In my opinion, it was easier for me to have a dual battery system in the vehicle. Only for ease of use. It does not have to be manually carried, it will be charged as required without having to remember to do it, I use an extension lead from the vehicle for the camp shower and lighting, etc.
Downside is, I have to run the lead, whereas a battery pack can be taken to the spot, although it is heavy.
We have a second battery in our vehicle which has two extra cigarette lighter outlets fitted in; one for the fridge and one for charging up our other gadgets. We have found it very useful as when we stay in an unpowered spot, it cuts out before the fridge runs it completely down and it then gets charged up as we drive. We can run a fridge for 4 days and also a few 12v lights with no dramas. We don't have to worry about starting the car again as it is on a completely different system to the main car battery. Although I don't know much about power packs, we found the second car battery to be very economical and sufficient for our needs.
A vehicle alternator will not charge a lead acid battery to 100% of its rated capacity (if you want references see Collyn River's books.) The problem is that a deep cycle lead acid battery requires 14.5V to charge it and alternators generally start at about 14.2 V and drop their voltage when the motor compartment warms up - this only takes a few minutes.
You can overcome this in a couple of ways. If the second battery is under the bonnet you can install a gel-cell or AGM battery which require less voltage input to charge - although this will still only improve the situation a little. The other way is to install a battery booster or recharger (two terms for the same thing) which will boost the alternator voltage. If the battery is in the cool and not under the bonnet then the booster is the only way.
PeterD
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PeterD Nissan Navara D23 diesel auto, Spaceland pop-top Retired radio and electronics technician. NSW Central Coast.
