I have a loose NL battery pack (with split-charge isolator) sitting at the back of the Troll connected to my main battery with a brad harrison/10mm cable. Going on a long trip with 2-3 days at sites with limited driving. I have a second national luna box/battery. The main power users are 2 x 40 l Engel fridges. Some stupid Questions:
Can I simply connect the 2nd DCycle battery to the same 10mm wire running from the main DCycle to the crank battery and hope the alternator/cable can cope? I can then have battery 3 as a backup? Ore am I being doff,yes I know
Longer term I will look at a more permanent solution: DC-DC charger or Solar panel/both.
Your 10mm wire is too thin over that distance. Your voltage drop will be too much for your batteries to charge properly. Measure your voltage on your current system. It should not exceed 0.1v max. Anything more than that means your wire is too thin for the length of the cable.
Absolute minimum over that distance with a solenoid system is 16mm, but 25mm is preferred by most people. Instead of replacing the 10mm wire with a thicker one, you can just add another 10mm wire.
I say it is too thin for a solenoid system, but if you use LiFeP04, AGM or Gel batteries, a solenoid system is not going to do it either. Then you have to use a DC to DC charger instead. If that is fitted to the rear of the van, the 10mm wire from the main battery to the DC to DC charger is OK. The DC to DC charger (I am talking of an intelligent DC to DC charger like the Victron Energy Orion-Tr Smart 12/12-18A (18 amp) or the Victron Energy Orion-Tr Smart 12/12-30A (30 amp) charger) can be set for the voltage required. It is also an intelligent charger, and starts off by bulk charging, then float charging and eventually maintenance charging. You can set it and monitor what is is doing via the Victron APP downloaded to your smart phone from Google Play Store.
LiFeP04, Gel and AGM batteries have to be charged at 14.4 to 14.5 volts on the bulk charging stage, and different voltages on the different stages, only a smart DC to DC charger allows you to program the charger. If you on the other hand just use your old system and modify you voltage regulator to charge 14,5 volts, you start frying the standard electrical components of the vehicle, like globes, meters, gauges and on-board computers...
Shout if you need more help.
I run two AGM/Gel Hybrid batteries on my 2008 3.0Di with one 100ah battery in the engine bay and a second 100ah battery in the rear of the vehicle I have a (in the rear mounted) Victron Energy Orion-Tr Smart 12/12-18A for the alternator charging and a SmartSolar MPPT 75/15 (12/24V) for my solar panel
Thank you for the prompt and comprehensive reply Malcolm even though most went over my head
To summarise there is no easy, short-term solution and I'm just going to leave the second DCycle battery at home. Longer term, I will give you a shout for advice, thank you - sounds like a lekker December project - will also give me time to save up.
Regards
John
JohnBoyZA wrote: ↑11 Aug 2020 06:00
Thank you for the prompt and comprehensive reply Malcolm even though most went over my head
To summarise there is no easy, short-term solution and I'm just going to leave the second DCycle battery at home. Longer term, I will give you a shout for advice, thank you - sounds like a lekker December project - will also give me time to save up.
Regards
John
Theoretically as Malcolm puts it, yes
in an emergency it will work, but not long term as there is not enough amps coming through that skinny wire to keep both batteries at full capacity, so it will just likely destroy the battery before long. Solar panel all the way John! Do it, do it now! I bought mine around 5 years ago and never looked back, even in the middle of summer in the Richtersveld, fridge stayed frozen and I could charge the aux batteries at the same time.
Thank you Rhett! Had a look at a few solar panel options today from R1700 to R7000. The latter looks really cool, gel based concertina type, but no budget for that yet. Will start a separate thread when I'm ready for that.
On a side note, discovered that where the cabling exits into the vehicle at the back the wires are actually much thinner, maybe 5mm and probably done to make it easier to bend. I quickly checked the voltage from the thinner wire and the thicker wire behind the inspection hatch and they are both produce 14.2V when the engine is idling (see pics). Is this because the thin wire is so short? Should I be concerned? I have been using this setup for 13 years.
Thank you
John
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Ok let me give my 2 sents worth ..
Spend the money on a c tech and you will not be sorry ...the 10mm cable is more than ok for 25a at a time . But your alternator can not charge a deep cycle period . You will get it to 70% . You need at least 14.8 on the battery for it to get to 100% . Remember the more your crank battery get full the lower the voltage goes . For now you can plug one in at a time and rotate . C tek has solar mppt in already ...solinoid systems only work ok with crank batteries
John, how are you measuring the wire, as that looks way thicker than 5 or 10 to me? Remember, the number refers to cross-sectional area of the copper, not diameter.
In other words, you need to measure the diameter of the wire (excluding the insulation), halve it, square the answer and then multiply by PI (3.124). 25mm wire thus has an actual diameter of 5.6mm.
Secondly, the voltage drop will only become evident if you are drawing significant amperage. Thus you need to run the fridge for a few hours, and then see what voltage you get.
Thank you Tinus, still can't get my head around the DC-DC charging concept, but as you've guessed electrickery is not my game ;-) Will leave this project till December.
Bliksem Peter, didn't know that about the cable area vs diameter Measured voltage without drawing any power, will do similar test when fridge is running and let you know.
even though most went over my head 
I bought mine around 5 years ago and never looked back, even in the middle of summer in the Richtersveld, fridge stayed frozen and I could charge the aux batteries at the same time.
