials in the electrochemical series does have a temperature coefficient.

I think a fuse line of something of that nature would be a sensible precaustion ion series with each bank. I thought I read that the OP was going to add a 350A in series with each positive terminal.

I do agree with you on this point, in practice a failed bank would be overcharged with nominally 14V across 5 cells. Many batteries would cope with only a raised leakage and gassing, where the cell will eventually dry out. Detecting the reduced terminal voltage on discharge would be problematic and the OP ought to periodically check the terminal voltage of each bank under discharge.

I guess a charger of the type you mention relies upon a too low a terminal voltage for too long a time. During the main charging characteristic, the terminal voltage is 13.2V. Where there is a shorted cell, the faulty batter will take a charge at voltage up to 2.2V below this where this bank will be charged in preference to the others. However if it's fully charged when the other banks aren't, as is likely, then the charger won't see the faulty cell.

I hope the OP has good ventilation especially whilst charging!

If the OP has the resources, he can place shunts and ameters in the leg of each bank to make sure all is well.

[NT]

If you work out the resistance of each size of copper you'll see that such massive blocks are totally unnecessary, and more about marketing and profits.

NT