Thread: Defender second battery location

Ctek dual might be cheaper option than Traxide and victron mppt

The ctek can only output 20amps. It will be slower to bulk charge for sure, but better towards full charge.

IME with the ctek250, I am always fully charged by next camp spot. And it continues to charge on solar. So it's not a bad bit of kit for ~250 buck.

Also when starting up after a good run down on the batteries , I have bulk amps off the alternator going to the starter battery as priority. And 20 amps dedicated to the aux.

Some batteries have better absorption rates than others. AGM batteries will generally absorb around 50amps in a downwards taper. So by the time the starter is full off the alternator , the aux is only calling for <20amps anyway. That last 10% to full is where the DCDC excells. The ctek temperature awareness and 5 stage charging map helps also to get as much as you can into the top end of the battery.

Its working well for me anyway. So not a complete waste of time and money.

Hi Rexy and this is just a suggested setup but will easily give you the best option.


First off, being a Land Rover, there is absolutely no advantage to fitting a DC/DC device, other than being able to use unregulated solar panels, but if you permanently fit the solar to your vehicle, you can only get power from one source at a time, meaning the other is wasted.


You are far better off using a dedicated solar regulator, and then you can be recharging your batteries from the alternator and the solar at the same time while driving.


As Tombie suggested, if you use one of my DT90 isolators but an Optima D34 Yellowtop, a 55Ah battery, you will have a system that will give you around 80Ah of accessories power and a recharge time of up to 6x faster than any DC/DC device can do.


You will also have over discharge protection for both the cranking battery and the auxiliary battery, again, something no DC/DC device can offer.


If you use a DC/DC device, you will need to have that 105Ah battery to get the same amount of usable power, but now you will need to drive for MANY more hours to allow the DC/DC device to recharge a low auxiliary battery.


My suggested setup would be about the same price but would give you a far superior system.

How can a 300tdi with a 65amp alternator charge the aux battery 6 times faster than an aux charged from a dedicated 20 amp DC?

And at what point in terms of SOC does a yellow top or common AGM stop taking on more than 20amps? Will the fluctuating 13.x volt alternator output on a typical TDI be quicker to full charge once the battery's amp acceptance drops to <= 20 amps?

Would a battery last longer if charged at a regulated voltage with temperature consideration and proper float? Or constantly force fed by a poorly regulated alternator?

For sure, the Ctek 250 does not provide low voltage protection, so it is not a stand alone dual battery solution. And it will limit the charge rate potential to your aux battery. You still need to add a vsr, and ideally have the relay switch all your aux loads to alternator whilst driving to get all of the dcdc charging capacity. But the charge stages are correct, it adds solar and you might see advantages to charging your aux with a little less urgency. Pros and cons.

I have had the ctek 250 in the TDI Defender for what must be at least 4 years now. It has been submerged up on the Cape and rattled over the gun barrel. It's proved to be reliable, and the battery it is looking after now is the longest lasting deep cycle I have ever owned.

I have no issues with the DCDC charge rate. It takes around 2 hours of driving to bounce back from a long night out camping. Less or no time at all if solar had its time. 100AH AGM.

I will admit it was been a bit of a faff to get the low voltage cut out and aux load switching set up properly to support the ctek. But on the other hand solar was built in and ready to go.

Fridge will be the only thing drawing power while driving. If I need to I can either plug it into the ciggy socket or an Anderson off the house battery while driving.

Fridge has a low voltage cut out but I do have a vsr here I can run the other equipment through. Ctek does have low voltage cutout on the house battery so wont drain it flat.

Its a ute without racks, isn't really anywhere to add a solar panel while driving.

I just want something simple that will give me extended battery life as it wont be used all the time. Not looking to add 4 specific controllers when the ctek can do all those things already.

Drivesafe, if I pull up at a camp how will a 55ah battery give me the same usable power as a 100ah battery with your system? Once the cars off (which is where a second battery comes in handy) your system plays no part I would have thought?

Hi again Rexy and you will have to pardon me as I have just assumed you would have had the 100 amp upgrade done before any dual battery set up was carried out and this depends on intended use.


If you have not done the upgrade, I would suggest going down that road first.


Hi manic, and with the standard 65 amp alternator still fitted, the last think you need is an energy wasting device like a DC/DC charger fitted.


For they need a lot more amps in to a DC/DC unit than comes out, particularly if the voltage runs as low as you make out. I have not done much monitoring of the 65 amp alternators but feedback shows they usually run in the low 14V area.


Rexy, to your question of a 55Ah battery providing up to 80Ah while camping.


Unlike ordinary VSR type isolators and DC/DC devices, where you only have access to your auxiliary battery, my isolators allow up to 50% of the cranking battery to be used to assist in the powering of accessories while camped.


By using an Optima Yellowtop, you will have no problem fitting it alongside your cranking battery and still have plenty of room for the isolator as well.


If you do still have the 65 amp alternator, this setup will still give you much faster recharging especially as the Optima is a rapid charge battery.


For example, if you have been camping for a while and both batteries are down around the 50% SoC mark, the two batteries will draw everything available from your alternator but as the two batteries charge, your cranking battery will draw less as it charges up passed the 75% mark, but the optima will continue to draw as much current as it can until it get to around the 90% to 95% mark, when it too, will start to reduce its charge current draw.


This simply means you have a far more efficient use of the limited charge power available.


The other advantage it that if you do not drive long enough to get both batteries up around the 95% charged mark, when you turn your motor off, the Optima, being in a higher state of charge, will slowly back discharge into the lower charge cranking battery, helping to keep it in a better state of charge.


There is a lot more to it but that’s the basics.


Again, depending on how much use you intend for you Defender and you intend to use it, like if you are planning long trips with lots of free camping, if you have not upgraded to a 100 amp alternator, this might be the first thing to consider.


If you are only planning the occasional weekend away and lots of short trips around town, then the smaller alternators is fine but the Optima will still be a big advantage over more conventional auxiliary batteries in this type of usage.

Reading these posts, I realised I forgot to mention I have a Traxide 160 dual battery controller fitted. This means the fridge in the back can draw half of the 120 amp AGM second battery plus half of the starting battery or about 110 amps total. The 80 watt solar panel on the roof feeds the starting battery, so as it is charged the Traxide keeps the two batteries connected.
Incidentally, an ammeter plugged into the Anderson plug on the rear bumper tells me it is sending about 8 amps to the camper trailer. As the fridge-freezer is drawing about 4 amps it means about twice as much is going in as is being used while driving. When camped we plug in extra solar.

I dont want to draw any power from starter battery. Problem is with remote locations come back and car wont start. If I was going to do that i'd just get the biggest starter I could fit under the seat.

Originally Posted by Rexy

I dont want to draw any power from starter battery. Problem is with remote locations come back and car wont start.

Hi Rexy and sorry mate but there is no grounds for that statement when using one of my isolators.


If you were to come back to your vehicle and could not start it with a battery at 50% then there is every likelihood that your cranking battery is already stuffed and whether it was at 50% or 100%, when you left it, it’s still stuffed.


Most people are unaware that the CCA rating of their battery is based on a battery tested at 40% SoC and at a temperature of 0 degrees. So again, if your battery won’t start your vehicle when at 50% SoC it is already stuffed.


Furthermore, because my isolators keep the batteries together, every time you start your motor, while most of the energy needed to start the motor will still come from the cranking battery, the load is shared over the two batteries and and this has a multitude of advantages over a normal single battery setup.


Because the starting load is shared, you motor will start easier and this in itself helps to reduce ware and tare on the starter motor.


Because the starting load is shared, the cranking battery is not worked as hard while starting the motor, and this will help to extend its operating life.


Because the starting load is shared, the cranking battery will not be discharged as low as with a single battery system, and this means the cranking battery will back to a fully charged state in a shorter drive time. Again this will help to extend its operating life.


All this helps to extend the cranking battery’s life spans and as pointed out earlier, by allowing the auxiliary battery to keep the cranking battery in a better state of charge in the first place, again this will make starting the motor easier and again this will help to extend its operating life.


When using one of my isolators, your cranking battery will eventually die, but it is likely to be in use for years longer than with any other setup or where there is no setup at all.


There is no down side to the way my isolators work but there are many upsides to using them.