secondary chargeable 12v system

Hi there, Been a long while since I last posted here, but people have always helped out, so hoped someone could help out with this... Here in the UK I have a Toyota Previa, which is converted to a basic campervan. Space is obviously very tight, and there isn't room for a hefty leisure battery, or even the smallest one! Currently I use a jump-start pack that works well for 2days or so, and can be easily charged from the Previa's main battery whilst driving via the 12v cigarette lighter socket. This is fine, however I want a much neater install now. The 12v system generally runs just a small water pump when water is required, and is also used to power a 12v socket for charging of phones overnight whilst camping. We have also used a 12v-240v inverter so that we could use/charge a laptop. What I want to be able to do, is use the same concept of the jump start pack, but with a small 12v battery such as these; http://www.ebay.co.uk/itm/150948665552?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649 My issue is I have no idea if the batteries above will have enough capacity, or how I can charge the batteries from the Previa 12v socket whilst the vehicle is running? If anyone has any low-cost ideas for this application, please let me know your thoughts. I'm going round in circles with this project at the moment, and don't want to waste any money on failed ideas. Do really need this setup to be completed v soon as I have a trip planned. PLEASE help! Many thanks in advance JamesThanx 4 your time

For secondary/aux batteries it's generally better using one large battery than multiple smaller batteries. That's because of the complication of having paralleled batteries permanently connected - they should only be paralleled when charging and - assuming they are in good condition - when being used. Connection during charging is no problem. If the alternator has a charge light (or similar is, or is not, charging circuit) a relay can be used. (I call that a UIBI; search here or google "oldspark uibi".) The only consideration is that the circuit must be capable of powering the relay's coil. If not, a transistor or MOSFET buffer needs to be added. Alternatively a voltage sensing battery isolator (aka smart isolator) can be used, but they are more expensive. A single UIBI or voltage-sensing isolator can be used to drive multiple relays where there are more than 2 batteries. Hence your two aux batteries can be isolated when not charging. By adding diodes and a switch, the aux batteries could be manually connected when required. That adds the current of the relay coil compared to a hard-wired aux battery interconnection, but that should only be up to ~250mA = 3W which should be insignificant compared to loads. Alternatively a current sensing circuit could be added to automate the aux battery interconnection, but that can be complex. And AGM types (like your linked Lucas batteries) can be more hazardous if thermal runaway were to occur. I never hardwire parallel batteries - except when OEM as with large machinery (eg, 4 12V batteries in series/parallel for huge engines etc).    Of course you might decide to keep both aux batteries hard-wired to each other (maybe disconnecting when unused for long periods). Many people think that is fine when both batteries are the same (batch, history, age). But if one fails, it will damage the other(s). Campervans etc usually have dual batteries with auto isolation to protect against discharging the main cranking battery. As to battery capacity, that depends on your loads and how long you want your aux batteries to last wrt both discharge time and lifetime (ie deeper discharges and longer between recharges means shorter lifespan).