24V battery charger recommendations needed

Welcome to the forum Xallero,

I like to do an overall system math check--I am all for a "balanced" system design. Load->Battery Bank sizing; Battery bank->Charging requirements; loads+battery bank+location/sun->solar charging requirements.

Since I don't know your daily loads (by season)--We can skip to the battery bank itself and make some guesses.

A good place to start is a battery bank that supports 2 days of storage and 50% maximum "planned" discharge (makes daily solar charging "easier", longer battery life, good surge power for starting motors/pumps, etc.).

• 390 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,989 WH per day battery draw

That works well to support your DC fridge (may be around 1,000 WH per day), plus LED lighting, RV style water pump, cell phone charging, some laptop charging).

For a Flooded Cell Lead Acid storage battery (FLA), typically suggest around 5% rate of charge (for sunny weekend/non-winter, suggested EQ charging current of 2.5-5%) cabin use, 10%-13% for minimum for full time off grid (10% is usually the minimum recommended factory rate of charge), and 13% works as a cost effective maximum. For folks with gensets and/or poor winter harvest (or shading from trees, etc.), 20%-25% rate of charge maximum (with 13%+ rate of charge using a remote battery temperature sensor to keep bank from overheating).

• 390 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 734 Watt array minimum
• 390 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,469 Watt array nominal
• 390 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,909 Watt array "typical" cost effective maximum
  

And then based on your loads/seasonal usage/location. Fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Fredericton
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 45° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.67	3.59	4.52	4.69	4.56	4.75
Jul	Aug	Sep	Oct	Nov	Dec
4.68	4.73	4.32	3.37	2.39	2.15

Lets toss the bottom three months (assuming using genset when weather/load support needed with poor sun):

• 1,989 WH per day * 1/0.52 off grid AC system eff * 1/3.37 hours of sun (break even October) = 1,135 Watt array

Suggest for base loads (loads that need power 24x7 like fridge and lights)... Use only 50% to 65% of predicted solar harvest (other "optional loads" when sunny weather/running genset):

• 1,100 WH per day (fridge+lights) * 1/0.52 off grid system eff * 1/3.37 hours of sun (Oct) * 1/0.65 base load fudge factor =966 Watt array "base load" 65% of harvest
• 1,100 WH per day (fridge+lights) * 1/0.52 off grid system eff * 1/3.37 hours of sun (Oct) * 1/0.50 base load fudge factor =1,255 Watt array based on 50% of base load harvest.

What is the estimated harvest for October (long term daily average) for your 1,100 Watt array:

• 1,100 Watts * 0.52 off grid system eff * 3.37 average October sun = 1,928 WH per day

Overall, you have a pretty well balanced system... You could justify adding some more solar to 1,469-1,909 Watts array total.

Now to genset charging. 10% and 20% charging numbers:

• 390 AH * 0.10 rate of charge = 39 amps
• 390 AH * 29.0 volts charging * 0.10 rate of charge = 1,131 Watts (10%)
• 390 AH * 0.20 rate of charge = 78 amps
• 390 * 29.0 volts charging * 0.20 rate of charge = 2,260 Watts (20%)

For a standard genset, suggest minimum loading at 50% (below 50% for standard genset (not inverter-generator), fuel economy falls (output power usage drops, fuel usage remains about the same GPH/LPH). And suggest not loading a gense with more than 80% of rated load for longer life, fewer problems (residential type gensets). Inverter-generators can run down towards 25% loading and still be "reasonably" fuel efficient.

• 3,000 Watts * 0.8 genset derating * 0.85 "average" battery charger eff * 1/29 volts charging = ~70 Amps maximum DC charging current from genset
• 3,000 Watts * 0.5 genset derating * 0.85 "average" battery charger eff * 1/29 volts charging = ~44 Amps maximum DC charging current from genset

So, around 44 to 70 amp @ 24 volt battery charger is a good range (decent fuel efficiency when bulk charging, not overheating genset/alternator/wiring during hours of battery charging).

The minimum suggested battery charger output--Would be around 5% rate of charge:

• 390 AH * 0.05 rate of charge = 19.5 Amps (so you can EQ batteries when needed)

To charge from 50% to 80% your battery bank, a 5%/10%/20% battery charger would take:

• 80%-50% = 30% capacity recharge
• 30% / 5% rate of charge = 6 hours
• 30% / 10% rate of charge = 3 hours
• 30% / 20% rate of charge = 1.5 hours

And, based on your 3kW genset, a 5% rate of charge will use (very roughly) 2x more total fuel than 10%+ rate of charge (genset+battery charger not very fuel efficient when well below ~44 Amp/11% rate of charge).

Suggest a good genset usage of charging battery bank from 50% to 80% when you are there... And charging to near 100% when locking up for winter (if you leave solar charging system on and have sun for a week after you leave, solar could take 80% to 90% State of Charge to "full" battery bank (batteries should not be stored below 75% SoC, however daily cycling between 50% and 80% SoC is OK--Just reach >90% SoC once a week to once a month). FLA batteries need recharging about once a month if stored at 25C/77F... At 0C/32F or below, can go 4 months (or longer if colder) without charging.

A suggestion is a 1% to 2% rate of charge solar panel(s) hung on side of building/vertical tilt/etc. to shed snow and avoid driving as much as practicable to help float batteries for long winter. All other loads/AC inverter/etc. should be turned off--Especially if panels are snow covered all winter.

Battery chargers--There are ones that are good for bulk charging (charge at rated current for hours at a time without overheating). And there are (more expensive) charge controllers that can do bulk/absorb/float/temperature compensation better (cold FLA battery bank need higher voltage when they are cold/very cold for full charging).

An example of a "simple" bulk charger (good for your 50% to 80% charging:

https://www.solar-electric.com/ioten25amp24.html
https://www.solar-electric.com/ioen40amp24v.html

And an example of a very nice battery charger (does everything, and has PFC--Power factor corrected AC input--"simple" battery chargers are notorious for having "poor power factor"--AC loading current is higher, but power usage is "relatively" lower--For smaller gensets you can run larger PFC based battery chargers vs the non-PFC versions).

https://www.solar-electric.com/xantrex-804-2420-truecharge2-20-amp-battery-charger.html
https://www.solar-electric.com/xantrex-804-2430-truecharge2-30-amp-battery-charger.html

Lots of stuff here... Before going into exactly what you want/need for a battery charger--Your thoughts and/or correction for my guesses?

-Bill