Battery max charge rate?

Re: Battery max charge rate?

You are probably helping your customers rather than hurting them. Solar panels are so much cheaper than a decade ago, and they last for 20+ years--It is hard not to have a larger array vs more generator run time in poor weather.

In the end, if your customers are happy and sucessfully running their homes--And the batteries are lasting--I am not going to complain.

With a larger array (greater than ~13% rate of charge), I would highly suggest installing a Remote Battery Temperature Sensor. Both optimizes charging (cold batteries use higher charging voltage) and better safety (hot batteries need lower charging temperatures--With high charging currents and a cold charge controller--Internal Batt Temp Sensor--Hotter battery, lower charging voltage, possible thermal run-away). Not all charge controller support a RBTS--So that will be part of the hardware selection process. With an RBTS, you can charge upwards of 20-25% rate of charge pretty easily/safely (as always, review battery mfg. manuals for details).

In general, we have a set of rules of thumbs that help design a "balanced" solar system. 1-3 days of storage and 50% maximum discharge. 2 days + 50% discharge (4x daily load) seems to be a good compromise. There are lots of reasons that it ends up working out well--Including better surge capacity (starting well pumps), usually keeps daily cycling between 25% and 50% nominally for good battery life (cycling less than 10-20% or deep cycling below 50% can reduce battery life). Also, when the sun does shine, a larger battery bank can store the available good solar days more quickly (batteries that are less than 80% state of charge are more efficiently charged and can take higher charging current than 90% or higher SOC--more heat/hydrogen production).

Also, when discharging batteries, C/20 works out pretty well for most installations. I.e., 5 hours an evening/morning discharge for 2 days to 50%. If you do a C/10 rate that is 5 hours of discharge to 50% capacity--C/10 rate battery is less efficient (apparent battery AH capacity is less at C/10 vs C/20 rates).

For batteries, the rules of thumbs I like to use (based on flooded cell batteries, but still works well for AGM too):

• C/20 = Battery AH capacity used for charging/disch. arging rules of thumbs
• C/20 = "Average" discharge rate
• C/10 = "Heavy" discharge rate
• C/8 = "Max continuous load
• C/5 = "Short Term" very heavy loads (well pump? for a few minutes at a time)
• C/2.5 = "Max Surge" load (few seconds to minute or so)
• C/20 (5%) = Minimum rate of charge (charge during day, use power at night)
• C/10 (10%) = Nominal rate of charge--Works well for most people and some loads during day
• C/8 (13%) = Max "cost effective" rate of charge--Batteries quickly recharged and back in float long before sun sets, supports more daytime loads, RBTS not needed (still helpful)
• C/8-C/4 (13-25%) = Usually used for Generator chargers--Quickly recharge a battery bank to >80% SOC--Then let solar continue charge to float (RBTS highly recommended).
• 100 amps -- Try to keep nominal DC current at or below 100 amps
• 60-80 amps -- Generally the maximum charging current for most larger solar charge controllers

For example--I use 3.3 kWH per day for a very energy efficient home with Energy Star Refrigerator, lights, well pump, washing machine, laptop, radio+TV. About the minimum size recommended for a "near normal" electric life. Here how the numbers work out:

• 3,300 WH per day * 1/0.85 AC Inverter Eff * 1/24 volt battery bank * 2 days of storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank

That works out nicely with 3 parallel strings of 6 volt ~220 AH golf cart batteries (3parallel * 4series = 12 batteries). This is great for a less expensive "starter" battery bank. Last 3-5 years and let people learn how to take care of the bank. Normally, I suggest one series string as ideal, and not more than 3 parallel strings (more strings, more connections, more cells to check/fills, more things to go wrong, harder to keep battery currents balanced, etc.).

At a 10% rate of charge, that is ~65 amps of charging current--Just about the maximum a single 60-80 MPPT charge controller can manage... You can always go 48 volt battery bank to use a larger array and keep the costs/simplicity of one solar charge controller (and uses larger AH capacity cells to reduce number of parallel strings).

Next is sizing the solar array for size of battery bank--A larger battery bank needs a larger solar array to properly charge. Generally 5% is the absolute minimum charging current (charge during day, use power at night)... But for a daily cycling off grid cabin, a 10%+ rate of charge is highly recommended:

• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,237 Watt array minimum
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,474 Watt array nominal
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,217 Watt array "cost effective" maximum
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.20 rate of charge = 4,949 Watt array with RBTS
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.25 rate of charge = 6,186 Watt array with RBTS not recommended to exceed

And there is sizing of the array based on reasonable estimate of minimum sun... I typically use PV Watts or Solar Electric Handbook for "hours of sun" per day by month. Using PV Watts for Atlanta Georgia, fixed array, tilted 34 degrees from horizontal:

Month    Solar Radiation (kWh/m 2/day)
1      3.86     
2      4.67     
3      5.21     
4      6.17     
5      5.95     
6      5.81     
7      5.82     
8      5.83     
9      5.21     
10      5.51     
11      4.42     
12      3.72     
Year      5.18

Typically, we use 4 hours of sun minimum. In much of the US, people get 4+ hours of sun for 9+ months of the year. Use genset for winter/poor weather backup. Atlanta seems to have lots of sun, so lest use 3.72 hours of sun (long term average) for December "break even" estimate array:

• 3,300 WH per day * 1/0.52 end to end system eff * 1/3.72 hours of sun = 1,706 Watt array minimum

So, based on the above, the array should be around 1,706 Watts to 3,217 Watts max--With 2,474 Watt array as a healthy nominal size. A nominal array in December would generate an average of:

• 2,474 Watt array * 0.52 system eff * 3.72 hours of sun = 4,786 WH = 4.8 kWH per day

Again--A pretty balanced system that is capable of running significant daytime loads too (irrigation, daytime laptop/at home job, etc.).

And what would a 647 AH @ 24 volt battery bank battery bank "support"?

• 647 AH * 24 volts * 0.85 inverter eff * 1/20 discharge rate = 660 Watt averge loads (over two days, 5 hours per day)
• 647 AH * 24 volts * 0.85 inverter eff * 1/10 discharge rate = 1,320 Watt average heavier loads (microwave oven, irrigation pumping)
• 647 AH * 24 volts * 0.85 inverter eff * 1/8 discharge rate = 1,650 Watt max continuous loads (10+ hours)
• 647 AH * 24 volts * 0.85 inverter eff * 1/5 discharge rate = 2,640 Watt you really don't want to ever exceed steady state loads
• 647 AH * 24 volts * 0.85 inverter eff * 1/2.5 discharge rate = 5,280 Watt maximum surge power

You can see the above numbers also help you define the range of inverters that would work well with this battery bank... A 1,500 watt to ~2,500 Watt AC inverter would do well... 1,500 watt average maixmum load. Most good inverters can support 2x maximum surge or (5,280 watts / 2 2,640 max supported AC inverter.

Numbers are intended to be conservative and support loads both in first year of battery bank life and ~3-5+ years later too.

Also, numbers for solar are within 10% accuracy... I carry out digits to avoid round off error, and so you can see/reproduce my math.

-Bill

Re: Battery max charge rate?

One thing I would suggest if you are gong to "over panel" an installation. Batteries really do well with long term/moderate charging currents. They really do not "like" 4-6 hours of maximum charging current.

In the "olden days", you could stretch the hours a day of good charging current by using a one or two axis tracker. You do have the added expense of the hardware and maintenance issues.

But, today, with the much lower costs of solar panels... There is the idea of virtual tracking. Basically facing 1/2 of the array south east and the other 1/2 of the array south west. This gives you several more hours a day of charging current without the costs/hassles of a mechanical tracking system.

You can play with PV Watts and optimize the system (different tilts and compass headings) for your location to get you more hours of charging and, potentially, longer battery life.

You can also look at the energy usage and adjust the array to better meet the needs--Is this a 3 season home/cabin? Do they use A/C and need more summer power? Do they need more power in the winter (more lighting, fans for moving warm air around, etc.)?

-Bill