Need help with system!

javid

I really could use some constructive criticism with my newly installed system.  I should have asked these questions before initial set-up I realize, but better late than never I guess.  Unfortunately, I am one of those people who suffer from analysis paralysis – so for a lot of projects I need to dive right in and fix it up later – I know that is not ideal, but I have found this is only way I get started…

48 Volt system.  Charge controller is Midnite Solar Kid.  Inverter is Exeltech XP600.  Solar Panels are Solar World SW255 (nameplate Voc 38.7 and 9.05A) – 3 of them in series.

Batteries are 6-year old Interstate DCM0075U (12v 75AH AGM) in 2 strings – so total of 150AH @ 48V.  Batteries are from some backup system – before installation their resting voltage was 12.7-12.8, so I think they still have some life in them.  I could use some guidance on what absorb and float setting to put into Kid.  The spec sheet for the batteries says “Float 13.5-13.8.  Cycle 14.4-15.0”.  Currently I have them set for Absorb 57.6V (120 minutes) and Float 54.0V.  I don’t believe these batteries should be equalized, so I set that at 57.6V as well.

My current loads on this system are minimal – running/charging a couple of laptops, charging phones, charging some cordless tools and running a few fans when it gets hot in cabin.  Additional loads that will be added soon are a small 7cubic foot chest freezer that will be used as fridge.  No additional loads expected – this is for seasonal cabin.  I expect daily loads to be around 800WH when freezer gets added.

Wiring is as follows:

-        Panels to Kid CC – 50’ of 10awg PV wire going through 15A MNEPV Breaker on + wire (breaker in BigBabyBox)

-        CC to pos and neg bus bar in Big Baby Box – 10awg wire with positive going through 30A MNEPV Breaker.

-        Batteries connected in series with 4AWG wire and each string connected to pos/neg bus bars with 6’ of 8awg wire.  30A MRBF fuses between positive string and 8awg wire.

-        Inverter connected to pos/neg bus bars with pos wire going through 30A MNEPV breaker – 8 AWG wire for this.

I have attached rudimentary wiring diagram – hopefully it makes sense. 

Currently, panels are not grounded – they are on temp mount practically on the ground.  I have current ground set up with 8 AWG copper to 8’ ground rod in moist area right outside shed.

Questions:

-        Big Red Flags?

-        How are my CC settings so I don’t ruin batteries?

-        Sizing of panels to batteries to loads balanced?

-        Are my wiring sizes correct?

-        I am wondering if my current inverter will be able to handle starting small chest freezer – I went with smallish inverter because of small constant drain (8w).  Will my wiring sizes impact this at all or will inverter limits be the issue?

Sorry this is so long!  Thanks for any help!

Estragon

I don't see any big red flags.  Do you have a generator for longish spells of gloomy weather?

I prefer heavier wire from CC to bank to minimize voltage drop (which can fool the CC into thinking it's charging at a higher voltage than it really is and lead to chronic undercharge).  If this circuit is fairly short, with the currents involved in your setup the 10awg may be okay though.  You may want to confirm by checking voltage at the CC output terminals and the bank while the CC is near max output.  If they're the same, no issue.

Is there a remote temp sensor on the bank?

I'm dubious about a 600w inverter for a freezer.  As a fridge, the duty cycle (and daily watt-hours) will be reduced, but running and starting current will be the same as a freezer.  Starting current required might be too high?  Seems like most folks around here use min 1000-1500w inverters for standard AC compressor fridges/freezers.

javid

Thanks a lot for looking over my system and providing input!

I do have a honda EU2000 generator - not sure where I would hook up to the system though - still to be figured out - usually gloomy weather here is late fall, so I have some time.

There is a remote temp sensor on the bank from the Midnite Kid.

The wire run from the CC to BigBabyBox (where pos/negative bus bars are located) is 18".  From Bus bars to batteries is 6' of 8awg wire.  I will compare voltage next time during bulk cycle.

What do you think of my settings for Kid? - absorb and float and time for absorb?  Maybe because I am not using much power these days, but it doesn't seem like CC is putting much into batteries and often spends a lot of time in float.  I want to avoid chronic undercharge - any way to check for that?

Thanks for your help!

Estragon

Unfortunately, it's harder to catch chronic undercharging with AGMs.  Watering and checking/logging SG with flooded can be a pain, but it does help a lot in catching charging issues early.  With relatively low usage and a long float, you're likely fine.  If/when you start cycling more deeply, you may want to nudge the absorb voltage up a tenth or two.

The way I use my generators on my 12v bank is normally to only bulk charge with relatively cheap "dumb" chargers, and let whatever solar there is do absorb to the extent possible.  Running the bank down to <50% SOC regularly, and/or letting it sit at <80% for long periods is what really hurts.  Absorb starts at ~80-85%, so I stop the genny when charge current starts dropping.  My 48v inverters have 3-stage chargers built in, but I normally just run a bulk with them as well.  In fall/winter, in really gloomy weather and short days, I make sure to get a full absorb one way or another at least weekly or so.

The honda has DC output, but it's pretty useless for charging batteries of any size.  You'll likely want to get an AC charger to plug in to it.  I use Iotas , but pretty much anything that will do ~15-20adc would do.  That's only ~2aac, so you'll want to put the honda in eco mode once it warms up if no other loads.

mike95490

I would guess surplus/used AGM's that are 6 years old, are pretty much end of life.  AGM don't last as long as flooded, and backup batteries are not really designed for daily cycle use (even if they have a sticker on them)

javid

Good news - the chest freezer has been installed for 12 hours and seems to be working okay so far with smallish inverter..

I know batteries may not last long - I got 9 of them for $250, so not much to lose and gives me some time to figure things out and make some mistakes.  Plus, with my low loads, it seems like they should be able to work for a while.   I can probably get $125 for them in scrap value... 

My next batteries will be Lithium - in fact I have two ebikes each with 48v 24.8 ah batteries in them that I would like to integrate into the system at some point for added storage.  Not sure if that is even possible...

Estragon

Good to know about the inverter (that it will start a chest freezer). 

One thing to watch out for is if the bank gets to a lower state of charge, voltage into the inverter could sag during compressor cycle startup below the low battery cut-off, in which case the inverter may stop.  Some will auto-restart, some need manual rebooting.  Not sure about yours.  Anyway, just something to watch out for before you fill it with a bunch of expensive food.

javid

Thanks for the feedback about making sure the inverter doesn't do the low battery cut-off - I will read up in the manual..

BB.

600 Watts is very close for "reliably" starting a compressor fridge/freezer... And closer to 1,200 to 1,500 Watts for a full size energy start refrigerator/freezer.

As Estragon warns, starting surge current support may be an issue as batteries get discharged--Making sure you have short/heavy cables with less than 0.5 volt drop (for 12 volt battery bank) or 1.0 volt max drop (for 24 volt battery bank), helps too.

Also, for flooded cell batteries, suggest that battery bank AH be at least 200 AH per 1,000 Watt of inverter rating (i.e, 1,500 Watt would be a minimum of 300 AH @ 24 volt battery bank)--To reliably supply surge current over time, state of charge, temperature, age, etc.

AGM, Li Ion, and other batteries can have better, or worse surge capabilities.

-Bill