What's so special about dual battery controllers?

[TheOtherLeft]

Hiya all,

I'm wondering what's so special about dual battery controllers compared to a BEP VSR?

The Redarc's etc operate to the same principles as the VSR but have some fancy features such as the ability to link both batteries in parallel for starting. I think the Projecta operate the same was as well.

Given that the BEP VSR can be bought for a little over $100 what's the difference between this and the other units in terms of battery charging?

[-Scott-]

Starter batteries can accept an essentially unlimited charge current, and modern alternator systems are designed to deliver large currents.

Deep cycle batteries don't like large charging currents, and can be overcharged by a simple solenoid based charging system.

Some electronic controlled systems will limit the charging current into the second battery to avoid doing damage.

The leading (read "most expensive") devices operate like a multi-stage battery charger, and will limit initial charge currents to protect the second battery, but still bring your battery back to full charge faster than an alternator and solenoid combination.

If you are using two starting batteries, you probably don't need anything more than the VSR type controller.

[drivesafe]

Hi TheOtherLeft, it all depends on how you actually want to use a dual battery set up.

While I agree in part with Scott’s reply, it is over simplifying the subject ( no offence meant Scott ).

The intended use of a system should govern how you set up.

For a starter I do not rate AGM batteries as being better than good old Flooded Wet Cell batteries (FWC), including Deep cycle types.

Now if you are fitting a second battery to run you accessories while camped out for a few days and you intend to fit the battery under the bonnet then you should seriously be considering nothing other than a FWC type battery ( or a very expensive Automotive grade AGM, like an Optima ) because of both the heat and high voltage. A FWC battery will take both so no special charging device is needed.

If your planning on using a fair bit of power while camped up then you will probably need two ( or more ) auxiliary/house batteries and this means, because of space restraints in the average engine bay, they will have to be fitted else where.

Now with a different location, AGM batteries are now fine and both the size of your battery capacity and the remote location means that you can charge straight off your alternator, no matter how big it is, you are not going to overcharge the batteries.

Furthermore, the more power you use while camping and the lower you take your batteries, AGM or FWC, you will SAFELY charge the batteries off your alternator and will charge them quicker the a DC-DC step up device can do.

In both situations, a standard VSR type will look after your batteries better than a DC-DC step up device can.

Next, you now need to look at the different advantages different VSR type isolators offer.

Most simply switch on and off when the motor is running or stopped.

Some offer an emergency jump start feature.

Some offer the ability to winch from a second battery.

But if your after the quickest recharge set up, and sorry this is going to be a spam but facts are facts, my SC80 offers the fastest
recharging set up of any Dual battery system, including faster than DC-DC step up device.

So TheOtherLeft, while you are asking the question of the difference in DBS, what do you want from a DBS set up?

[TheOtherLeft]

Thanks for the reply fellas.

Drivesafe - are you saying all DC-DC chargers aren't worth it? If so then is it just the case of ensuring adequate gauge wiring (ie fatter then 6AWG) is enough to get the second battery (which is mounted in the rear of the vehicle) fully charged, assuming of course you drive long enough to give the alt enough time to charge the batts?

[drivesafe]

Hi TheOtherLeft and you hit the nail right on the head.

For any system to be able to fully charge an auxiliary battery, no matter where it is, you need to drive long enough to allow what ever system is being used, to do it’s work.

It’s this time factor that the SC80 has over other set ups, but I have spammed enough.

6B&S cable is large enough for just about every dual battery set up but TheOtherLeft, if you have thicker cable, all the better for it.

Cheers, Tim.

[-Scott-]

Tim, no need to apologise. My response was simple, because I was only providing an example of circumstances under which a more complex system could offer benefits. I also accept that what I consider to be (technically) an excessive charge rate for some battery types is probably a theoretical problem only, and of no practical significance.

However, as I have explained more than once in the past, I'm here to learn, as are many other members. I don't want to "learn" incorrect, over-simplified, or mis-represented information, and I doubt anybody else does either.

I agree that pretty much any commercially available dual battery system will allow the auxiliary battery to fully recharge - eventually. To me, there are only two significant issues to worry about. These are time to recharge, and how "kind" the system is to the auxiliary battery (the ability of the charging system to match the battery manufacturer's recommended charging profile.)

This is a can of worms, as the performance of any particular battery management system is affected by the installation method, but there are also some constraints inherent in some systems.

I have no doubt that the SC80 is a well designed and well supported solution and wouldn't automatically advise against it. Depending on the user's requirements, it could easily be the best solution.

However, for some systems, installed properly in comparable configurations, I believe that a product such as Redarc's Smart Start will recharge the auxiliary battery faster than any solenoid based system, while producing a charge profile that is better suited to the battery. More expensive, undoubtedly. If money were no object, I would install this system tomorrow, without hesitation. If I were gifted an SC80, I would wait until my current system fails (again).

I'm happy to be proved wrong (again), but I want some evidence - not just your word for it.

TheOtherLeft: One comment on charge current. The Redarc unit I linked above has a nominal maximum charge current of 20A. At this rate, a 100Ah battery (large) would be (theoretically) charged from 50% discharge in 2.5 hours. In reality, none of the battery management systems discussed here will fully recharge any battery in anywhere near 2.5 hours - so don't be too concerned about maximum charge current. It's unlikely that a battery in the rear of your vehicle will ever draw more than 20A of charge current, and certainly not for long, so a 20A "limit" isn't significant.

[TheOtherLeft]

Thanks for the info Scott. Why is it there is a 20amp limit? Is it the wiring, alternator or dual battery controller because I thought the second battery, like any other electrical load will suck up as much current as it will allow???

[drivesafe]

Hi Scott and I actually TOTALLY agree with all you have posted, but even though my SC80 is a relay ( solenoid ) type isolator, it is NOT your run of the mill isolators.

I’ll explain in a little more detail of how it works then you be the judge.

The primary difference between the SC80 and all other isolators is that, while other isolators literally disconnect the auxiliary battery from the system once the motor has stopped, the SC80 does not.

The SC80 allows 50% of the cranking battery’s reserve capacity to be used in conjunction with the auxiliary battery to power your accessories while camping.

This unique way of operating actually allows the system to meet the very points you suggested as being ideal.

Some comparative examples. If you have a single 100 Ah auxiliary battery and you use around 40% of it’s capacity and then recharge it using a 20 DC-DC device, you will take about 3 hours to fully recharge then battery.

With an SC80 set up using the same amount of power ( 40 amperes ) your standard alternator running at around 13.2v will take about 3 hours and if the alternator is running at 14.0v, it will take a little over 2 hours to replace the power and the reason for the shorter charge time that with the SC80 set up, you are only taking 20% from two batteries not the 40% from one battery.

Now say your a heavy power user and you take your single battery down to 20% SoC ( 80 amperes used ). Your DC-DC device will take at least 5 hours to fully charge the battery, whereas the SC80 set up will take around 4 hours with an alternator voltage of 13.2v and around 3 hours with an alternator voltage of 14.0v and again the reason is that with the SC80, you only need to bring the batteries up from 60% SoC not 20% SoC.

And the advantages of the SC80 goes WAY beyond this.

The first and obvious is that because your not discharging your batteries as low as you have to do with a set up using a DC-DC device, your batteries are likely to have a longer life span.

You are also helping the batteries life span when using ( discharging ) the batteries because you are splitting the current drain over two batteries so the work load on each battery is less.

Also, if you do need more usable power, with a DC-DC device and a single battery, bad luck you have all there is. With the SC80, if you have an 80 Ah cranking battery and the same 100 Ah auxiliary battery, even if you only take the auxiliary battery down to 30% SoC, you still have a total available usable capacity of 110 Ah, 30 more than the other set up and an SC80 set up will cost you loads less than a DC-DC set up.

Do the figures yourself and you actually have more chance of harming a battery using a DC-DC device than you do with an SC80 set up.

Now if you find that with either set up you still need more power and add another 100 Ah battery to the set up. While the SC80 might take up to 20% longer to fully charge all the batteries, a DC-DC set will take around 90% longer.

The SC80 will literally pay for itself just in the battery longevity it gives.

[-Scott-]

Thank you Tim. I stand corrected.

I had forgotten that your SC80 is more than a simple isolator, and I was hung up on simply re-charging the second battery. I had forgotten the discharge part of the cycle.

I'm surprised by some of the charge times you quote but, fundamentally, you're right. Discharging and re-charging two batteries in parallel offers significant benefits.

Personally, I'm too conservative to be completely comfortable with drawing down my starting battery like that, although I'll have to think about it some more. You've also given me an idea for playing with a Redarc system. Thanks.

Thanks for the info Scott. Why is it there is a 20amp limit? Is it the wiring, alternator or dual battery controller because I thought the second battery, like any other electrical load will suck up as much current as it will allow???

The Redarc unit I linked (with the 20A limit) is a DC-DC unit - it's design limits it to a nominal 20A (300W, buried somewhere in the specifications).

A lead acid battery will "suck" as much current as it wants, based on its state of charge and the voltage applied. If the voltage is fixed, the charge current drops off as the state of charge increases. If you want to increase the current again, you need to increase the voltage.

[drivesafe]

Hi again Scott, the info on charging is based on tests I’ve carried out and I used Flooded Wet Cell (FWC) batteries for the tests.

The reason I primarily use Flooded Wet Cell batteries for the testing, is that this type of battery is the slowest battery to recharge when using an alternator so I can get worst case scenarios.

When I have used AGMs, the recharge times are even shorter when charging from an alternator but because of the set current limitations of a DC-DC device, there is no significant difference in recharge times for any type of battery.

While your spot on the money about having to limit the charge current to some batteries, this is only limited to AGM type batteries. Flooded Wet Cell batteries are literally self regulating and it make no difference how much current you have available and how high the charge voltage is, a Flooded Wet Cell sets it’s own charge current levels that are quite safe for Flooded Wet Cell type batteries.

AGMs on the other hand can easily be damaged if the charge current and the charge voltage are too high.

The saving grace for most set ups using AGM batteries is that the battery is usually fitted somewhere else in the vehicle or in a caravan or camper trailer.

In these set ups, the long cable runs, no matter how thick the cable is, the length of the cable acts as a quasi voltage and current regulator that acts to protects these batteries.

[drivesafe]

Scott, your response to taking the cranking battery down to 12.0v ( 50% SoC ) is common but the SC80 has been working like it does for over 20 years now and there has never been a problem.

In fact the average user seems to get better than average life spans out both the auxiliary battery and the cranking battery.

The reason for the longer life span of the auxiliary battery is easily explained, as in the previous reply.

From loads of feedback from RVers using my gear, indicates they get above average operating life spans but I can’t explain why.

[midi73]

Hi guys. I have read this post with interest. I am trying to sort out what the best isolator is. I seem to know enough about electrics to get me into trouble lol, but not enough to sort this sort of thing out.
Drivesafe. are you saying that a flooded (deepcycle) battery would be better than an agm?
I will tell you gurus what setup I want, and could you please advise.
I have the early model 80 turbo diesel so I have the luxury of being able to fit 2 aux batteries under the bonnet. I want to run a fridge off these batterys 24/7. Obviously the alternator charges when I am driving, but I am not always going to be driving long distances to get a full charge, so I want to set up an 80w solar panel (should be enough to stay ahead of the fridge) to charge during the day and possibly hook in a battery charger to keep the battery topped up at night. My query is, which is the best isolator setup for this. Drivesafe is yours good for this? would a redarc do? or would I be best going to a rotronics independant charge isolator, or is that more for agm batterys? Where do I connect in the solar panel and the battery charger. Phew lol, so many questions.
Also Drivesafe, I happen to have one of your sc40 isolators. What is the difference between it and the sc80.
thanks guys. Sorry for the hijack, although it will prob help to answer some questions in the origional post.

[drivesafe]

Hi midl73, as you’re planning on adding two auxiliary batteries, your SC40 is not going to meet your needs. The SC80 is a higher capacity version of the SC40 and the SC80 is designed to charge any number of auxiliary/house batteries while the SC40 is designed for charging a single auxiliary battery.

Because the second battery is going to be under the bonnet, I would go with a flooded wet cell (FWC) battery. They tolerate the heat and higher voltage found under the bonnet, much better than AGMs will, unless they are automotive grade AGMs and a lot more expensive than standard AGMs.

A deep cycle FWC batteries would give you 20% more usable capacity over a cranking batteries but you can use cranking batteries as your auxiliary batteries.

Any form of DBS that separates batteries will actually take much longer to charge the batteries over the way the SC80 works and while they work in older vehicles, DBS that separates batteries can actually cause heaps of problems in some newer vehicles.

A standard solenoid/relay type isolator will do a good job but combine deep cycle batteries with the way the SC80 works, which will give you around 30% more usable battery capacity than any other isolator, and you will have the best possible dual battery set up for what your planning to do.

The SC80 will give you ability to stay longer between charges and will replace the most used capacity in the shortest drive time, which fits in with what you have posted.

[drivesafe]

BTW midl73, forgot to post about your Solar connection.

With the SC80, you can connect to either the cranking battery or the auxiliary batteries and all your batteries will be charged.