Cheap but good dual battery control?

Joined: 11 Jan 2004 Location: Adelaide

drivesafe wrote:

-Scott- wrote:

No.

Where does this myth come from? Rolling Eyes

Under some circumstances, some users may find that their alternator never fully recharges their batteries - because they drain the batteries too far, and don't allow the alternator sufficient time to FULLY recharge. Don't take this situation out of context.

For most people, with most vehicles, the alternator WILL fully charge the battery - if given enough time. My Pajero, with factory stock charging system, will fully charge BOTH of my batteries - no problem.

Your a brave person Scott, when I saw the post I had to bite my tongue and ignore it as inevitably it would start the same old argument.

Hi aford, Scott is absolutely correct, this is a MYTH and a myth that has some truth to it but that part of the myth relates to vehicles that are used as shopping trolleys not vehicles that get a few hours constant driving.

Like Scott, I have 20 years experience in the field of designing and manufacturing dual battery controllers and spent a fair amount of time developing these systems around how lead acid batteries and alternators ACTUALLY work not around how many people THINK they work.

There is no reason why a vehicle, with a properly maintained electrical system, can not get ALL the vehicle’s batteries charged to at least 95% SoC using nothing more than it’s alternator.

Cheers.

Brave or stupid - it's often a fine line. Very Happy

I figure these tech forums are for people to obtain advice. Unfortunately, much of the advice isn't great, but some incorrect advice seems more prevalent.

At the end of the day, people will believe what they want to believe. If they don't get both versions, they will believe the version they do get - right or wrong.

I find it unfathomable that people believe a modern automotive charging system is not DESIGNED to keep a battery charged.

Yes, a more complex (and more expensive) system can (under the right circumstances) charge a battery significantly faster than a typical automotive system. More complex systems can do MUCH better things for a battery than what's under my bonnet. But I try not to allow this "70-80%" line go un-answered.

I note you say "at least 95%" - I am prepared to bow to your superior experience. Very Happy

Cheers,

Scott

Joined: 25 Sep 2006 Location: carina hts

fair enough but im just refering to the average driver which is probly 90 percent or more of people who dont drive long distance often

Joined: 11 Dec 2004 Location: Gold Coast

Hi Scott, the reason I say 95% is because this is the maximum that one can practically expect to get out of a battery.

95% is as far as I’m concerned, a fully charged battery because the most accurate measurement you can get of the SoC of a battery is still only 95% accurate and I don’t care by what means anybody chooses to charge a battery, by an alternator, a battery charger or a solar panel, there is no realistic way to prove that a battery is 100% charged, short of removing a battery and taking it to a laboratory to have it analysed.

Cheers

Joined: 11 Dec 2004 Location: Gold Coast

Hi Scott, just a clarification on last night’s post ( Sorry, I needed some sleep so didn’t explain that so well )

The reason for saying 95% is a fully charged battery is because, even with the best measuring equipment ( and this includes load testing ) and the best conditions, while the battery is in the vehicle, there is going to be up to a 5% margin of error, so a reading that shows a battery has a State of Charge ( SoC ) of 100% could mean the actual SoC is anywhere from 95% to 105% ( batteries can hold higher SoC when hot ) so as in my last post, short of removing the battery to test it, which is totally impractical, if you calculate on only being able to access 95% of the batteries capacity, you won’t come up short when you actually use the battery.

Cheers.

Joined: 12 Dec 2002 Location: Wangaratta

drivesafe wrote:

Hi Scott, just a clarification on last night’s post ( Sorry, I needed some sleep so didn’t explain that so well )

The reason for saying 95% is a fully charged battery is because, even with the best measuring equipment ( and this includes load testing ) and the best conditions, while the battery is in the vehicle, there is going to be up to a 5% margin of error, so a reading that shows a battery has a State of Charge ( SoC ) of 100% could mean the actual SoC is anywhere from 95% to 105% ( batteries can hold higher SoC when hot ) so as in my last post, short of removing the battery to test it, which is totally impractical, if you calculate on only being able to access 95% of the batteries capacity, you won’t come up short when you actually use the battery.

Cheers.

Ahh engineers Very Happy

(reads like something my dad would write) Not having a go I know exactly waht you mean ... I just find it ammusing.

Joined: 12 Dec 2002 Location: Wangaratta

chimpboy wrote:

I already have dual batteries on my Maverick, using a single VSR.

However I recently found a PRC4427 "voltage sensitive switch" from a landrover discovery in my big box of goodies. It apparently cuts in at 13V and cuts out again at 12.1V, so it would do a decent job as a dual battery controller.

Jason .. you have a pic of what on elooks like so I would know it without looking at the part No ?

Joined: 25 Oct 2003 Location: Melbourne Australia

love_mud wrote:

chimpboy wrote:

I already have dual batteries on my Maverick, using a single VSR.

However I recently found a PRC4427 "voltage sensitive switch" from a landrover discovery in my big box of goodies. It apparently cuts in at 13V and cuts out again at 12.1V, so it would do a decent job as a dual battery controller.

Jason .. you have a pic of what on elooks like so I would know it without looking at the part No ?

I haven't taken a picture of the one I've got, but I will get one in the next day or two and put it up.

Joined: 14 Jan 2004 Location: somewhere out there

sounds like what I've got in my 40..

Joined: 25 Oct 2003 Location: Melbourne Australia

shorty_f0rty wrote:

sounds like what I've got in my 40..

That's the same as what I've got. It'll set you back about a hundred bucks though.

The voltage-sensitive switch I was talking about in the original post comes from a landrover or range rover and looks like this:

Looking at how light the wiring is, I would wire it to a run of the mill 12V 30A relay, then to one of those $15-$30 supercheap starter solenoids.

Provided you get the voltage-sensitive switch for under $20 from a wrecker somewhere it's not a bad deal on a dual battery setup imho.

Joined: 21 Sep 2006 Location: Jabiru

chimpboy wrote:

I haven't used the basic solenoid setup myself, because I don't think it does what a dual battery setup needs to do - ie automatically charging the primary battery fully before charging the secondary battery. There are some situations where you can end up with two flat batteries, which I think defeats the purpose of having dual batteries. I would describe it as a step up from just joining the two batteries together permanently, but not a true dual battery system. I am not shooting anyone down though, I think the basic set-up has some value as a cheap option.

But anyway, if you did want to do what you've described, it's pretty simple. The wire that Acebryan says you should hook up to "ACC" power can also be connected by a switch to battery power. Then when you flick this switch, you trip the solenoid and your two batteries are connected no matter what.

Will this be enough to start your car through? Maybe; normally a two-battery setup uses reasonably heavy, but not incredibly heavy, cable between the two batteries. It's not normally going to be cable that's as heavy as your starter motor cable. It's also a lot longer than your starter motor cable. But it would probably work for an emergency and I would install a switch like that if I were using that system. If your dual-battery cable is as good as a jumper lead then it should do it.

Two things though.

Firstly, if I ever needed to start from my aux battery, I would just use a set of jumper leads across the two batteries, not the dual battery connection. For me this keeps things a bit simpler, and it's crazy not to carry jumper leads in any car imho anyway.

Secondly, I would not install a simple on-off switch for the emergency start situation. I would install a two-position switch as follows:

Terminal 1: Accessories power
Terminal 2: -> to the solenoid
Terminal 3: "always on" battery power

This means that you can switch between the batteries being connected all the time, or only when you have the key turned to Accessories or Ign. This way you won't be sending power back to your accessories from the aux battery when you desperately need all the power to start the car.

Or you could have a three-position switch that gives you a manual "off" position for the solenoid.

Normally you will leave it in the "accessories" position if you want it to work that way. But you can use the "on" position for starting, and the "off" position if your aux battery is very flat, and you don't think you'll be driving for long enough to charge both batteries.

Cheers,

Jason

Would you be able to draw up a basic wiring schematic for this setup?

I am trying to work out 1) what you are describing and 2) how to put in a switch that disconnects the aux battery from the main when the car is started aka a manual overide switch.

Any help appreciated.
Thanks

Joined: 21 Sep 2006 Location: Jabiru

I think i've found what i'm looking for.

http://www.lcool.org/technical/90_series/DualBattery/battery.html

Joined: 25 Oct 2003 Location: Melbourne Australia

The switch has three positions:

On (1): batteries remain connected no matter what the car is doing
Off: batteries are always disconnected
On (2): batteries connect/disconnect when you turn the key

Joined: 21 Sep 2006 Location: Jabiru

Swoit!
Thanks mate