why 24v battery system?

[Flagbag]

In the sticky by Sunking, 'Solar off grid battery design' step 5 is:

5 DETERMINE BATTERY SIZE and VOLTAGE

· First determine battery Voltage. This is based on Panel Wattage
· 0 to 600 watts = 12 Volts or higher
· 601 to 2000 watts = 24 volts or higher
· 2001 to 5000 watts = 48 volts or higher
· 5001 watts and higher = Forget it or higher than 48 volts. You cannot afford it.

We have panels rated at 800W on what is a 12V battery bank, but we're willing to change since our batteries are due to be changed.

The skicky suggests 24v for 800W panel size. I'm just trying to understand 'why' a 24v system..? Or, put another way... What one loses if continuing to use a 12v system with 800W panels?

[SunEagle]

In the sticky by Sunking, 'Solar off grid battery design' step 5 is:

5 DETERMINE BATTERY SIZE and VOLTAGE

· First determine battery Voltage. This is based on Panel Wattage
· 0 to 600 watts = 12 Volts or higher
· 601 to 2000 watts = 24 volts or higher
· 2001 to 5000 watts = 48 volts or higher
· 5001 watts and higher = Forget it or higher than 48 volts. You cannot afford it.

We have panels rated at 800W on what is a 12V battery bank, but we're willing to change since our batteries are due to be changed.

The skicky suggests 24v for 800W panel size. I'm just trying to understand 'why' a 24v system..? Or, put another way... What one loses if continuing to use a 12v system with 800W panels?

The higher the panel wattage of your system the more amps it generates to charge your batteries. That requires a higher amp rated charge controller.

Also going with a higher voltage battery system (ie. 12v to 24v) will allow you to use a smaller wire size and longer cable runs due to less of a voltage drop.

Quick calculation: 800 watts / 12volts = 67amps or 800 watts / 24volts = 33.3 amps.

[sensij]

Also, the battery design will be more reliable and/or less expensive by going to a higher voltage, and generally the panel wattage correlates strongly to battery capacity.

Let's say you have four 6V batteries, with 208 Ah each. The total energy capacity is 4 * 6 * 208 = 4992 Wh no matter how they are wired. If you go 12 V, you would need to put them two in series, in parallel. For a 24 V system, all four batteries would be in series. With the same cyclic load on each, the 24 V battery is likely to last longer. Note that C/10 in this case is 500 W, which is right at the decision point between 12 V and 24 V. More capacity will need more wattage to charge, generally pushing into 24 V territory.

In your current system, you have 12 x 6V batteries, wired for 12 V. That is six parallel strings, way too many to keep balanced, which will ultimately hurt the battery life. If you stay with 12 V, and want similar energy capacity (~15 kWh), the only real alternative is to put six 2 V batteries in series, with something approaching 1250 Ah. Better than 6 parallel strings, but expensive, although at $350 / ea for the 1110 Ah battery, not a terrible choice (if 13.3 kWh is enough).

Switching to 24 V, you can get that same 15 kWh by putting in two strings of four 6V 312 Ah batteries (8 total). Only two parallel strings will be much better than six strings, although you'll still be looking at $2500 for good batteries.

[Flagbag]

Thanks SunEagle and Senji for the great replies and analysis. While still getting my head wrapped around it to follow your math and examples, I wanted to reply now and thank-you. I am seeing the benefits of a 24v system over the 12v we now have and seeing the cost/benefit relationship. If nothing else you are teaching me why my late step father went the 6V golf cart battery route (twelve total rigged for 12V system); and that was understandably to save money at the expense of reliability and longevity of the batteries.

Thank-you.

Now I am trying to correlate this knowledge about battery choices with what we have, and will continue to use as a PV array.

Our batteries are due to be changed within the next year, so we have to meet that expense either with new 6V Golf Carts (to save money short-term) or the better but more expensive alternatives you so kindly present, above.

Knowing I have to get new batteries but cannot afford to change the PV array, does that change anything? Especially, I wonder about having different sized panels. I have tried looking-up info about mis-matched panels and am finding conflicting opinions.

	Pm	Vmp	Voc	Imp	Isc
Panel #1	100W	17.5V	21V	5.71A	6.4A
Panel #2	100W	17.5V	21V	5.71A	6.4A
Panel #3	205W	18.4V	22.8	11.15A	12.1A
Panel #4	205W	18.4V	22.8	11.15A	12.1A
Panel #5	205W	18.4V	22.8	11.15A	12.1A

again, Gracias Amigos