Upgrade AGM to Lithium

[Tlovell]

When I bought my motorhome, it was already set up with batteries and solar. I will post a picture of the current battery bank. There are four 170 watt panels on the roof and a Xantrex Freedom SW3012 inverter/charger. There are ten 6 volt AGM, 224ah batteries that make a 12V, 1120ah bank. Being lead acid and can only be discharged down to 50%, my usable ah are more like 560. I've already been told that the 760 watts of solar are going to struggle to charge that bank but the motorhome has an onboard generator to help with that. I am considering buying twelve 3.2v, 280ah cells from Amy Wan to create a 12v, 840ah bank. Besides increasing my solar panels, do you think the rest of the components will be ok with just the battery swap? I think a DC to DC charger should be added? I'm am new to all this solar, batteries, and Lifepo4 stuff so please be gentle with me...

 

[snoobler]

Welcome to the forum.

760W / 12V = 63A. That is pretty marginal for an 1120Ah AGM bank, but it's at the low end of "okay." That of course assumes you actually have a 60A+ solar charge controller, and the one in the picture is only 40A, so you're limited to about 40*14V = 560W as your batteries near full.

LFP is much more flexible with charging current. It can be very happy with low charge current, and it can generally accept at least 420A of current for the config you're proposing, so there is no need to increase your solar. If you do choose to increase your solar, you should add another or upgrade your existing charge controller.

One of the most limiting things with LFP is the BMS. I recommend you actually construct 3X 4S batteries, each with their own BMS for redundancy.

You'll need to confirm your AC-DC converter is compatible with LFP. If the peak charge current is at or below 14.6V, and the float voltage is at or below 13.6V, you're good. Very common to have a 3 stage with 14.4V bulk, 13.8V absorption and 13.2V bulk.

A DC-DC charger is prudent assuming you wish to charge from the alternator.

 

[MisterSandals]

four 170 watt panels

760W / 12V = 63A.

My math says 680w solar
And 680w / 12v = 56 amps

small difference I know but it’s nice to work with good numbers, especially on edge conditions.

you're limited to about 40*14V = 560W as your batteries near full.

 

[snoobler]

I just went with the "760 watts of solar"...

 

[Tlovell]

Sorry, that's on me. At one point I said 760 watts of solar. I wasn't sure if I should make 3X 4S batteries with a bms for each or one large battery with one bms. I was thinking 3X batteries would be better, so thanks for confirming that. How large of a bms with a 12v 280ah battery do you think?

 

[snoobler]

BMS size depends your loads, e.g., a 2000W inverter would need 2000W/12V/.85 = 196A (0.85 is for inverter efficiency), so a 200A BMS would be a minimum on a single battery, but 100A BMS on 3 batteries would likely work fine.

 

[chrisski]

If not already installed, it’d be nice to have a batttery monitor. That way you can get some data to see if the panels output the power you want. When I did my install, I ended up adding more panels throughout the first year.

I had 600 watts of panels on the roof of my RV and they never came close to pushing the 36 amps to the battery tat in theory they could. The flat angle of the panels prevented max output and at times they were shaded with the air conditioner or other vents. I may have seen 20 or 25 amps tops.

I have a 440 ah battery bank, which is less than you have with those 10 batteries, and in the shortest days of the year and if I used 165 amp hours of battery, I’m not sure that would be enough power. With 800+ ah of battery bank you’re building, I would think more panels are good.

My first solution was 400 watts of portable panels, which produced up to 30 amps by tilting and pointing at the sun. That brought total power out to about 50 -55 amps. Those were difficult to deploy and keep from blowing away, so I added 350 watts more of solar panels on the roof, and that brought my roof only power to 40 to 45 amps and if I put the portable panels out I got 74 amps.

I’m building a lithium bank next, and I have some plans to add a few more more panels to support that.

 

[GSXR1000]

your converter will charge the batteries but once it goes to float it will need to discharge to below 50% before it will charge again..... or turn converter off then back on... like have no power then start your generator