Thread: Dual Battery Time Again

Originally Posted by drivesafe

Hi rovernutter and while there is always going to be a voltage drop in long runs of cable, particularly when it's to a caravan or camper trailer.

But if you run decent size cable, like 6B&S twin ( 13.5mm2 x 2 ) between the cranking battery and the house battery, if the house battery is low, the voltage drop is more likely to be greater at the battery when using a DC/DC device than it is when using decent cable.

From comments posted, it appears that this subject gets debated on a regular basis. So I will try and keep this brief.

There is a formula that calculates voltage drop. Voltage drop equals the length of the cable in metres multiplied by the current draw in amps multiplied by copper resistance of 0.017. This is then all divided by the cross sectional area of the copper part of the cable in square millimetres.

What most people do not take into account is that the length is the total length, there and back, of the cable to the main battery.

From what I have found is that by the time you weave the cable around the engine bay, down to floor level, to the back of the car and then back up to the battery, you at looking at a bit less than 4 metres. So there and back is 8 metres in total.

As people were talking about a 40 amp charge rate. I will use this in the formula and the 6B&S cable diameter.

So the voltage drop will be (8x40x0.017)/13.5 or .4 volts.

If you were running it to a caravan, you can easily double the length of cable and the voltage drop would be (16x40x0.017)/13.5 or 0.8v.

I simply gave up on trying to run a cable big enough to my camper to charge the battery is any sort of reasonable time.

This is just from my experience. It will vary from vehicle to vehicle. It will depend on whether you have an old type alternator or the newer variable voltage ones. It depends on what voltage you are getting at the main battery, etc, etc.

The new style alternators only run at 14.4 volts for a short period of time after starting the car. They then drop back to around 13.5 volts. This makes charging aux batteries a pain. I think this is why DC/DC chargers are becoming more popular.

There are just too many variables to give a simple answer that will cover all vehicles. So I am not about to recommend any particular setup.

Hi again rovernutter and you, like most people, have fallen for the supposed voltage drop advantage DC/DC devices are claimed to resolve.

You just posted up some nice math but you only gave half the story.

I have carried out test on my own vehicles and on other, and with low batteries, usually two and sometimes three, there will be more than a 1.0 volt drop.

But these same tests have in excess of 60 amps being drawn by the low batteries and the high amperage is the reason for the large voltage drop.

If batteries are drawing 60 plus amps, the amount of voltage drop becomes irrelevant.

As for how alternators work in deferent vehicles. Here is a testimonial from one of my customers, and it covers the three main types of alternator operations found in Land Rovers, plus what happens when you use a DC/DC device in similar circumstances.



This is an E-mail I received late last year and demonstrates the need to get RELIABLE info for equipping a vehicle with a DBS that actually works and not rely on what so called experts in this field claim.

These guys fitted their own DBS and had no problems yet the fifth guy got his DBS "professionally" installed, and look at the results.

Note, the D2 has a constant voltage alternator, the D3 has a variable operating voltages, and the D4s have Regenerative Braking Voltage operation.

This covers just about every type of alternator operation.

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Hi Tim

Sorry for the long email ? in the second half of the email my mate with a 200 series wants to know if you can help him

It seems like only yesterday that we installed your dual battery kits in 4 vehicles (D3, 2 x D4, D2) ready for a big trip. We've been back a while so I just wanted to give you an update on how the battery kits went.

In short, they were brilliant. We installed all 4 kits you sent us in one day. Aside from the fiddly process of removing trim pieces and working in the tight confines of a Land Rover engine bay the installation was straightforward with your instructions.#
Throughout the trip the kits never missed a beat, and others on the trip had lots of electrical issues.

On a few occasions the battery guard did it's job cutting out the fridges, but this was usually when we were camped in one spot for a few day (very little or no driving) in very hot weather (making the fridges run almost non-stop). It took surprisingly little driving to provide a good bulk charge back into the batteries.

We travelled the Tanami Track, Gibb River Rd, Kalumburu Rd, Mitchell Falls NP Road and the Great Central Road and nothing rattled loose or played up in any way.

Our friends in a 200 series Land Cruiser, on the other hand, did not fare so well when it came to electrics. Following local advice he had a Ctek DC-DC charger installed professionally by a very large and well known auto electrician here in Newcastle. They also installed an auxiliary circuit for his fridges and accessories etc connected to the aux battery.

In short the system never worked properly. Initially the DC-DC was installed in a very hot part of the engine bay making it cut out once the engine warmed up. We repositioned it behind his grill but even then he only got a useful charge on big driving days. On days were we did short trips around camp he was always having problems with a flat aux battery (including the extra aux battery on the trailer). To further add to his woes the auto elec did NOT install any form of low voltage protection on the circuit for his fridges and accessories. He ran his batteries far too low a few times and it didn't take long for the aux in the engine bay and the aux on the trailer to both die completely.

He couldn't believe how well our systems worked. So he was wanting to find out a little more:

Feel free to use my feedback as a testimonial if you wish.

Thanks


NOTE, the Toyota now has one of my systems installed and all is working properly.

Tim, I have never used your products and therefore cannot comment on them. I am in no way commenting on your products or knowledge.

I can only comment from my own experience. It is not from information from people trying to sell their products.

The thing that killed me most was the ECU controlled alternator. With the relatively low voltage they switched to, combined with the voltage drop to the camper, there was no way that the camper battery was going to charge in any reasonable timeframe. I doubled the size of the cable and it made bugger all difference. That is when I started to do the calculations to work out what size cable I would need. Just was not practical.

DC/DC charger works well and will charge the battery during a normal travel day, with the fridge running.

Obviously you have had different experiences, I can only comment on mine.

In regard to your email from your customer, it should be noted that the Toyota had a voltage variable alternator, I do not believe that the Discos do. Therefore, the DC/DC charger would offer no benefit to the discos. The issues they appeared to have was with the unit failing to work rather than the concept of the DC/DC charging system. Maybe the unit didn't like the heat, that it was installed badly, or it was a rubbish brand. I do not know why it didn't want to operate, so can't comment.

Yep correct, Toyotas have a basic variable voltage system, and this one had an Alternator Voltage Booster Fuse installed when my kit was installed.

Also, Land Rovers not only have variable voltage operating alternators, they will operate even lower than any Toyota does.

This does not alter the fact that you still do not a DC/DC device in these vehicles and with more than 4,000 D3, D4, RRS and quite a few L322 RR plus a very large number of Defenders equipped with my gear, and no one has the problems you are having.

Originally Posted by drivesafe

Yep correct, Toyotas have a basic variable voltage system, and this one had an Alternator Voltage Booster Fuse installed when my kit was installed.

Also, Land Rovers not only have variable voltage operating alternators, they will operate even lower than any Toyota does.

This does not alter the fact that you still do not a DC/DC device in these vehicles and with more than 4,000 D3, D4, RRS and quite a few L322 RR plus a very large number of Defenders equipped with my gear, and no one has the problems you are having.

I suppose I should be more specific.

The Toyota Land Cruiser had a Temperature Compensating Alternator. It just works off the engine temp. While the motor is warming up it puts out full voltage. Once it is at running temp it cuts back on the voltage. It is not that smart, like the one on my car. So while it is at running temp it only puts out the lower voltage and therefore takes considerably more time to charge the battery. Such systems can benefit from a DC/DC charger.

The LR3/LR4/RRS have a Regulated Voltage Control (RVC) alternator. It is a lot more sophisticated and monitors the load on the system, voltage, and a number of other factors. It will therefore take into account the extra draw of the auxiliary battery when working out its output voltage. Therefore this system should charge both batteries quicker than the old style alternators and should not need a DC/DC charger for batteries, particularly those under the bonnet.

I am also aware that people make units to trick the temperature type alternators to put out a higher voltage when the motor is warm. I did not use one of these as I was unsure of any damage that adding the extra voltage to the higher cold voltage would do. It also runs the higher voltage when the batteries are fully charged. I only need the extra voltage a few weeks of the year when the camper is on the back of the car.

But it is all about horses for courses. What suits me will not suit many others. I do not believe that there is a one size fits all solution.

But I do not wish to turn this into another debate about batteries, so I will leave it at that.

Well that was a very interesting read, thanks Boyz.

Don't worry too much about it being a debate, I think it's really good you both listed all the Pro's & Con's

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Originally Posted by drivesafe

Hi Dave and thanks for all that info, and while I agree with most of it, the difference between what you get in the way of available accessories battery capacity is quite a large amount more than you covered.



Next, and Dave, this is one I disagree with you on. You can use dissimilar batteries in a parallel DBS setup and you will not have any problems that are different to problems you can have when using identical batteries.

for the first snippit...

Agreed, but I didnt really need to see the point in delving any further into it than just the simplest benefit in the simplest perfect world environment assuming that the DCDC allowed the batteries to remain connected down to the 50% threshold on the main battery (most dont), its still going to burn amps shunting the power across. when I sat down and did the numbers across an amps meter the calculated amps that most DC/DC chargers burn up in a 24 hour period is enough to run my engle for 3 hours. by comparison the milliamps an SC80 burns in the same 24 hours might run a a pair of my pole marker leds for a couple of hours.

I've just gone back and re-read the post. I made an absolute lunch out of writing out what I was trying to put across... I left out the bit about "assuming the DCDC remained connected for the same length of time as the traxide"... I'll go back and edit it in in blue, on an edit no I wont, I made the post 2 days ago so its locked


for the second snippit

as for the battery thing. I wont deny that you can do it without problems but there are minor benefits that can be had if you stay on the same tech type for the batteries and there are some drawbacks if you regularly push the limits of friendship on the batteries of course theres some majorly huge pitfalls to fall into if you go wildly dense about crossing technology platforms in batteries without taking into account the variation in the optimal charging and discharging requirements. Call it me being lazy, easier to advise that the best results can be had if you stay on one platform.

People who know much more than me have already posted, so I'll just support some things from my experience.
Starting batteries are designed for quick bursts of power, which is what you need when winching, whereas second batteries such as deep cycle and AGMs are designed for slow draw down for a fridge and similar demand.
Connect the winch to the starting battery, which will be a wet cell, because it will provide large bursts of charge in a short time. Keep the engine running while winching and you won't have a problem.
Install an AGM second battery (mine are 120 in the vehicle and 100 in the camper) for a slow drawdown to run your fridge.
You probably don't need a DC-DC charger in the vehicle. After all, an alternator should make at least 80 amps, and then its a matter of how much your wiring is rated to send power from the first to second battery. Mine has 60 amp fuses. Most DC-DCs only power to 20-30amps, so its only a gain if there is a lot of current loss. For example, I did install one in the camper, simply to boost the power running all the way down from the front of the vehicle through the trailer plug and into the camper.
If you want to go Redarc they have a simple dual battery isolator for about $150 from memory. We had one in the Disco, which died in the desert, and bought another one in Alice. Cheap, simple and does the job.
If you want something better, the Traxide seems to be unusual in that it allows the fridge to draw on both the second battery and part of the starting battery, but ensures it still has enough to start the vehicle, so that gives more available battery reserve and so more running time.
We have a Traxide in the Defender and its great. Our first and second batteries are connected with dual 60 amp fuses, which seems to work fine, but I know Drivesafe says you can go higher if its is done correctly.
As for solar, most solar kits come with a regulator, so you don't also need a solar DC-DC to charge off solar, as the regulator does the job. Just fit Anderson plugs and charge the battery directly.
Carry a 240 volt multi-stage charger if travelling and, when mains power is available, hook it up and give your system an overnight boost, which should see it become fully charged (about 12.9 volts).
I hope that helps and I have got it right. Seems to work for me.

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