DC to DC Charger with Isolator?

[Luke70]

Two years ago a built my own portable battery supply. It was being used mainly as a power source for a Dometic refrigerator and occasionally for my laptop and other electronics. Basically it is a Dewalt tool box that holds a 12v, 100Ah AGM deep cycle battery. It was being charged from a battery isolator under the hood ( DB Electrical 626-01000 160 Amp Two Battery Isolator). This isolator splits the amperage from the 160 Amp alternator. So I was sending 80 Amps to the deep cycle battery. Apparently not great for a deep cycle batteries (too many angry pixies and nothing to tame them). And maybe not so great for the alternator it seems.




It looks like the best way to do this is with a 20A DC to DC charger off the starting battery and deleting the isolator. All well and good, that is my plan. However, does do any harm to just connect the DC to DC charger to the supply already coming from the isolator? Personally I cannot see any down side but I am also the guy that set it up poorly the first time. It would save me only a little effort, but mostly for my education, I would like to hear what you all think. Thanks.

 

[john61ct]

Does the DC-DC include VSR/ACR switching functionality?

Not all do.

If not then leave the isolator.

If yes then it is redundant, more to go wrong

 

[burleyman]

How have you monitored charging current and voltage to the 100AH AGM portable? Most 100AH AGM as well as typical lead-acid group size batteries will only accept about 40 amps maximum at mid-fourteen charging volts, then amps typically decrease to thirty or less rather quickly. The battery won't be fully charged until charging amps fall to the single digits at mid-fourteen volts, and that can take hours of driving. That charge profile has been around for many years, but requires relearning for lithium.

My 130 amp alternator has been charging a non-lithium house battery though a relay since 1994. Accurate to tenths digital voltmeters and Hall Effect dc ammeters are inexpensive through online suppliers for monitoring charging while driving.

Adding high continuous DC amp loads like an inverter or lithium batteries can overtax the alternator. A DC ammeter will show that.

If you don't know charging amps, an alternator failure might be just a coincidence.

 

[Martinjmpr]

My DC/DC charger (Renogy 20A) has a switch that needs a hot wire in order for the charger to run. I ran that wire to a fuse tap for a circuit that is only energized when the ignition is on. In essence, that's my "isolator" that prevents the DC/DC charger from drawing off the battery when the truck isn't running.

I think more expensive chargers may have a voltage sensing relay that shuts off the charger if voltage drops below charging voltage, but mine doesn't. IMO the isolator is redundant but I can't see any harm that would come from it unless something fails.

I put a manual "isolator" on my DC/DC charger's + connection at the battery, just a simple "top terminal disconnect" with a knob, similar to this:


The reason I did it is because my F-150 is also my DD and when I'm using the truck in DD mode, I don't want the DC/DC charger to be powered (because I'm not using it, and because it does generate some heat.) So, when we get back from a camping trip, I unscrew the little knob and throw it in the glove compartment, and the charger doesn't get any 12v power. When I load the truck up to go on a camping trip, I screw the knob in place and then the charger is powered, but only when the ignition is on.

 

[john61ct]

Then the isolator is redundant.

Top deep cycling AGM like Lifeline can accept 0.8C and higher current in early Bulk stage.

Yes it quickly goes lower as SoC rises.

0.4C and up can last for a half hour or more.