Replace failed RV fridge with "apartment" fridge & inverter?

Thank you for the follow up. I get it now....

have you thought about a 12 volt fridge

Hi,

If the wiring is properly balanced you can do more than 2. If you wish to understand the "why" surf here:

correctly interconnecting multiple twelve volt batteries

It is true that the more cells there are, statistically there may be more chance of a failure. However, 4 of my house batteries are 9 years old, and 3 are 5 years old.

Recommended charge rate from Trojan is 14.8 volts.

500w/14.8v = 33.78 amps.

If it is a flat install that will be pie in the sky as far as amps go.

As far as having too much coming in from the panels, get a charge controller that allows for diversion loads--and use the excess power to run a water heater element. I do this manually myself--but I'm an energy pig so I rarely am in the position to not use every erg my panels are bringing in.

PNW_Steve wrote:
That does bring up another question:

What is the logic behind avoiding parallel battery setups?

I have been told "2 in parallel if you must but no more".

it is very seldom ... that there is capacity you can't/wont use

your thinking of only the batteries..and calculated charge

your not thinking real world use

lets say you could get 30amps avg out of that 500w 41amp calculated

when the fridge was cooling, you would only see 20amps charge, ten amps going to the fridge
water pump tv ? PC ?
bad cloudy day or two..and you will need all the solar you can get for the batteries IF you are not there to run the generator

in an RV install there is not such thing as too much solar
maybe more than avg use, but NOT more than can be used or sometimes needed
simply not enough roof space for an over large system
i have 505w and wish i had 1000w

Trojan recommends 10 to 13%. Even 13% is hardly a number the Trojan will not accept, and besides, a portion of the solar current is going to go straight into any appliances running when the solar can indeed some close to that recommended range.

There are reports of these large 12v Trojans requiring higher voltages for longer, compared to GC batteries, to max out the SG.

I have a single 12v group 31 ( claimed 130 A/h) that will happily gobble up 65 initial amps when depleted. It recommends a 10% rate, which would be 13 amps. My solar can just do this for a few hours on a cool June21st, and with everything electric turned off.

Charging rate maximums, IMO. apply to Gel batteries, and to lower end AGM's which say no more than 30%. When the next discharge cycle begins just before sundown, gosta do everything one can to get them near 100% before then.

That does bring up another question:

What is the logic behind avoiding parallel battery setups?

I have been told "2 in parallel if you must but no more".

Battery manufacturer recommended charge rate of <10% of the 20 hour discharge capacity.

With 2 Trojan T1275 batteries in parallel I will have a combined 20 hour rate capacity of 300 AH.

10% of 300 = 30 amps max.

I said that the 500 watt system would produce more than the batteries could use. NOT that it would "overcharge" them.

500 watts @ 12 volts = 41.67 amps.

41.67 is somewhat greater than 30.......

If I am going to go with the 500 watt solar I will need a battery bank that can take the higher charge rate. Either AGM batteries or larger flooded deep cycle. 4 T105s in series (24 volt system) would be suited to a 22 amp charge rate.

500 watts @ 24 volts= 20.83 amps

I understand that the 500 watt system will only produce 500 watts under ideal conditions but I would think it best not to purchase and install capacity that I cannot use.

What batteries are looking at? I really don't understand how you could have enough battery AH for the fridge if it won't accept the current from a 400 Watt solar array, maybe 20 Amps or so. (Isc X Voc)

I have 4 GCs that will yield supposedly 484 AH. The theoretical C20 charging rate is 96.8 Amps. (484 X 20%) I have my Xantrex inverter/charger set for 96 Amps.

My 420 Watts of battery seldom provide much over 20 Amps, about a 5th of max charge rate.

If I were to run a residential fridge in our rig (the plan if/when the Norcold gives up the ghost) I'd add more panels to give us somewhere around 600-750 watts total. I would also consider adding a couple of AGMs and feed them separately with a couple of panels and inverter.

over charge is NOT an issue
batteries are self limiting .. within correct applied charge voltage
charge controller will Not over charge batteries

what is at issue is batteries ability to supply power during insufficient solar output and not fall below 50% of capacity
and then be fully recharged during the Solar Day, while at the same time Solar is also supplying the power to keep the fridge going

basic example if fridge uses 100Ahr daily
given 5hr max charge rate
solar must supply fridge for 5 hrs and in that 5 hours also recharge 19 hrs of battery use, which will require more than 100ampHrs
don't forget lights, water pump, TV, mobile device chargers etc..
total use plus 10% minimum for recharge conversion losses

if you take out 150ampHrs your charging must equal at least 165ampHrs

this is a very simplified example
solar will begin at sun up with low batteries and low solar
but will taper off before sundown because the low solar late afternoon hrs cannot produce enough to add More charge to batteries that have been charging all day

but at 7am with 3amps solar the batteries will still be discharging while the fridge is running and only charging when the fridge is Not actively cooling, if the inverter pulls 9amps running the fridge (9-3=6) the batteries will be supplying 6amps of the needed power at 7am

the amp numbers may not match your location panel wattage, but the example is correct

at solar noon max power will be applied while fridge is not cooling,
But will only receive partial charge rate while fridge is cooling ..(Max - fridge use) left over goes to batteries
so batteries get full solar rate about 50%~60% per hour the other 40~50% they get less than full rate

500w will not overcharge your batteries, but a larger battery bank that can more easily accept the full rate when available will be more efficient and work better and last longer