Will both batteries discharge at the same time?

[Yoshimura]

Originally Posted by hbn7hj

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

On the Sterling Power charger, the input voltage is ignored if the Ignition input is > 4V. The Low voltage trip (default 10.0V) can be configured.

[Yoshimura]

Originally Posted by markopolo

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.

I assumed all 12v to 12v chargers were boost/buck type.

[Yoshimura]

Originally Posted by markopolo

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

Battleborn 100Ah.

My inverter/charger is set to 38A (probably its maximum). For the DC/DC charger, Battleborn recommend .5C charge current so probably between 30A and 60A.

[hbn7hj]

Try it and let us know how it works.

[Yoshimura]

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.

The DC/DC charger doesn't care where the input voltage come from. It will simply convert it to the appropriate voltage to charge the lithium.

[Yoshimura]

Originally Posted by hbn7hj

Try it and let us know how it works.

I still need to do some homework.

[Yoshimura]

Originally Posted by hbn7hj

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.

Since I never boondock for more than one night, I will let the DC/DC charger charges the lithium when the engine is running (using the ignition input) or when on shore power. However, I was wondering if I could easily use the AGM capacity when boondocking if ever needed.

[Yoshimura]

Originally Posted by Yoshimura

I still need to do some homework.

I might also simply install a second AGM or even simpler, make a custom Voltstart (remote starter + voltage comparator).

[@Michael]

Originally Posted by Yoshimura

Since I never boondock for more than one night, I will let the DC/DC charger charges the lithium when the engine is running (using the ignition input) or when on shore power. However, I was wondering if I could easily use the AGM capacity when boondocking if ever needed.

That where the A/B switch suggested by @hbn7hj comes in. Put the load on the common side of an A/B. At bedtime, switch the load to whichever bank has juice to last the night.

My setup is really close to what you propose, except I do not have an inverter. I've used it for a year & it works OK.

The AGM and lithium batteries in my coach have overlapping charge profiles, so in theory I could use the same charger to (individually) charge both banks without re-programming. Put an A/B switch in front of the inverter/charger, set a single charge profile that fits both battery types and switch the charger/inverter between banks with the A/B.

[markopolo]

I did some testing on combined lead acid & LFP batteries with diodes and without -> https://www.classbforum.com/forums/f...tml#post109324

Seems like a good match to me. Let the lithium battery do most of the work. The lead acid battery should last a long time.

For discussion, what are the issues / concerns / problems with paralleling the two different battery types?

[GallenH]

hbn,

have you seen this: https://www.ohmmu.com/product-page/gen2group31

right in our backyard.

[GallenH]

Originally Posted by @Michael

That where the A/B switch suggested by @hbn7hj comes in. Put the load on the common side of an A/B. At bedtime, switch the load to whichever bank has juice to last the night.

My setup is really close to what you propose, except I do not have an inverter. I've used it for a year & it works OK.

The AGM and lithium batteries in my coach have overlapping charge profiles, so in theory I could use the same charger to (individually) charge both banks without re-programming. Put an A/B switch in front of the inverter/charger, set a single charge profile that fits both battery types and switch the charger/inverter between banks with the A/B.

Thanks for the detailed info! How long have you been up and running?

[GallenH]

Sorry Michael..........reread your post and see you've been going with it for a year. So I'm assuming that all is good. Nice blog BTW

[rowiebowie]

Originally Posted by Yoshimura

I might also simply install a second AGM or even simpler, make a custom Voltstart (remote starter + voltage comparator).

Why not ditch the AGM's and just go all lithium. Simpler (must be if I can do it) and lithiums are smaller/lighter.

[Yoshimura]

Originally Posted by rowiebowie

Why not ditch the AGM's and just go all lithium. Simpler (must be if I can do it) and lithiums are smaller/lighter.

When lithium stop charging, energy in the alternator must go somewhere and the AGM is used to absorb this energy. Otherwise, your alternator diodes will burn out. From you schematic, it seems you use our engine battery.

[hbn7hj]

Originally Posted by 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?

The answer to the original question is no.

[rowiebowie]

Originally Posted by Yoshimura

When lithium stop charging, energy in the alternator must go somewhere and the AGM is used to absorb this energy. Otherwise, your alternator diodes will burn out. From you schematic, it seems you use our engine battery.

Ah, but the alternator has the engine battery to absorb this energy. Plus, my DC to DC charger is only 40 amps max and ramps down from what I've observed. Shouldn't be a problem.

[markopolo]

The OP wants increased low to mid range DC current capacity for coach loads and wants to use 6AWG wire.

If that's the only goal then a wiring change to the previously posted schematic and adding a 40A or 80A Victron Combiner would accomplish it. Modified schematic attached.
screen_shot_2020_07_18_at_12_00_40_b89d3af21908ad1def738f5026e9b7e670f525bc.png

Note: fusing not shown, 6AWG typically rated at 50A

Also note, this does nothing to help the inverter side of things.

In this example, the LFP battery will supply the house loads (excluding the inverter) until it's nearly depleted and then those loads will be seamlessly supplied the AGM battery. No user intervention needed. Low to mid range DC current capacity would be doubled. The combiner would presumably cause a .3V voltage drop but higher nominal voltage of the LFP makes that much less noticeable in the coach.

It would probably cost $1,000 less to add a second 100Ah AGM instead of the LFP assuming the original AGM is in great condition. The combiner and the DC-DC controller are not needed for that.

[Yoshimura]

Originally Posted by markopolo

The OP wants increased low to mid range DC current capacity for coach loads and wants to use 6AWG wire.

If that's the only goal then a wiring change to the previously posted schematic and adding a 40A or 80A Victron Combiner would accomplish it. Modified schematic attached.
Attachment 9552

Note: fusing not shown, 6AWG typically rated at 50A

Also note, this does nothing to help the inverter side of things.

In this example, the LFP battery will supply the house loads (excluding the inverter) until it's nearly depleted and then those loads will be seamlessly supplied the AGM battery. No user intervention needed. Low to mid range DC current capacity would be doubled. The combiner would presumably cause a .3V voltage drop but higher nominal voltage of the LFP makes that much less noticeable in the coach.

It would probably cost $1,000 less to add a second 100Ah AGM instead of the LFP assuming the original AGM is in great condition. The combiner and the DC-DC controller are not needed for that.

This is very interesting.

The LFP will initially supply the loads because of its higher nominal voltage and flat discharge curve?

Thanks for the schematic!

[markopolo]

You're welcome!

The LFP battery will supply the power from 13.4V down to 12.8V where sharing of the load begins. Battle Born's automatic cutoff appears to be 10V so it is likely that the sharing would continue until the Battle Born reaches that point. It seem unlikely that you would go that low before recharging.

With the voltage drop caused by the Combiner you should still see 13V to 12.7V in the coach for a long time.