DC DC charger Li and AGM?

[MarcMassanari]

I have AGM starter battery and Li house bank. Can I use a Dc Dc charger to charge both while underway?

 

[wholybee]

yes.

 

[eagleray]

I have AGM starter battery and Li house bank. Can I use a Dc Dc charger to charge both while underway?

I'm cuious how best to configure this as well. I know lead starter and lithium deep cycle are common, but it's hard to picture this setup w/out trial and error. You can't just wire AGM to lithium in parallel, so there needs to be a junction where all supplies (shore, solar, alternator, other?) can route to either the lithium or the AGM, and the junction needs logic to determine which is which and use a matching charge profile. That's a charger's job, but most chargers don't have DC input AFAIK. I assume if you have Victron's laundry list of gear, the MultiPlus or other inverter+charger combo solves this puzzle, but assuming you have a more open-source hodgepodge of gear, what device(s) can pull this off?

Off the top of my head, I'd assume a typical multi-bank charger dedicates an output to each of the chemistries, but I'm stuck on how to get DC input into a random multi-bank charger. I guess you'd have to run the MPPT and the alternator and any other DC supply into the inverter input which would then output to the charger? That seems wasteful. It's DC from most supplies. I wouldn't want to have to invert it and then convert it back again if it's possible to avoid that. Am I missing something? Are there multi-bank chargers that do have DC input other than Victron?

 

[wholybee]

It works like this. Connect all of your charge sources to one battery. Typically the start battery. Then, use a dcdc charger, which has a 12V input, but can be programmed with a correct lithium profile, to charge the Lithium. The DCDC charger is smart and only turns on when it senses the start battery is being charged, and will not drain the battery. This is typically done to avoid alternator upgrades required to charge lfp with the alternator.

It can also go the other way, if you have upgraded the alternator, all charge sources can go to the LFP, and the DCDC charger charges the start battery. This will charge the lfp with the full current of the alternator, instead of the limit of the charger which is much lower

How it looks to wire is exactly the same as an ACR, if you have used a charge relay before. A DCDC charger will drop right in place of an ACR, but instead of just closing a relay, it is a multistage charger.

 

[wholybee]

Another option, whick "looks" more like what i think you are picturing, is an agrofet isolator. This device has a single input, and either 2 or 3 outputs. Charge sources go on the input, and outputs to the banks. Typically only the alternator would connect to the input, and other sources direct to battery. An agrofet is not a charger. It switches current to one or both batteries and at the same time isolates them so they are not parallel. It relies on the fact that lead acid can be charged with a lithium profile, so you set your charge sources for lithium.

 

[dsb]

If you are limited on space there are a few dc to dc chargers that also have inbuilt MPPT, allowing you to also connect solar.

The brands I’m currently looking at are Evedrive, Renogly and Kisae.

I’m still in the process of deciding if I go with two dedicated chargers or a single combined unit, the main advantage I can see of the combined unit is they take up less space and would be simpler to connect than two units. some of them also allow both batteries to be charged from solar, which could be useful while the RV is in storage. Although I’m yet to work out if any of them allow you to set a separate charge profile for each battery.

Having separate units might better if you have the space or are looking at a larger system as you will probably have more control over each component.

 

[mikefitz]

It's not complex, the alternator charges the starter battery and adjusts the charge conditions to keep it fully charged. A battery to battery charger is connected between the starter and lithium house battery. I recomend the Victron Tri Smart , a 30 amp unit.
The battery to battery charger converts power from the starter battery to a suitable charge profile for the house battery. When the alternator stops charging the battery to battery charger switches off.