24v solar system question - from Greece!

yiannis5

Hello all,

This is my first post - congratulation for the site, it is very informative. I am a solar beginner and have recently installed a small off-grid system. I am located in Greece and the installation was done by a local firm. Basically I would like to get an experts opinion on whether the system was correctly installed and balanced. What we have so far:

1. 4 X 6v Trojan t105RE batteries 225 Ah (20 hour rate) connected in series so if I understand correctly we have a 24v battery bank at 225Ah

2. 4 X Solarworld poly panels 230 watts each ( not sure how they were connected, I think it is a combination of series and parallel!) - the charger displayed a maximum voltage of 64 volts so far so I assume they are connected 2 in series and 2 parallel?

Specs for the panels

Maximum power 230 Wp
Voc 36.9
Vmp 29.8
Imp 7.72A

3. Solar charger is a MPPT Tracer

45A charge current
20A discharge current
150 VDC

4. Inverter is a Victron Multiplus 24v/3000W with a 70A charger

5. I also have a Honda EU26i generator.

Can someone let me know if this system is balanced or needs a bigger battery or solar bank? If I add another set of 4 Trojan batteries will I have better autonomy and will I also need to add more solar panels or will I have enough power from the panel side?

Many thanks for sunny Greece!!

Yiannis

Cariboocoot

Re: 24v solar system question - from Greece!

Welcome to the forum Yiannis.

You've got your system specifications correct: 225 Amp hours @ 24 Volts, panels in series & parallel for 60.6 Vmp 15.44 Imp array.

It's pretty easy to check on the panel to battery ratio. The total array size is 4 * 230 Watts for 920 Watts total. Using an MPPT controller you can probably expect: 920 * 0.77 (typical efficiency) / 24 = ~30 Amps of peak charge current which is more than enough for your batteries (13% rate in fact; more would probably not be economical).

At 25% depth of discharge you'd have roughly 1.2 kW hours AC available. Whether this is large enough for your needs depends on how much power you use daily. Also you will have plenty of 'extra' power available from the panels once the batteries are charged because of the large array and plenty of sun in Greece!

The easiest thing for you to do is to get a hydrometer and check the specific gravity of the batteries after they are fully charged and again first thing in the morning before charging begins. That way you'll know how much capacity is being used overnight.

If necessary you could increase the battery bank by 2X without having to increase the array size. Peak charging would drop to 6.6%, but it should still work especially if the depth of discharge is reduced significantly.

yiannis5

Re: 24v solar system question - from Greece!

Many thanks for the speedy reply! I am thinking of getting another set of 4 X 6v 225 Ah batteries soon as I fear in the winter I might not have enough autonomy. I also understand that it is better to add new batteries to the existing ones sooner rather than later so they are more 'even'? I installed the system only last week.

On a separate issue I have some more questions

1. According to the specifications of the Trojan battery the equalize charge is at 30.96 V. However reading at the manual of my solar charger the equalization voltage is set at 29.6 V and this setting is not configurable by the user. I have a battery monitor installed and I noticed that despite my low daily power usage the voltage of the battery bank will drop to 24.6 V when dark and will be around 24.2 V early in the morning. FYI my power requirements as I currently stay in a caravan are

Inverter, 15w at no load X 24 hours
DC breaker in the caravan, 20w X 24 hours
2 X 2.8w LED lights used for very little in the night for about 2 hours max
LED TV at 20w for max 2 hours
12v water pump in the caravan used maybe 10 times a day for a few seconds

I wonder if there is something wrong with the batteries as the voltage drops significantly and they need to be equalized? Also I am not sure if the batery monitor voltage indicator only is adequate to judge the actual capacity and SOC of the batteries?

Any help appreciated! Maybe by getting another set of 4 batteries will cover my power needs in a better way?

Yiannis

vtmaps

Re: 24v solar system question - from Greece!

yiannis5 wrote: »

I have a battery monitor installed and I noticed that despite my low daily power usage the voltage of the battery bank will drop to 24.6 V when dark and will be around 24.2 V early in the morning.

I wonder if there is something wrong with the batteries as the voltage drops significantly and they need to be equalized? Also I am not sure if the battery monitor voltage indicator only is adequate to judge the actual capacity and SOC of the batteries?

Get an hydrometer. Then you will know whether your batteries are getting charged properly. I suspect that your charger is not adequately charging your batteries. The battery monitor my be deceiving you. Battery monitors are useful, but they must be calibrated to the battery. If your battery monitor says your batteries are charged and your hydrometer says they are not charged, the hydrometer is correct.

If your tracer charge controller is not able to charge the batteries (wrong voltage or wrong amount of absorb time) then use your inverter/charger to charge the batteries.

GET AN HYDROMETER! Time is of the essence... your batteries may be permanently damaged (sulfation) if you let them remain in a discharged state for several weeks.

--vtMaps

Cariboocoot

Re: 24v solar system question - from Greece!

You've got perhaps two problems:

29.6 is not high enough for an equalization charge, but you shouldn't need to be doing this yet (Trojan recommends checking specific gravity for too much variation before doing an EQ charge). If that's all it will allow, the charge controller should be changed. That is in fact just about what a 24 Volt system with Trojan batteries will Absorb at.

Load consumption looks like;
Inverter 360 Watt hours, caravan power (probably does not draw 20 Watts all the time) 480 Watt hours, LED's 11 Watt hours, TV 40 Watt hours. At that point you're up to 890 to 900 Watt hours. This should not be severely affecting the state of charge of the batteries as it is less than 25%.

That means either the loads are much higher than you expect or the batteries are not getting charged fully. You are right that Voltage alone is not a good indicator of state of charge: it really only works with batteries at rest (no current in or out for a couple of hours minimum) which is not practical on a working system.

Adding more batteries is probably not the solution here, because if the ones you have aren't getting properly charged more will just be worse. As before, get a hydrometer and check the SG of the cells. If the charge controller has adjustments for Float level, turn it up to 27.6 if possible.

Also, does this charge controller have a remote temperature sensor? If not it would be a very good idea to get one. I suspect the controller is the culprit here, with its lack of flexibility to accommodate the Trojan's Voltage levels.

yiannis5

Re: 24v solar system question - from Greece!

Cariboocoot wrote: »

You've got perhaps two problems:

29.6 is not high enough for an equalization charge, but you shouldn't need to be doing this yet (Trojan recommends checking specific gravity for too much variation before doing an EQ charge). If that's all it will allow, the charge controller should be changed. That is in fact just about what a 24 Volt system with Trojan batteries will Absorb at.

Load consumption looks like;
Inverter 360 Watt hours, caravan power (probably does not draw 20 Watts all the time) 480 Watt hours, LED's 11 Watt hours, TV 40 Watt hours. At that point you're up to 890 to 900 Watt hours. This should not be severely affecting the state of charge of the batteries as it is less than 25%.

That means either the loads are much higher than you expect or the batteries are not getting charged fully. You are right that Voltage alone is not a good indicator of state of charge: it really only works with batteries at rest (no current in or out for a couple of hours minimum) which is not practical on a working system.

Adding more batteries is probably not the solution here, because if the ones you have aren't getting properly charged more will just be worse. As before, get a hydrometer and check the SG of the cells. If the charge controller has adjustments for Float level, turn it up to 27.6 if possible.

Also, does this charge controller have a remote temperature sensor? If not it would be a very good idea to get one. I suspect the controller is the culprit here, with its lack of flexibility to accommodate the Trojan's Voltage levels.

Thanks again!

1. Yes, the charge controller has a temperature sensor - is there specific temperatures I should be looking when charging? According to the solar chargers specs the battery parameters at temperature of 25 Celsius are

Float voltage = 27.6
Boost voltage = 29.2
Equalization voltage = 29.6
Equalization duration = 2 hours

Above figures cannot be altered...

Maybe the solution here is after I get accurate readings through a hydrometer to perform an equalization through my Inverter / Charger since it is totally configurable? Also just a thought but I read somewhere that new batteries take up to 100 discharges / charges until they reach full capacity?

Also what is the correct overnight voltage for my batteries taking into account my daily loads? Loads that are running overnight is the inverter / charger / caravan power and that is all!

Cariboocoot

Re: 24v solar system question - from Greece!

yiannis5 wrote: »

Thanks again!

1. Yes, the charge controller has a temperature sensor - is there specific temperatures I should be looking when charging? According to the solar chargers specs the battery parameters at temperature of 25 Celsius are

What the temp sensor does is adjust the charging Voltage (usually the reading, not the set points) to compensate for temperature differences above/below 25C. When the battery is hotter it needs less Voltage to achieve full charge, when colder it needs more.

Float voltage = 27.6
Boost voltage = 29.2
Equalization voltage = 29.6
Equalization duration = 2 hours

Above figures cannot be altered...

That is a problem. "Boost" Voltage is being used for what we'd normally call Absorb Voltage. The base line for this on a 24 Volt system is 28.8. An EQ Voltage that is only 0.4 Volts higher than Absorb is useless. Likewise you want to be able to control the length of time for both Absorb and EQ. It almost sounds like this unit does not EQ, but instead they have used the term in place of Absorb. However, the lack of adjustable settings is a problem no matter what you call them.

Maybe the solution here is after I get accurate readings through a hydrometer to perform an equalization through my Inverter / Charger since it is totally configurable? Also just a thought but I read somewhere that new batteries take up to 100 discharges / charges until they reach full capacity?

This is a very good idea. There is nothing that can take the place of correct SG readings for judging a battery's state of charge.

Also what is the correct overnight voltage for my batteries taking into account my daily loads? Loads that are running overnight is the inverter / charger / caravan power and that is all!

Some general info about deep cycle batteries: http://www.solar-electric.com/deep-cycle-battery-faq.html
The Voltage is not going to be an accurate indicator of SOC on an active system. But the quick drop to 24.6 and overnight fall to 24.2 sounds like capacity has been compromised due to undercharging. When charging stops (no more Float) you can expect battery Voltage to drop right away, but it should be more like 25 or more Volts, not less. The overnight loss should also be minimal, unless the current consumption in that time is highest for the day.

yiannis5

Re: 24v solar system question - from Greece!

Thanks for the fantastic info and all the help. Not sure if this is allowed within your forums guidelines but would you be able to recommend a good hydrometer for me to buy so I can accurately check my batteries?

Yiannis

yiannis5

Re: 24v solar system question - from Greece!

Cariboocoot wrote: »

That is a problem. "Boost" Voltage is being used for what we'd normally call Absorb Voltage. The base line for this on a 24 Volt system is 28.8. An EQ Voltage that is only 0.4 Volts higher than Absorb is useless. Likewise you want to be able to control the length of time for both Absorb and EQ. It almost sounds like this unit does not EQ, but instead they have used the term in place of Absorb. However, the lack of adjustable settings is a problem no matter what you call them.

Forgot to say, the charger manual also mentions

over voltage reconnect 30v
charging limit voltage 31v
high volt disconnect 32v

...if it makes any difference!

Yiannis

BB.

Re: 24v solar system question - from Greece!

Sure, people are more than welcome to post links to good quality hydrometers (and as answers to other hardware related questions).

Here is a nice discussion about Hydrometer types and some links to product.

Refractometers to measure battery specific gravity?

Since you are in Greece, our European members my be able to give you better sources that us over here.

-Bill B. Moderator

Rybren

Re: 24v solar system question - from Greece!

I'm a real neophyte when it comes to solar, so take my thoughts with a grain of salt.

I initially looked at getting the Tracer CC - they're inexpensive, a lot of dealers carry them here in Canada and they are quite common on ebay. However, as you noted already, most of the settings are fixed (and are less than optimal) so you're stuck with whatever they decided. I ultimately purchased a Classic 150 - it is significantly more expensive than the Tracer; however, it is extremely flexible and can handle more than I ever plan on throwing at it. There are other controllers that would meet your needs as well - and would be better than the Tracer.

-Jerry

westbranch

Re: 24v solar system question - from Greece!

your vigilance is paying off. My interpretation is that you are desperately in need of a MUCH better CC, as you do not want to have to purchase new batteries every year or so. Victron, Midnite Classic are 2 to consider, you need a CC on steroids, not a 90 pound weakling...

Being in Greece you might see what forum user NIGTOMDAW (Spain) or STEVENDV (Spain) can put you on to. Try a PM ( personal Message) buy going to their profile.

hth