Sizing Fuses for 6 to 36 Volt Storage Battery Test Supply

In testing low voltage D.C. devices rated at numerous high current ratings under "no load" conditions, I've opted to build a test station using the old fashioned high current battery capacity of lead acid storage batteries for their "pure" Direct Current attributes.

I realize I could have gone the route of feeding a high current welding transformer via a suitable variable auto-transformer with a rectifier, etc. for a smooth "ramping up" of the required voltage.

I also understand the test batteries will need to be alternated from time-to-time to equalize their time under demand.

Because it's not a perfect world, and most of the time you just have to go with
your last idea, as being your best idea, I have to finish what I started.

The attached pencil drawings of the control circuit, and heavy current circuit are illustrative, and are not finished documents.

The sequence of battery connections in each "stepped" configuration are as follows:

Batteries 1, 2, and 3 are paralleled for providing 6 volts.

Batteries 1, 2, and 3 are paralleled, and connected in series with paralleled batteries 6, 7, and 8
for providing 12 volts.

Batteries 1, 2, and 3 are connected in series to provide 18 volts and so on... up to 36 volts.


Here's what I need help with:

If the maximum capacity of the final output contact(s) illustrated in the attached
drawing(s) are rated at 200 amps, how should I approach sizing the individual fuse ratings
for each 6 volt battery at their respective terminal post to protect them in the event of relay contact failure, excessive current draw during a test, and so on?

Thanks to all, in advance.

John

YouTube URL: http://www.youtube.com/user/ElectroMechApparatus

d'Arsonval said:

...how should I approach sizing the individual fuse ratings for each 6 volt battery at their respective terminal post to protect them...

Click to expand...

The answer will depend on the specifications of the battery itself, as that is the device being protected by the fuse. A lead acid battery can deliver a whopping current without damaging itself much, but you need to be concerned with preventing excessive discharge. That shortens the life dramatically.

What would be the maximum current you want to draw from these batteries?

if say 80amps is what you need, then put 80Amp fuses on each battery.

Maximum amount of current the final output solenoid(s) are rated for are 200 amps.
So... perhaps that's what my fuse rating should be.

The current demand will very depending on what is connected to the circuit at any given time.
So, a 200 amp fuse (rated appropriately) at each battery may be what's required.
It took a "jog" on the brain to get me figuring this out.

Any other thinking on this, I'll appreciate.

Thanks,

John

Nope, if you want a fuse at the battery to protect the battery, the proper size depends on the BATTERY and not the load. If the load is protected by a 200A fuse and that current level is OK for the battery, then you really don't need to separately protect the battery.

Thanks again for your helpful replies.

I'm politely not convinced one fuse would protect each of the six batteries in the event of a relay contact failure
based on the earlier attached diagram(s). Hence, the reason for wanting to provide individual battery protection.

"A lead acid battery can deliver a whopping current...", as [wayneh] indicates.

And that's what will be used. (Lead acid storage batteries capable of providing hundreds of amperes.)
If the final output contact is rated at 200 amps... the circuit design is limited by that capacity rating because more than 200 amps passing through it would cause its integrity to be diminished. So, perhaps I've answered my own question.

Any other thoughts will be helpful.
Thanks.

John

YouTube URL: http://www.youtube.com/user/ElectroMechApparatus

One thing you'll want to avoid is a scenario where the failure of one component or fuse causes increased stress elsewhere, leading to a cascading failure blowing all your fuses at once.

I'm Calling Your Bluff wayneh.

Your reply to my original post on this site held no useful information.

I found this information much more ""useful" from another site... with more credits behind its POST:
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If you intend to have 200 amps going through each output contactor, meaning 200A output at each voltage 6V-36V, then I am not sure about the rating of the other, interconnecting contactors.

For example, at 6V you have batteries 1-3 in parallel with the final output through the 200A contactor 15. For a 200A output, each battery will output 67A. Wouldn't this mean that contactors 2 and 5, rated for 100A each, will have to carry 134A.

For 12V, batteries 1-3 are in parallel and also in series with the paralleled batteries 4-6 with the final output through the 200A contactor 16. For a 200A output, each battery would output 67A. Contactors 2, 5, 8, and 12, rated for 100A, will have to carry 134A and contactor 7, also rated for 100A, will have to carry 200A.

For 18V, batteries 1-3 are in series with the final output through the 200A contactor 15. For a 200A output, each battery will output 200A, meaning that contactors 3 and 4, rated for 100A, will have to carry 200A.

For 24V, batteries 1-4 are in series with the final output through the 200A contactor 18. For a 200A output, each battery will output 200A, meaning that contactors 3, 4, and 7, rated at 100A, will have to carry 200A.

For 36V, batteries 1-6 are in series. One problem here is that your control circuit omitted contactor 7 that is necessary to tie batteries 1-3 in series with batteries 4-6. Assuming that contactor 7 was added, for a 200A output, each battery will output 200A. Contactors 3, 4, (7), 10, and 11, rated for 100A, will have to carry 200A.

Beyond the problems with contactor ratings, a 200A output at each voltage with 200A fuses on each battery will mean that for the parallel connections for 6V-12V, the paralleled fuses will be greatly oversized compared to the contactor ratings and the anticipated load and will not offer proper protection against overcurrents.

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If we assume 200A output for 6V-12V and 70A output for 18V-36V, then 70A fuses can be used for each battery. This still leaves the problem with the ratings of contactors 2, 5, 7, 8,and 12 when used in the 6V and 12V connections. However, this does eliminate the problems with the ratings of the rest of the contactors as used in the 18V-36V series connections. Of course, the problem of missing contactor 7 for the 36V connection remains.

Finally, all of the above analysis is based on continuous current ratings. There will be an inrush current as each voltage is initially applied. This would imply that the fuses require a time delay element.

Correction to my previous post: For the 12V connection and 200A output, contactors 2, 5, 9 (not 8), and 12 are overloaded at 134A and contactor 7 is overloaded at 200A.

The maximum output that can be achieved using the existing design and equipment without overloading any contactors follows.

6V = 150A output : 50A per battery in parallel with greater output limited by contactors 2 & 5.

12V = 100A output : 33A per battery in parallel with greater output limited by contactor 7. Changing contactor 7 to a rating of 150A (or 200A) allows an output of 150A, 50A per battery in parallel, with greater output limited by contactors 2, 5, 9, & 12.

18V - 36V = 100A output : 100A per battery in series with greater output limited by all of the contactors in series except the 200A output contactors.

There is no single fuse rating for the batteries that will allow maximum output at each voltage while still protecting the contactors, and the motor being tested, from overload or short circuit.

Use of 50A fuses will reduce the 18-36V output to 50A. Use of 100A fuses will leave the 6-12V output unprotected.

If you used a shunt overload on the device output terminals, you could achieve 100A rating for all voltages without overloading the contactors. In this case, you could use a fast acting fuse on each battery to protect against short circuits within the device. Of course, the fast acting fuse would need to be large enough to allow for the inrush as each higher voltage is selected.

If you want 200A output at every voltage, the only way to achieve this is to change all of the contactors to 200A rating except 1, 6, 8, and 13. Of course, you will still have a problem selecting a fuse that will protect both the 6-12V parallel battery connections and the 18-36V series battery connections. A shunt overload on the output terminals for overload protection and fast acting fuses on the batteries for short circuit protection would still be required.

I'll finish by adding that for a device designed to be a testing power supply, an output voltmeter and ammeter would be a nice feature.

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wayneh's contribution to the original reply here was useful from the standpoint that it led me to seek info elsewhere.

Not a slam. Just a fact.

John

d'Arsonval said:

wayneh's contribution to the original reply here was useful from the standpoint that it led me to seek info elsewhere.

Click to expand...

That was precisely the point.

You choose a fuse to protect something from over-current in that something. Your initial question was how to choose a fuse of the proper size to protect each battery, and my suggestion was to first read the specifications for the particular battery being used. I'm fairly certain you are no closer to the answer, and the post you quoted is useless in this regard. (What fuse exactly are you planning to put on each battery?)

Just a fact.

If your relay contacts are rated for 200 amps you should be limited the current to a level below this rating, least you weld contacts shut every time you get a fault.

If this was a standard electrical system the NEC proscribes a current limiting device at least 1.25 times larger then the normal maximum current to prevent false tripping. That means a circuit handling up to 80 amps would get a 100 amp fuse.

Batteries can provide an extremely large current into a short. Big. And being DC things can arc and keep current flowing. You must use a fuse with a DC rating, and also with an AIR (Ampere Interrupting Rating) above the maximum short circuit current the battery can provide.

From "PV Power Systems" by John Wiles: "A single 220 amp hour, 6 volt, deep discharge, lead acid battery may produce short-circuit currents as high as 8,000 amps for a fraction of a second and as much as 6,000 amps for a few seconds in a direct terminal to terminal short circuit." (I can provide a link to that if anyone desires.)