Battery Setup BMV

[PaulF007]

As batteries are been discussed nowadays here another question to figure.

If you have a battery with a 150ah c10 rating and you have 8 of them would you then , on a 48v system , set your BMV to 300 ah. 
I have noticed that some batteries have a c20 rating how would that measure against a c10 rating?

Regards

Paul

[Chris Hobson]

Ok let's make sure we understand exactly what a C rating is. The C rating as defined by de Coulomb is the continuous current draw that the cell (battery) will maintain. So a C1 rating is the number of  Amps delivered during a 1 hour discharge, where the battery is completely discharged (1.75V per cell). A 2 hour discharge rate (written as C/2 or 0.5C) is the continuous draw over two hours and so on. One might imagine that this relationship is linear but is not due to Peukert's effect it is not constant. Discharge efficiency is related to the discharge rate (high discharges have high inefficiencies) and intrinsic losses. AGM and Gel batteries achieve better efficiencies than flooded LAs.

Plonky eloquently explained a difficult concept here so I am not going to repeat it. 

 

So if a battery does not have a C/20 (0.05C) rating it was just manufacturer was lazy. There should not be more than 1or 2 Ah difference between a 0.1C rating and a 0.05C rating.

We use the same discharge rates to describe charging rates. Makes sense.

[PaulF007]

Yes thank you @Chris Hobson

Also , after looking at the speck sheet you can derive c20 from it as well 

So as I understand it 

C10 will be the amount of continuous current you can apply in 10 hours time until a cell gets , in my case , to 1.80v
So the table above shows you

C10 = 17.2 a x 10 hours = 170 ah bank

and 

C20 = 9.14 a x 20 hours = 180 ah bank

From what I could understand , from the BMV manual , is that you use C20 when you setup the BMV unless you are going to put loads , again in my case , higher than 15 a continuous  then you should use the C10 rating that is 170 ah.

That correct? 

[Chris Hobson]

2 hours ago, PaulF007 said:

From what I could understand , from the BMV manual , is that you use C20 when you setup the BMV unless you are going to put loads , again in my case , higher than 15 a continuous  then you should use the C10 rating that is 170 ah.

That correct? 

Hi Paul your C/10 C/20 calcs are spot on. I don't remember the C/10  C/20 recommendation. I have briefly looked at the manual but cannot find it (it is Friday  and I apologise if my brief look is/was too brief). Very few of us are going to have continuous high (in your case 15A) loads.

[SilverNodashi]

On 5/5/2017 at 8:01 PM, PaulF007 said:

Yes thank you @Chris Hobson

Also , after looking at the speck sheet you can derive c20 from it as well 

So as I understand it 

C10 will be the amount of continuous current you can apply in 10 hours time until a cell gets , in my case , to 1.80v
So the table above shows you

C10 = 17.2 a x 10 hours = 170 ah bank

and 

C20 = 9.14 a x 20 hours = 180 ah bank

From what I could understand , from the BMV manual , is that you use C20 when you setup the BMV unless you are going to put loads , again in my case , higher than 15 a continuous  then you should use the C10 rating that is 170 ah.

That correct? 

As an interesting by-note, Omnipower used to have a 150Ah battery a few years ago. Now they have a 200Ah battery. The interesting part is: both are the same dimensions + weight which either means they lie with their marketing, or their technology improved. Why is this of relevance? I often feel that we're being lied to about the real figures of these batteries. 

[pilotfish]

22 minutes ago, SilverNodashi said:

Omnipower used to have a 150Ah battery a few years ago. Now they have a 200Ah battery

The previous OPS range was 60/120/180/260 at C100 and 50/100/150/200 at C10

The current OPR range is 60/120/180/240 at C100 and 50/100/150/200 at C10

The anomaly was the previous 260Ah rating at C100 which I believe was an error that they couldn't simply change for business reasons, hence re-branding and correction to 240Ah at C100 in the new range.

There are some other minor construction changes which results in very small mass changes, but nothing worth mentioning.

[SilverNodashi]

2 hours ago, pilotfish said:

The previous OPS range was 60/120/180/260 at C100 and 50/100/150/200 at C10

The current OPR range is 60/120/180/240 at C100 and 50/100/150/200 at C10

The anomaly was the previous 260Ah rating at C100 which I believe was an error that they couldn't simply change for business reasons, hence re-branding and correction to 240Ah at C100 in the new range.

There are some other minor construction changes which results in very small mass changes, but nothing worth mentioning.

I guess the set I saw was mislabeled then. It's definitely labeled as 150A

[pilotfish]

I have no doubt that this has been asked and answered but I cant find the answer anywhere...

My battery bank has a C10 rating of 490Ah and C20 rating of 539Ah (from Peurkert) - which one should I using when setting up Battery Capacity in the BMV-702?

 

[Chris Hobson]

6 hours ago, pilotfish said:

I have no doubt that this has been asked and answered but I cant find the answer anywhere...

My battery bank has a C10 rating of 490Ah and C20 rating of 539Ah (from Peurkert) - which one should I using when setting up Battery Capacity in the BMV-702?

 

490Ah Is the standard Ah rating of those batteries. In smaller battery banks the discharge under C20 conditions does not approximate operating conditions. My 260Ah bank C20 Watt rating is somewhere between 620 and 650W. Shave off 50W for the inverter and you are somewhere between my active night time use and my overnight base load. So my usage is more in line with C10. In larger vented battery systems where the user can measure the specific gravity (SG) of cells many folk notice a disparity between SOC based on SG reading and those based on a battery monitor. This is easy to explain in that their usage pattern is more inline with C20 discharge rate. 

I am sure the reported cycle life is based on a C10 DOD. Thus I would stick with the standard knowing that you have some spare capacity because you have a large bank your usage is more likely to mimic a C20 discharge. When your batteries start failing you can compare your C10 usage against the rated lifespan if Alzheimer's  has not already kicked in by then. We eagerly await  your report back in 2030. 

[pilotfish]

Oldtimers has kicked in already... never mind "by then"!

Thanks for the feedback, I expect my usage to be more in line with C20 so I will use that capacity for now and change it if new info is obtained.

I just think that if the BMV applies the Peurkert formula to calculate the correct SOC then it should be off a known base - so they really should specify which rating to use when entering the battery capacity

[TheOracle]

6 hours ago, pilotfish said:

Oldtimers has kicked in already... never mind "by then"!

Thanks for the feedback, I expect my usage to be more in line with C20 so I will use that capacity for now and change it if new info is obtained.

I just think that if the BMV applies the Peurkert formula to calculate the correct SOC then it should be off a known base - so they really should specify which rating to use when entering the battery capacity

That is also how I understand it, my manual say you must preferably use the 20 hour capacity rating. 

[pilotfish]

On 9/2/2017 at 5:05 PM, TheOracle said:

That is also how I understand it, my manual say you must preferably use the 20 hour capacity rating. 

Manual...what manual?

I must admit that I went straight to "Full Setup Details" on page 17 and dived in from there, but now I have perused the manual at a bit more of a leisurely pace and I still don't see anywhere that it says "enter your battery capacity at C20" or something similar.

Anyway it seems to be working great in conjunction with ICC so I am very happy despite my bitching!

[Mark]

Section 5.1 mentions the "normal rate" being c20 when calculating the Peukert exponent.  I think this is where the reference to c20 comes from.

 

Interesting link here: http://forum.solar-electric.com/discussion/comment/233688#Comment_233688