# # of Panels to Charge Batteries

Posts: 4Registered Users
I bought a home with an existing 5 year old 48V off grid system. It's comprised of nine 24V, 190W evergreen panels, wired in 3 strings of 3 panels each which go to a combiner box, an Outback Flexmax 60 w/RTS, VF 3648, Mate & Hub. The batteries are 16, 6V Surrette S530, 400 ah wired in 2 strings of 8 - series/parallel.

What I would like to know is if my meter reads 49.2V in the morning, which I think is 75% of battery capacity, do I have sufficient panel power (6 hours of direct sun daily) to recharge the batteries to 100% using the setpoints of 58.8V absorb for 2 hours and then 52.3V float for the remainder of the day (as per surrette instructions)? Also how many panels would I need to be able to equalize ([email protected] hours) the batteries on panel power alone without a generator?

Right now the panels do not seem to be producing enough power to bring the batteries to the absorb stage. I don't know whether that's because of poor battery condition or because of not enough panels or a combination of both. When speaking to the previous owner it sounds as if he only equalized the batteries once or twice a year and he was using an absorb setpoint of 57.7 for 1.5 hours.

If the batteries are in poor condition, heavily sulfated, is it possible to bring them back with equalizing them for 5 hours 2 days in a row? All of the battery cables/connections have proper size wiring and are perfectly clean.

Also how can I tell if one or more of the panels are not functioning correctly?

I know it's a lot of questions but I'm kind of on my own out here and I don't want to make a mistake and replace a component that doesn't need replacing since everything is expensive, especially the batteries.

• Posts: 17,615Banned ✭✭
Re: # of Panels to Charge Batteries

Welcome to the forum.

Let's run some basic calculations and see how your array compares to your battery bank.
The array is 9 * 190 Watts or 1710 Watts and the battery bank is 800 Amp hours @ 48 Volts.
What you can expect from the array in terms of charging power:
1710 * 0.77 / 48 = 27 peak charging Amps.
That is no place near the minimum 5% for that battery bank, which would be 40 Amps.
In short, the system as is can not function properly over the long run. What you've been seeing is a very low depth of discharge with accompanying replacement of "used Amp hours" which appears to be recharging but in effect is always in a deficit state leading to sulphation and premature battery death.

What you do: connect a generator to the inverter and bulk charge the batteries in the morning. This should be done with an Absorb Voltage setting of 58.4 and an Absorb time of four hours. Watch the current to the batteries; when it drops below 8 Amps (with no loads) you can stop the charging. Then check each and every cell's specific gravity with a hydrometer. If there's a variance of more than 0.010 then run an equalization charge at 60 Volts for an hour and see if there is any improvement. Repeat this until no improvement is seen. That is the best you will get, and any additional charging will not help. Be sure and watch the water levels as well; if below the plates to begin with, add some distilled water to cover the plates. After the Absorb charge fill the level to proper height before doing the EQ.

At that point you will be able to tell which are the eight best batteries. Use them, leave the others out of the mix. You are way low on panel here for the batteries. the 27 Amps expected will be about 6% charge rate on 400 Amp hours (one string) and will be barely enough.

Basically you need about 2X that much panel for the full battery capacity. I suggest you re-evaluate your loads and see how much battery you actually need, then work from there. If this deficit charging has been going on for a long time the batteries will be toast and need to be replaced. There is no way to restore them.
Re: # of Panels to Charge Batteries

Time to buy some "toys". :roll:

First, I would suggest a DC Current Clamp Meter. Sears in the US has a very nicely priced \$60 unit that is good enough for our work here (actually a full function AC/DC current clamp meter with standard DMM functions too). You can clip the clamp on any wire in your system and measure the DC current. For example, you have three parallel strings of solar panels. In full sun, measure the current for each string and make sure that all are properly sharing the load. It is not unusual for a poor electrical connection to cause one string to stop producing--leaving the owner wonder why the batteries are not getting properly recharged.

Next, do you have a way of monitoring the battery bank? I would suggest a battery monitor (Victron is a very good European brand). Running without a battery monitor is a bit like running your vehicle without a gas gauge. It will be a big help to understand both your loads and the power you harvest from the array.

Lastly, if you are using an AC inverter, get some sort of kWH (kilo watt hour) meter. They are very common and cheap these days--Like this European version of the Kill-a-Watt meter.

Basically, as Marc/Cariboocoot says--You do not have enough solar array for such a large battery bank (you almost have a large trickle charger sized solar array)... In the olden days--the standard answer for people that wanted "more power" was to install more batteries. But--that was not the correct answer then, or now. But with solar panels costing much less these days, adding more panels is now much cheaper and the better answer.

In the end, we like to have a "balanced" system design. Measure and quantify your loads--Then size the battery bank to support those loads. The rest is then straight forward math to size the array, AC battery charger, backup genset, etc...

A too large battery bank is a huge money pit. You have to feed it with more panels and more fuel for your genset. Plus the bank will probably not last as long and will require a bunch of money to replace.

Since you already have what you have... Your best bet is to get the battery bank back to full charge (and using a hydrometer, measure and log the specific gravity in each cell, if the spread is 0.015 to 0.030 or greater, then you need to equalize too). Get the battery bank back over 80-90% state of charge right away (battery banks with low charge sulfate faster, batteries die faster). And make sure your charging systems (solar, genest, etc.) are working correctly.

In the mean time, start logging your energy usage so you can, A) work on conservation (always a very good idea--much cheaper to conserve vs generating your power). And , with that data, we can size the battery bank, and the balance of system for an optimal configuration.

Can you give us your Generator size and AC battery charger ratings?

When using the genset, the typical suggestion is to start it early in the morning and get the battery bank to absorb set point (~59 VDC). At that point, you can leave the generator run for up to 2-4 hours (you may want to do that the first time) and/or turn off the genset when the battery charging current starts to taper down--and let the solar array finish up the charging (from ~9am to 3pm or so)--That will help save fuel costs.

Note that generators are most efficient between 50% and 100% of rated load. As the rated load falls below 50%, the generator typically still draws ~50% of rated fuel flow, while supplying less electrical energy. Hence becoming less efficient.

Running a 7kW genset with an AC charger drawing 1kW or less (as an example), you are flushing fuel down the drain. In some cases, having two gensets is a better option... The large genset for running shop tools, etc. And a smaller genset supplying a "reasonable size" AC battery charger (perhaps ~10% or so of bank AH capacity--or in the 5% to 13% range--You can go to 25%, but it will quickly fall back from maximum output current as the battery becomes more charged).

Note about equalization--That is raising the battery voltage to 60-63 volts or so and holding it. Then measuring the specific gravity of the low cells every 30-60 minutes until they specific gravity stops rising. Then stop equalizing. At that point your battery bank is equalized (by the way, equalization is hard on a battery bank, that is why you only do it 6-12 times a year maximum--depending on vendor, and stop equalization when the cells stop rising in S.G.).

So, first fully charge the battery bank (raise voltage to ~59 volts and hold for 2-4 hours--you should be doing this a couple times a week in normal operation to get your bank >90% state of charge, then start equalization). And equalization does not require "that much current"--Probably 5% of battery AH capacity maximum (and one reason why we recommend 5% of bank capacity for minimum charger ampere rating). Note, not very many AC battery chargers really do have a "real" equalization voltage setting--You may have to do this with your solar array (turn off all loads, if needed, to reach 5% rate of charge).

By the way, can you tell us the maximum charging current/voltage you typically see with your solar array, and your genset (see if everything is working as expected)?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 4Registered Users
Re: # of Panels to Charge Batteries

Thanks Cariboocoot and BB. Looks like I'm going to have to downsize my bank to one string of 8 batteries. After I absorb and equalize to your specs I'll pick the best 8 to use.

As far as my panels go on the FlexMax 60 display right now at high noon and 95 F it shows 63V and 22A in. I checked each string of three panels at the combiner box, with the breaker to the controller off. Each string shows 82 - 83 volts. When I turn on the breaker and check each string at the controller they show 82 - 83 volts and each string shows 7.12 - 7.18 amps. Is there a way to check the total wattage coming from the panels? Or the total wattage coming from each individual panel? There were painters working on the roof where my panels are located and maybe it was just a coincidence but all the problems with the charging started the day after they were working up there so I'm wondering if there is also a problem with one or more of the panels.

The VFX3648 inverter will only take up to 20 amps from an ac source for charging (my 5kw generator) and the rest has to come from the panels. The Mate shows that I'm buying 1.2 - 1.3 KW from the generator. Using the Mate I can change the number of amps that the inverter accepts from a generator from anywhere between 1 and 60. I imagine that 20 goes towards charging and the rest to running the ac loads. If I set it to accept any more than 12 amps it trips the 20 amp breaker on the generator when an ac load comes online so I leave it set to 12. The continuous battery charge output of the VFX3648 is 45 amps.

As far as a DC Current Clamp Meter is concerned I don't think that there is one for purchase withing 500 miles of where I live. I forgot to mention that I do have a Bogart TriMetric Battery Monitor.

If you have any more questions send them along and I'll do my best to answer them.
Re: # of Panels to Charge Batteries
briti1 wrote: »
As far as my panels go on the FlexMax 60 display right now at high noon and 95 F it shows 63V and 22A in. I checked each string of three panels at the combiner box, with the breaker to the controller off. Each string shows 82 - 83 volts. When I turn on the breaker and check each string at the controller they show 82 - 83 volts and each string shows 7.12 - 7.18 amps. Is there a way to check the total wattage coming from the panels? Or the total wattage coming from each individual panel? There were painters working on the roof where my panels are located and maybe it was just a coincidence but all the problems with the charging started the day after they were working up there so I'm wondering if there is also a problem with one or more of the panels.

If I understand you correctly--You did all of the work to figure out wattages:
• 63 volts * 22 amps = 1,386 watts from array
• 63 volts * 7.12 amps = 489 watt from string "a"
• 63 volts * 7.14 amps = 450 watt from string "b"
• 63 volts * 7.18 amps = 452 watt from string "c"
• 9 * 190 Watt panels = 1,710 watts STC rating
• 1,710 watts * 0.77 typical panel+controller losses = 1,316 Watt to battery bank typical maximum

So, your system looks to be operating correctly (note that it is not unusual for some charge controllers to read and average of 5% higher than a calibrated set of meters will read--sometimes even 10% high).
The VFX3648 inverter will only take up to 20 amps from an ac source for charging (my 5kw generator) and the rest has to come from the panels. The Mate shows that I'm buying 1.2 - 1.3 KW from the generator. Using the Mate I can change the number of amps that the inverter accepts from a generator from anywhere between 1 and 60. I imagine that 20 goes towards charging and the rest to running the ac loads. If I set it to accept any more than 12 amps it trips the 20 amp breaker on the generator when an ac load comes online so I leave it set to 12. The continuous battery charge output of the VFX3648 is 45 amps.

I am assuming your running a 50Hz 230 VAC system?

5kW system:
• 5,000 watts / 230 VAC = 21.8 amps nominal

In the US, we would us a:
• 21.8 amp nominal * 1.25 wiring/breaker derating = 27.25 amp rounded up to 30 amp breaker
• 20 amp draw * 1.25 = 25 amp breaker/wiring

So, I would suggest reviewing the AC wiring and breakers and think about upgrading to 25 or 30 amps. Or at least 25 amps. Most breakers/fuses are designed to blow at 100% rating (may take minutes to hours) and not blow at 80% of rating (1/0.80 = 1.25).

You could charge that battery bank with 5% to 13% rated current very safely (this is the total ~59 volt DC battery charging current of AC+Solar charging):
• 800 amp*Hours * 0.05 = 40 amps minimum
• 800 amp*Hours * 0.10 = 80 amps nominal
• 800 amp*Hours * 0.13 = 104 amps cost effective maximum

So, free your system. Put at least a 25 amp breaker in the 230 VAC line to the inverter (always check the user/installation manuals for details). And raise the inverter/charger's share of AC charging current:
• 12 amps * 230 VAC = 2,760 VA
• 17 amps * 230 VAC = 3,910 VA (say you add 5 amps to present 12 amp setting).

That will bring your estimated charging current to (note, numbers always are approximate):
• 12 amps * 230 VAC * 0.80 charger efficiency * 1/59 volts charging = 37 amps
• 12 amps * 230 VAC * 0.80 charger efficiency * 1/52 volts charging = 42.4 amps (note, lower battery voltage can increase amps from chargers)
• 17 amps * 230 VAC * 0.80 charger efficiency * 1/59 volts charging = 66 amps

You might also check if the Inverter/Charger has an AC input breaker setting. Some inverter/chargers have a setting to limit the input current to prevent tripping breakers on smaller gensets (in your case, I would still go to 25-30 amp AC breaker and wiring--no reason to run a genset at less than rated power--Of course there are 5kW gensets with 4kW of continuous output rating--So if the Outback inverter/charger has a programmable maximum AC input limit--that will keep the genset at the correct power usage and not pop your 20 amp breaker--especially if increase the AC charger's input current allowance).
As far as a DC Current Clamp Meter is concerned I don't think that there is one for purchase withing 500 miles of where I live. I forgot to mention that I do have a Bogart TriMetric Battery Monitor.

By switching the combiner box breakers on and off--you did pretty much exactly the same thing and showed that your solar array is operating fine.

Questions for us?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 17,615Banned ✭✭
Re: # of Panels to Charge Batteries
briti1 wrote: »
Thanks Cariboocoot and BB. Looks like I'm going to have to downsize my bank to one string of 8 batteries. After I absorb and equalize to your specs I'll pick the best 8 to use.

As far as my panels go on the FlexMax 60 display right now at high noon and 95 F it shows 63V and 22A in. I checked each string of three panels at the combiner box, with the breaker to the controller off. Each string shows 82 - 83 volts. When I turn on the breaker and check each string at the controller they show 82 - 83 volts and each string shows 7.12 - 7.18 amps. Is there a way to check the total wattage coming from the panels? Or the total wattage coming from each individual panel? There were painters working on the roof where my panels are located and maybe it was just a coincidence but all the problems with the charging started the day after they were working up there so I'm wondering if there is also a problem with one or more of the panels.

The VFX3648 inverter will only take up to 20 amps from an ac source for charging (my 5kw generator) and the rest has to come from the panels. The Mate shows that I'm buying 1.2 - 1.3 KW from the generator. Using the Mate I can change the number of amps that the inverter accepts from a generator from anywhere between 1 and 60. I imagine that 20 goes towards charging and the rest to running the ac loads. If I set it to accept any more than 12 amps it trips the 20 amp breaker on the generator when an ac load comes online so I leave it set to 12. The continuous battery charge output of the VFX3648 is 45 amps.

As far as a DC Current Clamp Meter is concerned I don't think that there is one for purchase withing 500 miles of where I live. I forgot to mention that I do have a Bogart TriMetric Battery Monitor.

If you have any more questions send them along and I'll do my best to answer them.

What's going in to the controller isn't as important as what is coming out. With the MPPT function both array Voltage and current will vary over the day.

20 Amps AC on a 3648 is about 50 Amps on the DC side. Crank it up and let the generator do the Bulk charging; it's about double the current potential of the PV's.
There are two setting on this: AC IN current limit (which include charging and loads) and CHARGER current limit (which is confusingly also in AC Amps). Max the settings on both. leave the loads off, charge the batteries at full power (they'd take over 80 Amps DC if it was available; it isn't).

Alternately you could disconnect one battery string and recharge them one at a time.

I wouldn't worry too much about getting a DC clamp-on Ammeter or battery monitor at this point.
• Posts: 4Registered Users
Re: # of Panels to Charge Batteries

I'm running a 120 VAC 60Hz system.
• Posts: 3,738Solar Expert ✭✭✭✭
Re: # of Panels to Charge Batteries
briti1 wrote: »
Looks like I'm going to have to downsize my bank to one string of 8 batteries. After I absorb and equalize to your specs I'll pick the best 8 to use.

Question: Do you have a remote battery temperature sensor? That is important (to prevent thermal runaway) especially in a warm climate where you are going to be doing some hard charging. (hard charge = long absorb and/or equalization)

I think that, as mentioned, sulfation is the likely problem. There are other possibilities such as a shorted or high resistance cell. I think in a situation like this it is a good idea to use a refractometer or hydrometer to check your specific gravity and use a digital voltmeter to check voltages on each abttery BEFORE starting a hard charging regimen. If there is an obviously bad battery you might want to disconnect the string it is in before starting a hard charge.

Another thing to do (even after checking each battery) is disconnect one string while charging the other. Its possible (with both strings connected) that one string will be taking all or most of the charging current. That could lead to overheating. While charging, check the batteries frequently for overheating. Don't rely on the remote temp sensor (if you have one) because it isn't connected to each battery.

good luck, --vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Posts: 4Registered Users
Re: # of Panels to Charge Batteries

Ok, here are the SG's for the 16 batteries. It's as accurate as I can get using a turkey baster style.

5 batteries read: 12.65-12.65-12.65

The ones above I'm keeping unless someone suggests otherwise.

the 8 others read:

12.65-12.50-12.55
12.55-12.60-12.55
12.50-12.65-12.65
12.55-12.55-12.45
12.50-12.50-12.50
12.65-12.35-12.50
12.65-12.40-12.25
12.50-12.35-12.50

I hate to throw (recycle) batteries away so are any of the remaining 8 in good enough shape to give to someone who wants to run either a small 24v or 12v system and have no batteries at all?

In order to maintain the health of my new setup what would be a good absorb setpoint and time to use? What would be a good float setpoint and time to use? And last but not least what should I use for the equalize setpoint and time. Will the panels that I now have be sufficient to equalize the new 400AH battery bank with out using the generator?

For the next few years I'll only be here 3 months out of each year so I would like to keep the batteries in as good as shape as possible. When I'm gone the only loads on the system will be a Conserv refrigerator which uses 1.5Kwh daily and a 110v 1/2 hp ac pump for my irrigation drip lines which uses 10 amps to start, 5 amps to run and which starts and runs for 1 minute about 40 times in a 2 hour period, every other day. I can set the timers so the 2 hour sessions start at 1pm and end at 3pm so the panels are still producing power while the pump is running. Does it sound like my panels/batteries will be able to handle the loads and still remain healthy? I'd like to put off the expense of purchasing 16 new batteries and 9 more panels until I'm living here full time.

Once again thanks to all for your help. It's very much appreciated.
• Posts: 17,615Banned ✭✭
Re: # of Panels to Charge Batteries

The one battery with the 1.225 reading is probably in trouble. Chances are the other will respond to equalization.

You have just barely enough panel to support 400 Amp hours, but you may need to do Bulk charge from the generator from time to time.

A refrigerator and a water pump; two of the toughest loads you can have. 1.5 kW hours and a pump that can draw hundreds of Watts while running. Ouch.
1710 Watts of panel may be good for 3.5 kW hours AC per day. The batteries (at 400 Amp hours) should be good for 4.8 kW hours per day at 25% DOD. This is almost in balance.
1.5 kW hours for the 'frige, and about 500 Watt hours for the pump if the figures are solid. It would be better if you could measure the power consumption with a Kill-A-Watt rather than try to calculate it; there are sometimes nasty surprises with pumps. If we can count on those numbers your usage would be 2kW hours per day and both the panels and batteries have capacity well above that. It should work, and you don't need the extra batteries. This may change when you get to live there full time.
Re: # of Panels to Charge Batteries

If you could shut down the fridge when you are not there, it would help a lot.

Another issue is running loads when you are not there. All it takes is some bad weather or a charger/load/timer hiccup and your battery bank is drawn dead-murdered if you like.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 1,571Solar Expert
Re: # of Panels to Charge Batteries

Before you throw batteries away, you can try increasing the absorb time to 3 hours daily. Does the charge controller currently switch to float mode during the day?
If the system can get through a 3 hour absorb every day, then the batteries must be charging just fine.
• Posts: 348Solar Expert ✭✭✭
Re: # of Panels to Charge Batteries

I would bite the bullet and get some more pv now. This may delay the purchase of new battery's .And once you do replace your battery's you will need more pv any way to keep them healthy.
Regards Peter
2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . victron battery monitor . 24 volts 450 ah surette batterys . off grid  holiday home
• Posts: 17,615Banned ✭✭
Re: # of Panels to Charge Batteries
BB. wrote: »
If you could shut down the fridge when you are not there, it would help a lot.

Another issue is running loads when you are not there. All it takes is some bad weather or a charger/load/timer hiccup and your battery bank is drawn dead-murdered if you like.

-Bill

To avoid this, reprogram your VFX3648's low Voltage disconnect to shut down at 48 Volts. The batteries should then never be drawn below rechargable level. True, your irrigation timers will be wrong and the refrigerator will shut down. But hopefully that won't matter as much. Once the panels are able to push the battery Voltage above reconnect (about 50 Volts) it will all start functioning again. Usually a cold, closed 'frige can stand several hours without power before the temp rises too high.