Thread: Defender second battery location

Hi Rexy and sorry mate for raising a ruckus in your thread, but as stated earlier, you originally did not say you already had the Ctek so I was trying to give you a warning to the drawbacks of DC/DC charging.


Just a question for you. Did you get any advice on that size unit before buying it, because you will have to derate it down to 20 amps output as it is way too big for your 100Ah battery you were saying you wanted to get?


If you do not derate it, you will stuff the battery.


Again, if you do not derate it, you will need to upgrade your alternator or you will have some very interesting night driving.


Why did you not just keep the Ctek?

Gets better, in a previous post you said all alternators are self protecting.
You also banged on about how your kit can provide way more amps than a ctek now the 40amp dc to dc is providing too much. The graph you posted earlier showed the 25amo dc to dc adjusted amps according to charge and battery capacity.

I understand you are selling a product but really if it were so great it would sell itself. To me it’s an overpriced and complex vsr with none of the features I need. Anyway this thread wasn’t for you to advertise it was to find out how I can work around the limited room. You never attempted to answer that just shove your product down the throat of everyone.

Originally Posted by Rexy

.. .... ...
We all want certain features and don’t want others. Draining any power from starter is one.
.. .... ...

I am not offering advice or criticism of any system, but I wonder if people who are worried about drawing some power from the starter battery before switching to the aux battery are being paranoid.

I realise that no-one wants to be stranded with a battery that won't do the job, but my experience suggests that 50% SoC should be more than enough.

When I had a 7 year old battery that was on its way out, it wasn't going to be a problem at home if it failed to start one morning. So I checked the voltage each morning for a few weeks before I started the engine. During that time, my multimeter read 10.5 volts, yet the engine still started .

My understanding is that 10.5 volts is probably less than 10% SoC and certainly a long, long way below 50%.

I wouldn't head off on a trip with a battery that dropped to 10.5 volts overnight with no load, but I can't believe that a good battery with 50% SoC would have any trouble starting.

When I had the Series III, I had even less concern about low voltages because after two consecutive nights running the fridge in the NT, all I had to do was get the crank handle out. Ah! the good old days.

Originally Posted by Rexy

Gets better, in a previous post you said all alternators are self protecting.
You also banged on about how your kit can provide way more amps than a ctek now the 40amp dc to dc is providing too much. The graph you posted earlier showed the 25amo dc to dc adjusted amps according to charge and battery capacity.

I understand you are selling a product but really if it were so great it would sell itself. To me it’s an overpriced and complex vsr with none of the features I need. Anyway this thread wasn’t for you to advertise it was to find out how I can work around the limited room. You never attempted to answer that just shove your product down the throat of everyone.

Rexy, mate, if you actually understood what I had posted, and this is my fault for not explaining it better, but you hit the nail on the head.

All alternators are self-protecting against overloading.

And as I posted, they do this by dropping their output voltage when ever the load is greater than they can produce.

Has the penny dropped yet????

Even during the day, because that DC/DC unit is way to big for your alternator's size, if your auxiliary battery is low, unless you can keep the revs up high enough so your alternator can produce its maximum current at all times you are going to have problems.

With a low auxiliary battery, that size DC/DC will try to pull more current than your alternator is turning out, so your alternator will protect itself by dropping its output voltage. As are all alternators designed to do.

Unfortunately the lower voltage will cause the DC/DC unit to try compensate for the continually lowering voltage by trying to pull even more current and again this even high current draw will cause the alternator to lower its voltage further again.

All you have done is created a catch 22 situation.

But there is a fix.

When the alternator voltage gets too low, the the current difference will be made up by the device drawing on your cranking battery.

So while you are driving, unless you keep the revs up, you will actually have a situation where you will be DISCHARGING YOUR CRANKING BATTERY WHILE YOU ARE DRIVING.

So my isolators are no good because they allow you to use the surplus energy from your cranking battery, while camped, but your setup now, which actually discharges your cranking battery while driving, is much better.

Thank you for that education.

BTW, what I was on about when I posted that your night time driving was going to be interesting. The first time you drive at night with a low auxiliary battery, even if you keep your revs up, you are going to have a situation where you headlights are going to get duller and duller.

When this occurs, the "ONLY" way for you to be able to continue driving is to turn the DC/DC unit off, and this usually means you have to disconnect it.

BTW Rexy, you were half correct, I was promoting my isolators, but I was also trying to warn you about the drawbacks of using DC/DC devices, particularly in your specific situation, with such a small alternator.

All the best to you.

Originally Posted by manic

Riiiight. I think everyone on this thread knows the difference between a fuse and a CB. This answer adds nothing. The 50amp limit still applies. Just an unnessesary attempt to discredit the conversation. Thanks for that!

I am really interested to hear how "The 50 amp limit still applies"?

I’ve got an aftermarket alternator. It’ll be fine. Have another at home from a disco as well

Now Rexy, isn’t this something I suggested to you in the first place.


See I haven’t wasted my time and everybody elses. You do take notice of good advice.


Seriously, Rexy my posts were not just about flogging my isolators. I sell DC/DC devices.


The thing is though, no Land Rover needs them and regardless of what make of vehicle it might be, any vehicle with such small capacity alternators should never have a DC/DC device of any form used to charge an auxiliary or house battery.


Anyway, fitting the bigger alternator will give you a safer setup than what you have now.

BTW Rexy, just incase you think my warning was a tongue in cheek remark.


It was not an off the cuff remark.


As posted earlier, once you decide what battery you are going to use, READ THE INSTRUCTION MANUAL and then setup the DC/DC device according to what they state.


Do not use the unit until it is set up correctly, otherwise, and regardless of whether you change the alternator, you will cook a battery if the setting is incorrect.