Keeping batteries charged over the winter

Rybren
Rybren Solar Expert Posts: 351 ✭✭
I'm in the process of trying to help out a neighbour at my cottage.  He has 6x230W roof mounted panels that charge a ~550 AH 24V battery bank.  The property will be vacant for the winter and no one will be around to clear the snow off the panels (they are set at a fairly shallow angle of 30 to 40 above horizontal and we get a lot of snow)

All loads will be disconnected from the battery bank prior to leaving and he wants to be able to raise one or two panels to a near vertical position to shed the snow. He has reconfigured one panel so that the snow will fall off the roof and not accumulate.

I'm concerned that a single 230W panel (Vmp 29.80V Imp 7.71A) won't be enough to keep the batteries charged and that he'd be better off with 2. It would be almost impossible to set up more than 2 panels so that the snow wouldn't accumulate at the base.

Thoughts?
Thanks.

Comments

  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    < 1ma/AH is typically enough to maintain charge on a disconnected battery.   So less than 1A for that battery bank.

    I am available for custom hardware/firmware development

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    I am not the expert here... But the typical talking points:
    • have battery bank fully charged before leaving--At room temperature will last 30 days between charges (flooded cell). Will not freeze unless ridiculously cold (below -50F if near full charge).
    • If AGM, fully charged AGMs should last 6 months between charges at 75F/25C. If if partial charged, generally no damage from freezing (must thaw to recharge/discharge).
    • generally, the rule of 2x change in life/behavior for every 10C change, when batteries are cold, they will self discharge much slower... At 25C-20C or 5C, instead of 1 month between charging (self discharge), 2^2 = 4 months between charging.
    • Float with 1% to 2% rate of charge at 25C, at lower temperatures, less float current needed (note: at 2% or higher float charge, really need a charge controller to avoid over charging). 
    • 550 AH * 35 Volts Vmp (assuming PWM controller) * 0.01 rate of charge = 193 Watt array minimum (if forklift/traction batteries, recommend 2% minimum float array because of higher self discharge).
    • Turn off all loads to battery bank (any loads, like an unloaded AC inverter, etc.) can take bank flat pretty quickly (obviously depends on loads, sun, array size, etc.).
    • Mount panel(s) on vertical surface high on side of building to avoid drifting snow (if possible).
    • Generally, recommend Vmp-array~35-40 volts for PWM controller... A Vmp~30 volt array (typical for larger Watt panels, although other Vmp ratings can be available) is usually too low to properly charge a 24 volt battery bank (lead acid bank needs near 30 volts to float if below zero). As panels heat (cells>25C), the array voltage falls.
    • A pair of "12 volt" (Vmp~17.5 volts per panel) ~100-140 Watt panels in series may be a better fit on side of cabin and deliver better charging voltage (or 4x 140 Watt panels for ~2%+ rate of charge.
    • Larger MPPT controllers may take enough power during dark (say array is snow covered for 3 months) to discharge a battery bank. A Midnite Classic max draw is 2.5 Watts: 2.5 Watts * 24 hours * 90 days * 1/24 volt bus = 225 Watt*Hours or almost 1/2 the bank capacity. Having a smaller PWM controller with ~200-400 Watt array (Vmp-array~36 volts), would run less of a risk of discharging the bank during heavy snow seasons. In cases like this, if MPPT controller--Might be better to simply turn off and let the battery bank get "cold"--And recharge (genset/solar) when return after the place is accessible.
    Where is the place (roughly)--Different places have different snow accumulation patterns and "stickiness".

    Questions, corrections from people who know what they are talking about, etc.?  :*

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    Thank you for your thoughts and comments. Here is some amplifying info...

    -Battery Bank is FLA (Surette 550)
    -Controller is an Outback MPPT, no PWM available
    -Site location is just north of Ottawa, Canada
    -Winter Temps can hit -40 but -10C to -30C is more typical with occasional periods above freezing
    -Total snowfall typically exceeds 10 feet with accumulations of 5 or more feet common.
    -Battery Bank will be fully charged before departure and site will be unoccupied for ~4.5 months


  • 706jim
    706jim Solar Expert Posts: 514 ✭✭✭✭
    My island system would have the panels snow covered for two to three months in winter. Batteries were charged before the panels were covered. Those 12 GC's lasted 19 seasons. So I'm thinking you could just leave them alone so long as they were charged.

    BTW, I (and others here) have taken the panels off the roof. With panels on the ground, you have a lot more options as to placing them vertical (or even beyond vertical to catch sun reflection off of the snow) plus the relative ease of checking output, connections etc.
    Island cottage solar system with 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 30th year.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    https://www.solar-electric.com/outback-power-flexmax-fm80-150-mppt-charge-controller.html
    • Standby power consumption: typically < 1 Watt
    So, taking that number:
    • 1 watt * 24 hours * 30 days per month * 4.5 month snow = 3,240 Watt*Hours
    • 3,240 WH / 24 volt bus = 135 Amp hours
    • 135 AH / 550 AH battery bank = 0.25 = 25% discharge
    So, if there is a risk of ~4.5 months of snow on panels, then leaving the FM X0 MPPT controller connected may not be the best idea.

    If using 2x 140 Watt "12 volt" panels in series:
    • 2 * 140 Watt * 1/17.5 volts Vmp = 16 Amps
    Get 2x ~140 Watt panels (mounted vertically) and a decent 20 Amp PWM controller, would be one solution... Set up to no equalize charging (don't want to boil batteries dry when nobody is using power/there to check the electrolyte levels.

    https://www.solar-electric.com/midnite-solar-brat-pwm-solar-charge-controller.html ($90 Midnite Brat 30 amp PWM charge controller)

    Should consume much less than 1 Watt in "dark"/standby... But probably would not hurt to ask Midnite or NAWS.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    I have 450 ah of FLA batteries (12 vdc)on an off grid installation that can sit vacant for 6 months in the winter.  I have two old 63 watt panels wall mounted on the building. The batteries are always happy in the spring, after decades of doing this.  Consider that keeping the batteries in float takes little power.  I think your 250 watts is going to be plenty.

    Tony
  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    Thank you everyone.  I will pass on the info to my friend.

    This is the first year that it will be an issue for him.  In previous years his brother would spend the winter at the lake and would routinely clear the panels of snow, but the brother will be 'snowbirding' it this year and won't be around.