I've done umpteen 1 week to 1 month trips touring and use a dc-dc charger. No dramas at all and it gets the battery up to 100% SOC and giving me longer run times.
Previously when I used just my alternator , id be seeing 11.9v a lot sooner , without fail. (I never ever take a battery below 11.9v). Auto SParky mates of mine (who are part of my 4wd'ing crew) say it's closer to 70% SOC than 80% with alty
Sure , an alternator will charge a flat battery to 70-80% SOC a little faster than dc charger will , but then doesn't take it to sweet spot of chemically healthy 80-100% (which you get to retain in following days if you don't indeed use it). At the end of the day I'd rather have ability to extend run time and maintain battery up around 80-100%. This means in the days following a full charge I have more runtime available before going flat , and therefore will be further along the charge percentage compared to if i had only been at 70-80% to start with.
My starter always settles to around 12.7 overnight , Aux will be sitting around 13.1v overnight (assuming no load).
I just prefer to be always hovering 70--100% SOC (which is realistic) , than 50-70/80%.
In essence, think of the battery as a glass of water ... the bottom 50% you shouldn't tap into (damages the battery) ... I'd prefer to be able to fill my glass of water to the top, even if it takes a bit longer (never ever been issue for me mine seems to do fine) than to fill it really quickly but only to 70% (80% is a very optimistic figure). At end of the day the runtime outweighs the perceived slightly slower charge rate. Assuming I drink half a glass of water per night , I'm still ahead on the next driving day because I had full glass to start with. Taking a battery below 50% greatly reduces its life.
If you can hold more than a non dc-dc setup "in the tank" then you're stilll going to be ahead "on the next drive"
I also believe in the starter battery only ever being the starter, not part of a smart solenoid scenario. Aux / house battery can be raped no worries but that starter is for engine only. It rarely ever deviates from its SOC and is never used for house duties. (Even stereo system draws power from aux circuit and no need for key ignition).
In terms of speediness of charge (to the 70-80% mark, only the 80-100% mark is relevant for DCDC) ... My alternator is 80amp , a redarc 40amp dc charger is chewing half the alty output ... Frankly don't think with all that other loads sucking on the alty that you're going to get much more than 40amps dumping into a battery even with alty directly connected.
Dc chargers are also 3 or 5 stage and set to the battery type (important for non standard batteries) , giving a healthier and longer life for the batteries that are constantly in discharge / recharge use scenarios. They're temperate aware and kinder to battery chemistry in charge / discharge cycling.
In bulk charge mode it hovers between 13-14v lots of amps, then up to 15v when hits absorption phase (90-100%, less amps). , then floats at 13.3. Almost always mines on 14.9 to 15v (90-100% SOC). Unless I've had a heavy time with amps / subs / fridge working hard / mrs watching DVD with inverter on, then she will bulk charge for couple hrs on next drive.
My vote goes to using dc-dc chargers. (And Ac-dc chargers when appropriate such as access to genny power).
At end of day if you're going to be sucking power galore for days on end with no driving you're going to need some other source of power. (Solar / genny /240v : Ac-dc 7 stage charger)
At a min , get yourself an Ac-dc charger so u can top them up to 100% before venturing off. (I take mine with me and plug in when stopover at family farm for a few days of no driving , with fridge and freezer full and still running when enroute on way up north as part of a longer trip)
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