Battery Charging terms explanation

[steveh57]

Just been reading a statement regarding the charging of a TN 110 Ah Lithium Battery and trying to get a grip of the terminology involved.

"Electrical Characteristics"
Nominal Voltage 12.8v

Nominal Capacity 110Ah@ 0.2C

" Charging mode - At 0℃~45℃ temperature, charged to 14.6V at a constant current of 0.2C5A, and
then,changed continuously with constant voltage of 14.6V until the current was not
more than 0.02C5A"

" Charging Current - 21.6 Amps"

"Max, Charging Current - 54 Amps"


Ok for arguments sake lets use 100Ah to make the sums easy , the nominal capacity and the term 0.2C5A are what I need an understanding of please?

 

[eddie]

http://web.mit.edu/evt/summary_battery_specifications.pdf

That will help you

 

[steveh57]

So if we use a 100Ah battery is the nominal capacity @0.2 C 20A for 5 hours? The term 0.2C5A - does that mean charge at 20 Amps for 5 hours????

 

[funflair]

So if we use a 100Ah battery is the nominal capacity @0.2 C 20A for 5 hours? The term 0.2C5A - does that mean charge at 20 Amps for 5 hours????

The nominal capacity in ah is measured at a given discharge rate usually 0.2C or C5 or 100/5 so 20ah, the slower the discharge the higher capacity they can quote so C20 would be higher nominal capacity for the same battery, and it's all to do with Peukerts law.

And they might also quote a C rate for charge current as in your original post so for your 100ah example charging at C5 would be 100/5 so yes you are right 20amps

 

[Raul]

I have come across this terminology before, not many use it. My understanding is you charge with 0,2C until current tails off to 5A at 14,6v and then float at 14,6v with no more than 0,02C.
They mingled up some data in the sheet as well, the legend colours on the graph is not right.
I will take this with a pinch of salt, any LFP can absorb until 2% tail off current then stop charging, no float.

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[autorouter]

Ok for arguments sake lets use 100Ah to make the sums easy , the nominal capacity and the term 0.2C5A are what I need an understanding of please?

C is the battery capacity in amp-hours (Ah). If you discharge a battery at a low rate over a longer time you get more capacity than discharging at a higher rate over a shorter time. So there are different C values.

C20 is the usual measure, when the battery is discharged over 20 hours. Other common rates are C5 (5 hours) and C100 (100 hours). So if you're a manufacturer giving recommended charging values, you have to be specific about which rate you are talking about.

0.2C5 is 0.2 x the C5 rate. If the battery C5 capacity is 100Ah, 0.2C5 is 20 amps.

" Charging mode - At 0℃~45℃ temperature, charged to 14.6V at a constant current of 0.2C5A, and
then,changed continuously with constant voltage of 14.6V until the current was not
more than 0.02C5A"

Start with a constant current of 0.2C5 (20A for a 100Ah battery). The voltage will start low, and gradually rise as the battery charges, until it reaches 14.6V.

At that point, switch to constant voltage of 14.6V, and the current will gradually drop as the charging nears completion.

When the current has dropped to 0.02C5 (2A for a 100Ah battery) the charging is complete, and the charger should be turned off.

 

[steveh57]

Thanks everyone

 

[steveh57]

Just thinking (dangerous I know) that I could modify the Victron MPPT settings for Lithium and enable more use of the available solar energy. Unlike Lead acid a Lithium battery doesn't mind not being fully charged and indeed it may be beneficial long term At present I've got an absorption setting of 14.6V, 30 mins max, 2% tail and a float of 13.5V . As I understand the battery voltage then has to fall to 13.4 (bulk offset 0.1v) before any solar power is utilised.

Accepting that the battery may never be fully charged, are there any disadvantages ( provided there is solar power available) of dropping the absorption voltage to 13.5 and reducing the bulk offset to say 0.01V. My logic states at this point with a draw on the battery some or all will be provided by Solar.

Also will this screw my smart shunt readings up as I wondering how you workout out the SOC with absorption and float at the same voltage

 

[Raul]

For the starter your float is a tad high. But if you have consumption al the time is OK. I had 13,8v absorb with 30mins and 4a tail current. Was not enough for my usage. To bring it u to 97-98% I used absorbed to 14v 40mins and 4a tail current for a 400ah bank. Float regardless of usage at 3,35vpc or 13,4v. Any higher you are charging not floating. To sustain the load, it does not need to drop in bulk, it can happily hold in float constant voltage and supply power to loads almost the full rated output of charger.

 

[autorouter]

Also will this screw my smart shunt readings up as I wondering how you workout out the SOC with absorption and float at the same voltage

No, the voltage levels are irrelevant for this type of SOC meter. Basically it counts the electrons flowing in and out of the battery, that are causing the chemical reaction inside the battery. It does this by reading the flow rate, ie amps, at regular intervals, and calculating the charge, minute by minute. I could tell you the number of electrons in one amp-hour of charge, but it's a bit large, so we normally just count the amp-hours.

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[steveh57]

Thanks , I'm using about 9-11W in storage to power the Raspberry 3+B, 2 serial convertors, USB GPS etc. So does that class as continuous consumption. However it seems sensible for me to drop the float to 13.4. The 14.6V Absorption I got off the spec sheet which copied into my first post- is that worth bringing down a bit as well, having read Raul comment regarding usage?

 

[Raul]

If in storage, absorb 13,6v and float 13,4v in my case, but I have a constant drain of at least 20w I think. That’s rpi , 3 serial converters, camera and router. If you don’t have high drain, reduce absorb time to 5mins.
The only think will interfere with the shunt is charged voltage setting versus absorb setting on the chargers. This will determine when to reset the clock to 0. I would not worry about it as long as the charged voltage in the shunt is set high enough to prevent synchronisation .

 

[steveh57]

Done as suggested,whilst in storage. I'll keep an eye on the smart shunt output.

 

[steveh57]

I had the charged voltage set too low , seems for lithium 13.8/13.9 are possibly better values?

 

[andy63]

and it's all to do with Peukerts law.

Hi Martin... you mentioned it ...but just to be clear...peukets law doesn't as far as I know apply to lithium ion technology..not found a great deal on the subject and someone may be able to tell me otherwise
Andy

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[funflair]

Hi Martin... you mentioned it ...but just to be clear...peukets law doesn't as far as I know apply to lithium ion technology..not found a great deal on the subject and someone may be able to tell me otherwise
Andy

I don’t know either way for sure Andy but what you say sounds right from my little understanding of Lithium as a power source.

 

[andy63]

my little understanding of Lithium as a power source.

Well you have enough of it...Time you were on the case
Andy

 

[Raul]

Hi Martin... you mentioned it ...but just to be clear...peukets law doesn't as far as I know apply to lithium ion technology..not found a great deal on the subject and someone may be able to tell me otherwise
Andy

It does apply, but not the same values like lead or other chemistries. The worst peukert effect is on nickel batteries whit efficiency as low as 60-70%. The LiFePo4 is about 97% with a peukert of 1,03. That if you keep charge-discharge to max 0,3C. At 1C peukert will be more. It’s not linear. Even Lipo, and Lico has a degree of peukert but tiny compared to lifepo4. There is no battery that has 100% efficiency on round trip, and the more you raise the rate the worse it gets.

 

[andy63]

There is no battery that has 100% efficiency on round trip, and the more you raise the rate the worse it gets.

Thanks Raul.. I understand the above .. just the little reading I'd done suggested that with lithium technology been so efficient and the rates of discharge having what I thought was a negligible effect on capacity even at higher rates , peuketes theory was largely discounted especially on newer batteries..
Ta..
Andy

 

[Pausim]

For LiFePO4 Victron suggest a Peukerts exponent of 1.05(1.25), a charge efficiency of 99%(95%) and a temperature coefficient of 0.5%(1%) - figures in brackets for lead acid. All of which make LiFePO4 much better at giving back the Ah you put in. Cold lead acid batteries with a high rate of discharge are particularly poor performers.

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