To much charge voltage?

Les49 · September 2016

High all, I'm installing a 24 volt off grid system and I'm trying to get my charging voltage down to a comfortable level. During solar noon I'm generating around 43 open volts and through the charge controller at the battery bank I'm getting around 33.5 volts. As batteries are expensive I'm a little concerned about doing damage to them. Ideally 28 to 30 volts would be fine. Anyone have any thoughts on charging at this high voltage?
Thanks for your input, Les

Marc Kurth · September 2016

Les,

Which charge controller are you using?
Which batteries?

At 33.5v continuously, you are likely destroying your battery bank.

Les49 · September 2016

This is with a Chinese 80 amp controller that is programmable but it's not even showing the current so I'm sure the current is to high as well. The Batteries are Duracell 230ah 6volt (golf cart) 4 in series for a 24 volt bank. It's a growing system and I wound up with 16 24volt 320 watt panels. I do have an Outback 60 amp controller that should be here by Friday. It will handle 8 of the sixteen panels at 48 volts. I have stopped trying to use the Chinese controller. I disconnected all but one panel and the voltage was still up around 31.5 volts and probably climbing........... Instead of charging the batteries they seemed to discharging at that high current and voltage. Sooo it's off line for the moment till the Outback get's here.

Marc Kurth · September 2016

Les,

Are your Duracell batteries the standard flooded type with caps, or sealed AGM's? Even the 31.5v is already too high. It really sounds like your controller is shot and the battery bank is getting cooked.

I'm not sure what you mean by "they seem to be discharging at that high current and voltage." Give us a little more info and collectively, we can likely offer good input! Bunch of smart people here (not me) who can help.

Marc

littleharbor2 · September 2016

Correct me if I'm wrong but even if hooked directly to a 24 volt battery bank wouldn't the voltage output of the panels get pulled down to something in the batteries normal uncharged, semi charged or fully charged range? Are you sure you aren't measuring the vmp of the panels in an unloaded condition? Can a 24 volt battery even get up to 33.5 volts? Were your batteries at a rolling boil before you unhooked the CC?

jonr · September 2016

I'd also be suspicious about what most volt meters will read with a pulsed voltage.

mike95490 · September 2016

What's the quality / calibration of the meter ? Check it against a resting automotive battery, should be 12.5v or so.

Les49 · September 2016

OK, to answer your question about the battery type....yes they are flooded lead acid type and what I mean by discharging is the % of charge shown on the controller drops down each time I shut the system down. Which I've come to believe the controller is whacked. Each individual battery reads 6.4 volts and the bank reads 25.7 volts after a 1 hr. rest.

I'm using three different meters one of which reads about 1 volt lower than the other two. So far the more accurate two agree with the volt reading on the controller, And yes littleharbor2 the charging voltage is pulled down from the panel open voltage of 40+ volts to the battery voltage of 26+ volts but then very quickly rises to 30+ volts and the battery bank goes into a rolling boil at which time I cut the solar input.

My multimeter only reads 10 amps so I've not been able to get a total current reading from the 8 panel array. Individually the panels have a short circuit reading of 8.5 amps and spec out at 9.15 max current. But if they are all putting out what they are supposed to I should be getting somewhere close to 73 amps at the controller. The 80 amp controller should be able to handle this......but obviously it can't.

I am going to reconfigure the panels to 48 volts to drop the current down and I should be able to comfortably charge with the Outback Flex Max 60.

Photowhit · September 2016

We haven't discussed the type of controller, is it a PWM or MPPT? Are the panels in strings of all parallel?

Math, charge controllers are rated on their output amperage, a 80 amp MPPT charge controller could only handle about 2500watts of array if it limited the output to 80 amps, I wouldn't count on a 'cheap' charge controller to do that! 2500 watts at 24 volts = 2500/24=104 amps! normal output of panels is 75% so about 80 amps.

Your Outback 60 amp should handle about 1900 watts of array, it will limit output. 1900 watts/24=79amps 75% = @60 amps. About 6 of your 320 watt panels.

BB. · September 2016

You may also not have large enough cable from the charge controller to the battery bank (voltage drop).

The other possibility is a 230 AH @ 240 volt battery bank would nominally be charged with a 5% to 13% rate of charge... Or 13% * 230 AH = 29.9 amps maximum (more or less) recommended rate of charge. It is possible that with your ~80 amps of charging current, you could be dumping too much current into the battery bank and pull it >30 volts charging (especially if the battery bank is near fully charged).

Are you planning on putting another 2-3 strings in parallel? Not many folks "over panel" their first off grid power system (usually they have too many batteries and/or too much load current for the battery bank to supply).

I suggest that you get a DC current clamp meter (this one from Sears--AC/DC current clamp DMM is "good enough" for our needs and around $60).

Another handy tool is a specific gravity meter (hydrometer). That is one "good thing" about flooded cell batteries--You can use the hydrometer to understand your battery bank's state of charge and how it is doing.

Be careful--It is very easy to "murder" your battery bank (over or under charging is a common issue, as is over discharging with your loads).

-Bill

jonr · September 2016

With a PWM controller, I'd use a 20% de-rating (for temperature and panel variations), 30% with a cheap one. Could well be that the mosfets are now shorted. An Outback MPPT won't be damaged with any number of paralleled panels.

Les49 · September 2016

The controller is/was a Sun Yoba Solar 80 PMW and the panels were wired in parallel (6-320 watt 24 volt Panels) of which I now have 8 rewired in series/parallel giving me an open voltage of 83 volts. which should equate to 1280 watts 26.6 amps which the Outback 60 should be able to handle. Tell me if I'm wrong but shouldn't the Outback be able to handle all 16 panels in series/parallel? That would be 2560 watts and 53.3 amps @ 48 panel volts. The recommended max for the Outback 60 is 3000 watts @ 48 volts
Wire size from the panels to the controller was 10 awg and the size to the batteries was 4 awg which was as large as the clamp terminals would except on the controller. I could go larger from the combiner box to the controller but I have an inline 150 amp watt meter which has 10 awg leads for both the positive and negative input and output.
I do have an AC DC current clamp meter......it's the one that reads one volt low, and also a hydrometer......very handy indeed.

In the late 80's, early 90's I lived off the grid surrounded by National Forest near Colville, WA for about 5 years with a very simple 12 volt system. 6-45 watt panels (in parallel) 270 watts and a 30 amp Trace controller, 8-100 ah 6 volt Trojans and a 800 watt inverter (don't remember the brand). A 5kw Generac generator for the heavy stuff, propane for cooking and refrigeration. -12 degrees in the winter with 1 to 3 feet of snow..........that was enough! And the reason I live in southern AZ now.
Jonr I believe you are correct! The mosfets are most likely fried!

Thanks to all of you for your input. .
Les

Photowhit · September 2016

Les49 said: Tell me if I'm wrong but shouldn't the Outback be able to handle all 16 panels in series/parallel? That would be 2560 watts and 53.3 amps @ 48 panel volts. The recommended max for the Outback 60 is 3000 watts @ 48 volts

Well, you are correct, but NOT correct, or misunderstanding somewhere.

Previously you stated; "I'm installing a 24 volt off grid system..."

So if you are installing a 48 volt system, you are correct, if you are installing a 24 volt system, which you said earlier, use the numbers I've provided. No way you can get 48 volts out of "Batteries are Duracell 230ah 6volt (golf cart) 4 in series for a 24 volt bank."

As I stated earlier, the charge controllers are rated at "output amps" If you are outputting at 24 volts, they max out at amps for your 24 volt system.

Photowhit · September 2016

Les49 said:

Wire size from the panels to the controller was 10 awg and the size to the batteries was 4 awg which was as large as the clamp terminals would except on the controller. I could go larger from the combiner box to the controller but I have an inline 150 amp watt meter which has 10 awg leads for both the positive and negative input and output.

10 gauge wire shouldn't carry more than 30 amps and you will have serious losses in voltage drop without larger wire. Your Outback will give you a display of watts and amps, I'd dump the inline meter.

Manual for your China Solar charge controller;

http://www.chinasolarregulator.com/user-manual/solar60-80.pdf

Photowhit · September 2016

Les49 said:

The controller is/was a Sun Yoba Solar 80 PMW and the panels were wired in parallel (6-320 watt 24 volt Panels) of which I now have 8 rewired in series/parallel giving me an open voltage of 83 volts. which should equate to 1280 watts 26.6 amps which the Outback 60 should be able to handle. Tell me if I'm wrong but shouldn't the Outback be able to handle all 16 panels in series/parallel? That would be 2560 watts and 53.3 amps @ 48 panel volts. The recommended max for the Outback 60 is 3000 watts @ 48 volts

Some how I missed that there is some basic calculation problems here...

Watts are a measure of "energy" and the old adage "energy cannot be created or destroyed" works here. If you have a string of 6, 6 panels in parallel, 2 strings of 3, or 3 strings of 2, you will still have 6 x 320 watts for 1920 watt array.
... 16 panels, 16 x 320 = 5120 watt array.
This works so long as the panels are distributed evenly.

I revisited this to add a link to Outback's String sizing tool;

http://www.outbackpower.com/outback-support/string-sizing-tool

Les49 · September 2016

Thanks Photowhit, I have tried opening the string sizing tool several times and it will not unzip. You were right about my confusion between the battery voltage and the solar input voltage........:) I know that it all falls back to battery voltage in sizing the system but for some reason it just wasn't sinking in. Got it now thanks!

So let me ask you this........other than wire sizing and run distances, is there any advantage to a 48 volt array over a 24 volt input? And especially for charging a 24 volt battery bank? Eventually I want to increase the battery bank to 48 volts, but for now the down line equipment is set up for 24v.

As it turns out the Outback 60 guide states: the Maximum PV input wattage per charge controller for a 24v battery bank is 1500 watts so that drops me back to 4 panels in either configuration.

softdown · September 2016

My cheap Chinese controller simply blazed away at a constant voltage of ~31 volts...as memory serves. As long as the sun was good, my ~1650 pound forklift battery was simmering with only 360 watts charging it. Some felt it impossible to damage that battery with only 360 watts. BB thought differently...I put an American charge controller on it that costs 12 times more.

Plus there are a lot of Chinese PWM controllers that call themselves MPPT controllers. At least I didn't fall for that one.

Les49 · September 2016

softdown said:

Plus there are a lot of Chinese PWM controllers that call themselves MPPT controllers. At least I didn't fall for that one.

Yeah, I checked into the so called MPPT, Chinese, controllers and found out that their only claim to fame is accepting either a 12 or 24 volt PV input. They have flooded the market with these things and it's pretty hard to get much info about them. Youtube can't keep up with them all and it seems that they pass or fail by a vast variety of opinions, which may or may not mean much.

The price of these things is what is the temptation. But as the old adage goes...........you get what you pay for, (in most cases).

Trial and error can be expensive even if the items involved are cheap.

Photowhit · September 2016

Les49 said:

So let me ask you this........other than wire sizing and run distances, is there any advantage to a 48 volt array over a 24 volt input? And especially for charging a 24 volt battery bank? Eventually I want to increase the battery bank to 48 volts, but for now the down line equipment is set up for 24v.

The MPPT type charge controller actually need to have some 'head space', voltage above the intended charging voltage. I think about double the output voltage is idea, charging should be around 29-30 volts and I think they suggest 30% above, or 30% above system voltage. I'm not sure which, but in this case it would be important since the 35 volts would be about 30% above system voltage.

Remember that inverters require a set input voltage, so if you switch system/battery bank voltages you would have to switch out the inverter as well. I know this too well, as I should have a higher voltage battery/system for my new larger home, but my battery may last 15 years (forklift battery). If I put in a new larger wattage inverter for my current battery and my battery ends up showing signs of failure, I'll have to buy both. So I'm stuck using an undersized inverter for now. I hope it's a long and fruitful suffering!

softdown · September 2016

Les49 said:

softdown said:

Plus there are a lot of Chinese PWM controllers that call themselves MPPT controllers. At least I didn't fall for that one.

Yeah, I checked into the so called MPPT, Chinese, controllers and found out that their only claim to fame is accepting either a 12 or 24 volt PV input. They have flooded the market with these things and it's pretty hard to get much info about them. Youtube can't keep up with them all and it seems that they pass or fail by a vast variety of opinions, which may or may not mean much.

The price of these things is what is the temptation. But as the old adage goes...........you get what you pay for, (in most cases).

Trial and error can be expensive even if the items involved are cheap.

"Get what you pay for" is a cute, trite, and over-simplified saying. It is almost impossible to know the quality of the electronics one is buying. The mark-up with electronics is often pretty incredible as well.

My late uncle was pretty savvy. He designed Colorado's oldest, still in use, transformer station (or was it a power station?). He did seem to simply buy the most expensive electronics from what I saw. Other times, he refused to buy while lamenting the gigantic mark-up found in many electronics. For example, I read that a $700 I-Phone costs a couple bucks to make.

To much charge voltage?

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