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Battery charging voltage


Noobie
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I am sure that this has been discussed in depth before, but here you go anyway...

Need some advice after commissioning my Axpert 3k 24v inverter.

I have 2 x OmniPower 260aH batteries in series and wanted to make sure I had set the inverters charging voltages correctly.

The batteries spec is:

Cycle: 14,4 - 14,9V

Standby: 13,6 - 13,8V

I have set the battery type to user defined in program 5

Program 12 (Setting voltage point back to utility source) I am not sure what voltage to set this to, I would like it to be at 50% DOD?

Program 13 (Setting voltage point back to battery mode) Again, not sure what to set it to here?

Program 26 (Bulk charging voltage) I was thinking I should set this to 29,3V

Program 27 (Floating charge voltage) I was thinking I should set this to 27,4V

Program 29 (Low DC cut off voltage) I was thinking I should set this to 24V

 

I am worried that if I dont set it up correctly the batteries will not last as long

I would really appreciate some advice here

 

 

 

 

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Hi Noobie

 

You are going to have to in a week or two change your forum alias as you definitely will not be a newbie any more  :D.

 

This is my second attempt at replying to your post. The first was an eloquent, informative masterpiece I deleted accidentally trying to include some useful references. So here goes again I hope I manage to remember all the information I had included in the first version.

 

Your Omnipower batteries are very similar to the CB Solar batteries I have. Just looking at their size and shape I suspect that they are made in the same facility with the Omnipower batteries being manufactured to a higher standard than the CB Solar as there is a big difference in DOD versus no of cycles.

 

Since they are sealed batteries your major concern is not to overcharge them and boil away electrolyte. It cannot be replaced (well not in a conventional manner anyway) and once the battery runs out of electrolyte it is dead. 

 

Thus I would recommend to keep your bulk charging at the bottom end of the battery spec to keep below or close to gassing voltage. The battery does have the ability to store some gas but I do not know the quantity or up to what pressure before the gas bleeds off via the one-way valves.

For reference here is a table of gassing voltage and from my records I have seen that my battery temps are usually 2-3 degrees warmer than ambient temperature.

 

post-822-0-32622900-1453054611_thumb.png

 

The spec for float is 27.0V for 20 and 25oC (http://www.sinetech.co.za/assets/img/omnipower-ops/OmniPower%20Battery.pdf).

 

Determination of SOC from battery voltage is an educated guess at best so the setting of program 12 and 13 is going to be determined by your needs and a little bit of fiddling to gain experience. You may find that DOD of 50% is too low  and if you could be at 20-30% DOD you batteries are going to last a lot longer. 

 

Here is a table of SOC versus rested battery voltage which is not accurate in a solar inverter system as the batteries are invariably not in a rested state.

 

 post-822-0-89842100-1453054664_thumb.png   

  

 

Plonky was doing some calculations an at current electricity tariffs it would seem not cost effective to discharge ones batteries as the reduced cycle life was not offset by the power saving.

 

Program 29 should remain at the default of 21V for the simple reason that the upper range of this value is still too low to protect your battery and changing the default makes the Axpert SOC reading wildly inaccurate. It is inaccurate anyway but at least there is some semblance of accuracy at 21V.

 

I am going to post this and then look for other useful references so as not to have a repeat performance.

 

Edit : Aargh no! My lekker little tables have been turned to horse$h*t!

 

Edit: Fixed.

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I have a 24V system. My batteries are probably similar to yours, I have two Victron 200Ah AGM batteries in there at the moment. I got them secondhand from a UPS, where they spent their whole life floating, so they are good as new as far as I'm concerned :-)

 

I have my absorption set to 28.8V, which is on the lower end of your range. Float is set to 27.6.

 

Regarding a voltage to go back to grid at: I have an assistant setup on my Victron that does pretty much the same thing. I have mine set to 24.7V at the moment. Now what happens is that at my typical load, it reaches that voltage at around 9PM (5AM in the morning when I'm not here) and then when the load is taken off the batteries they bounce back to 25.2V (12.6V per battery), which is 85% SoC or thereabouts. If I set it to 24.3V, then it goes down to 55%-60% SoC. How do I know? Well, either you use a BMV monitor which will tell you, or the next day you tell the inverter to stay on the grid, and you look at the kwh counter on the MPPT charge controller (which is separate on mine). When I do this, I see around 2kwh coming back in the next day, which is around 50% of my storage capacity.

 

So you'll have to experiment a bit, but I expect a value between 24.3V and 24.7V will do it for you.

 

Cost of storage. Well, work out lifetime kwh by multiplying the Ah capacity by the nominal voltage by the DoD by the cycle life, so for example on these Victron AGMs:

200 * 24 * 0.5 * 900 = 2160 000wh = 2160kwh over its lifetime.

 

Now divide the cost of the battery by that number. Now I got these batteries for 3k, so my storage cost is R1.40 per kwh. I'm going to kill the batteries in two years, but that's fine :-)

 

If the cost of storage is more than R2 per kwh, then never cycle the batteries, go back to grid at 90% SoC or earlier. This you can correlate with your electricity cost. At the moment in Cape Town, on the Domestic tariff, electricity above 600kwh costs R2.13 per unit, and battery storage make sense if you have the right batteries, but for most people, at R1.86 per kwh (on the lower tariff) batteries don't make sense yet.

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Thanks Chris,

 

Your post has helped me out but at the same time got me asking more questions :D

 

I have programmed the inverter as follows:

Program 26 (Bulk charging voltage) 28,8V - as suggested, its at the lower end of the spectrum but hopefully this will prevent gassing.

Program 27 (Floating charge voltage) 27V - as suggested according to the OmniPower spec sheet at 20-25 degrees Celsius.

Program 12 (Setting voltage point back to utility source) I have set this to 24,5V, which I believe is roughly a 50% DOD. Am I correct in assuming that this means that when the battery voltage drops below 24,5V it will switch back to Eskom supply to charge the battery and supply power to the connected load?

Program 13 (Setting voltage point back to battery mode) This is set to 27V, which was the default settingAm I correct in assuming this means that the inverter will only switch back to battery supply once the battery voltage gets to 27V?

Program 29 (Low DC cut off voltage) Now this is a tricky one... :wacko: If I'm correct this setting will tell the inverter when to disconnect the batteries to prevent discharging them to 0% DOD. If I leave the setting at default of 21V this means it will only disconnect the batteries when they are at 0% DOD?

Surely I should set this to 24V which is about 40% DOD? I was thinking that because I  have a BMV 702 monitoring the batteries, I wouldn't rely on the Axperts SOC reading, or am I on the wrong path here?

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I have a 24V system. My batteries are probably similar to yours, I have two Victron 200Ah AGM batteries in there at the moment. I got them secondhand from a UPS, where they spent their whole life floating, so they are good as new as far as I'm concerned :-)

 

I have my absorption set to 28.8V, which is on the lower end of your range. Float is set to 27.6.

 

Regarding a voltage to go back to grid at: I have an assistant setup on my Victron that does pretty much the same thing. I have mine set to 24.7V at the moment. Now what happens is that at my typical load, it reaches that voltage at around 9PM (5AM in the morning when I'm not here) and then when the load is taken off the batteries they bounce back to 25.2V (12.6V per battery), which is 85% SoC or thereabouts. If I set it to 24.3V, then it goes down to 55%-60% SoC. How do I know? Well, either you use a BMV monitor which will tell you, or the next day you tell the inverter to stay on the grid, and you look at the kwh counter on the MPPT charge controller (which is separate on mine). When I do this, I see around 2kwh coming back in the next day, which is around 50% of my storage capacity.

 

So you'll have to experiment a bit, but I expect a value between 24.3V and 24.7V will do it for you.

 

Cost of storage. Well, work out lifetime kwh by multiplying the Ah capacity by the nominal voltage by the DoD by the cycle life, so for example on these Victron AGMs:

200 * 24 * 0.5 * 900 = 2160 000wh = 2160kwh over its lifetime.

 

Now divide the cost of the battery by that number. Now I got these batteries for 3k, so my storage cost is R1.40 per kwh. I'm going to kill the batteries in two years, but that's fine :-)

 

If the cost of storage is more than R2 per kwh, then never cycle the batteries, go back to grid at 90% SoC or earlier. This you can correlate with your electricity cost. At the moment in Cape Town, on the Domestic tariff, electricity above 600kwh costs R2.13 per unit, and battery storage make sense if you have the right batteries, but for most people, at R1.86 per kwh (on the lower tariff) batteries don't make sense yet.

Thanks Plonky,

 

Just saw your reply now!

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Program 29 (Low DC cut off voltage) Now this is a tricky one... :wacko: If I'm correct this setting will tell the inverter when to disconnect the batteries to prevent discharging them to 0% DOD. If I leave the setting at default of 21V this means it will only disconnect the batteries when they are at 0% DOD?

Surely I should set this to 24V which is about 40% DOD? I was thinking that because I  have a BMV 702 monitoring the batteries, I wouldn't rely on the Axperts SOC reading, or am I on the wrong path here?

I like your settings but read more and improve your understanding especially the replacement costs versus grid usage at night. It may in the long run be more cost effective to run of Eskom at night. I am off grid so I have not really looked at it from that angle. Plonky has given you very good advice based on his experience.

 

 

Since you have a 702  you could use the VIctron relay to activate a contactor using  24V battery power (fused). The contactor should then disconnect the load from you inverter in the event of your battery SOC dipping below a pre-determined threshold. Since you do have a 702 and therefore have a accurate SOC you can set program 29 to whatever voltage you want to disconnect at, bearing in mind that under a large draw battery voltage can drop quickly and not be indicative of SOC. Remove the large load and battery voltage is restored even without the battery being charged.

 

I never understand why Voltronic do not Coulomb count (Amps in versus Amps out). Yes it is problematic but with a bit of software tweeking they could have a system that was nearly accurate and could be programmed to synchronise under set conditions to keep it accurate.

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Some useful references as promised.

 

Charging VRLA Batteries.pdf

 

The Truth About Batteries - POWERTHRU White Paper.pdf

 

and one more which I am not permitted to upload I see if I can change it to a pdf

 

Edit managed to convert it to pdf. Sorry about the blank first page but cannot seem to get it to format properly.

 

Battery Types and Comparisons.pdf

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  • 3 years later...
  • 1 year later...

Hi guys, I have 4 200amp gel batteries. What is the best setting for these to be on bearing in mind I have a 5kva 48v easy power inverter and I’m referring to option 12  and 13 which is the setting voltage point back to utility source when selecting SBU

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10 hours ago, Help me said:

Hi guys, I have 4 200amp gel batteries. What is the best setting for these to be on bearing in mind I have a 5kva 48v easy power inverter and I’m referring to option 12  and 13 which is the setting voltage point back to utility source when selecting SBU

Ask your supplier for the data sheet for the batteries and use those settings..  

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