DC-DC charger setup question

[DCH109]

I would also disagree about DC-DC chargers being 'terrible'. In some situations they are not necessary, but in may they are. In most modern vehicles the alternator is controlled by an ECU and the operation is dialed in to charge the starting battery and provide for the stock loads while minimizing parasitic loads on the engine. They are not designed to charge an extra house battery and they will quickly drop back to a lower voltage, providing very little charge to the house battery. Modern Toyota alternators are definitely this way - they rapidly decrease output to 13.6-13.7V. If you want to get a decent amount of power form these to a house battery you either need to over-ride the ECU or get a DC-DC charger.

Almost all DC-DC converters only come on when the car is running, so they do not draw down your starting battery.

While I have little knowledge of a DC-DC charger other than what was in my old camper, I will agree and disagree with your post above. However most ECU's are smart enough to handle this. My LR3 is one of the worse energy hog out there, if the main battery is low the Rover will drop into limp mode (well known issue). That said, I have run a 2nd battery in one for years using a custom made isolator without any issue what so ever. I have been out camping for days on end and the 2nd battery was always recharged fully once back on the road (yes I checked this for month before hand). Also ran the same setup in my FJCruiser without any issues. Maybe this is something in new Toyotas. I have never tried moving from one camping spot to another after a week, so this is a possibility in this case, but camping 1 place each day with a fridge and all accessories off the 2nd battery, never been an issue.

 

[Rando]

It sounds like you don't need a DC-DC charger, which is great. My post was addressing the idea that DC-DC chargers were 'terrible' and served no purpose, by pointing out there are cases were they definitely help. If you are aux battery and starting battery are of the same type and both under the hood with short wire runs, then you are probably fine with just an isolator as they appear as one battery to the alternator/ECU.

However, with the Toyota LIN controlled alternators, they will rarely provide voltages above 14.4 and when they do it is not for long, because a starting battery that rarely goes below 95% SOC just doesn't need it. If you add a long wire run, and a large, deeply discharged, AGM battery at the other end (say a camper battery) then you won't come close to getting it fully charged. Chances are it won't get through the absorption phase of charging even with a DC-DC charger, but it will do better than just the alternator as it will at least have a chance of sitting at 14.x volts for some time. In these situations you either need a shore power charger or solar to get the battery back to 100%.

I now have lithium in my camper and plenty of solar, so this is all a non-issue, but it was something I thought about with an AGM battery in the back of a modern Tacoma.

 

[Heyfish]

I have a similar setup I'm working on to put a aux battery in the bed of an F-150. I am basing it on the Renogy DC-DC MPPT charger for future solar panels. I have attached the basic diagram from the Renogy manual showing the main parts from the starting battery/alternator to the DC-DC and aux battery. If I am connecting a fuse block to the aux battery, does there need to be a fuse between the aux battery and the fuse block? Would that just protect the fuse block? If it is needed, what size fuse? The fuse block has a capacity of 100A so I would assume something similar. At this point I'm only going to be running a water pump, dometic fridge and maybe some extra house lights so the total draw is nowhere near the 100A capacity of the fuse block.

 

[dreadlocks]

you should have a master fuse on the battery post rated large enough none of your normal loads would not trip it, but not larger than the wiring is capable of providing between your battery and your fuse box... you could wire your solar charger up to the fuse box if yeh want and its close to the battery.

 

[Heyfish]

Any significant pros/cons between these two types of fuse? Seems like the terminal fuse would save a connection by going directly from the conductor to fuse/terminal as opposed to needing another section of conductor for the ANL fuse.

 

[dreadlocks]

I use the bluesea terminal block my self, great design and hard to beat IMO..

 

[dreadlocks]

in a pinch the terminal block can be removed entirely whereas an ANL block spliced in not so much unless you are wise enough leave your self enough slack to bridge the gap, presuming you cant get any fuses and have rectified the situation.. but really your master battery fuse should never really pop, the fuse box downstream should start blowing easy to obtain blade fuses long before this one pops.. the main battery fuse is more to prevent the rubble of your vehicle from catching on fire after an accident..

however having said that, I do carry a spare master fuse in my trailer even though I never expect to use it.. but I do that for lots of things thanks to my time in the BoyScouts.

 

[Heyfish]

Thanks for the feedback!