Electric mower used as additional solar storage.

[mrmrm]

I just bought a Cub Cadet RZT-S Zero electric lawn mower off craigslist with dead batteries. I'd like to replace the 4 bad AGM batteries with LifePo4 to increase the storage for my home when the mower is not in use.

Current Setup:
2x 5000es
3x EG4 48v 100ah rack mounted.

Mower new batteries:
4x Vestwoods 12v 100ah batteries (these are the only ones that fit the group 27 agm brackets in the mower)

Question:
I assume I'm good just paralleling the "mower pack" into my EG4 rack bus bar. I understand there will probably be some inefficiency or loss due to slightly longer cables going to the mower? But my main question is if I need to match the voltage of the 2 packs (15.3kwh EG4s and 5.12kwh mower pack) when hooking them up to be charged and used for the rest of the week.

In other words can I just wire in an anderson connector to my EG4 pack, unplug the mower, use it, and then just plug it back in so they can balance? Or would the lower voltage of the mower pack then do damage to the EG4s?

 

[Hedges]

Better have them pretty close to same voltage before paralleling. Otherwise, even if not massive current, could still be well over allowed charge rate.
They could of course be balanced through a resistor, but with power dissipation.
Ideal solution is inverter having multiple battery inputs (I see that for high-voltage battery inverters, not low voltage.)
Another would be DC/DC converter, in this case 48V to 48V.
Simplest is to run the high one down on inverter until its voltage matches the low one. But, probably mower is fully charged, and inverter battery is not. So instead wait until it is charged up to the same. Store mower partially charged? May be best for battery lifespan anyway. Especially if yard shed is hot.

 

[mrmrm]

@Hedges That's what I figured. Any opinion on what a "safe" range would be to parallel? I'm assuming within .1 volt is recommended.

 

[Hedges]

IR ~ 0.17 milliohm per cell x 16 cells = 0.0027 ohms
50A * 0.0027 ohms = 0.136V

Yeah, about that.
Actually, two sets of 16 cells in series, so twice the resistance and half the current.

 

[NorMI]

I have a Ryobi 48V that the original lead finally kicked after the third year, and I put Battleborn's I had left over from an old project in their place. The problem I had (and anticipated) was that turning on the key would close the contactor and charge up the caps in motor controller, immediately tripping the BMS on the 4 battleborns. So I put a precharge circuit across the contactor to protect the BMS. I would not parallel batteries from different age/batch/mfr, sounds like a recipe for disaster. I'd just eat the losses and use a DC/DC buck boost converter to use the mower as a bidirectional slave pack for your main system, that's what I plan to do when I get my next 48V system set up - plus I ordered a 15S rack mount battery and the mower is 16S, so it's really the only feasible option in that specific case - but realistically using a DC/DC interface is the only thing I'd personally ever do to slave together dissimilar packs like that anyways...

 

[Hedges]

Or high-current diodes could be used to parallel two packs onto one load.

 

[cs1234]

If you want to go some distance and don't mind some conversion losses a small inverter and an ac/dc charger would work also.

 

[wattmatters]

Neat idea to use the extra capacity for double duty.

Instead of V2H, you'll have M2H.

@Hedges That's what I figured. Any opinion on what a "safe" range would be to parallel? I'm assuming within .1 volt is recommended.

And that's pretty easy to achieve with LiFePO4 cells as the voltage is so flat between 20% and 80% state of charge. They will naturally self balance with the other battery packs when in parallel. Assuming of course packs in parallel are all a 16s cell configuration.

I would have a suitably rated DC disconnect in line so you are not plugging in a "live" Anderson connector.

 

[cs1234]

I don't know, I like the idea of a small inverter and ac/dc charger myself. Then you have a backup "generator" always ready to go, and it's on wheels.

 

[NorMI]

Meanwell makes a awesome bidirectional charger/power supply that I've been eying up for a similar project, since my Magnum MS4448PAE's are bidirectional they play well with V2G stuff like this: https://www.meanwell.com/newsInfo.aspx?c=1&i=984 Certainly a cool option, I was considering this would be more appropriate for greenfield work.

The main thing with paralleling unmatched batteries is the uneven load share, with the lowest IR/ESR string(s) taking more of the brunt. But, the mower in this case as the same nominal capacity, and on a longer lead it will just "chip in" while the rack batteries on their shorter leads serve as the primary, so in this case the simple parallel just takes some pressure off the house batteries in theory, while during charge/discharge they won't be at wildly different state of charge as you move up and down throughout the day at least. IF it was say a 50AH mower and 100AH house battery strings, very different story.

 

[fromport]

Meanwell makes a awesome bidirectional charger/power supply that I've been eying up for a similar project, since my Magnum MS4448PAE's are bidirectional they play well with V2G stuff like this: https://www.meanwell.com/newsInfo.aspx?c=1&i=984 Certainly a cool option, I was considering this would be more appropriate for greenfield work.

Did you see the video and the phase difference between charging vs backfeeding?


Current 180 degrees out of phase with Voltage.
That is not going to work nicely in any residential setup imo.

 

[NorMI]

Figures they'd cheap out on the regen, what a bunch of $&%@! ah well...

 

[mrmrm]

@wattmatters Yes that's a good point about the disconnect.

At this point I'm exploring 2 options:

1. Just match voltage and parallel them as planned. I guess I'm still not sure what damage I'm risking. Damage to my mower cells? Damage to my main EG4 rack? Just reduced number of life cycles in the mower pack?

2. Buy a growatt 3000 and just have a separate charger/inverter to be used as a redundant backup.

Leaning towards #1 at the moment. I know I'm not the first person to parallel dissimilar battery brands. All the cells are new at this point.

 

[Vigo]

So, there are actually numerous lifepo4 batts with the group 27 footprint, it's just that most of them will be larger than 100ah. I have some 140s that are 'group 27' shaped.

As far as paralleling them.. with lifepo4 there are more concerns which has already been mentioned. But i've been paralleling my 48v lead acid golf cart directly to my 48v lead acid house pack for months with NO management/protection of any kind whatsoever. But the stakes are lower, i guess. The golf cart never leaves my ~5acres and if it had a range more than 2 miles i'd never know it. It usually travels half a mile and gets plugged back in. Also it's a hideous scabbed together POS with no framerails left (unless you count the reinforcements i welded on) and the 48v pack is 4 group 27 marine batteries, total $400 at full retail and they already half paid for themselves in other use before i ever put them in the cart. So as long as i don't think they're ruining my house batteries i almost don't care how 'bad' they get just being paralleled in.

I think if you aren't trying to incorporate high c-rate charging as one of the advantages of having the mower hooked to the house, i would just do it through AC devices. I have this type of arrangement at my house between large 48v and 12v systems. The 12v can charge from the 48v at ~600w, and it can feed back to the 48v side at ~1000w. It's controlled through a silly assortment of timer switches, voltage window switches with delay, etc etc. But there are inverters at both ends, running power supplies/chargers. So it's all DC to AC to DC either direction.

If you don't need a fast charge rate on the mower you can just permanently install a small battery charger onboard and leave it. Add a timer switch to only charge during the day. If you want it to feed back to house you can use an inverter and a large charger/power supply. One thing i've done is buy an adjustable power supply and set it to the voltage i want power to start transferring. I.e. hook up a power supply set to ~48.4v (i really did use this number so this is something i've done and measured) to your 48v pack. When your pack is above 48.4 the power supply does nothing but idle consumption (4w on the one i used) and when your house pack drops below 48.4 it will start contributing power until it maxes out its current rating at a few tenths of a volt difference. So at 48.3 it might be doing less than half its current rating, but by 48.0 it's 'all in' doing whatever its current limit is, and when house pack rises about 48.4 it resumes doing nothing. I've done about 1000w max continuous in the 12v to 48v direction, but working on taking that up to like ~1700w continuous and adding more controls based on how much solar the 12v system is getting blah blah blah. I don't want to write too long an essay but i've done stuff like this between my 48v and 12v systems and with enough renditions and tinkering it works fine. If anything, 48v to 48v would be slightly less inefficient.

 

[wattmatters]

1. Just match voltage and parallel them as planned. I guess I'm still not sure what damage I'm risking. Damage to my mower cells? Damage to my main EG4 rack? Just reduced number of life cycles in the mower pack?

I don't really see the problem. Provided the batteries are all 16S configuration (and polarity is correct!) then when connected in parallel the packs will sort themselves out and self balance. Even if SOC is quite different but within that 20-80% range they will be fine.

Just have a suitably rated in-line fuse (and/or disconnect as discussed) and the fuse will prevent any damage to batteries in case of current overload. The BMS's in each battery will also shut off a battery in case of an over current scenario.

If you look past the schtick, have a look at these two experiments on paralleling batteries:

i. paralleling a full LiFePO4 battery with an empty one:

ii. paralleling a 304 Ah battery with a 5 Ah battery:

I can see a suitably rated DC disconnect (e.g. 125 A) NoArk or something like that mounted on the wall near your main home battery system, with one side connected to an Anderson plug, the other connected to your system's main DC bus bar. Add an HRC fuse in line for redundancy. This sort of thing:

NOARK Electric: DC MCCB based Switch Disconnector Ex9M1SD DC125 3P

 

[mrmrm]

@wattmatters Agreed. I've seen both of those videos and even @Will Prowse has several videos with frankenstein mixes of different brand and quality cells in parallel. The worst I can see is that it's possible is my vestwoods batteries would be drawn down faster because of the slightly longer cables, correct?

 

[Delmar]

I don't know, I like the idea of a small inverter and ac/dc charger myself. Then you have a backup "generator" always ready to go, and it's on wheels.

Ditto, my self-propelled power station is unbelievably handy because hell will eventually freeze over (see pic). Add a solar panel roof for comfort shade and to extend the range. Plus you have a backup inverter should your primary inverter(s) go POOF.