Thread: Defender second battery location

Originally Posted by drivesafe

EDIT :- here is the graph for the test above and note the time taken before the DC/DC device went into float!

Attachment 155360

That graph is as expected. The bulk charge to 75% is obviously slower than an alternator that can supply more than 20amps. But still, it's only a couple of hours. And the DCDC only needs to charge one battery.

Where's the comparison graph for a 65amp TDI trying to get two depleted batteries to true 100%?

And why is it that when the yellow top got over 75% of the charge , current demand fell below 20 amps?

Long drives are common for me when I take the Defender out on a trip. The morning camp sees a solar precharge so I would rarely ever need to recover from as low as 11.8v. I'm more often charging up from >=12.2v and on float before next camp. From 75% to float, a wobbly TDI alternator is unlikely to beat a dedicated 20amp DCDC.

But for those who regularly discharge to low voltage cut off , have no solar and take short drives - a 20amp DCDC would obviously be a poor choice.

Hi Manic and a bit of fudging the facts there mate.


First off, and the obvious, my specs are for a 55Ah battery and you have a 100”ish”Ah battery but there is no difference in the charge capacity of the two DC/DC devices.


So your battery will take at least 4 hours and because of its larger capacity, will need to be charged to around 85% SoC before it goes into the Absorption stage. Your setup will still require another hour or so, to get that battery up around 90 to 95% charged


With one of my isolators and the 65 amp alternator, even if driving at night, with all the lights draw power as well, both batteries will easily be way over 75% in under two hours.and after less than three hours the two batteries will be around 90 to 95% charged


This puts my setup’s charge capability two hours better than yours in a vehicle with a 65 amp alternator in a worst case drive time.


During the day, with the lights turned off, the drive time needed is even shorter.


Regardless of what charging source is being used, very few people will ever drive long enough to get the batteries to 100% and this means a DC/DC device could not get to the float mode anyway.


Or are you saying you drive for 9 hours straight every time you have been at a camp spot?

With one of my isolators and the 65 amp alternator, even if driving at night, with all the lights draw power as well, both batteries will easily be way over 75% in under two hours.and after less than three hours the two batteries will be around 90 to 95% charged

This puts my setup’s charge capability two hours better than yours in a vehicle with a 65 amp alternator in a worst case drive time.

OK so obvs no graph, but what do you base that on? 30 amps to each battery and 5 amps for the lights - or what?

With the dcdc you have 45 amps for the starter battery and accessories, and 20 charging the aux.

Or are you saying you drive for 9 hours straight every time you have been at a camp spot?

Often 6 but very rarely 9. And after a night I dont get close to battery cut out. In the morning there is solar. And so no, it doesn't take me any where near 9 hours to get back up to float!

A TDI alternator IME having owned a few, usually fluctuate between 13.5v and 13.9v. And when under heavy load I have seen them drop as low as 13.1v!

Here is a graph that demonstrates what voltage drop can do to charge potential/rate

Worst case 13.5 average
[IMG]https://www.nationalluna.com/wp-content/uploads/2018/09/National-Luna-Excessive-Discharge-Zone-Charging-curve.jpg
[/IMG]

Best case 13.9v average
[IMG]https://www.nationalluna.com/wp-content/uploads/2018/09/National-Luna-Excessive-Discharge-Zone-Charging-curve-02-1024x664.jpg
[/IMG]

And that's on a 100amp alternator, which the TDI does not have.

I think this helps to explain how I have been able to get a better SOC on my aux with the DCDC than I used to get straight off the TDI alternator. With 4-6 hour drives.

Hi Manic and first off those graphs are next to useless.


All it states is that they used a 115Ah AGM battery. What brand, what type?


With the Optima 55Ah Yellowtop, discharge to 30% ( same as their test level ), that 100 amp alternator, operating at 13.5v, would have the Optima up around 95% charged in under the first hour.


Now to your TDi300. With the lights on and on high beam, you would have a load of between 15 and 20 amps, leaving around 40 amps available to recharge the two batteries.


This is a worst case scenario.


Because the batteries are in a low state of charge ( cranking battery at 50% and the Optima at 30% ) the total load being applied to the alternator would be greater than it’s total capacity of 65 amps.


This would cause the alternator voltage to drop, but it would still provide all available current to the two batteries.


Regardless of the voltage, the two batteries collectively, will be drawing a little over 40 amps, and this will continue for about 30 to 45 minutes.


The alternator voltage will have now risen to 13.5 ( OR HIGHER ) as the batteries are charging and reducing their current draw on the alternator and this is where the batteries will go into the Absorption stage of the charge.


After about two and a half hours, the two batteries will be up around 90% and over 95% within three hours.


During the day, with no lights on, you could be over 95% in less than two and a half hours

RexY
I’ll only weigh in to the controller Debate is that in direct comparison with a Victron solar controller the solar side of your DCDC is disappointingly poor.
Not sure how that plays into your plans
But with a load of a single fridge I would at least consider mounting a 120W panel to the raked roof good charge controller and big AGM and then just having a simple switch to connect to TDI if needed

I run a fridge permanently standalone in the back of my ute with no need for any charge between vehicle wiring and solar system. I do have an off/1/2/both switch so can link into the crank battery if needed but that is more for redundancy to use the solar to charge the Cranker if ever needed.


Now to answer your actual question!!!!

Yes you can fit two N70s in a TDI battery box but it is tight
That is what I use to run
Also won’t be much room at all for much else

But as you have a Ute (if it’s a tray) consider this setup
Using that dead space between outrigger and tray I have a 130ah AGM
It is bigger than N70

At times I’ve run two fridges constantly but in those occasions i doubled my solar panel input from 130W to 250W

Anyways - pics attached

S

Have to agree with that. The ctek250 has a solar charge controller but it's not a serious one. It could easily be better, but my unit is quite old now. I don't know if they improved the mppt tracking on the newer models, but the original which I have works well enough to supply a good deal more amps than I draw at camp. So has me covered.

For a more serious solar set up, say on a well kitted out camper/caravan I would certainly run a dedicated solar charge system. I have a 40amp EPEver charging Tesla lithium for a static caravan, and that of course is way beyond the requirements of a small dual battery system that runs a radio some led lights and a 40litre Engel.

Which Victron do you use?

For this setup just the baby
100/15 with BT dongle attached
It is stored in waterproof enclosure (bolted to Al tray chassis for heat sink)
Onboard of that battery

Amazes me how efficient it can be with 130W of panel
Fridge running at beer temp in full sun she will hit float by mid morning

With fridge at freeze temps it obviously hammers things a bit harder

S