Will both batteries discharge at the same time?

[Yoshimura]

I'm considering adding a lithium battery and a DC/DC charger to my Roadtrek Simplicity (see attached schema).

When boondocking (assuming inverter is turned off and engine is not running), will both batteries discharge at the same time when using DC appliances?

[@Michael]

Originally Posted by Yoshimura

When boondocking (assuming inverter is turned off and engine is not running), will both batteries discharge at the same time when using DC appliances?

By default, no.

If your DC-DC charger is a typical B2B charger, it will charge the lithium battery only when the AGM side's voltage is above a threshold. That threshold is high enough that it'll only charge the lithium when the AGM is being charged by your solar, converter or alternator.

On some B2B's, you can lower that threshold so that it will charge untill the source side is discharged to a lower voltage, in which case you can drain the AGM's down quite a ways before they stop charging the lithium.

[GallenH]

Interesting. I’m watching to see what others with more knowledge than me say about the setup because I’m contemplating the same type of mixed system. And I have a few questions.

Is the home alternator a 2nd alternator? Does it have a programmable charging profile? If so, how would that affect the DCDC charger since I’m thinking that they’d be different on the AGM vs. Lithium.

Same question with regard to your Inverter/Charger.

[hbn7hj]

The two systems do not work well together. Either run the coach on the lithium battery or the AGM battery, not both. They can be charged together with separate chargers.

We have found no way to dumb it up so Grandma could run it.

[Yoshimura]

Originally Posted by GallenH

Interesting. I’m watching to see what others with more knowledge than me say about the setup because I’m contemplating the same type of mixed system. And I have a few questions.

Is the home alternator a 2nd alternator? Does it have a programmable charging profile? If so, how would that affect the DCDC charger since I’m thinking that they’d be different on the AGM vs. Lithium.

Same question with regard to your Inverter/Charger.

Yes, this is a second alternator but with an internal voltage regulator. So no charging profile (... has a set point of 14.7 volts cold and 14.2 volts hot if batteries are at full charge).

Inverter / charger has a charging profile appropriated for the AGM battery and the DC/DC charger will have a charging profile appropriated for the Lithium.

[Yoshimura]

Originally Posted by hbn7hj

The two systems do not work well together. Either run the coach on the lithium battery or the AGM battery, not both. They can be charged together with separate chargers.

We have found no way to dumb it up so Grandma could run it.

Both are not connected together so I'm not sure I understand your comment. Lithium will power the DC and the AGM will power AC though the inverter. And each battery has it own charger.

[hbn7hj]

The lithium battery does a better job of powering the inverter than does the AGM battery due to less voltage drop. You are losing a major advantage of having a lithium battery.

I gather you have not built and used this?

[Yoshimura]

Originally Posted by hbn7hj

The lithium battery does a better job of powering the inverter than does the AGM battery due to less voltage drop. You are losing one advantage of having a lithium battery.

I gather you have not built and used this?

I just want to increase my evening / night DC capacity and this seems a very easy way. Particularly the fact that I could wire everything inside using 6 AWG cables.

No, not build yet. But a online member has this layout.

[Yoshimura]

Originally Posted by @Michael

By default, no.

If your DC-DC charger is a typical B2B charger, it will charge the lithium battery only when the AGM side's voltage is above a threshold. That threshold is high enough that it'll only charge the lithium when the AGM is being charged by your solar, converter or alternator.

On some B2B's, you can lower that threshold so that it will charge untill the source side is discharged to a lower voltage, in which case you can drain the AGM's down quite a ways before they stop charging the lithium.

Make sense.

If the DC/DC charger is 50A and the Inverter/Charger is 15A, 35A will be drained from the AMG until the battery volage drops below the stop threshold, right? Then, the 15A will go to the AGM until the voltage reach the start threshold (assuming you could set two different threshold). Hum... could this cycling be a problem?

[hbn7hj]

You can try it. The best way is to put in a battery switch to choose between powering everything by lithium or AGM.

As it is I would expect your inverter to alarm low voltage pretty quickly. Run the inverter from the lithium and there will be no low voltage inverter alarms.

[GallenH]

Hi hbn,

I'm still confused. Whether or not it's coming from shore or alternator, wouldn't you need a different charge profile for the AGM and the lithium. As I recall from your early work in this area, you had a separate shore charger for each. I can't remember what you did coming off the alternator.

best.glenn

[Yoshimura]

Originally Posted by GallenH

Hi hbn,

I'm still confused. Whether or not it's coming from shore or alternator, wouldn't you need a different charge profile for the AGM and the lithium. As I recall from your early work in this area, you had a separate shore charger for each. I can't remember what you did coming off the alternator.

best.glenn

The lithium is charged by the DC/DC charger which has the proper charge profile for lithium. The AGM is still charged the same way as before.

[hbn7hj]

I charge by generator but I did put in a DC to DC charger limited to 40 amps from the alternator.

The suggested schematic loses two lithium advantages. One it runs the inverter better and two the lithium could be used to charge the AGM.

I suggest the person that came up with the installation let us know how it worked. I can duplicate it by flipping a switch but my DC to DC charger to the lithium will not charge without the engine running.

[hbn7hj]

Originally Posted by Yoshimura

The lithium is charged by the DC/DC charger which has the proper charge profile for lithium. The AGM is still charged the same way as before.

No DC to DC charger that I know of will do that with the engine off. Mine will not.

[Yoshimura]

Originally Posted by hbn7hj

No DC to DC charger that I know of will do that with the engine off. Mine will not.

Some chargers (for example Sterling Power) have an Ignition input that is normally connected to the ignition feed. You just need to connect it to 12V.

[hbn7hj]

Originally Posted by Yoshimura

Some chargers (for example Sterling Power) have an Ignition input that is normally connected to the ignition feed. You just need to connect it to 12V.

Mine has that, too. It still won’t charge till the voltage is high enough. (Kisae)

[markopolo]

Originally Posted by Yoshimura

.................... When boondocking (assuming inverter is turned off and engine is not running), will both batteries discharge at the same time when using DC appliances?

Some DC-DC charge controllers are boost/buck devices so they can take the lower voltage from the AGM and boost it to a higher voltage suitable to charge the LFP battery in the absence of a charge source. If the DC-DC charge controller is not that type then no, both batteries won't discharge at the same time when using DC appliances if the inverter is turned off or engine is not running. The LFP functions at a higher voltage than the AGM so the LFP supplies the power until nearly depleted.

[markopolo]

The schematic shows 200AH LFP battery. What LFP capacity are you thinking of using?

[GallenH]

So I still don't understand (not unusual). If you have a charger that's setup for AGM 3-stage charging profile and the output of that charger was also fed to a DC->DC charger whose profile was setup for lithium. Would the changing output of the AGM charger have any impact on the DC->DC charger as the former goes from stage to stage?

thx.glenn

hope you're not here and out of the hot weather, hbn.

[hbn7hj]

Originally Posted by GallenH

So I still don't understand (not unusual). If you have a charger that's setup for AGM 3-stage charging profile and the output of that charger was also fed to a DC->DC charger whose profile was setup for lithium. Would the changing output of the AGM charger have any impact on the DC->DC charger as the former goes from stage to stage?

thx.glenn

hope you're not here and out of the hot weather, hbn.

65 degrees in rain is the good news. The bad news is I have to be in Phoenix tomorrow.

I just don't think the system, as presented, is very good. Switch the batteries and it would be better but the inverter/charger probably doesn’t have a lithium profile. After depleting an AGM battery it will take 6 hours or more to charge it to 100%

There is a loss in keeping the DC to DC charger on all evening.