Electrical & Wiring - older vehicle theory vs. modern ideas

[Chorky]

So some questions for the community about electrical theory. I think I just overloaded and fried my brain with some engineering stuff for other things, or having a brain fart, but in figuring some electrical mods I seem to have confused myself.

The general question regards wiring needs for electrical load amounts and determining that gauge. Naturally there are the formulas for figuring amp rating, wattage conversions, etc... I am planning on adding two batteries to my already dual system (diesel) for a variety of tasks typical to a camper (but custom). One would naturally think that a small gauge wire (large wire) would be desired to go from the factory batteries to the new batteries for high current flow. However, what brings this to question is the factory wiring harness on the truck for camper/trailer's. Upon looking up the EVTM, I realized again that the factory harness has a charging wire with a 30a fuse specific for charging trailer/camper batteries. Now, one would think that if the camper/trailer batteries are drained to, say, 70%, and the engine started, that the potential power going from the main batteries through the factory charging wire would inherently be greater than 30A, but maybe I am wrong in this?

So, I suppose a better question is how does one determine the power flow (amperes) from one fully charged battery (or an alternator at 14.5v) to a secondary battery that is discharged at 70%? Or, what is the determining factor that dictates amp flow between wire when there is no real resistance other than that of pushing power through another battery? I would think it would push max amperage/voltage as the voltage regulator in the alternator would sense this, but am I wrong? Is there a reason why only a 30A circuit is used? I have fully drained my trailer batteries before, and never blown that 30a fuse, which just seems very strange to me....

To continue with this, my specific rig ('97 F350 7.3l) currently has a charging wire that I cannot quite sort out, short of cutting apart the wiring harness loom. I was hoping to install a mean green alternator at 200A for heavy loads and run a small gauge charging wire, but noticed that the alternator wire seems to split in the loom and run several places, which makes no sense... In addition to being split into two 12ga fuseable links that tie into the starter relay before jumping across the terminals to another single 8ga wire going directly (I think) to the main battery. Now I think I am wrong, and that the wire from the alternator just gets tied into the loom with a bunch of other wires for protection and is not spliced anywhere, as the wiring diagram would suggest....but wanted to reach out and see if anyone knows specifically....

Diagram:
https://www.bing.com/images/search?...rging+diagram&selectedindex=0&ajaxhist=0&vt=0

I would like to note that some things in the EVTM don't quite match the vehicle it would seem.

So a final question is if someone was running things typical of a camper, would it be sufficient to just utilize the existing wiring harness, and tie the 30a circuit directly into a solenoid that goes to two camper batteries, or would it be better to run a single small gauge wire to those batteries for a safety factor (knowing older vehicle harnesses are generally considered inadequate). Or, would it be better to bite the bullet and figure out how to put on a second alternator and use that specifically for the custom camper? And what if someone had typical camper type things drawing power over that 30a circuit, and also had a trailer hooked up that had additional loads? Seems to me that the factory wiring is inadequate.

 

[dwh]

The battery to battery flow is throttled by resistance. The resistance is plotted on a curve by state of charge (SoC). The lower the SoC the higher the resistance...and the higher the SoC the higher the resistance.

So you get the most amp flow in the middle. Generally around 40%-60% SoC.

http://www.smartgauge.co.uk/nosurge2.html

Stock automotive systems aren't really designed to charge batteries, they are designed to service the vehicle loads, while also maintaining a battery that is almost never drawn down below 99% SoC.

 

[DaveInDenver]

OE harness wiring is sized based on tolerable voltage drop, heating and insulation type. The original design assumed a few things to allow the engineer to select a wire that probably doesn't meet the standard NEC-derived charts you'd expect for a wire size. Those charts assume the worst case, which is indefinite current duration using a low temp insulation.

How much a wire heats up is a function of current, resistance and time. As @dwh says, this results in the charging circuit in your truck *probably* being assumed to only see an inherently limited current for some initial length and as the battery charged the current would taper. Coupled with an insulation that can tolerate higher temperature means what is stock is going to be just barely adequate but not ideal.

Since copper is expensive, heavy and takes up space the OEM has to build the harness with a lot of assumption and as you probably know they don't always stay on the good side of the adequate edge, e.g. vehicles do under charge, melt wires or in the extreme have wiring harness fires even without any aftermarket modifications. So I wouldn't take anything stock as the last word. If it doesn't make sense then you've run into a compromise the harness designers made.

 

[ducktapeguy]

Now, one would think that if the camper/trailer batteries are drained to, say, 70%, and the engine started, that the potential power going from the main batteries through the factory charging wire would inherently be greater than 30A, but maybe I am wrong in this?

I don’t know what type of batteries you’re using, but I have a aux battery connected to my main starting battery through about 12ft of 10 ga wire and I’ve never seen it pull more than 9-10 amp even when fully discharged to near 0%. Even then it drops pretty quickly as the voltage drop decreases. Even with a fridge load it never draws more than 13 amp measured with my inline power meter.

 

[Chorky]

The battery to battery flow is throttled by resistance. The resistance is plotted on a curve by state of charge (SoC). The lower the SoC the higher the resistance...and the higher the SoC the higher the resistance.

So you get the most amp flow in the middle. Generally around 40%-60% SoC.

http://www.smartgauge.co.uk/nosurge2.html

Stock automotive systems aren't really designed to charge batteries, they are designed to service the vehicle loads, while also maintaining a battery that is almost never drawn down below 99% SoC.

So I guess the better question then is where is the resistance coming from? If you have 2 sets of batteries. One set is connected through a large 4ga wire (such as the 7.3 as is stock), and another set that is connected via a small 18ga wire, naturally the second set would have a wire meltdown much quicker than the first set with the large wire. So where does the resistance between the two come from to 'throttle' that flow? So in instances where people are say, buying a smart solenoid (forget the company name) that is capable of handling 85 amps to charge a secondary battery they install in their bed for random accessories such as a fridge, etc.. At some point that second battery will be very low on voltage compared to the primary. so when the engine is running again, and the alternator is pushing max amperage to charge the system, what is preventing that 85a solenoid from frying when the alternator can push over 100a?

To add to that, then engineers who designed the wiring system for a camper must have known that those with campers would draw the camper battery down well below 99%. So if that's the case, that must also be the reason they chose a 30a circuit for the charge circuit. But, again, what prevents (other than the fuse blowing) the alternator trying to push over 100a through that 30a circuit to recharge the camper battery?

OE harness wiring is sized based on tolerable voltage drop, heating and insulation type. The original design assumed a few things to allow the engineer to select a wire that probably doesn't meet the standard NEC-derived charts you'd expect for a wire size. Those charts assume the worst case, which is indefinite current duration using a low temp insulation.

How much a wire heats up is a function of current, resistance and time. As @dwh says, this results in the charging circuit in your truck *probably* being assumed to only see an inherently limited current for some initial length and as the battery charged the current would taper. Coupled with an insulation that can tolerate higher temperature means what is stock is going to be just barely adequate but not ideal.

Since copper is expensive, heavy and takes up space the OEM has to build the harness with a lot of assumption and as you probably know they don't always stay on the good side of the adequate edge, e.g. vehicles do under charge, melt wires or in the extreme have wiring harness fires even without any aftermarket modifications. So I wouldn't take anything stock as the last word. If it doesn't make sense then you've run into a compromise the harness designers made.

So I agree that assuming stock harness design is adequate is asking for trouble and why I'm trying to ensure I have adequate wiring for heavy loads. I thought I had it figured out until last night - as is the usual case with most things for most people I think. In any case, your statement about the amount of charge for the length of time is important that I don't think many consider, and I too forgot about it until you said it so thanks for the reminder. Could this help explain the situation mentioned above? How a 30a circuit could get away with charging a camper battery down to 70% or less? So how would one go about calculating the amount of time required to charge a battery of specified capacity to full charge from a specified discharge amount (say that 70% for example), and then turn that into the potential catastrophic failure timeline of the factory 30a fuse.



As a side note the reason I bring this up is of course to ensure adequate wiring and charging ability for future vehicle plans; however, I, along with many people, last year spent some time off grid in my travel trailer, where over a 3 day period brought my 2 6v trailer batteries down to, oh...11.2v probably. Myabe even below 11. Not thinking anything of it when it was time to leave, just hooked up to the truck and took off. Now, thinking about the wiring, and looking back at the situation, how can one explain how a 30a circuit would not fail attempting to charge 2 nearly dead 6v batteries? I must be missing something here...but not sure what that is. I would assume that if you took a fully charge battery, and touched a 30a circuit from it to a nearly dead battery, the current alone would blow that circuit. So why did this not happen to myself, and countless other people, and what does this say about the actual needs for electrical wiring? Hoping all that made sense to you guys.

I don’t know what type of batteries you’re using, but I have a aux battery connected to my main starting battery through about 12ft of 10 ga wire and I’ve never seen it pull more than 9-10 amp even when fully discharged to near 0%. Even then it drops pretty quickly as the voltage drop decreases. Even with a fridge load it never draws more than 13 amp measured with my inline power meter.

So this is the exact situation I'm trying to figure out. Not just the fact that in yrou case you never pulled more than 10 amps to charge your aux battery. But WHY is that the case? And what calculations would one need to do to ensure they have sufficient wire to charge that system. So for me, I plan on adding 2 aux batteries to the truck's existing 2 starting batteries. So given normal amp requirements of a 12v fridge, water pump, heater, lights, etc..all the normal stuff. What calculations are needed to make sure that said wire from the main batteries to the aux batteries are sufficient? Naturally it relies on what the amperage pull will be of course, so how is that part calculated? It does not seem to follow the normal logic of calculating amps with given voltage and resistance.... Oh and I'm just currently using standard AGM factory replacement batteries for the truck, and plan to add 2 more of those for the aux system for purposes of redundancy knowing that they don't have as much capacity as other options....

 

[javajoe79]

If it never popped that 30a fuse then the battery was never accepting 30a. I think it's that simple. If for example you had a high load hooked to the aux batteries, like a winch, and you used the winch while the aux batteries were connected to the truck through that 30a circuit, it would blow at that point.

 

[dwh]

So I guess the better question then is where is the resistance coming from?

The resistance comes from the batteries. And the wiring.

If you have 2 sets of batteries. One set is connected through a large 4ga wire (such as the 7.3 as is stock), and another set that is connected via a small 18ga wire, naturally the second set would have a wire meltdown much quicker than the first set with the large wire.

Maybe, maybe not.

So where does the resistance between the two come from to 'throttle' that flow?

When you're talking about connecting two batteries, and the current that flows between them, you have to remember that the batteries are not capacitors - they are resistors. Current *will* flow between them, but slowly, even with big wire, because the batteries both resist. With small wire, it will still flow, but the small wire adds even more resistance and slows (throttles) the current flow even more. So it's not like you have some fixed current value, like 100a and that is what will flow regardless of the wire size. With big wire 100a might flow, with smaller wire it might be only 1a.

So in instances where people are say, buying a smart solenoid (forget the company name) that is capable of handling 85 amps to charge a secondary battery they install in their bed for random accessories such as a fridge, etc.. At some point that second battery will be very low on voltage compared to the primary. so when the engine is running again, and the alternator is pushing max amperage to charge the system, what is preventing that 85a solenoid from frying when the alternator can push over 100a?

Ding! Common misconception alert!

Alternators don't "push". So they don't "push max amperage". Loads, such as lights and batteries, suck. Hoover Dam is capable of producing 2,000 megawatts. But even with all turbines spinning, if the only load connected is a 40w light bulb, then Hoover Dam produces 40w. There is a very important concept in electricity that most people seem not to notice - "potential". Hoover Dan has a 2,000 megawatt "potential". Your alternator might have a 1500w (100a x 15v) potential (at least when cold, most alternators have both cold and hot ratings, generally, the potential generally goes down around 20% when hot), the solar on the roof of my camper has a 300w potential.

But that's just the potential. What actually happens depends on the situation at hand. Again, connecting two batteries with a small wire might not overload the wire, because the total resistance of the circuit (batteries + wire + voltage drop) limits the potential. So even if the circuit has a 100a potential with big wire, it might only have a 1a potential with small wire, and only 1a will flow.

To add to that, then engineers who designed the wiring system for a camper must have known that those with campers would draw the camper battery down well below 99%. So if that's the case, that must also be the reason they chose a 30a circuit for the charge circuit. But, again, what prevents (other than the fuse blowing) the alternator trying to push over 100a through that 30a circuit to recharge the camper battery?

The resistance of the wire size plus the wire length, plus the resistance of the aux battery limits the total potential. In this case, apparently to less than 30a, since you've never blown the fuse. Doesn't really matter that the aux battery is dead or near dead. That just means that in the beginning, the high resistance of a dead battery will limit the potential even more.

A dead lead-acid battery doesn't draw more current, it draws less current. It won't draw as much as it can until it reaches a SoC where the resistance is as low as gets for that battery. Usually around 40%-60% SoC. Then, as it gets full, the electrolyte can't absorb as much as quickly - it usually takes more time to get that last 10% or 15% into the electrolyte than it took to get the first 85% or 90% into it to reach a full charge. So you might have a 4 hour bulk stage, then it takes 6 or 8 hours of absorb stage to reach 100%.



[broken up to finish in the next post due to the stupid, retarded, social media brain-dead twittweet mindset that limits posts to 10k characters)

 

[dwh]

So I agree that assuming stock harness design is adequate is asking for trouble and why I'm trying to ensure I have adequate wiring for heavy loads. I thought I had it figured out until last night - as is the usual case with most things for most people I think. In any case, your statement about the amount of charge for the length of time is important that I don't think many consider, and I too forgot about it until you said it so thanks for the reminder. Could this help explain the situation mentioned above? How a 30a circuit could get away with charging a camper battery down to 70% or less? So how would one go about calculating the amount of time required to charge a battery of specified capacity to full charge from a specified discharge amount (say that 70% for example), and then turn that into the potential catastrophic failure timeline of the factory 30a fuse.

Good luck with that. The math gets to be unwieldy. First of all, you have to know the absolute capacity of the battery, but that changes (goes down) over time as the battery gets used / ages. Then you have to know the total resistance of the circuit under varying conditions - and the conditions do vary. The amps flowing through X wire with battery Y will be Z if the battery is at N SoC but as the SoC rises, the amps flowing will change (for a dead battery, first the amp flow will rise for a while, then start dropping). Then you have to figure the voltage drop (resistance vs. potential), but that also changes as the situation changes. The voltage drop of a 20' length of #4 wire will be X with a 30a load at 12v, but will be Y with a 10a load at 14v.

Then you have to factor in the fuse. Fuses are not on/off switches. They are heating elements that can stand a little heat for a long time, a medium amount of heat for a medium time or a lot of heat for a short time. You have to exceed the rating by X amount for Y time to make it blow. Only a serious overload, such as a dead short, will cause them to blow instantly (and technically, even then they aren't instant, there's some microseconds involved).

As a side note the reason I bring this up is of course to ensure adequate wiring and charging ability for future vehicle plans; however, I, along with many people, last year spent some time off grid in my travel trailer, where over a 3 day period brought my 2 6v trailer batteries down to, oh...11.2v probably. Myabe even below 11. Not thinking anything of it when it was time to leave, just hooked up to the truck and took off. Now, thinking about the wiring, and looking back at the situation, how can one explain how a 30a circuit would not fail attempting to charge 2 nearly dead 6v batteries? I must be missing something here...but not sure what that is. I would assume that if you took a fully charge battery, and touched a 30a circuit from it to a nearly dead battery, the current alone would blow that circuit. So why did this not happen to myself, and countless other people, and what does this say about the actual needs for electrical wiring? Hoping all that made sense to you guys.

In that situation I wouldn't be looking for the 30a circuit to blow when the aux batteries were dead. I would be looking for the circuit to maybe blow when the batteries got to 50% charged and were at their lowest resistance. That's the point where you'll see the most amps flowing. Then, if that situation goes on a while (hours), it would be entirely possible that the fuse would eventually overheat.

Except that the resistance of the wire and the batteries is obviously enough to limit the potential amp flow to 30a or less, since the fuse didn't blow.

So this is the exact situation I'm trying to figure out. Not just the fact that in yrou case you never pulled more than 10 amps to charge your aux battery. But WHY is that the case? And what calculations would one need to do to ensure they have sufficient wire to charge that system.

I'm just an old retired electrictian (24,000 hours on the books) and network engineer. The way I do it is KISS. I just figure out the max amps that could potentially flow, and size the wire for that (and obviously, size the fuse to protect that size wire). If you have a 100a alternator, you won't see more than 100a so there you go, done deal. Voltage drop is a bit of a red herring. The guys over on the solar forums make a big deal out of it "You gotta keep it below 3%!!!", which is fine for some things, but a bit goofy when talking about battery charging, since the voltage/amperage/resistance/voltage drop is a constantly shifting moving target. It's a whole lot easier to just figure out the max potential, and size for that. 99.9% of the time the actual system will be running at far less than redline, so sizing for max will always be adequate.

(Of course, there is still that factory alternator->battery wire to consider, which might only be a #8 wire with a 50a fusible link...)

So for me, I plan on adding 2 aux batteries to the truck's existing 2 starting batteries. So given normal amp requirements of a 12v fridge, water pump, heater, lights, etc..all the normal stuff. What calculations are needed to make sure that said wire from the main batteries to the aux batteries are sufficient? Naturally it relies on what the amperage pull will be of course, so how is that part calculated? It does not seem to follow the normal logic of calculating amps with given voltage and resistance.... Oh and I'm just currently using standard AGM factory replacement batteries for the truck, and plan to add 2 more of those for the aux system for purposes of redundancy knowing that they don't have as much capacity as other options....

Again, doesn't really matter. You have a 100a potential source, you size the wire to supply 100a and you're done. Oh, you can add up the loads - 5a for the fridge (but only when the compressor is running), 5a for the lights, etc., etc. - but no matter what loads you have, you aren't going to exceed the max potential of the source (especially when driving - you probably won't be running the microwave off the inverter while driving)...so size for that.

So most of those loads aren't going to be supplied by the alternator. Most of the time, the aux loads will be supplied by the aux batteries, and the aux batteries will be charged by the alternator while driving.

Right now, your factory system is sized to limit the charge potential to 30a or less, if you can live with that as a charge rate while driving, then you are good to go with what you have. It can take 12-24-36 HOURS to get an aux battery fully charged from driving (depending on the potential vs. the size of the battery bank), upsizing the wire to shave off some of the resistance and improve the flow potential, might cut that time in half. So it's question of if it's worth doing. If you typically drive only 2-4 hours per day while traveling, no matter how big the wire is, it might never be enough to get the job done. If you drive 2 hours then camp for 3 days, then it certainly won't be enough and probably not worth bothering. You'd be better off investing is a good shore power charger to use at home, and maybe a small generator to run the shore charger at camp.

 

[Chorky]

Well this is a ton of awesome information! Thank you for sharing! :beer:


quotes edited for space....

The resistance comes from the batteries. And the wiring.
With small wire, it will still flow, but the small wire adds even more resistance and slows (throttles) the current flow even more. So it's not like you have some fixed current value, like 100a and that is what will flow regardless of the wire size. With big wire 100a might flow, with smaller wire it might be only 1a.

But that's just the potential. What actually happens depends on the situation at hand. Again, connecting two batteries with a small wire might not overload the wire, because the total resistance of the circuit (batteries + wire + voltage drop) limits the potential. So even if the circuit has a 100a potential with big wire, it might only have a 1a potential with small wire, and only 1a will flow.

The resistance of the wire size plus the wire length, plus the resistance of the aux battery limits the total potential. In this case, apparently to less than 30a, since you've never blown the fuse. Doesn't really matter that the aux battery is dead or near dead. That just means that in the beginning, the high resistance of a dead battery will limit the potential even more.

A dead lead-acid battery doesn't draw more current, it draws less current. It won't draw as much as it can until it reaches a SoC where the resistance is as low as gets for that battery. Usually around 40%-60% SoC. Then, as it gets full, the electrolyte can't absorb as much as quickly - it usually takes more time to get that last 10% or 15% into the electrolyte than it took to get the first 85% or 90% into it to reach a full charge. So you might have a 4 hour bulk stage, then it takes 6 or 8 hours of absorb stage to reach 100%.

This is very interesting. So basically, what I understand from this is the age old theory many people have that 'if you have a powerful alternator you need a huge wire' is not necessarily true, and that in fact depending on the situation, sometimes having a high output alternator may not even be of much benefit.

Good luck with that. The math gets to be unwieldy.




I'm just an old retired electrictian (24,000 hours on the books) and network engineer. The way I do it is KISS. I just figure out the max amps that could potentially flow, and size the wire for that (and obviously, size the fuse to protect that size wire). If you have a 100a alternator, you won't see more than 100a so there you go, done deal.

(Of course, there is still that factory alternator->battery wire to consider, which might only be a #8 wire with a 50a fusible link...)

So most of those loads aren't going to be supplied by the alternator. Most of the time, the aux loads will be supplied by the aux batteries, and the aux batteries will be charged by the alternator while driving.

Right now, your factory system is sized to limit the charge potential to 30a or less, if you can live with that as a charge rate while driving, then you are good to go with what you have. It can take 12-24-36 HOURS to get an aux battery fully charged from driving (depending on the potential vs. the size of the battery bank), upsizing the wire to shave off some of the resistance and improve the flow potential, might cut that time in half. So it's question of if it's worth doing. If you typically drive only 2-4 hours per day while traveling, no matter how big the wire is, it might never be enough to get the job done. If you drive 2 hours then camp for 3 days, then it certainly won't be enough and probably not worth bothering. You'd be better off investing is a good shore power charger to use at home, and maybe a small generator to run the shore charger at camp.

Hey, I'm a researcher, so keeping it simple is the hardest thing to do sometimes haha. That being said, I totally agree that most the loads will be supplied by the batteries and the batteries recharged via the alternator, unless some thiungs are on while the engine is running. For example if I had a 50a air compressor running with the engine on, then there is a potential for a 50a flow. And it adds up if other things are going as well.

So what this makes me think is to size the wire necessary for a 200a alternator (which I am planning to install, as the current one is already 180a I believe), and then just call it good not worrying about the rest since the math sounds a lot more difficult than necessary. Also, about the main alternator wire, this is the one I am trying to figure out what to replace with mostly. It is a 6ga with 12ga fuse links I believe. But that all being said, also, the primary plan was in fact to utilize the alternator to charge the 'house' batteries after a few days of camping. Now solar could be added at a later date, but I'm skeptical of its functionality in my region with the given costs. So, with what you said above, it sounds like if I were to use the house batteries for 2 days without driving (camping), or even for a half of a day running that 50a compressor and some other tools, the likelihood of charging those batteries to even 90% after 4 hours of driving is, well, not likely? This may explain why after 5 hours of running the trailer on the genny the batteries will still below a full charge. But, carrying a genny with me is sort of out of the question so that makes me want to rethink some situations.

With that in mind....It makes it sound as if any use of a winch practically requires a vehicle to be plugged into a trickle charger once returned home otherwise the batteries may never recover. Do I understand this correctly?


Thank you again for all the input!

 

[Ari3sgr3gg0]

Here's a picture of why trusting factory wiring isn't great. This charge wire is for my 1989 F250 85 amp alternator. There were two spade connectors on the 2g alternator that had 10 gauge wires coming off of it. Those tied into this one teeny tiny wire. Headlights on, fan on, and wipers caused all kinds of grief before I finally tore into factory wiring and re did it. Ran a 4 gauge wire with 100 amp fuse and no more issues.

For your 3g just run a 2 or 4 gauge wire with fuse for less head ache and worry. There are plenty of diagrams online for how they are supposed to be run, super easy and less than an hour to wire up.
Now regarding your 30 amp confusion, the factory wire has so much voltage drop going back there that there isn't much juice flowing back that way. On eBay you can snag up 4 gauge jumper cables for cheap, aka good wire source. Run that new cable back to your trailer batteries for charging and you'll notice a big difference in state of charge. Of course you also need to see what size charge cable is inside the trailer as well. RV makers are notorious for making everything wire related too small. Basically if you want good bulk charging in a two to four hour drive for your deep cycles you need to up the cable size and fuse size.

 

[Arjan]

We re-wire our Land Rovers as I like to know what is going one and I have different standards than most car builders.

With everything LED these days, most wiring is 2,5 mm2 direct feed to the consumer (lights) and dedicated earths. We measure sometimes a voltage drop of about 0,05 volts over 6 mtrs. of cable and that has never been a problem. Bigger consumers have a 4 mm2 cable and, again, never problems. Winches, startermotors etc. get 35mm2 or 50 mm2 and quality crimps.

The KISS systems is really the way to go - 2 battery systems can be a real cause for some serious problems - one we see with clients vehicle in about 80 % of the break downs where the added electrics cause vehicles to stop. Quality wiring costs money as good cable is not cheap put it pays for itself once installed and it just works.

We link 2 batteries with a very simple HD relay (HELLA 4RA003.437.081) that closes 20 min. after start up and opens again when the engine stops. Start battery is 100 % seperated from the living one so no problems

If you'd like to know what amps you're talking about, get an amp meter and you have real world info. We have a small one that slots into a standard blade fuse holder and works very nice.