Battery Circuit Breaker

[RAC68]

Hi All,
I am back looking at installing a manually reset circuit breaker or fuse onto my battery and initially anticipated use of a 300 Amp 12VDC unit. A friend questioned the 300 Amp as too low and suggested I go higher amp unit. Since my amp meter does not register high enough to do an actual reading, I was wondering if anyone had an idea of what the starter max draw would be for a BJ8 using the original starter at a low of 30F with 20W50 oil.

Also, I was told that breakers or fuses could be put in series to increase amperage handling…a 300A+100A would support around 400Amps. I have just heard this and would also like to know if this is true.

I understand this is not a common thing to measure so I would appreciate any ideas.

Ray (64BJ8P1)

 

[John Turney]

Hi All,
....

Also, I was told that breakers or fuses could be put in series to increase amperage handling…a 300A+100A would support around 400Amps. I have just heard this and would also like to know if this is true.

I understand this is not a common thing to measure so I would appreciate any ideas.

Ray (64BJ8P1)

If you put circuit breakers in series to one with the lowest setting will trip at it's setting. In this case, 100A. One could, theoretically, put circuit breakers in parallel and increase the load carrying capacity, but in practice, getting each branch to carry thesame load is difficult.

To measure high currents, typically a shunt is used. It is a heavy piece of metal with a known low resistance, such that you can measure the voltage drop across the shunt to indicate voltage. A common shunt would have a voltage drop of 0.100 volts at 100 amps, 0.200 volts (200 millivolts) at 200 amps, etc. One can also make an approximate shunt by knowing the length and gauge of wire between the battery and starter solenoid, calculate the resistance, and measure the voltage drop there while starting.

 

[57_BN4]

Hi Ray,

It may be quite hard to properly fuse the starter circuit because fuses respond to the average current over time. It might be fine for ages and then one time when the engine takes a little longer to fire up because it is flooded hot in traffic and now the fuse blows... Some good fuse info on this page https://www.bcae1.com/fuses.htm

You should be able to pick up a reasonable 600A DC current clamp meter for under a hundred bux which will give a pretty good indication of the actual starting current. Have a look on Amazon.

Andy.

 

[RAC68]

Hi John/Andy and thanks for your responses.

As some may remember, I have been after doing something to protect from the result of short circuit between the starter solenoid and battery ever since a good friend lost his car to fire from such an incident. Everything else is well protected (hopefully) and I have surveyed the cable to make sure all is still well. However, it is still a vulnerable rout so I am still looking.

John, I couldn’t see how placing fuses in series would but since I am not up on this technology and read it as part of my research, I thought I would tap into the Forum for clarification.

Andy, I went to the web site article mentioned and it brought up a good point. I have been looking for the electrical draw of the unit when I am really protecting the cable.

Does anyone know what gauge the original BJ8 battery cable is? I seem to have a list of everything but, and am looking to find out what max amperage the cable is rated for. By knowing the amperage, a fuse selection can be made for some lesser value and still stay within the operating draw of the starter.

From Healey documents, I understand the draw of the starter to be around 450 amps max. Although an actual test would verify this value, at this point, it’s an acceptable value to go by.

Again, thanks guys for the information.

All the best,
Ray (64BJ8P1)

 

[andrea]

Hi Ray
Typical European electric motor for starter draw around 100 A 1000 W+/- at 12 volt or 200 A 2000 W+/- at 12 V for big cars
it drawn more at the beginning of the works
Battery wire cooper on our cars have 10mm diameter, I don't know the correspondent gauge

 

[John Turney]

With Andrea's measurement, you can find the corresponding gauge and other data here: https://www.powerstream.com/Wire_Size.htm.

Note that the maximum amps listed is for continuous operation. One of the reasons starters are as small as they are is that early engineers figured out that the starter only has to work for a short period of time and not continuously. Thus, they could use smaller wire sizes and smaller starters than the numbers in the table would indicate.

 

[RAC68]

Well, now I am confused. Every set of new battery cables I have reviewed seem to be created from 4 gauge copper multi-strand cable with a conductor of 5.19mm. Andrea provided an actual conductor measurement of his cable at 10mm diameter which translates to 100 gauge (European) and between 9.2mm (2/0) and 10.4 (3/0) AWG. When measuring the original cable in my Healey, I found the total cable diameter (conductor + insulator) is around 9 mm and would probably fall as 4 gauge when subtracting the insulator. So, given all these numbers, what is the correct cable gauge for our Healeys?

Max Cable Amperate:
From the web, there are a number of charts that vary in their indication of max amperage for a given distance and gauge. Although some of the deviations can be attributed to the amount of voltage drop (resistance) produced by the cable, I have found no explanation for the total difference and expect it is in the area off number of cable strands and conductor composition. Although all max current estimates are stated to be conservative, Max Amperage was specified in 2 categories, Chassis Wiring and Power Transmission. Reviewing the gauges under Chassis Wiring (not bundled as is our battery cable), in one chart without a cable length specified, 4 AWG max current is 135A and the 2/0 AWG rating is 283. However, reviewing another chart on another web site, the same 2 gauges tested under the more realistic considerations of a 15’ length at an ambient temperature of 85F degrees, show a 4 AWG cable has a max of 108A and a 2/0 AWG passes 344A. Although it seems logical that, at length, max amperage would decrease as reflected in the 4 AWG ratings, what happened to the 2/0 gauge rating?

Starter Max Lock Draw:
Now, when reviewing the spec of the Healey starter, max Lock Current draw (the high amperage at startup) is stated at from 450A - 470A at 7.0V – 7.4V. When considering the amperage max, it appears that a voltage drop of 41% was considered when stating amperage to 7 volts. After performing a voltage drop on my present cable connection, the cranking voltage drop was less than 1 volt and went from 11.18V to 10.22V. This would indicate that resistance in my connection was well within spec and also shows that instead a max Lock Current draw of up to 470A, a value of 340 Max Lock Current would be more appropriate for 10.22V.

Conclusion:
So, now I have a starter Max Lock Current draw of 340A and (what has been stated as conservative) a 4 AWG max amperage @15’ of 108A cable capacity. If a 25% overflow capacity is the conservative consideration of a cable rating, my 4 AWG cable will handle a max of 135A current flow and will still fall far low of the 340A max Lock rating of the starter.

If this cable currant capacity has handled the starter’s draw, in all situations for 49 years without fault, it is reasonable to conclude that the starter will not draw 340A and even 135A would be high. As a result, and considering that a circuit breaker can also pass a short term current overflow of around 25%, a 100A breaker should be able to protect the cable and allow full starter function.

What am I missing?

Ray (64BJ8P1)

 

[steveg]

If this isn't done properly it becomes yet another single point of failure. Looks like there are plenty of cheapo 100 amp breakers out there, but this Bussmann gets good reviews: https://tinyurl.com/blur86x

 

[andrea]

Hi Ray
you are on the right way,
100 A breaker will be critical only if your car won't start and you have prolonged time with electric motor starter ON,
when this occour you can bridge the breaker(have one right strap ready for this evenience)
probably steveg Breaker, that have also a fast reset system, are one good solution
Cheers (very interested at the result obtained)

 

[57_BN4]

Ray, wire gauge current ratings are a varied thing... The ratings usually quoted are for continuous operation without thermal rise sufficient to damage anything. Many factors come into cable ratings so it is hard to use generic sparky's tables to compare to your particular application. You need the conductor area and surface area (diameter of strand and number of strands) to get the closest AWG number but even that is not accurate as the current capability is affected by the surface area so a lot of small strands will carry more current than a few bigger ones as in the Healey cable. The insulation thickness is irrelevant in this calculation.

Something to consider too: If the cable chafes through somewhere and shorts to ground, it may not draw enough to blow a big fuse but still catch fire because the energy is going to be concentrated in just one or two strands that make contact with the chassis. Perhaps a more reliable/complicated way to fuse the starter circuit would be to put a second starter solenoid next to the battery and have a 60A fuse in parallel with it so that for 99% of the time the entire vehicle is protected by a 60A fuse and for the times when you are cranking the engine, the 60A fuse is bypassed by the extra start solenoid. If there is a fault with the main cable then it will show up when cranking and you are in control of the situation.

Andy.

 

[drambuie]

I know this sounds over simplistic ....but why don't you just insulate the entire battery cable from beginning to end with heavy duty plastic conduit or some other form of extra insulation like a split rubber hose or fuel line that has the proper I.D. For a neat installation. A simple mind is a terrible thing to waste!:eagerness:

 

[RAC68]

First, Drambuie, when starting this, I never expected what a pain it was going to be. I expected the starter, being an electric motor, had a max initiation draw (max Lock) and all I had to do was pick a breaker that would handle the surge while being lower than the max current the cable could handle. That view is turning out to be more complicated to determine and, as Andy indicated, available data is not very consistent.

Andy, using a starter solenoid to bypass a battery cable circuit breaker is a very interesting idea. Andrea presented something similar when he suggested having a jumper available to bypass the 100A breaker for those very few times when longer harder starts (i.e. Cold Winter start with 20W50 in the engine) are expected. Your thought of having a bypass solenoid would allow the bypass to be triggered in parallel with the turn of the key and then fall back to a run rate breaker of much lower voltage.

You hit the frustration on the head. It is very difficult to find what breaker value would protect a given gauge of cable since length, temperature, and cable composition all effect capacity. Add to the problem is the % over-current of the rating and fuse/breaker and the simple becomes really complex for someone with basic knowledge.

Thanks again,
Ray (64BJ8P1)

 

[drambuie]

Hello Ray, First thing I want to say is...I really commend you for taking this topic into serious consideration and working hard to find a safety cutout circuit. To be honest you really had me wondering about my own old battery cable insulation and I fully intend to get under my car when the weather gets warmer and check every inch of my own battery cable.

I still remember my sisters old Pontiac tempest going up in flames as soon as she backed out of the drive way, I don't think people realize just how fast a fire can devour a car when a battery cable shorts out. All you can do is sand back and watch the car melt away, horns, lights, windshield wipers all going crazy at the same time as smoke bellows out from the hood and you watch the dash melt and paint on the hood begin to peal off from the intense heat! As you can tell from my discription, I have seen this first hand and it is still etched in my mind!

Ray, if you find a solution or not.....at least you got some people's attention like mine and in doing so may save some Healeys and other vintage cars in the future. Who knows, you may even end up marketing something! Good luck.

 

[57_BN4]

Just out of interest, today I saw my old starter cable and measured the conductors. It has 37 strands of 0.034" copper giving just under 22mm^2 area (sorry for mixing units!) which is equivalent to 4ga. Andy.

 

[RAC68]

Hi All,

With the help of 2 friends, I created a solution that was initially triggered by comments from Andrea and Andy. In effect, key ignition will trigger the starter solenoid and switch an SPDT battery relay from its normally closed lower current continuous operation position with 40 amp circuit breaker protection to the unprotected high amperage starter position. Once the key is released, the starter solenoid will open and release the starter and the battery relay will fall back to its low amperage, protected, operations position again.




Since the battery relay allows for an inrush (motor startup surge) of 400A, there should be no problem with difficult starts. Since the normally closed relay position can handle a constant 50 amps, there should be no issue using a 40 amp circuit breaker for protection.

The 40 amp circuit breaker was chosen as an amperage normal battery draw during normal operation but low enough to trip before starting a fire, however, there is no guaranty. Should something happen during the act of starting the car (with the relay and circuit in the unprotected position), the individual initiating the start would be able to release the key causing the battery solenoid to fall back to the normally closed protected position and causing a circuit breaker fault that will break the battery connection.

What do you think?
Ray (64BJ8P1)

 

[steveg]

Ray,
This looks like a good thing to do.
Do you have more details on your battery relay? What would be the amperage rating?

Some folks use Ford-type remote solenoids for this.

 

[LarryK]

If you put a new proper gauge cable on the car, fires probably will be limited. Most people run their wiring for 40 years and expect it to be free of corrosion and then add bigger batteries to overcome the drag of worn wiriing. Replace the old wires and renew and install a battery kill switch as they do in racing. The extra relays will not pass concours, so a switch will not be of harm. All my cars have a battery kill switch and all wires that become suspect are replaced. While car is in garage or storage or sitting long term switches are off. Does not take long to switch and if under bonnet, then you actually have to look under hood and possibly forsee any trouble. As far as amperage, what is your CCA on battery and amp per hour usage say on label. Relay for the max or go less and have possible reset, as CCA should not be less than what starter needs.

 

[vette]

after reading all the research that others have done here on this subject I just wanted to mention that I too have been concerned about a damaged battery cable. When I wired my Healey I moved my new battery cable up as high as possible on the frame rail. But still the concern was where it goes under the rear cruciform before it gets to the rear axle area. Since our Healeys have a battery cut-off switch I use it every time I park the car. And lock the trunk. Therefore I am not worried about a shorted cable while the car is at rest. When I rewired, I bought battery cable used in drag racing cars from Jegs Performance Products. It is # 1 AWG, with extremely thick insulation. Where it was unavoidable to pass the cable under the frame and across the weld seam of the frame rail, I bushed the cable with another piece of cut tubing just at the weld crossing. This is substantially stronger and more protected than the original construction of the car. With this situation and routine inspections I feel pretty confident of the systems safety. I will try to attach a pic of my installation.
Dave. View attachment 26435

 

[RAC68]

Larry/Dave, your suggestions are good ones and what you have done, I expect, will make your cars less prone to an electrical problem.

I am not promoting this approach to anyone and have no interest in whether it qualifies for concourse. I am looking for the group’s knowledge to help verify that my logic is correct and the approach has no major flaws. Does it look like it will work and is there anything I haven’t thought of that could result in an unexpected consequence?

A Kill switch is a good idea and new grade 1 battery cables will definitely reduce the voltage drop from the battery to the starter along with providing more substantial insulation than the original cable, however, these are the only alternatives we have presently.

I have never had a car electrical fire and maybe I am just lucky for that or maybe it is because, with all I have done in the cars, I have also installed a sufficient number of protecting fuses on all critical circuits and addressed the condition of the lines with respect to how I expect them to perform. The main power feed line of the car is the only circuit unprotected and happens to be long and somewhat exposed in its run under the car.

Steve, a friend helped me identify the Contactor (relay) @ (https://www.stancor.com/pdfs/Catalog_2006/Pg_054_55.pdf) and one distributor has it as a $65.88 item. I am hoping to find something a little less pricy.

When reviewing the specs, I have learned that “inrush” refers to initial surge current capacity (i.e. when the Starter Lock Current) and the side normally open but closed during startup has an inrush of 400 amps. The Normally Closed side of the relay will handle a constant 50 amps with an inrush of 200 amps so it would seem to be able to easily handle a 40 amp circuit breaker and the Healey’s normal running current. I have a 65 amp alternator and also expect the breaker to fault if it ever creates a surge greater than 40 amps as well.

So, back to my objective, does anyone see a flaw in or a potential unintended consequence in the approach?

Thanks again for your responses and as I always learn from them,
Ray (64BJ8P1)

 

[57_BN4]

Hi Ray, You'll need to consider the full output of the alternator going into a dead flat battery through your breaker- I'd shoot for 80 amp or so breaker. A 65A alternator may produce more than its rating when dead cold with a flat battery- they are very susceptible to temperature. I've seen a 40A alternator produce 55A for the first few minutes of operation.

Another issue is if the fuse/breaker goes while the engine is running then your alternator is not connected to the battery which may cause the system voltage to run away as in another recent thread. You could add a high current diode 'backwards' across the fuse which will prevent this but now it is starting to get messy. Perhaps there is a reason cars don't have any protection on the starting and charging circuits...

The double throw relay circuit you are proposing will momentarily disconnect the battery from the rest of the vehicle as the contacts change over- this may upset electronics like a modern radio if you have one. I'd try first with just a regular starter solenoid and a fuse in parallel with it. This may or may not work depending on the reaction time of both the fuse and the solenoid vs the original starter solenoid. If the original starter solenoid reacts much faster than the one in the trunk then for a short period starting current will try to flow through your fuse/breaker. I'd borrow a regular solenoid and try it this way and if the breaker or fuse regularly trips then investigate your double throw setup. Ford 'mousetrap' starter solenoids are pretty cheap at NAPA- I got one for my 1972 LTD last year and it was $16 from memory.

Andy.