earth bars / busbars

[Dave Plowman (News)]

No - common failure mode is a cell or two goes high impedance therefore dropping the charge current to that battery.

[Dave Plowman (News)]

When a battery fails it's usual for one or more cells to go high impedance thus reducing the current flow through it. And a fully charged battery doesn't stay at a high enough voltage to produce much of a current flow into another anyway - especially when they're all in parallel.

[The Natural Philosopher]

Not really. Charge level varies with applied charge voltage. A 'small capacity' battery will simply charge up with less current..

Same goes for discharge. Charge level is related to terminal voltage.

If a cell shorts out, you are in trouble anyway. Whether its one of a bank or not. Since your charger will be volatge limited, it will overcharge the rest.

[NT]

Its very necessary to fuse and ventilate this lot correctly, or as has been explained you will find yourself unable to breathe one day.

You need to look at a chart of time versus current for your proposed fuses. Pick the lowest possible fuse rating that will see your load currents and times stay below the fuses's curve. You'll notice that the nominal fuse rating will be far below the actual currents you draw. A 200A fuse is way too high, and as good as useless.

NT

[neverwas]

Purely from being shown round a few RVs in the USA, aren't there vast numbers of similar (and larger) series/parallel battery banks in use there, with a lot of experience of what does and doesn't work?

IIRC the guys who showed me theirs stressed the importance of the type of battery and of charger rather than the connections. And I vaguely recall they used copper welding cable.

[Fredxx]

Why's 200A too high, you're trying to protect the wiring more than anything else, and each bank would more that happily blow a 200A fuse. That's only the 1hr rate for each bank! He must make sure that the cable is sized to take the steady state fuse current.

Given that the OP must assume at the end point, that only one battery may be supplying all the current, each leg must be fused in excess of the expected max load. If the inverter has a high startting current, then the fuse must cater for this or be a slow blow device.

[T i m]

Understood, however I've also seen / had a cell go short (loose sulphate / plate material).

*Personally*, I don't, care as it won't be me sleeping over them. ;-)

T i m

[T i m]

When I was running the EV I built a 5 way 10-LED bar-graph type battery monitor setup to display the typical running (not charging) voltage on all 5 battery systems (4 x traction + 1 x auxiliary). I calibrated these to the real world / on_load / across battery (not wiring) voltages (10.5 to 13.5V or so) because as a pure EV with no regen there was no chance of charge getting back in once lost and away from the plug socket. ;-)

Whilst every effort was made to ensure the wiring and other switchgear was kept symmetrical it was interesting to see how one bank always seemed to collapse the soonest. Of course this was generally only 'noticed' when running at full speed and on 48V (4 x 6+6). [1]

I even re wired the banks to see if the reaction was a function of position within the system but it was not.

T i m

[1] From memory the throttle was connected to a cam that actuated micro switches. These in turn operated double ended contactors on the batteries and motor. The motor was run in series or parallel configuration. All legs were fused.

Speed 1 = 4 x 12V in parallel, motor field and armature windings in parallel. Speed 2 = as above but motor in series config. Speed 3 = 2x 12V in parallel with 2 x 1V, parallel motor Speed 4 = as above but series motor config. Speed 5 = 4 x 12V in series , motor in parallel. Speed 6 = as above but series motor config.

In practice these steps were nearly unnoticeable. You had the same speed options in reverse of course. ;-)

[Fredxx]

Interesting, the OP has 1m cables which may have enough resistance to equally share the load amongst the 3 banks.

Out of interest, was it the coolest battery which discharged first? Did you physically move the battery? Or was it always the same battery independent of physical position?

[T i m]

They are arranged in two bays, front (under the 'bonnet') and rear (under the rear bench seats). Therefore I should imagine per 'bay' the temperature would be fairly constant (there was no front 'grille' or any heat source etc).

Yes, I swapped the front right hand pair to the left hand position etc (as the wiring was fairly substantial and nearly 'built_in'). The sense wires were connected to the battery terminal clamps themselves.

At about 30mph on the flat and with no discernable head / tail wind it was drawing approximately 300A (as measured on a DMM in the cockpit connected to an ampere-clamp in a battery compartment).

T i m

[js.b1]

1 - Fuse 2 - Copper welding cable 3 - CRIMP copper tube connections (buy a BIG crimp tool off Ebay or local electrical supplies)

240V has GFCI/RCD to limit risk of fire. 12V really needs fuses at the battery for peace of mind.

Check the RV groups for what size fuses, I suspect 100A or less because you are not starting an engine with these things, you are simply powering an invertor. All it takes is an abraded cable due to vibration and you will be glad you fitted fuses. Electrical fires are not uncommon with 12V. Use large grommets (or ideally plastic cable glands) for any holes through panels and such like. If the batteries are vented, most likely, ensure their container is too. Usual safety stuff.

[Jules]

TBH, a lot of US RV owners seem to just flutter between campsites, all of which have AC power hookups. I suspect that truly taking an RV on the road or into the wilderness and having to rely on DC and batteries alone is very rare - so it's quite possible that knowledge of large battery banks is pretty uncommon.

Tapping into knowledge within the boating community might be useful, however...

cheers

Jules

[The Natural Philosopher]

Well I have a small 'RV' with two batteries. One for the vehicle, one for the back. Both charge happily in parallel off either mains hookup or the engine.

Yes, they probably have some kind of contactor so they are not in parallel for discharge..but that is simply to ensure the vehicle starts..;-)

[NT]

Do you know how much current a 200A fuse would need to see to blow in

1 minute? Its way beyond anything the OP indicates he's doing.

wiring plus batteries. If one battery goes bad, as it will inevitably do aty some point, you could get a massive current dump that would leave anyone in the building unable to breathe.

must we? Battery voltage has fallen to 10.7v at end point, so I think that precluding faulty batteries this situation wouldnt occur.

no, less. If you fused each leg at same as load current, there would bea very large margin between load current and fusing current. This isnt what you want, you need to minimise the risk of high speed acid gas production. If your peak load lasts only 10 minutes, you pikc a fuse that will supply it for a bit more than 10 minutes, NOT a fuse with a nominal rating equal or greater to that peak load.

A lot of people seem to fondly imagine that fuses pop if their nominal ratings are exceeded. This is far from the case.

yup. However a large fuse in the 50-100A region is unlikely to be remotely concerned by such a relatively small & brief surge. Quite a different matter with a little 2A thing.

NT

[Fredxx]

That might depend on the type and characteristic of the fuse. Probably 300 to 500A when new. We're talking of fault currents and using fuses to protect cables.

Generally most batteries go open circuit. If a cell does go short (rare) then we have already given the scenario for that. The charging current for

5 cells at 2.4V is not going to be that high. A 100A fuse would also be unlikely to blow in this scenario.

A faulty o/c battery won't be supplying any current. At 10.7V a faulty battery with a dead s/c cell won't either!

Most fuses pop at 150% of rated current when new after a period of time, somewhat less when they've aged a few years. A fuse will have a defined time load blow characteristic at a known temperature which creates addition variablity.

The most likely failure of a fuse is down to carelessness by the OP connecting when connecting and disconnecting terminals. The current capable of being supplied by the batteries in this situation is mindblowing. The appropriate fuse will blow PDQ if a cable is inadvertently shorted to ground.

If the OP wishes to protect cable downline from the bus-bar, then an additional fuse in this line would be an alternative. Again he needs to check cable ratings and inspect the manual for the inverter.

I've never been aware of more protection afforded to lead acid batteries in parallel than straighforward fuses. With the number of hybrid cars on the market, perhaps someone from this market sector can comment.

Not sure if a 50-100A fuse is going to much use for the OP when he's been running his inverter full tilt for a few minutes! He's quoting 150-200A.

[Dave Plowman (News)]

You can buy circuit breakers here:-

to the bottom of the page.

[NT]

I think that's way off. Do you have tables? Even MCBs are nowhere near that discriminatory or quick.

NT

[Fredxx]

Way off? What fuses do you use?

a look at page 71.

[gazz]

called a split charge relay,

i dont have one on my motorhome, as i have a second alternator on my engine, so the origional 55 amp alternator charges the start battery and runs the vehicle electrical loads (headlights etc)

the 90 amp alternator charges the house batteries, totally seperate systems, allowing me to have the house battery alternators regulator set for a 14.4 volt charge rate, wherre the engine batteries alt is the standard 14 volt jobbie.

i also have an emergancy start solenoid i put in, the connection from the 90 amp alternator to the house batteries is via some 40sqmm cable, so i connected between that cable and the engine battery with a 500 amp contactor,

pressing the botton on the dash connects the house batteries to the engine battery, which i had to use a fair bit last year as the alarm was unbeknown to me dying, and draining the start battery, so basicaly i can self jumpstart without getting out the drivers seat,

i have now ripped the crappy alarm out, and fitted a nice new bosch blue top start battery, so the emergancy start is back to being how it was put in for... emergancies, not every bloomin day,

and now i am using proper deep cycle batteries, i will not be jump starting unless it's an absolute emergancy, handily i just need 1 revolution of the engine to start it, so if i can roll the van a couple of meters i can bump start it.

[NT]

impressive compared to wire fuses

NT