Thread: 12v charger and dual battery

Definition of a hydrometer: A device used to transfer acid from a battery to your toolbox.

Originally Posted by vnx205

Definition of a hydrometer: A device used to transfer acid from a battery to your toolbox.

And chew out the bottom!!!

Jokes aside guys, I still cannot understand how an alternator with an internally sensed regulation system with two dissimilar battery types, different states of charge and very different charging requirements can charge them both correctly at the same time.

Again, I stand by my findings. A 35% increase in usable battery run time of my auxilary battery (not the starter) since replacing the Redarc SmartStart SBI isolator with a Ctek 250S 12V/12V charger. In fact Redarc themselves now manufacture a 12V/12V charger as their premium auxilary battery isolation solution. (Bit pricy though)

Reading SOC by using voltage to calculate a percentage will vary greatly with the capacity of the battery and will change as the battery deteriorates with age. Even temperature can effect the voltage readings. It is just not accurate. Having said that, low or quickly dropping voltage is a pretty good indication of an issue that needs attention.

A hydrometer, as with any measuring instrument, must be used in the correct manner to get an accurate reading.

In the I.T.C. and Comms industries the worlds best practice presently for measuring S.O.C. of batteries in "mission critical" UPS (Uninterruptible Power Supplies) is still the hydrometer. Most UPS manufacturers will only accept hydrometer readings in SLA disputes. I guess in the (real) automotive world the rule of thumb seems to be acceptable.

I have no barrow to push here or financial interests but I too am concerned about the amount of misinformation in the area of dual/multi battery set-ups. Many a night has been spent around the camp fire over a few beers discussing this very subject.

I guess it's like Ford/Holden or Land Rover/The Others.

Everyone to there own and hey, if it works for you, go for it.

Wozzle,

It's the magic of Ohm's Law (I=V/R)

http://www.batteryweb.com/manuals/techman.pdf

When connected in parallel, the current from a charger will tend to divide almost equally between the batteries. No special matching of batteries is required. If the batteries of unequal capacity are connected in parallel, the current will tend to divide between the batteries in the ratio of capacities (actually, internal resistances).

When charging batteries in parallel, where different ratios of charge are to be expected, it is best to make provisions to assure that the currents will not vary too much between batteries. Holding a small resistance in series with each battery is all that is needed

Voltage and Internal Resistance provides all the "smarts" you need.

On measuring SoC it's worth taking a look at section 4 of the Battery FAQ.

http://jgdarden.com/batteryfaq/carfaq4.htm

There is a fair bit of detail on various methods of measuring SoC.

and lastly there is an excellent article from a back issue of Home Power on the relationship between SoC and Voltage, just in case you remain unconvinced.

http://www.scubaengineer.com/documen...ing_graphs.pdf

cheers
Paul

There's some great reading in that lot. Thanks.

Especially interesting was the difference in charge characteristics of different batteries and the SOC due to temperature.

I throw this in for further discussion.
What happens to a Ca battery (terminal voltage of 14.7 volts) if fitted to a vehicle whose regulator is set at 14.1 volts for a lead acid battery?
Surely the Ca battery will never fully charge. How can it? In my understanding the batteries should be the same i.e. lead acid etc and matched to the regulated output of the alternator to ensure full charging. Jim

Hi again Woz, the State of Charge ( SoC ) has nothing to do with the amount of capacity in a given battery.

If you have a new 100Ah battery and you measure the voltage with a small load applied to the battery. If the voltage is say 12.4v then your battery has an SoC of 80% which means the battery has about 80Ah of capacity available.

If you have an old 100Ah battery and it has deteriorated to the point where only 50% of it’s capacity can be fully charged and you again get a reading of 12.4v, the battery is at 80% SoC but as only half the battery capacity can be charged, it means you only have 40Ah available in the battery.

SoC will not indicate whether a battery is capable of being fully charged to it’s original capacity, it just indicates the SoC of the part of the battery that can still be charged.

Now to charging batteries of different SoC off an alternator.

Lets say you have three batteries 100Ah with different states of charge, and it does not matter what type they are.

Battery A has an SoC of 11.58v, ( 20% ), battery B has an SoC of 12.0v ( 50% ) and battery C has an SoC of 12.5v, ( 90% ).

When the motor is started, you have an alternator voltage of 14.0v

This means you have 14v being applied to each battery and so battery A will have a voltage difference of 2.42v and will be pulling the highest current, say around 40+ amps.

Battery B will have a difference of 2.0v and be pulling around 25 amps and C with a difference of 1.5v will be pulling around 6 to 7 amps.

Battery C will reach a fully charged state in the shortest drive time, then B and if you drive long enough, A will also reach a fully charged state.

This does not happen with battery chargers or DC/DC devices because they have a limited current output and battery A will pull most, if not all of the charge current for some time and eventually B will start charging but in the initial stage of the charging, battery C may actually discharge into battery A, until battery A’s SoC rises high enough that it is no long a load on battery C.

Last but not least, if your battery is a 100Ah battery and not that discharged when you drive then yes your DC/DC device will fully charge the battery quicker than your alternator.

On the other hand, if your battery is in a low state when you start your day’s drive, it is actually more likely that you will not drive long enough to give any form of charging long enough to fully charge your battery and in the first few hours, your alternator will replace more of the used capacity than your DC/DC device can.

And just to throw a spanner in the works, if after using your set up for a while, you find you need more capacity than you have and you add a second battery, you will now have to drive nearly twice as long to give your DC/DC device time to full charge your two batteries.

Whereas, with an alternator, if you have two low batteries at the start of your day’s drive, your alternator simply doubles it current production to cover the bigger load and your two batteries will not take much longer to charge then when you were charging one low battery.

Excellent explanation, Drivesafe.

The only advantages that I thought I knew of DC/DC chargers are that you can select your battery type and if you have a calcium type which has a need for 14.7V/15V input, they will provide it (possibly only eventually given your explanation). Secondly, for maintenance charging, I thought they pulsed the charge to stop sulphonation. Is this correct?

It would seem logical to me from all this that you need to be careful in choosing two battery types which require a significantly different charging voltages.

Mundy

Gents while all you knowledgable people are discussing this thread I have a question regarding the set up of my dual batteries.
I originally had an ARB system fitted to power my fridge and they connected this to the second battery along with CB radio etc.
One of the land rover mechanics I frequent in the northern suburbs informed me that the spare battery was only there to give the crank battery the power should it be discharged by using the accessories...and then proceeded to wire everything to the main battery.
I haven't had any issues with either set up and was wondering which one was correct. Any comments would be appreciated.