Defender second battery location

Every situation is different and I'm no
expert so I can only say what works for me, both on our current Puma Defender and previously on a 300Tdi Discovery.
I don't see any advantage to a DC-DC in a vehicle, but I do believe they have a place in caravans and campers, where the amps coming down the trailer plug could be boosted when the batteries are low and need a big initial charge, before dropping to lower amps after they hit around 80% charged. The last 20% takes much longer to reach. I have seen my 240 volt Projector charger sit on 0.5 for many hours overnight before going on float.
If an alternator is making 60-100 amps, why would you want to limit it to 25-40 amps by using a DC-DC?
My Defender has fat wiring and 60 amp resettable fuses between the starting and second batteries, so any DC-DC would actually limit the amps reaching the second battery.
I believe AGM batteries need about 13.5-14 amps to charge. Lithium need 14.7.
Starting batteries have thinner plates to give a greater initial boost to start the vehicle and faster recharging, while deep cycle batteries have thicker plates for slower discharge, needed by fridges, and take longer to recharge.
As for Traxide vs other DBCs, I have no brief for anyone, but I can only say when you camp for a week without mains power, it is an advantage to be able to draw some power from the starting battery, while knowing there is still plenty left to start the vehicle.
Hope that helps.

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If an alternator is making 60-100 amps, why would you want to limit it to 25-40 amps by using a DC-DC?

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Consider the OP. 65amp alternator, two batteries. Potentially with only 25a per battery to spare with the alternator running flat out right from cold start.

Now consider a DCDC charging one of those batteries at 20amp at up to 14.7v on a 5 step process that can take the battery all the way to float. Whilst the starter still gets its 20-30amps at a wobbly 13.5 to 13.9v from the alternator.

With the above set up, and after a good day the road, the DCDC wil be at a higher SOC than the alternator charged battery, and with less stress all round.

Even if you isolate the starter from discharge and point a 100amp alternator at your depleted AUX, you might find your battery type and its SOC at start rarely call for more than 25-30amps bulk.

I hear the yellow tops can suck it up though. Drive safe wil know - how many amps will they sink at 50%SOC?

Also consider

- the DCDC can continue to charge with the engine off (solar)
- the DCDC is temperature aware and can adjust the charge rate to reduce thermal stress on the battery.
- Many deep cycle AGMs max out at 30amp bulk charge, which can be easily met by 40amp DCDC chargers
- DCDC chargers are getting cheaper,
- Lithium batteries are getting cheaper.
- Faster bulk charges put more thermal stress on the battery
- Higher current demands put more stress on the alternator.

Why limit to 20-40 amps? I'm not saying you should, but some applications don't need to charge any faster than that - and some already can't. So if the DCDC charge rate suits the OP, then it comes down to cost.

What the OP was looking at:

Ctek 250 with built in solar + LVD + 100ah AGM battery

And now the proposed set up for fastest bulk charge:

VSR with built in LVD + solar mppt + 100amp alternator upgrade + two 55ah yellow tops

Hi folks and some clarifications.


Rexy, it was not until well into this thread that yo stated you already had a DC/DC unit. As such I and a number of others simply tried to warn you not to buy one because they are so useless, particularly with regards to your small alternator.


While not stating that you already had one, my mistake for not enquiring to this fact, you ( and Manic ) posted up posted up claims of how better DC/DC chargers perform when those claims were based on nothing more that the usual DC/DC advertising misinformation.


So Rexy please don’t shoot the messengers for trying to help you.


Manic, for some strange reason you just do not want to understand how DC/DC devices and alternators FACTUALLY work. You are just blindly your system is the best and everybody should be using similar.


First off, if you do actually drive for 9 hors just about every time you drive while on a trip, then you would fall into a VERY rare range of drivers, with less than 0.1% of people driving those sorts of hours on a regular basis, and I don’t just mean Land Rover drivers, all drivers.


Even when on long trips, most people will very VARY rarely drive for 9 hours while doing a trip around Australia, so you are doing well driving 9 hours all the time.


The vast majority of people will rarely drive more that 5 hours a day and even less when towing a caravan.

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Originally Posted by manic

Now consider a DCDC charging one of those batteries at 20amp at up to 14.7v on a 5 step process that can take the battery all the way to float. Whilst the starter still gets its 20-30amps at a wobbly 13.5 to 13.9v from the alternator.

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As has already been demonstrated, if the auxiliary battery is low, you will need to drive for many hours before the battery gets to the absorption stage of the charge, so you can forget all the other stages.

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Originally Posted by manic

With the above set up, and after a good day the road, the DCDC wil be at a higher SOC than the alternator charged battery, and with less stress all round.

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Again, nothing but pure fabrication. If the auxiliary battery is low, no one drives ling enough to get a battery to 100% and how the hell can you “stress an alternator”, that is pure pulp fiction. All alternators are self protecting against overloading them, so again how do you “stress an alternator”

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Originally Posted by manic

Even if you isolate the starter from discharge and point a 100amp alternator at your depleted AUX, you might find your battery type and its SOC at start rarely call for more than 25-30amps bulk.

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I will cover this irresponsible garbage further down in this reply.

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Originally Posted by manic

Also consider

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Originally Posted by manic

- the DCDC can continue to charge with the engine off (solar)

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As has already been pointed out Ctek units are renowned for their gross inefficiency when used as solar regulators. You are far better off having a decent independent solar regulator.

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Originally Posted by manic

- the DCDC is temperature aware and can adjust the charge rate to reduce thermal stress on the battery.

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If you use the correct batteries, temperature will not be a problem.

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Originally Posted by manic

- Many deep cycle AGMs max out at 30amp bulk charge, which can be easily met by 40amp DCDC chargers

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This is nothing but total garbage. If a battery is designated as having a maximum charge current tolerance of 30 amps and if some poor unsuspecting person follows your “EXPERT” advice and they fit a 40 DC/DC devices to charge it, in just a couple of trips, if they get more than one trip in, they will have destroyed their battery. Manic, you have no idea what you are taking about because the DC/DC manufacturers themselves state NOT to use higher current chargers in these types of setups.

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Originally Posted by manic

- Faster bulk charges put more thermal stress on the battery

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Both cranking batteries and Optima Yellowtop batteries have no charge current limitation and Optima specifically state this on their website. So once again, another ignorance based claim.

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Originally Posted by manic

- Higher current demands put more stress on the alternator.

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As covered above, all alternators are self protecting against overloading them, so again how do you “stress an alternator”

Everyone take a step back.

The OP has decided what he’s doing and now knows an N70 should fit.

Let’s leave him to his own devices (pun intended). And let’s go back to our daily life.

Hi Tombie and yep I should let it go but when a self-proclaimed expert "advises" people on how to set up their systems so they can destroy the batteries, I had to warn others about this garbage.

BTW, my claim is not an exaggeration. When the first these 40 amp DC/DC devices came on the market a few years back, within months of their release, the different manufacturers were issuing warnings to suppliers and end uses about not using them on certain types and sizes of batteries because they were already cooking the crap out of heaps of batteries.

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Originally Posted by drivesafe

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As far as I can see no where on this thread has anyone claimed to be an expert.

All my comments should be taken in context as they pertain only to:

1) My experience with the ctek-250
2) The OPs setup, goal and requirements - which is similar to mine.

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Originally Posted by drivesafe

.First off, if you do actually drive for 9 hors ....

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Please read what I have posted. It does not take me 9 hours to charge my aux. Not even close. Your comparison does not match the use case. You are looking at a user who has solar and rarely starts up with a battery in a state of deep discharge. Yet you still claim 9 hours and have yet to concede that when it comes to the absorption stage of the charge cycle a DCDC can reach a higher SOC that an alternator with low voltage output.


I have stated clearly that I regularly do 5-6 hours of driving per day when out using my dual battery set up. And suggested that my set up would not be suitable for those who have larger battery banks, and/or frequently deep discharge, and/or don't do a lot of driving, and/or don't have solar.

Your calculations are based on full discharge up to an unspecified SOC on the back of a high output alternator that can sustain enough amps+volts to reach your SOC target (unspecified). My comments are in context of low discharge cycles, solar assisted, with 4-6 hours drives that add a sustainable 20amps and 14.5-7 volts. My SOC target is 100% float - and it gets there!

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how the hell can you “stress an alternator”, that is pure pulp fiction. All alternators are self protecting against overloading them, so again how do you “stress an alternator”

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More load on the alternator must cause more wear and tear. Whether or not that toll is enough to make a significant difference to alternator/belt longevity - I don't know for sure. And again consider the application. For example a 200tdi. The belt set up on these vehicles can barely hold on when its alternator is being asked for all its amps! The 300tdi is better, but still putting max load on the alternator every start up will surely take a toll. I feel happier when my alternator isn't glowing and my belt isn't screaming. If I need to make the alternator work that hard, I will have it work that hard. But with my use case it does not need to, and so there is an additional benefit there with a 20amp limited charger in this case. It spreads the charge across my drive. I am happy with that, and the alternator and battery don't complain either.

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Originally Posted by drivesafe

As has already been pointed out Ctek units are renowned for their gross inefficiency when used as solar regulators. You are far better off having a decent independent solar regulator.

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Yes, but if the Ctek unit provides enough amps to meet my requirements even when overcast - that's all that matters. Context: small dual battery set up, low power demands. 4 years and I have no need to upgrade.

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Originally Posted by drivesafe

This is nothing but total garbage. If a battery is designated as having a maximum charge current tolerance of 30 amps and if some poor unsuspecting person follows your “EXPERT” advice and they fit a 40 DC/DC devices to charge it, in just a couple of trips, if they get more than one trip in, they will have destroyed their battery. Manic, you have no idea what you are taking about because the DC/DC manufacturers themselves state NOT to use higher current chargers in these types of setups.

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I agree with you there on the 40DC, my comment could mislead. To clarify, I am not suggesting people go out and buy a 40amp to charge a 30amp limited battery. The point I was trying to make is that the DCDC's on offer can meet or even exceed max charging requirements. I will admit that I was under the impression that batteries would only take as much current as they need, and that DCDC's adjust output to meet that requirement similar to an alternator. But I trust, as you are the resident expert on charging, that this is not the case.

Also, I think you are being a bit sensationalist there with 'a couple of trips'. The DCDC will have a temperature probe on the battery, and should be able to detect over charge. On the flip side I have left an overcharged battery by the side of the road, bulging at the seams, hissing - for fear of imminent explosion. Whether it was the battery or alternator regulator at fault I suspect an appropriately sized DCDC would have prevented the battery reaching this extreme state.

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Both cranking batteries and Optima Yellowtop batteries have no charge current limitation and Optima specifically state this on their website. So once again, another ignorance based claim.

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I have asked you, as the resident expert on this thread, a couple of times already - How many amps will a yellow top consume off the alternator when at 50% SOC, if given a source of unlimited amps? Whilst the website states no charge limit - you might be misleading the reader into thinking that the battery can absorb as many amps as your alternator can throw at it. Whilst your VSR has a 50amp fuse, I suspect that for many batteries that limit is not well beyond the capabilities of a 20-40amp DCDC charger.

Optima also state on their website that their batteries will last longer if charged slower.

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Originally Posted by manic

Please read what I have posted. It does not take me 9 hours to charge my aux. Not even close. Your comparison does not match the use case. You are looking at a user who has solar and rarely starts up with a battery in a state of deep discharge. Yet you still claim 9 hours and have yet to concede that when it comes to the absorption stage of the charge cycle a DCDC can reach a higher SOC that an alternator with low voltage output.

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So every time you go camping, the suns always shining and you get most of your used charge replaced before you even start driving which is a best case scenario.


Nothing like looking at the world through rose coloured glasses.


In a worst case scenario when the weather is poor and your solar is of no use and with a low battery, then your will need to drive for 9 hours to get your battery charged enough for the DC/DC device to into float mode.


BTW, what advantage is there if your DC/DC device goes into float mode? Going into float mode doesn't protect your battery, it's the actual drive time that protects all batteries.


Now I don’t ware rose coloured glasses and I always give my customers info on a setup based on worst case scenarios, so they have a genuine idea of the limitations of their system.


But there is nothing stopping someone using the same size solar panel you are using and if they used the same amount of battery capacity that you use and then drove to their next stop.


But in this case, because they would be using a separate solar regulator, and it is highly likely to be way more efficient that what you are using, when they started their drive, their batteries are going to be in a higher state of charge.


Then because they have been sharing the load over their two batteries, again, their batteries are going to need less that half the amount of charge your single battery needs.


Once again, their batteries are going to reach a higher state of charge hours before your battery will.


My isolator come with a 50 amp auto resetting circuit breaker but the fundamentals are the same for automotive fuses and circuit breakers and once again, you obviously don’t know how either type of device works.

I'll let those more expert than me argue the finer details and just focus on what I think is the key question for me: do I have anything significant to gain by fitting a DC-DC between the starting and second AGM batteries?
So far I can't see any significant advantages for my vehicle. As I said above, I think there is an argument for a DC-DC in a caravan/camper, although I don't currently have one fitted in our camper.
I think changing from a PWM to a MPPT solar regulator might give more benefits than a DC-DC, but that's another argument.
I haven't noticed any mention of the difference between a traditional alternator and a smart alternator in this discussion. A traditional alternator just pumps out a steady output, in our Defender's case 14 volts. A smart alternator decides when the starting battery is charged and then cuts it's output, which can cause problems for charging a second battery.
Is this a factor affecting if a DC-DC would be beneficial in the OP's case?

Hi DiscoMick, can I suggest you start a separate thread as the OP has the info he requested and then your own thread can go in directions of its own as needed.

Yeah yeah the old rose tinted glasses again. And another attack on the solar.

There have been numerous firmware if not hardware revisions to the ctek250. The later ones have improved MPPT . I have not compared, but it is likely that the concern you have with it is now out dated.

I have a number of mppts here and can tell you that some of the cheap Chinese 'crap' is just as good at maxi-point tracking as some of the premium gear. In so far as amps recovered in same conditions on same panel are near enough identical.

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My isolator come with a 50 amp auto resetting circuit breaker but the fundamentals are the same for automotive fuses and circuit breakers and once again, you obviously don’t know how either type of device works.

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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!

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Look this isn't rocket science. And we can boil it down to:

The alternator can out out more amps.

The DCDC can put out more volts (context TDI alternator)


And with my alternator:
The DCDC can get to 100% faster than the alternator can. And my alternator can get me to 85% faster than my DCDC can.
But my alternator cannot get me to 100% - even after 9 hours!

The better set up for someone with less than ideal alternator voltage output would be a hybrid set up, where bulk is supplied direct from alternator and absorb to float is done via DCDC.

Ultimately the decision comes down to cost, needs, reliability.

My needs have been met, the unit cost me nothing, and it has been reliable.

If the unit left me wanting I would have ripped it out long ago.

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On my next trip I will log the charge cycles on both starter charged direct from alternator, and aux by DCDC.

And maybe then you will see how even on a grey day, it is still looking rosey! [emoji41]

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If you want a cheap and basic set up on the TDI, there is a VSR behind the instrument cluster that is used to enable the heated rear window function when alternator output has been detected. A connector on the loom can be found on the engine side of the bulkhead (black, silver stripe - from memory).

Run a line from that to a suitably rated relay in your battery box, and use that to link your batteries when alternator is running. Protect it with a fuse or resettable CIRCUIT BREAKER!

Then add an Anderson plug so you can plug your solar panel in with a panel mounted mppt.

Hook up your aux circuit to the aux battery with a fuse/cb and LVD.

If you add a switch between VSR trigger and relay you can manually link and disconnect the batteries. As well as auto switching on alternator output detection.


Job done