Noob needs help sizing for 24v battery bank charging.

BB. wrote: »

OK... It looks like these are 12 volt @ 130 AH batteries.

You run them at 24 volts--Lets see what a "24 volt" solar power system would look like.

First, normally I would suggest 5% to 13% as being a "reasonable" solar rate of charge--You can go lower, but for best battery life, you would need to monitor their state of charge and probably plug them into a higher capacity charger once a month or so...

Any, solar charging:

• 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.05 rate of charge = 203 Watt array minimum
• 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.10 rate of charge = 405 Watt array nominal
• 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.05 rate of charge = 527 Watt array "cost effective" maximum

Obviously, you are looking for a low cost solution, so a pair of "12 volt" ~100 Watt panels would be a good start at a "standalone" solar charging system (note: "12 volt" panels have Vmp~17.5-18.6 volt ratings each).

Using PV Watts for Milwaukee Wisconsin, array mounted flat to roof (OK for summer, but for winter, may be issues with low sun angle):

Month    Solar Radiation (kWh/m 2/day)
1      1.89     
2      2.61     
3      3.48     
4      4.79     
5      5.86     
6      6.55     
7      6.10     
8      5.44     
9      4.28     
10      2.94     
11      1.80     
12      1.44     
Year      3.94      

Tossing the bottom three months, we get 2.61 hours of sun per day (February). Lets say you are really "hard" on your batteries and discharge them to 20% State of Charge (near dead):

• 24 volts * 130 AH * 0.80 charging capacity = 2,496 Watt*Hours energy to recharge
• 2,496 WH energy / (203 W array * 0.77 panel+controller eff * 2.61 hours sun per day) = 6.1 days to recharge

If you only discharge the batteries by 50% (really better for longer battery life):

• 24 volts * 130 AH * 0.50 charging capacity = 1,560 Watt*Hours energy to recharge
• 1,560 WH energy / (203 W array * 0.77 panel+controller eff * 2.61 hours sun per day) = 3.8 days to recharge

So--Just a back of the envelope calculation, if you could fit two solar panels on the top of your truck--It appears that you have the makings of a stand-alone recharging system that would work for ~9 months of the year.

You would have to shop around to find ~100 Watt panels at $1 per Watt--Typically they are closer to $2 per Watt (plus a lot of money for shipping and handling sometimes--Always check the price to your front door).

And a question if you can fit two ~100 watt panels on the top of your truck.

Another way to go would be to recharge from your truck--But it may be better to use a 12 VDC to 24 VDC battery charger or a 120 VAC inverter to 12 VDC battery charger.

Normally, car/truck alternators are not designed to recharge at the higher voltages that deep cycle batteries like to see (~14.75+ volts for flooded cell batteries at room temperatures)--And vehicle alternators, even if they are rated at 150 Amps, they tend to fold back on their output current as the alternators get hot.

Also, how long is the drive? You are looking at 4-6 hours (or more) to recharge your battery bank if 50% discharged. Of course, you can do heavy charging for a couple of hours when driving, and let the solar array finish charge the next 3-5 days.

Anyway--That is how I would start. How much did I guess right? Does this look like anything you can use?

-Bill