Upgrading Existing Solar: Changing house batteries to lithium

[ADDvanced]

Hey boys, quick question on what I thought would be simple, but am now realizing it's a bit more complex. First off, my Skoolie is more of a cabin, I rarely drive it. It's parked off grid, and I use it mainly in winter as a ski cabin.

EXISTING:
200 watts of solar, connected to
Renogy MPPT Charger, charging
BOTH the starting batteries, AND twin 100AH AGM (battery maxx) batteries.

What's nice is when I leave my bus parked for months, I come back, and both the starting batteries AND the house batteries are still at 100%, since they're getting float almost every day.

I have now acquired 2 lithium 100Ah batteries, which will effectively double my house battery capacity, but I can't just drop them in since the Renogy charger is setup for lead acid/agm profiles. I realize you can't have li and lead connected to each other.

What's the best course of action here?

 

[Rucker]

Hey boys, quick question on what I thought would be simple, but am now realizing it's a bit more complex. First off, my Skoolie is more of a cabin, I rarely drive it. It's parked off grid, and I use it mainly in winter as a ski cabin.

EXISTING:
200 watts of solar, connected to
Renogy MPPT Charger, charging
BOTH the starting batteries, AND twin 100AH AGM (battery maxx) batteries.

What's nice is when I leave my bus parked for months, I come back, and both the starting batteries AND the house batteries are still at 100%, since they're getting float almost every day.

I have now acquired 2 lithium 100Ah batteries, which will effectively double my house battery capacity, but I can't just drop them in since the Renogy charger is setup for lead acid/agm profiles. I realize you can't have li and lead connected to each other.

What's the best course of action here?

A couple of ways to go. One would be to switch the Renogy over to LiFePo, connect up the new batteries, then connect a 12vdc to 12vdc trickle charger to the chassis and secondary AGM batteries....

Another would be to use a Renogy DCDC charger controller to charge the AGMs from the main bus bar off the MPPT controller. This is a little more expensive but would be better if the chassis and secondary batteries need to be replenished at a greater rate.

An incredibly simple but less efficient way would be to plug a battery charger into your inverter (assuming you have one) connect it to the chassis battery, and keep it on trickle charge.

 

[Papa Smurf]

I apologize, I just read this post. I cheaper/easy option is to add a battery disconnect switch

 

[ADDvanced]

So is there an easy way to trickle charge the starting battery bank (lead acid) from my house batteries (400Ah lithiums, always topped off due to solar panels)?

 

[Rucker]

Yes-there are a couple of low amp DC to DC trickle chargers out there. They pull from your 12VDC house system and trickle charge the lead acid.

I have a couple but have not yet wired them in, so I can't speak to how well they work, but the principle is dirt simple.

Let us know what you end up with.

 

[Jono14]

Plan for cold

Because you describe this as a "Ski Cabin", and because it was -15F yesterday (for me in Wisconsin) I wonder if you have addressed the issue of cold temps and lithium? This is one of the only areas where LFP batteries struggle compared to lead-acid.
If the lithium cells are below freezing (32 degF) then they cannot be charged. And batteries vary on how well they protect themselves in the cold.

The cheapest don't even sense temperature and assume the user will not try to charge when it is cold (fine if you live in a place that never gets cold). But this could be risky if you are not on site as the Renogy charger probably won't sense temps and just try to charge anyway. In the extreme, this can destroy the battery by creating internal Li "stalactites" that pierce membranes.

Better quality batteries have low temp sensing, low temp protection (turning off the MOSFET switches to block charging when cold), internal heaters that warm the battery when a charge current is applied, and bluetooth communication that allow you to see the temps and status of charging. External heating systems can also be created with heating pads, switches and thermostatic control. Even with "good" batteries, it is possible to get into a catch-22 situation where the battery shuts off the "discharge" MOSFETs (when below -4F) and then charge controller does not see a battery so it does not try to charge so the heaters don't get activated.

Also, if the day is cloudy or the panels get covered in snow, they may not produce enough amps to warm the batteries first and then charge them.

The nice thing about LFP batteries is their self-discharge rate is very low, so if you are going to leave them alone in the cold, better to just leave the SOC at 50-80%, disconnect them completely and let them sit - now cold wont bother them down to about -15F, maybe lower. They might loose 1-3% charge per month.

So this could impact your design. If your setup requires charging the LFP batteries and then they charge the LA bank, you will have to deal with them being functional in the cold. You might have one method when you are living in the bus (warm and want to max out the solar in) and one when you are gone and the system is cold (LFP batteries disconnected and Renogy SCC topping off the LA batteries directly).

 

[ADDvanced]

I'm using self heating batteries, and every time I come back to them, they're at 100% charged. I could disconnect them, I suppose, but why? If they are too cold to charge, the BMS won't charge them... until they warm up.

 

[Jono14]

Sounds good - you may be all set - and Rucker's trickle charger idea may be the best method.

 

[nikitis]

I apologize, I just read this post. I cheaper/easy option is to add a battery disconnect switch

I second his opinion. A $70 battery disconnect wired up can make your batteries last for 2-3 years sitting.

 

[ADDvanced]

I second his opinion. A $70 battery disconnect wired up can make your batteries last for 2-3 years sitting.

Lead acids?!??? This post was about my starting batteries (lead acid) vs my house batteries (lithium)