How to Maximize the Life of an Offgrid Battery?

[Greta Gremlin]

I'm trying to choose the correct battery size for an offgrid setup.
It seems the batteries will likely be the first to need replacing.
I've come across a lot of options for keeping a battery healthily charged.
Though I had some questions about the pros and cons of when to do this.



-Overpanel your charge controller to increase the time you have ideal charge rates on the battery.

Can be done as long as the panel array Voc and Ioc does not exceed that of the charge controller.
However you lose potential power from solar panels.
Will the excess current limited by the CC cause any serious damage from heat?
Coupled with longer sun hours(=longer excess current) and higher air temperatures, I'm most concerned about this during
summertime.
Are there any other concerns about overpaneling to CC?



-Properly sized panel wattage to battery.
If someone is using an oversized controller to panels (for future expansion), I can see there being issues with
undercharging and/or overcharging if battery is sized improperly.
-
MPPT output current=Panel Wattage/Battery Voltage
Is using the panels peak output accurate for sizing batteries?
What type of voltage should be used to size the batteries (nominal, float, speed charge, or something else)?
I've seen both nominal and float used, however if speed charging batteries is recommended, wouldn't that voltage be used?



-Equalize batteries periodically
If speed charge or maximum smoke is used, will the need to equalize batteries be as often?
If batteries are kept above 80%, will fewer equalizations be required?



-Charge the battery at C/10
What situations would call for sizing the battery to be charged at a higher charge rate than C/10?
Or when can hitting the Absorb phase earlier be good for the battery?

[Mike90250]

Figure you can achieve 70% of the STC nameplate panel wattage.
Use the coldest temp in your area to calculate the Voc for the array so you don't fry the 150v controller with 155V

[Sunking]

-Overpanel your charge controller to increase the time you have ideal charge rates on the battery.

Can be done as long as the panel array Voc and Ioc does not exceed that of the charge controller.
Are there any other concerns about overpaneling to CC?

There is no such thing as IOC, as current in an Open Circuit = 0-Amps. IOC = International Olympic Committee. I think you meant Isc or Imp which is not relevant to your question unless you are talking about a PWM controller which would be foolish. and a huge waste of money. .

You can over panel as much as your wallet can tolerate and survive stupidity, but does not gain you much and poorly spent money. Example lets say you are using 200 watt panels which you should be using at a minimum. So you have 5 x 200 watt panels each costing $200 each for a total of $1000. You decide to pinch pennies and only buy a 60 Amp Controller to save $100 on instead of buying an 80 Amp Controller. Now you have limited yourself to 60 amps of charge current instead of 80 amps. You saved $100 to screw yourself out of power. Very common practice for DIY. An RV application is about the only application where over panel can pay off. Stationary no gain accomplished, just poorly spent money. .

Only thing you have to match up is Voc as Mike eluded too. The rest your wallet will protest about if your brain is listening and you know what you are doing. The rest of your questions I am not going to bother with because you have missed the whole point. The system is designed for WORSE CASE. First step is to determine you worse case Daily Watt Hours needs be it Summer or Winter. Say you demand 5 Kwh in Winter, and 4 Kwh in Summer, you choose 5 Kwh. To maximize battery cycle life and get the most bang for your battery dollars is you size the battery to 5-days reserve capacity, so 5 days x 5 Kwh = 25 Kwh. To find the battery AH = Watt Hours / Nominal Battery Voltage.

25,000 wh / 48 volts = 520 AH = 1500 pounds = $4000 battery
25,000 wh / 24 volts = 1040 AH = 1500 pounds = $4000 battery
25,000 wh / 12 volts = 2080 AH = 1500 pounds = $4000 battery.

As for panel wattage it requires a minimum C/10 charge current, more if your Winter Sun Hours are less than 2.7 Sun Hours. So you know immediately the smallest panel wattage is C/10 x 13.3 volts for each 12 volts of nominal battery voltage. No matter what battery voltage you use will require 2766 watts or just call it 2800 watts is the minimum panel wattage required. Math proves it.

208 amps x 13.3 volts = 2766 watts.
104 amps x 26.6 volts = 2766 watts.
52 amps x 53.2 volts = 2766 watts.

If your Winter Sun Hours are less than 2.7 Sun Hours will require higher panel wattage. Now if you were to build the example above you would be a fool to use 12 or 24 volt battery. I know you may not understand that, but your wallet will and can explain it to you. If you used 12 volts would require 3 very expensive 65-Amp Charge Controllers. At 24 volts requires 2 very expensive 65-Amp Charge Controller, or at 48 volts only 1 very expensive 65-Amp Charge Controller. Ask your wallet, he will understand. Using 12 or 24 volts would also cut battery cycle life in half because you would have to use parallel battery strings. Instead of replacing $4000 batteries every 5 years, you get to replace them every couple of years. Again consult your wallet for in depth discussion. About the largest battery your back can handle is a 6-volt 520 AH battery weighing in at 190-lbs each. Regardless of system voltage of 12, 24, or 48 volts requires 8 x 6-Volt 520 AH batteries. At 24 volts requires twice the wire and hardware expense as 48 volts, and at 12 volts 4 times the wire and hardware expense as 48 volt. Both you back and wallet can explain it to you if you did not follow me. Both your back and wallet will break you into tears and severe pain if you do not listen carefully.

Short answer to your question if you did not catch it the first time is to size the battery to 5-day capacity. The rest is whatever it takes to meet minimum charge requirements.

[Greta Gremlin]

There is no such thing as IOC, as current in an Open Circuit = 0-Amps. IOC = International Olympic Committee. I think you meant Isc or Imp which is not relevant to your question unless you are talking about a PWM controller which would be foolish. and a huge waste of money. .

Yes, "Ioc" was a typo, "Isc" is correct.
I've read elsewhere that the Isc should not exceed charge controller limits, not just Voc.

https://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/#q4i_want_to_over_dimension_the_pv_array_how_can_i_calculate_the_maximum

Trying to get my facts straight without destroying a bunch of equipment.
Exceeded Voc = fried controller
Exceeded Isc = ?

An RV application is about the only application where over panel can pay off. Stationary no gain accomplished, just poorly spent money. .

Why would over paneling be applicable for RV?

The rest of your questions I am not going to bother with because you have missed the whole point. The system is designed for WORSE CASE.
Short answer to your question if you did not catch it the first time is to size the battery to 5-day capacity. The rest is whatever it takes to meet minimum charge requirements.

I'm understanding what you've said as
WORSE CASE=the bigger the battery to daily needs=the lower the discharging=the longer the battery cycle life

Since it is a race to charge the battery up everyday, my questions are centered around maintaining ideal charge rates.

From the stickies and other threads, I'm seeing C/10 as the ideal charge rate, the "sweet spot".
It allows you gain the most out of your battery's charging time.

https://www.solarpaneltalk.com/forum/off-grid-solar/batteries-energy-storage/351396-are-you-killing-your-batteries-part-2

If overpaneling, I assumed that would extend the time you could maintain that C/10 charge rate.
This way a 10 amp controller will charge 10 amps into 100ah battery for longer.

Following this, the right panel wattage is what I'm trying to figure.
Like Mike mentioned, expecting 70% of STC wattage.
Then add in losses from wiring and the charge controller.

If a 200 watt panel is expected to give me something like 140 watts.
A 150ah battery is only getting about 10 amps (C/15) instead of 15 (C/10) .
Doesn't this leave the battery undercharged?

If I size panel to 286 watts and then factor in losses, I get about 200 watts.
My 150ah battery is now being charged at around 15 amps (C/10)
But if under ideal conditions a bright sunny day in winter, panel hits closer to STC.
then from 280 watts I get around 20 amps for about a C/8 charge rate, which is still acceptable according to stickies.
Wouldn't overpaneling be a good option if my understanding is right here?

As for panel wattage it requires a minimum C/10 charge current, more if your Winter Sun Hours are less than 2.7 Sun Hours. So you know immediately the smallest panel wattage is C/10 x 13.3 volts for each 12 volts of nominal battery voltage.

Why is 13.3 volts used to size the panel wattage?

I've also seen C/12 recommended as the minimum, why is C/10 used as minimum in this case?

Only thing you have to match up is Voc as Mike eluded too.. .

alluded?

[Raul]

Greta Gremlin,

i use victron gear as well and yes there is a limit of ISC input on all victron controlers. In odd winter days if that ISC is exceeded by a small amount I get a error 34- input current exceeded. The controler shuts off to protect itself. Once the condition cleared, it resumes normal operation. However, I have reworked my strings in such a way that this does not happen again. The ISC permitted = panel rated ISC X number of strings installed.
Ex 18 panels on 6 strings of 3 with a ISC of 9.5a = a total ISC input of 57a . That is way below my 75a ISC limit , but way over paneled for my 38a @ C/10 requirement. I over paneled for winter harvest and peak charging current can be limit on the controler settings if needed. Another think I did that works for me is to avoid a sudden peak of power, I have the panels on 3 orientations so I spread the power thru out the day and maximise my harvest time. The way victron sizes the array permitted for a given controler is absorb battery voltage X max charging current rating of that controler.
Don't get confused by panel ISC and string ISC, I think that's where you got it wrong.

[HawaiiOffGrid]

Hello Everybody!

so ive been trying to follow Sunking's theory of "Maximum Smoke' to get the best out of my FLA batteries. (GC2 costco 6V 210AH. 2 strings of 8 =48V) I have 4000 Watts of panels (4 strings ) and only use about <3kwh of power a day. I have a FP1 3648 Outback inverter/FM80 CC. I live in Hawaii and even in winter get pretty decent sun. I recently changed my settings to Bulk/Absorb/Float=59.2. Absorb time=0. My problem is that i cant get it to BULK. it starts floating in the morning and the voltage on mate3 reads 59ish, but my SG readings are about 1.250. which is only 70% charged according to spec sheet for those batteries (100% is 1300) . Batteries are 5 months old and have never been taken down to 50%DoD. When i manually initiate bulk from mate3 it immediately switches back to float because voltage is high enough.

Appreciate any advice,
Thanks!