DC Solar Trailer Lead-Acid to LiFePO4 upgrade

So I searched the forum and did not find anything that i could figure out:

We took over an oil-field lease of 200 acres and while doing a site survey they discovered that the previous tenants had left (4) of those DC Solar Trailers (w/o generators) in place. Everything seems to be still working. Of course the issue is that all the lead-acid batteries - although still functional - are truly shot. Obviously they have not (never) been taken care of and I think all of them are on their last leg. And honestly - I just don't want to have to go out and get $400 of sulfuric acid to regenerate these and then spend any effort trying to super-charge them (ie blast them at 51 volts with 100A for 5 minutes and then have them rest for 5 and then do it again and again to see if they will hold a load)

And of course the trailer is terribly underpowered with those 250W panels ...

BUT the Sunny Island SMAs and the Outback 250 seems to all be working fine.

What I would like to do is drop these 250W panels and put on some 355W on (they barely fit the space) and then pull out the Forklift batteries and drop in some of these 48V 280Ah LiFePO4 batteries we got earlier in time that's just sitting in a warehouse gathering dust (long OSHA and mechanical engineer story on why we can't use these specific ones - but they are fully functional - or at least they were 4 years ago)

And we have plenty of 355Ws laying around since we are mostly using Bi-Facial 535's on most of the things we are doing now ...The issue is that I don't think that we can just swap these batteries out one for one ... reading some of these forums I know that we would need to update the SI SMA firmware and make some configuration changes -- and people mentioned something about a BMS comms cable??? ...BUT it gets real hazy on what the next steps are ... seems that anyone/everyone that has tried to do this has NOT came back and let anyone know if the suggestions(s)(s) actually worked or not ...

So wondering if anyone HAS been successful in converting these DC Solar to LiFePO4 and if so if they know the steps... or had suggestions .... or point us in the right direction ... funny that there is ZERO on YouTube ...

We would NOT be keeping these trailers - the site leads' intent was to either scrap these (which is a colossal pain for us) (literally its easier just to get an earth mover to plant these)(or just leave them for the next team like the last team did) ... but we are thinking of either calling the local county emergency response guys or church or whatever and see if they want them ...

I figure since we have the equipment laying around to easily swap out the tires, batteries and panels at literally zero cost to us - at least we could give whoever a working system and not one that's already on its last leg.

And NO we can't fix these and then sell them --

BUT if anyone has any suggestions or where they can point me to that would be great ...

I am including @Hedges @Higher_wire @SilverbackMP @OzSolar since they seem to be on most of these DC Solar forum responses ...

THX --

Four years sitting would be concerning for Lifepo4 batteries. If that's true, they are probably dead and damaged.

Otherwise it should be as simple as reconfiguring the Midnite CC's with Lifepo4 charging settings. Appropriate voltages and disabling any equalization.

The SMA's probably don't need anything changed if they are not attached to any grid power, so they're not doing any charging.

List them on Craigslist for free. They should be gone next week.

For Sunny Island, there are supported batteries on SMA's compatibility list (from BYD and others.)
Some people here use REC SI BMS for closed loop. there are some other brands.
Other people run other lithium batteries open loop using VRLA settings.

Precharge should be done to avoid blowing fuses due to inrush. You could turn on both SI breakers then precharge before hard connecting batteries. I'm thinking of backfeeding PTC fuses inside SI to precharge before closing SI breakers.

Recent reports of EG4 PowerPro working closed loop with SI, so that may interest you. Your ambient heat may but undesirable for battery life, perhaps air conditioning would be worthwhile.

The SI 6048US I got (presumably from DC Solar inventory) already have firmware for Lithium BMS.
Firmware update is easy. Download from SMA website to SD card. Insert in master SI, use menu to update.

Some cable to BMS required, should be easy data connections at each end according to manual.

You can also install a small AGM bank and configure max charge current appropriately.

I think those trailers have a battery shunt, which allows open-loop external SCC (or loads.) Could be with BMS closed loop, the shunt isn't needed.

You can add more SCC of course, but I would suggest using Sunny Boys set for offgrid, AC coupling.

You've been spending your entire professional life setting up off-grid PV. Why wouldn't you ship these to where you need power?

Having laid hands on 8 of these trailers (I worked on them, I don't own them), conversion to LFP is as trivial as any other lead-acid swap-out.

Inverter comms are not needed.

Set desired absorption/float.
Disable temperature compensation.
Disable equalization.

sunshine_eggo said:

Having laid hands on 8 of these trailers (I worked on them, I don't own them), conversion to LFP is as trivial as any other lead-acid swap-out.

Inverter comms are not needed.

Set desired absorption/float.
Disable temperature compensation.
Disable equalization.

Click to expand...

I haven't worked on them but this would have been my reply too

There are 2-3 Facebook groups specifically for DC Solar trailer owners. 2 members of those groups frequently “upgrade” these trailers to lithium.
I’d poke around there for additional details, it’s been the most info heavy source I’ve seen for these.

As a side note, I’d love to snag one from you if you ever need to remove one

hwy17 said:

Four years sitting would be concerning for Lifepo4 batteries. If that's true, they are probably dead and damaged.

Otherwise it should be as simple as reconfiguring the Midnite CC's with Lifepo4 charging settings. Appropriate voltages and disabling any equalization.

The SMA's probably don't need anything changed if they are not attached to any grid power, so they're not doing any charging.

Click to expand...

So LiFePO4 batteries have a discharge rate of less than 1% per month ... normally we see about 9% per year .... recently we replaced some of the very original CALB 3.2 LiFePO4 batteries that came in those big ugly blue plastic cases where the blue had faded so much it was grey and the solar panel that the batteries was hooked up to stopped working almost (6) years earlier when a hail storm had came through -- and everyone of those batteries still had a charge above 2.5 in them ...

Hedges said:

You've been spending your entire professional life setting up off-grid PV. Why wouldn't you ship these to where you need power?

Click to expand...

Thank you for the detailed information -- will follow everything you said ...

as for why we wouldn't ship them somewhere -- its very problematic ... the equipment does not "officially" belong to us so we would have to get corporate to put the trailers on the company insurance policy for us to haul it to any of our sites -- we are not allowed to TOW or move anything that is not ours ... -- first thing they will ask is for a purchase order or a rental agreement for the trailer and if we tell them that it was "found on site" they will just hang up ...

The second thing is if we do decide to use them where we need them they are typically woefully under powered from a solar perspective ... we have a welder here that will go into town and get 20' flat bed trailers for like 2K each, then come back and make a solar cage tilted at 40 degrees for 2 rows of 7 (455w) (14 total) panels each and we throw on a SOL-ARK and a bank of LiFePO4's and are done .... it would literally cost us more time and money to get these 8 year old DC Solar trailers up and running correctly then they are worth (if we used them officially) ...

BUT what i can do is drag them to the maintenance yard - have new tires thrown on them, the forklift batteries pulled, some old (new) LiFePO4 batteries no one can use laid down and some solar panels we aren't using thrown on at literally no cost and give these things to an organization that needs them -- or hell -- the security guys that's out in the middle of no where using generators 24/7 etc etc ...

Here's SMA's compatibility list for lithium batteries with closed-loop BMS communication.


But you're going to use batteries already on-hand. For open loop, you can explicitly set VRLA cell voltages for absorption and float (assuming 24s 48V, 23s 46V, whatever you say the bank is) and that will map to something per cell for 16s LiFePO4.

I'm less sure how low SoC shutdown will work with LiFePO4 that SI thinks is VRLA. Choice between 48V nominal, 46V nominal etc. may affect that. It has their intelligent scheme for estimating SoC based on current over time, voltage, probably instantaneous current when voltage is being measured. Not all clearly documented. Load-shed and shutdown settings are based on SoC.

It sounds like you only need a BMS to communicate with the SMAs.

We recommend the REC 2Q BMS. We offer a kit with contractor and shunt, or you can source your own to save $100. It has the comm cable, although you’ll need a short CAT5 extender if you put the batteries behind the axle. There’s a little bit of programming to do, but it takes maybe 10 min. Plus they’ve been very responsive to every one of our customers’ (aka our) questions, including AC coupling and grid export.

Daly’s smart BMS will also work and will be both cheaper and easier to wire. But also takes a bit of programming and we’ve had a few issues with them. Their team has been pretty responsive, though.

Without comms the SMAs will shut down after a few hours using LFP. They do have their own shunt (behind a cover on the driver’s side of the cabinet), which isn’t used when battery is set to “LiIon_Ext_BMS.”

The Classic 250 is good to 3200 watts. Midnite promised a few months ago that they’d have updated firmware to add lithium soon, but we haven’t seen it (and it’s not a big issue anyway).

Higher_wire said:

It sounds like you only need a BMS to communicate with the SMAs.

We recommend the REC 2Q BMS. We offer a kit with contractor and shunt, or you can source your own to save $100. It has the comm cable, although you’ll need a short CAT5 extender if you put the batteries behind the axle. There’s a little bit of programming to do, but it takes maybe 10 min. Plus they’ve been very responsive to every one of our customers’ (aka our) questions, including AC coupling and grid export.

Daly’s smart BMS will also work and will be both cheaper and easier to wire. But also takes a bit of programming and we’ve had a few issues with them. Their team has been pretty responsive, though.

Without comms the SMAs will shut down after a few hours using LFP. They do have their own shunt (behind a cover on the driver’s side of the cabinet), which isn’t used when battery is set to “LiIon_Ext_BMS.”

The Classic 250 is good to 3200 watts. Midnite promised a few months ago that they’d have updated firmware to add lithium soon, but we haven’t seen it (and it’s not a big issue anyway).

Click to expand...

I emailed offgrid and marine/REC in the fall with a few questions on their BMS specifically with DC solar trailer equipment (since its stated to work with sunny island but didn’t expand on its relationship with Midnite classics) and they said they would conduct some testing but never replied again.
Are you using all 3 in unison?

Higher_wire said:

It sounds like you only need a BMS to communicate with the SMAs.

...

Without comms the SMAs will shut down after a few hours using LFP. They do have their own shunt (behind a cover on the driver’s side of the cabinet), which isn’t used when battery is set to “LiIon_Ext_BMS.”

Click to expand...

If Sunny Island is unaware of external charging, it could think battery SoC is low based on Ah used even though voltage has remained high. (With comms, it should get current and/or SoC info from BMS. And be told what voltage or current is desired, allowing for better balancing.)

With SI set to "VRLA", shunt in battery cable and connected to SI sense lines, Midnight Classic connected to SI side of shunt so its current into battery is measured by shunt, the system ought work open loop.

Higher_wire said:

The Classic 250 is good to 3200 watts. Midnite promised a few months ago that they’d have updated firmware to add lithium soon, but we haven’t seen it (and it’s not a big issue anyway).

Click to expand...

Just suitable Bulk, Float, Equalization voltages and times?

Are you using all 3 in unison?

Click to expand...

We use REC for the CAN output. Haven't tried with Daly, though we've gotten them to work with Victron so I'm sure we could get them to work with SMA. The Midnite doesn't need any communication. Simply set voltage in the charge menu. We recommend 3.4v/cell for float and 3.5v/cell for absorb. Set equalization to the same voltage as absorb but turn auto-equalization off. Set temp compensation to 0.

With SI set to "VRLA", shunt in battery cable and connected to SI sense lines, Midnight Classic connected to SI side of shunt so its current into battery is measured by shunt, the system ought work open loop.

Click to expand...

This is a good idea and may work. Only question would be how SMA calculates SOC with LFP having a flat voltage curve. I'm sure they do coulomb counting + voltage + temp considering the charge output bypasses the shunt straight to the lead batteries.

Comms also allow the battery to tell the inverters to shut down prior to cutting power if there's an issue. We've had an over-charge where the Midnite started an equalize cycle (the Midnite must've reset, we definitely didn't miss a setting during commissioning............). The inverter flashed an over-charge fault prior to shutting down, which helped us figure it out.

IMHO, there is no NEED to have comms. Based on my experience, the built-in monitor is coulomb counting as it's shown high SoC for batteries with disturbingly low voltages.

We’ve never gotten a system to run for longer than a couple hours when setting to VRLA. If there’s a way to set it up from the inverter screen then I’d love to know the trick.