Another Dual Battery Wiring Question

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I'm with zzsstt on this one.

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Let's just agree to disagree then. Proper engineered insulation has a thermal gradient which reflects how it conducts heat. If you put a temperature probe into the insulation as you get closer to the heat source the temperature increases. If what you say is correct then there is huge radiation losses at a power station which strictly isn't true. Google if you want to check.

I make and made no representations about the heat shield for the battery. In theory I believe it can work, in practice who knows.

[zzsstt Junior Member Find all posts View Profile Close zzsstt]

The thermal gradient exists because energy moves through insulation based on temperature differential, thermal conductivity (or resistance) and distance. As the insulation thickness is increased (for any given insulation type), the resistance to heat transfer means that each successive "layer" is at a slightly lower temperature than the previous "layer". This in turn reduces the temperature differential for the next "layer", thus producing a gradient. However there comes a point where adding further insulation has a negligible effect, the temperature differentials are now so small that the additional insulation makes only a tiny difference. However at this point there is still an amount, albeit a small amount, of energy moving through the insulation. When the insulation contains a hot object, and the ambient temperature of the surroundings is low, this small amount of energy makes no impact because it is instantly absorbed by the massive "heatsink" of the surrounding atmosphere. All the above, when put together, shows that a power station or an insulated steam pipe will lose an amount of heat to the atmosphere without getting hot on the outside. How much heat depends on the thickness and efficacy of the insulation. It also shows why the maths that explains insulation theory involves not only the thermal conductivity and thickness of the insulation, but also the temperatures on each side of that insulation, and also why a steam pipe (or power station) will ALWAYS cool down when the source of heat is removed. If the insulation could ever provide 100% thermal isolation then the maths would only ever require the temperature on one side!

In the case of an insulated item in a hot environment, whilst the above still applies, each tiny amount of energy that passes through the insulation serves to warm the object (in fact this also applies in the paragraph above, but the planet is such a large heatsink that it is mathematically irrelevant). Over time the object will eventually heat up to the same temperature as the surroundings. How long that process will take depends on how effective the insulation is.

Insulation practice also involves cost - we could make an Esky that would keep ice frozen for weeks or months, but the cost and/or size would be prohibitive! On the other hand, health and safety and efficiency/profitability means that spending a bit more to insulate a steam pipe or power station is very worthwhile.

So the question is: do we think that a 4mm(?) thick wrap on the sides, but not top and bottom, of a battery in an engine bay provides enough insulation to give any significant protection? As before, my answer would be that it seems very doubtful!

Edit: Yes, lets just agree to disagree!

[brogers Advanced Member Find all posts View Profile Close brogers]

I think I might try and sumamrise the issue perhaps....and please correct me if I'm wrong

Some of us have placed an aux battery (of the AGm type or whatever) in the engine bay which is quite a hot location and batteries don't like heat
We wrap the aux battery partially with insulation in an attempt to isolate the battery somewhat from the engine bay heat..perhaps.
BUT...and I guess I hadn't though t about this...the battery itself generates internal heat when its being recharged so the added battery insulation will now act to slow heat loss from the battery.
But the insulation will also keep some of the heat from engine bay heat off the battery..

Heat from inside the battery now kept in the battery somewhat...heat from outside the battery kept outside somewhat..so what actually happens to temp of the battery?

mmm, so will the battery get hot and not work well or fail due to heat if placed in the engine bay? Who knows, I guess soemone will have to put a thermometer on the battery and take some measurements..

Two years and some big trips, including Mt Isa during mid Summer in 50 degrees ambient temperatures and my AGM (in insulation wrap) under the bonnett and so far still its working fine..so far anyway.

I was told by the folk who sell AGM batteries if the battery gets hot and fails it usually swells and "bulges" at the sides somewhat...no bulging at the sides of the battery just yet...well, not on the sides of the battery that is.

I have the ABR dual battery set up/isolator, and the isolator has saved us from a flat main battery on a few occasions , so far, so good for me anyway.

[zzsstt Junior Member Find all posts View Profile Close zzsstt]

AGM batteries don't like heat. No battery likes heat. Or cold. But then no battery likes vibration, deep discharges, uncontrolled charging, overcharging etc. etc.

People using batteries as domestic power supplies in off-grid systems have $1000's of dollars tied up in batteries. The chargers used have temperature sensors to monitor the batteries and adjust the charging rate to suit the temperature and vary the rate of charge, maximum voltage etc. to get the best performance and life from those batteries. They are carefully configured to charge at a maximum current (specific to the battery's capacity) in a series of stages as the batteries voltage increases until it is "full" (even the better workshop battery chargers can do this). Such chargers also can run regular automated equalization cycles on the batteries. The batteries are also carefully selected to provide sufficient power without ever being discharged more than perhaps 30-40%.

In comparison most 4WDer's chuck a (relatively) cheap eBay "marine" battery under the hood, connect it directly (via an isolator) to the alternator and hope it lasts for a couple of years, which it normally does! This is done with no regard to any differing voltage or current requirement that the auxillary battery may have when compared to the starter battery, and the battery is often sized on the basis of the amp-hour rating of the battery being 100% available for use.

Battery life is counted in discharge/recharge cycles, and the greater the depth of discharge the fewer cycles the battery will last. Quality deep cycle batteries are designed to provide a high number of deeper discharge cycles, starter batteries are designed to produce a burst of very high current but their life will be severly compromised by deep discharging. People will suggest that this is just marketing, and that all batteries are the same. There is some truth to that statement, but only in that the batteries that are marketed to 4WD'ers are not true deep discharge batteries - a "real" 200Ah deep cycle battery might weigh >60kg and cost >$600. That doesn't mean that deep cycle batteries are just marketing, it simply means that a 120Ah "deep cycle" battery that weighs 18kg and cost $120 on eBay isn't really very good!

Realistically it is far more likely that a battery will be killed (or mortally wounded) by vibration, a couple of >50% discharge cycles and uncontrolled charging than by the heat in an engine an engine bay. Or because it was cheap rubbish in the first place! On the other hand, if a $150 eBay battery runs the fridge removes the risk of depleting the starter battery when camping in the middle of a desert somewhere, then that's perfect. Is it really worth spending >$1000 on a top-of-the-range Redarc DC-DC charger and an expensive battery that may well be shaken to death or killed by an accidental 80% discharge cycle? Perhaps it's easier just to go the "cheap" route and replace it every year or two?!

The only important point here is that if we buy a cheap battery, ignore its charging requirements, discharge it without regard to it's life expectancy and install it in a harsh environment, then we probably should be happy that it lasts a year, not annoyed that it fails after three!!

(This post is thoughts and information, it is not targetted at any other post or poster!).

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

My view is that most auxiliary batteries die from over discharge and bad charging. Environment plays a part, vibration heat etc but pulling the battery down and only partially recharging it is the worst thing to do. I know a mate used to swear by marine batteries because they can take a pounding yet these lasted about the same time under the same use. And one of his so called marine batteries was also listed with the same specs as the run of the mill deep cycle battery. Also Andrew discovered that not all batteries are equal when he changed a new one like for like and the replacement was much better.

Anyway back to the subject, AK7 what are you going to do?

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Also Andrew discovered that not all batteries are equal when he changed a new one like for like and the replacement was much better.

Are you referring to me?

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Ummmm maybe?

[zzsstt Junior Member Find all posts View Profile Close zzsstt]

My view is that most auxiliary batteries die from over discharge and bad charging.

Given that the manufacturer of the vehicle puts a battery in the engine bay, normally with no insulation, and expects it to start the vehicle reliably for perhaps 5 years whilst being exposed to the same temperatures and vibrations as any auxillary battery that we might fit, I would surmise that you are totally correct. It is almost certainly "electrical abuse" that kills auxillary batteries if they die in a shorter timeframe.... or suicidal cheap batteries!

[Guest Close Guest]

Ummmm maybe?

Cause the first one was a dud right off the bat but I didn't know any better. It was an ebay cheapie too :P The new replacment was exchanged without hassle. It's an AGM and sits inside a battery box inside the cabin away from heat and isolated from vibration. It is also on a 240V charger 24/7 when the fridge is not in the Prado.

[AK7 Senior Member Find all posts View Profile Close AK7]

Anyway back to the subject, AK7 what are you going to do?

I'm going to focus in simple terms on just one aspect at a time. Perhaps we can keep this thread free from thermal dynamics and anecdotes?

Some people use the auxiliary battery as the starter...

1. How is that different to swapping the fuse power source to the auxiliary battery instead.
2. Is there anyone who has this configuration having any issues with it?
3. Aside from essential services (locks, etc) potentially causing problems after power loss, is there any other reason this configuration should not be used?
4. If power loss is such an issue for essential services (locks, etc), why don't I see dozens of threads about people having said issues when doing DIY dual battery config? Surely the batteries get disconnected during this time...

[zzsstt Junior Member Find all posts View Profile Close zzsstt]

The ABR approach is to simply relocate the standard battery to the other side of the vehicle. No changes are made to the factory wiring, all the factory fitted items are still connected to a single battery, the way Toyota intended, but that battery is located somewhere else. The ONLY reason for doing this is that (apparently) the standard battery "space" in the Prado is bigger than the space on the passenger side, so a larger auxillary battery can be fitted. It is done simply to make best use of the available space, and allow a larger auxillary battery to be fitted.

Auxillary batteries are fitted to provide a discrete power source for aftermarket items like fridges that run when the vehicle is "switched off", thus preventing them from depleting the "factory" battery. The first question, when discussing splitting the factory equipment between two batteries is simply WHY? It most unlikely that the standard factory wiring and accessories will ever flatten the standard battery, as everything is switched off with the ignition. So why attempt it at all?

The issues with doing it are, I suspect, many and varied. Not least is that if either battery is flat the engine won't start - the starter battery needs charge to churn the motor and the ECU etc. need charge to control the engine. So it would seem like a lot of work that fails to meet the aim of preventing aftermarket accessories from flattening the starter battery! You could, in an extreme case, flatten the auxillary battery with a fridge and then be unable to unlock the car because the locks were powered from the same battery!

Perhaps if you explain what you hope to achieve we can have a better understanding?

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Yep have to agree that I cannot see any reason why you would swap everything else over to the auxiliary. AK7 can you give a heads up why you think this is a good idea or why you want to? On the 150 the 'spare' battery space is quite small and as I understand it to fit even an N70 (12") battery you need to move the A/C piping. On the 120 I made a new battery tray and ran a 13" auxiliary battery so there was no way it was ever going to fit. Never considered swapping side for side so I did my battery box in the back thing and then the spare battery space became filled with air compressor.

I understand Derek's concept of putting something smaller but equivalent like an optima in the spare space and use that as the main battery relocating everything from L to R. And then using the larger space to fit as big an auxiliary as you can though still not sure a 13" or larger battery will fit but the other stuff is beyond me.

PS: thermodynamics and heat transfer are great, without it beer would be hot and we would be in the dark ages, like the UK. Eastop and McConkey!!!! In joke but anyone who has studied thermo at uni will probably understand. The bible!

[AK7 Senior Member Find all posts View Profile Close AK7]

I'm talking about seperating the starter motor and the fuse box. Reasons why or why not. Aftermarket items, sure, but why not include some of the factory items? You guys might not use them much but I may forseeably use them. I don't think anything is a good idea or a bad idea yet - that's what this thread is for.

Who the f'k is Derek?! :???: Sorry Derek...

I'm not concerned about battery size or potential heat issues - yet. I'm not up to that point yet and haven't even mentioned battery sizes... I know the auxiliary bay is smaller and that the AC pipe is an issue. I'm only interested in discussing the different dual battery wiring configurations and working out which is right for me.

My humble apologies if I sound rude. I know you guys have the best intentions of helping me cover all bases but I really need to just keep this thread on track please I've been accumulating the parts for this over the past six months. No rush. Wired myself but in a way that suits my needs. I need to have a clear picture in my head first.

Please can we just go back to basics and answer these questions:

1. Why do people swap the starter to the Auxiliary battery?
2. Is there anyone who has this configuration having any issues with it?
3. Aside from essential services (locks, etc) potentially causing problems after power loss, is there any other reason this configuration should not be used?
4. If power loss is such an issue for essential services (locks, etc), why don't I see dozens of threads about people having said issues when doing DIY dual battery config? Surely the batteries get disconnected during this time...

Yep thermal dynamics do help the world go 'round (in more ways than most would think) - my old man is a refrigeration engineer so I get my fix from him. This thread isn't about that though...

[zzsstt Junior Member Find all posts View Profile Close zzsstt]

All your questions have been answered in previous posts. But just for the sake of it:

Q1/ why do people swap....

Answer. They don't. Derek (who trades as ABR Sidewinder) sells an auxillary battery wiring kit that allows the "factory" battery to be relocated to the passenger side of the vehicle. This is done solely because the driver side space is smaller, so it allows a smaller battery (still perfectly adequate) to be used as the "factory" battery, and a larger "auxillary" battery to be fitted on the passenger side to run fridges etc. The factory battery, wherever it is located, is still used for ALL factory functions. The auxillary battery, wherever it is located, is only ever used for aftermarket products like fridges.

Q2/ Is there anyone with this configuration..

Answer. If you're talking about Dereks approach then there is no problem at all. If you are talking about what you propose (splitting the factory wiring between two batteries) I would seriously doubt that anybody has ever done it.

Q3/ Aside from essential service...... is there a reason it should not be used.

Answer. Because it serves no purpose and is a huge amount of work. (See also Answer 4, below)

Q4/ If power loss is such an issue....

Answer. I think you are confusing too many issues. Temporary disconnection is not the problem. The problem is that you are proposing splitting the basic vehicle functions between two batteries. That serves to create an additional single point of failure (if either battery is dead the car won't go). It also raises concerns of voltage control etc. - which battery does the management system look at to decide it is fully charged? Potentially the management system (hanging from the fuse box) will see a "full" battery and reduce the charge rate whilst the "starter" battery is still near death. I suspect there are many other potential problems, but as the configuration serves no apparent purpose there seems no reason to go looking for them!

I am still uncertain of exactly what factory fitments you would be using that would risk draining the battery. There's really only the radio, tiny inverter and ciggie lighter socket, all of which require the ignition to be left on unless they are rewired, and none of which are really designed for long term use with the car stationary. If you want to use an inverter for long periods, or anything powered from a ciggie lighter socket, it is surely easier and far less risky to install a conventional auxillary battery and leave the basic functioning of the car the way Toyota designed it.