Thread: 12v charger and dual battery

Hi,
From what I've been reading....
The DC-DC chargers can run using a thinner wire than is needed for a direct connection with a battery isolator.
They then step the voltage to what is needed for the battery state of charge and you can select the shape of the charging curve to suit the battery type being charged e.g. lead/acid, Calcium, Gel.
They work best as close to the second battery as you can get them to avoid voltage drop on the charging circuit.
They will stop drawing from the vehicle battery shortly after the alternator stops charging, so the main battery is protected.

Battery isolators are there just to stop your second battery drawing down the vehicle battery. Both batteries should be receiving the same voltage unless some thing is wrong like the wires are not substantial enough to avoid voltage drop or you have some other problem like faulty earthing.

cheers

There are a few reasons why many recommend a 12v charger. One is that some battery types (eg calcium) need at least 14v charging to reach a full charge and many alternators especially older ones, don't provide that output. Secondly, with some new vehicles now, they have their own sensing systems which, for reasons I don't understand, when you connect a second battery, cause problems with the charging of the batteries.

A third advantage is that the more sophisicated chargers can keep your second battery topped up without shortening its life. Many standard deep cycle batteries don't like having a high constant charge being applied with no draw down, over a prolonged period. From memory I think they also prevent sulphonation(?) occuring during recharge in the way they pulse the charge current.

I have a 2004 disco with a standard liquid lead acid deep cycle battery and simple voltage sensing isolator. My alternator outputs up to 14v (temperature and charge level dependent) and I don't have a problem. I manually isolate to avoid the constant charge problem. A good 12v charger is around $500 which is past my pain barrier.

Mundy

Please search for previous posts on this subject,,,

Originally Posted by Barney2803

Hi guys, I'm sure I've read on here about some guys putting a 12V charger in their dual battery setup to make sure the second battery is fully charged not just to the same level as the original but to it's full capacity??!!??

The main issue with deep cycle batteries in a "vanilla" dual battery installation is the alternator does not recharge them in the right manner, with the correct voltages. The alternator "sees" the starter battery and the deep cycle as one battery. This results, in most cases, to the deep cycle battery only ever charged to approx. 75% of it's capacity, not only reducing it's run time but also it's life.

I'm running a Ctek 250S 12V/12V charger INSTEAD of my Redarc isolator. I like this unit as it has temp sensor that looks after the second battery.

This fixed my second battery longevity issues and increased second battery run time by 35%.

I now run second hand AGM batteries like the ones the fire brigades change out every two years regardless of condition. I am still on my first one that I have had for five years now and it only cost $40 on Fleabay for 120AH capacity!!

Hi folks and I think there needs to be some facts introduced into this thread.

No matter what type of battery you use, or how many, alternators have no idea if a battery is fully charged or near flat, furthermore they can not and do not detect one battery as being fully charge and therefore decide not to charge another battery connected to the system if it is low.

This is pure fiction.

One of the greatest advantages alternators have of any other form of charging is that they are a constant VOLTAGE charger and apply the same voltage to all batteries in a vehicle.

What this means is that you could have three different types of batteries, at three different levels of charge and an alternator will supply the optimum charge current for each individual battery, charging each battery independently of any other battery is the set up.

Battery chargers, DC/DC devices and solar systems CAN NOT do this because they are all constant CURRENT chargers and will have the highest proportion of their charging current being required to charge the lowest charged battery in the system until this battery reaches a similar state of charge as other batteries in the system which means far longer charge times are needed.

Another point when using an alternator, no deep cycle batteries, and for that matter no lead acid battery get to 75% charge state and stops charging. This is not only a myth, it actually defies the laws of physics.

If a lead acid battery is not fully charged and you apply a voltage to the battery that is higher than the battery’s State of Charge ( SoC ) then the battery will continue to charge.

If a battery’s SoC is 12.3v and you apply a voltage of just 12.4v, the battery will continue to charge, all be it very slowly.

If you have a deep cycle battery that has an SoC of 12.35v, 75%, and your alternator is running at 13.2v, your deep cycle battery will eventually FULLY CHARGE, again slowly and if the alternator voltage is 14.0v, the deep cycle battery will reach a fully charged state in a much shorter time.

Please, take little notice of much of the advertising hype surrounding many of these devices.

Hi All,

To confirm Drivesafe's comments in my practical case. We have been traveling around OZ in our D2a for 5 Months with constant use of the following: 120AH Main Battery (Supercharge Allrounder) and a 120AH Deep Cycle AGM, linked with a Triaxide SC80. We run an Engel 40l fridge all the time. Other accessories are used only whilst driving such as an inverter.

Both batteries fully charge (ie above 12.7V, typically to 12.8V) with the Alternator only. We have only used a battery charger a couple of times and have not needed a solar panel yet either.

With this setup we can run the fridge for at least 3 days and both batteries will still be above 12V (ie 50% SOC).

So don't waste your money on a DC-DC charger.

These "Facts" are very interesting and quite contrary to what I was taught as an apprentice mechanic (a long time ago) and the data sheets provided by the manufacturers (not "advertising hype"). I am most happy to be corrected if wrong. We should never stop learning.

Originally Posted by drivesafe

Battery chargers, DC/DC devices and solar systems CAN NOT do this because they are all constant CURRENT chargers

I believe most Battery chargers sold today, DC/DC devices and solar systems (including the Ctek 250S Dual and Projecta DC20a) are multi stage chargers, normally 3 to 5 stages which have variable current, voltage and have the ability to pulse the current &/or voltage to aid reduction of sulphation.

Originally Posted by drivesafe

What this means is that you could have three different types of batteries, at three different levels of charge and an alternator will supply the optimum charge current for each individual battery, charging each battery independently of any other battery is the set up.

According to this logic an alternator can supply one battery with float current (say the starter battery) and another being supplied full charging current (say a deep cycle that has had it's charge depleted overnight running a fridge) at the same time all without any smarts in between. (Yes I am aware of the internal resistance of a charged battery V a depleted one). I don't believe this is possible. In my case the deep cycle AGM battery has very different charging requirements to my standard lead/acid starter battery.

Originally Posted by drivesafe

If you have a deep cycle battery that has an SoC of 12.35v, .......

S.O.C. (State Of Charge) cannot be measured by, nor should never be expressed in, voltage. Only the specific gravity of the battery's electrolyte will give you a true indication of the state of charge.


Just my 2c worth

Hi woz and again, this is one of the many misunderstandings people have because of the inadequate info supplied with things like multi stage battery chargers and DC/DC devices.

Just because a battery charger can charge a battery up to a given voltage, it dose no alter the fact that it is a constant CURRENT charger and contrary to what most people think, battery chargers and DC/DC devices can only reach these high voltage when the battery is nearly fully charged.

You can test this for yourself. Next time you have a low battery and you are going to charge it using a battery chargers and DC/DC devices, first put a small load on the battery for 10 minutes or so and then measure the voltage.

This WILL give you a fairly accurate SoC of the battery.

Then connect the battery chargers and DC/DC devices and measure the voltage again.

If the battery was at 12.0v before connecting, and if the battery chargers and DC/DC devices is set to charge at say 14.7v, it WILL NOT be charging at that voltage and will most likely be somewhere between 12.5v and 13.0v, depending on the size of the battery and the CURRENT capacity of the charging device.

As far as how an alternator charges different battery types and at different States of Charge, this I have already covered and you need to to a bit of independent research of your own.

Alternators have no idea what is requiring power, it could be a low battery or it could be that the driving lights have just been turned on, or the sound system’s volume has just been turned up.

No matter what has cause an increase need for power, as an alternator is not independently connected to each and every device in a vehicle, an alternator has no idea what is requiring power, it just supplies power on demand.