Jump to content

Omnipower Initial Charge Current


Bush Man
 Share

Recommended Posts

According to the staff at Sinetech and the Omnipower Care Sheet attached the Omnipower 150Ah (and other gel) batteries require an initial charge current of 36A, or 72A for two strings.  This translates to 24% of capacity, whilst the general wisdom on the forum would not exceed 13%. Two 150Ah strings would thus need about 4000W of panels to provide sufficient initial current, or a daily capacity (5hrs) of around 20 000Wh.

In my off-grid setup (48V, 8xOmnipower 150Ah, 5kVA Axpert) I rarely use more than 1500Wh per 24h day  (Energy efficient fridges and LED lighting only).  My 1500W panel array would be more than sufficient for that, providing 7500Wh per day less some losses.

Does that mean that in order to use Omnipower or other gel batteries I would need three times as many panels as I currently have, only for the sake of initial current? Would that also mean that for a low base load it doesn't make sense to consider gel batteries?

Omnipower Battery Care.pdf

Link to comment
Share on other sites

Hi Bushman,

 

My understanding of the initial current spec it that it assumes the battery are low or close to emply. Under 12V each. Since you are only drawing 1500wh, less than 10%, I think you can ignore the 36A requirement and just configure your Axpert according to the Float and Bulk Voltage specs.

 

More experienced forum users may have a different view

Link to comment
Share on other sites

21 hours ago, Bush Man said:

Does that mean that in order to use Omnipower or other gel batteries I would need three times as many panels as I currently have, only for the sake of initial current? Would that also mean that for a low base load it doesn't make sense to consider gel batteries?

I agree with Travis.

I have AGM batteries with a very similar charging recommendations.

58ba517e7dd75_Screenshot(36).png.fcfc13ad739a80f221f1d8df3b9308fb.png

If you look at the time period the charging recommendations are for a nearly depleted battery. 

I operate at battery temperatures higher than the 25oC for a considerable portion of the year and cycle my batteries more deeply than most (thanks Don for correctling me :P- I thought I was treating my battery fairly benignly) and if I crank the Amps up to what is recommended I can taste electrolyte in the air. A taste familiar to me from doing chemistry and the fume hoods  having been installed by van Riebeek's carpenters.

So I operate at about 13% and my battery bank is now just over 600 days old. It is starting to show its age. I must be mindful that at 30oC my battery bank ages twice as fast as those at 25oC.

What I really like is you battery suppliers recommendations. It is questions we get asked all the time and so for people that are browsing through this thread I am going to post the relevant section. 

58ba54ce5d788_Screenshot(35).png.f3329140a36c2cfdff00da0e1b4f016d.png

 

 

Link to comment
Share on other sites

Thanks, Chris

If one reads Sinetech's intro at http://www.sinetech.co.za/battery-care.shtml it says that bulk charging takes place up to 85% of battery capacity. Wouldn't it then be correct to surmise that at anything below 85% one needs a minimum charging current of around 25% of capacity?

I'll appreciate some thoughts that I can run past Sinetech when I (hopefully) go and collect my replacement 150Ah battery.  They were quite adamant about having sufficient panel array capacity to supply the minimum charge current.  Will revert after chatting to them.

Link to comment
Share on other sites

The Aussie, Dockarl, was adamant that one could open the taps so to speak (within reason) on GEL and AGM batteries and that the battery bank would accept the charge and would soon be in absorb and current is naturally limited due to the increased resistance of the battery bank.

Keep in mind one needs to be aware of gassing voltage which is about 2.39 V at 25oC and 2.36V at 30oC. Depending on the valve construction the back pressure is anything from 80 -200 mbar. Some VRLA have recombinant technology so very little gas escapes. By limiting charging rates one allows time for gas recombination. Sulphation on occurs below 2.07V so under normal circumstances a battery that is being cycled regularly or is on float should have no problems.

Ask the chaps at Sinetech why they are adamant for enough current for the minimum charge current. It cannot be to stir the electrolyte:D.

Link to comment
Share on other sites

Adding to what Chris said, my 2 cents, I have as other have too, come across where the battery supplier does not understand their own instructions given to their clients as they are merely passing on standard instructions given to them by someone who does not fully understand the instructions themselves.

Not saying this is the case here, just saying that you need to ask the right questions and if they cannot answer them with gusto based on in-depth knowledge of their product, then you can teach them.

Like my supplier. You can teach them nothing on Trojan batteries, but the moment you ask them solar / battery related questions, and they do not have the in-depth knowledge, they pass that onto a person who understands the question and the clients needs. Once that is resolved, they then sell the right battery with no doubts left.

Link to comment
Share on other sites

Went to Sinetech yesterday to collect my replacement 150Ah battery (thank you!) Sat down with Karl.  He's their senior technical sales manager and really seems to know his stuff - been doing batteries for about 20 years.

He remains adamant that one needs an intial charge current of around 25% of capacity for AGM Gel batteries, irrespective of the depth of discharge of the battery.  His words were "amperage is key".  He said that if there is not enough current, the impedence (or resistance) of the battery increases which leads to negative chemical processes in the AGM Gel batteries that he explained but that I didn't quite follow.  I mentioned that my three stage charger only does bulk charge up to about 12.5V on the Omnipowers, and then goes into absorption stage with a reduced current.  Karl said that even if the battery is at 12.8V, you still need the high initial charge current.

Karl's suggestion was that with my very limited base load of 1500Wh per 24h day (energy efficient fridges and LED lighting only) I would be much better off if I removed one of the two strings of 150Ah batteries and add 2-3 more panels to my current 1500W array for more current.  My depth of discharge would go up from 10% on two strings to 20% on one string, However, he says this would have a much smaller impact on these batteries' longevity than the shortage of current that I have at present.  I'm not sure how to calculate the amps currently produced by the panels (6 x 250 Watt Enersols, 2 strings of 3 each).  However, Karl says they do not even provide enough current for a single string of 150Ah batteries (36A required), let alone two strings (72A required). He says this will seriously reduce the lifespan of the batteries.

I would appreciate any thoughts.

Link to comment
Share on other sites

In comes Victron with their Gel and AGM Battery manual attached.  

15. Charge current
The charge current should preferably not exceed 0,2C (20A for a 100Ah battery).The temperature of a battery will increase by
more than 10°C if the charge current exceeds 0,2C. Therefore temperature compensation is required if the charge current exceeds
0,2C

Their maximum charging current for AGM Gel is thus 20%. They don't specify a minimum, as opposed to Omnipower's stated minimum of 25%.

Victron-GEL-and-AGM-Batteries-Manual.pdf

Link to comment
Share on other sites

2 hours ago, Bush Man said:

I'm not sure how to calculate the amps currently produced by the panels (6 x 250 Watt Enersols, 2 strings of 3 each).

@Bush Man, if I look at your panel spec sheet, your panels each do 30.8 volts at 8.1 amps.

 

(Connecting batteries in series, the volts go up (add the voltage of each battery), but the amps remain constant.)

(Connecting batteries in parallel, the volts stay constant, but the amps go up (add the amps of each string)).

 

Therefore 3 x panels in series = 92.4 volts at 8.1 amps.

Adding 2 strings in parallel = 92.4 volts at 16.2 amps.

 

Therefore if you double the panels you currently have with the same configuration, the volts would stay the same at 92.4 volts, but the amps would go up to 34.4 amps. 

 

Link to comment
Share on other sites

16 hours ago, Bush Man said:

I mentioned that my three stage charger only does bulk charge up to about 12.5V on the Omnipowers, and then goes into absorption stage with a reduced current.  Karl said that even if the battery is at 12.8V, you still need the high initial charge current.

12.5V x 4 = 50V - That is very low. The requirement for deep cell operation is 14.4V x 4 = 57.6V

16 hours ago, Bush Man said:

Karl says they do not even provide enough current for a single string of 150Ah batteries (36A required), let alone two strings (72A required). He says this will seriously reduce the lifespan of the batteries.

I tend to agree with Karl. As an example - You have a kettle with a 2000 watt element. You want to make coffee and switch on the kettle, but only have 1000 watt power available. Therefore you only get the water temperature up to 50 degrees Celsius, even if you run the kettle the whole day. You thus cannot make proper coffee. You need to increase the kettle element power to 2000 watt to be able to boil the water and make proper coffee.

On 3/3/2017 at 10:17 AM, Bush Man said:

My 1500W panel array would be more than sufficient for that, providing 7500Wh per day less some losses.

7500/10 hours of sun = 750 watt on ave. Your load is 300 watt. Available to charge batteries = 450 watt. 450/48 = 9.4 amps available to charge batteries.

You have 2 strings of 150 Ah = 300 Ah.

On 3/3/2017 at 10:17 AM, Bush Man said:

According to the staff at Sinetech and the Omnipower Care Sheet attached the Omnipower 150Ah (and other gel) batteries require an initial charge current of 36A, or 72A for two strings.

- You do not have sufficient number of panels to properly charge those 2 strings of batteries. You are basically trickle charging them.

Link to comment
Share on other sites

16 hours ago, Don said:

Therefore 3 x panels in series = 92.4 volts at 8.1 amps.

Adding 2 strings in parallel = 92.4 volts at 16.2 amps.

Thanks for all the input, Don!

Am I correct then in my understanding that the configuration of the panels is also critical in determining the Amperes produced?  In this example, I presently have two strings of three panels together producing 16.2 Amps.  If I reconfigured to three strings of two panels each I would have 8.1 x 3 = 24.3 Amps?  The Volts would then drop to 30.83V x 2 = 61.66V.  This is still within the 5kva 48V Axpert's range of 60-115V.

If I were to add more panels, would one then rather look at stringing them two together rather than three?

Link to comment
Share on other sites

1 hour ago, Bush Man said:

Volts would then drop to 30.83V x 2 = 61.66V.  This is still within the 5kva 48V Axpert's range of 60-115V.

That is correct. The volts however would be right at the bottom edge of the operating volt for the Axperts and not sure if they would operate properly. If you have some clouds, the voltage might drop below 60V and the inverter might just disconnect you PV altogether. I would say 3 panels is the minimum you would require in series. I would suggest to get another 3 panels minimum and add them in parallel to your existing panels to get the amps up to 24 amps.

Link to comment
Share on other sites

3 hours ago, Don said:

7500/10 hours of sun = 750 watt on ave. Your load is 300 watt. Available to charge batteries = 450 watt. 450/48 = 9.4 amps available to charge batteries.

Thanks again, Don!

Would the following calculation not also be correct to determine amps available for charging during sunlight hours?

Total load in a 24 hour day: 1500 watts

Average load per hour: 1500w/24h = 62.5W

Say 80W, as the fridges would be running more during daytime.

Now take the present 1500W panel array less losses = 1300W

Deduct the 80W base load, to arrive at 1220W available for charging.

Now calculate Amps: 1220W/48V = 25.4 Amps available for charging

Could one then take this figure further and say that 25.4Amps/300Ah of batteries translates to charging at 8.5% of capacity?  According to Karl 10-13% of capacity would be the norm for flooded lead acid batteries, as opposed to the stated minimum of 25% for AGM Gels.

 

Link to comment
Share on other sites

3 hours ago, Bush Man said:

Total load in a 24 hour day: 1500 watts

Watts is an instantaneous reading, not a reading over time. Watts over a period of time = kWh. You cannot divide watt by hours as you have done above. You can only divide watt by amps to give you volts or watt divide by volts to give you amps. 

3 hours ago, Bush Man said:

Now take the present 1500W panel array less losses = 1300W

Deduct the 80W base load, to arrive at 1220W available for charging.

Now calculate Amps: 1220W/48V = 25.4 Amps available for charging

That is correct. So for one moment in time when your panels do exactly 1500W, that would be true. That would also be the maximum and for a short period of time.

Remember, you start off with 0W at say 6h00. At 12h00 you may do 1500W on your 1500W panels if you are lucky. Then your solar production will start to drop down all the way to 0W at 18h00 again. 

The best would be to have a look at your battery charging amps graph for the day. That would tell you exactly what you are doing during the day.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...